Topics in Offshore Oil Production Optimization using Real-Time Data

Transcription

1 Hans Petter Beker Topcs n Offshore Ol Producton Optmzaton usng Real-Tme Data Thess for the degree of doktor ngenør Trondhem, June 2007 Norwegan Unversty of Scence and Technology Faculty of Informaton Technology, Mathematcs and Electrcal Engneerng Department of Engneerng Cybernetcs

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3 Abstract In all producton systems, producton optmzaton s mportant because t can reduce the cost of operaton and ncrease the producton. Ths thess s a contrbuton wthn the feld of producton optmzaton of offshore ol producton systems usng measured real-tme data. Four novel methods related to producton optmzaton of such ol producton systems have been proposed. Usng measured data, they are contrbutng to maxmze the total ol producton rate or the expected total ol producton rate of the ol producton system. Frst, a method optmzng the total ol producton rate from subsea wells where a model of the pressure nterconnecton of a common flow lne must be ncluded s proposed. The method uses a pecewse lnear approxmaton of the pressure drop n the flow lnes and wells enablng global optmzaton usng a branch and bound mxed nteger lnear programmng solver. Second, a method for optmzng the expected total ol producton rate by selectng wells for testng s proposed, usng real-tme data. The well testng gves nformaton on the gas ol ratos or the water cuts that s more accurate allowng an mproved prortzaton of the wells compared to the ndustry practce when a processng constrant s avalable. A method for calculatng stochastc dstrbutons of the gas ol ratos or water cuts s proposed. Thrd, a method handlng the uncertantes n the gas ol ratos or water cuts explctly for prortzng the wells when a processng constrant s avalable s proposed. The prortzaton was found to depend on the probablty dstrbuton of the gas ol ratos or water cuts, ol potental of

4 each well, and processng capacty. The method s able to handle all these uncertantes explctly by usng a user-provded probablty dstrbuton for each of them. Fourth, a method fndng the optmal sequence to open the wells when a lmted flow change rate nto the producton separator and from each well s requred s proposed. The method may be used to fnd a ramp-up sequence after a shutdown. The excess treatment capacty s updated usng the measurements of the treatment utlzaton n each tme step, allowng the treatment capacty to be fully utlzed.

5 Acknowledgments Frst, I would lke to thank my supervsors Professor Dr Ing Tor Arne Johansen and Dr Ing Olav Slupphaug. Tor Arne has been an nvaluable resource suggestng new ways of solvng the challenges I studed. I apprecate hs constructve commentng of my manuscrpts. Olav has been the source of most of the ndustral challenges studed n ths work. He has gven me valuable and requred background nformaton on the operaton of ol producton systems and challenges n producton optmzaton. Hs ndefatgablty commentng of my manuscrpts has certanly mproved the qualty of them. Wthout my supervsors, the thess would not be possble. The Research Councl of Norway, Norsk Hydro ASA, and ABB AS are acknowledged for fnancng ths work. In partcular, I would lke to thank ABB AS for provdng an nsprng workng envronment. It has been a source of many of the challenges nvestgated n ths thess. Several of the other professonals at ABB AS have been suggestng nterestng challenges to study, and ther help s also much apprecated. Hans Petter Beker Oslo, June 2007

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7 Table of Contents Abstract... Acknowledgments... Table of Contents... v 1 Introducton Offshore Ol Producton System Reservor Well Gatherng Network Processng Facltes Motvaton Summary and Contrbutons of Papers Paper I: Real-Tme Optmzaton of Ol and Gas Producton Systems: A Technology Survey Paper II: Global Optmzaton of Multphase Flow Networks n Ol and Gas Producton Systems Paper III: Optmal Well-Testng Strategy for Producton Optmzaton: A Monte Carlo Smulaton Approach Paper IV: Well Management under Uncertan Gas or Water Ol Ratos Paper V: Optmal Start-up Schedulng of Producton Wells Real-Tme Optmzaton of Ol and Gas Producton Systems: A Technology Survey Introducton Informaton Flow n Producton Optmzaton Data Acquston Control Producton Plannng v

8 2.2.4 Operator Strategc Plannng Reservor Plannng Well Model Updatng Processng Faclty Model Updatng Reservor Model Updatng Technology and Reference Cases Global Versus Local Optmzaton Producton Plannng Reservor Plannng Model Updatng Challenges Conclusons Global Optmzaton of Multphase Flow Networks n Ol and Gas Producton Systems Introducton Methodology Well Flow Lne Choke Outlet Boundary Connecton Objectve Constrants Case Study Conclusons Further Work Nomenclature Optmal Well-Testng Strategy for Producton Optmzaton: A Monte Carlo Smulaton Approach Introducton v

9 4.2 Monte Carlo Smulaton Calculatng Producton Error Dstrbuton of Ol Resource Rato Case Study Conclusons Further Work Nomenclature Well Management under Uncertan Gas or Water Ol Ratos Introducton Uncertanty Matters Low Processng Capacty Hgh Processng Capacty Comparson Proposed Method Case Study Conclusons Further Work Optmal Start-up Schedulng of Producton Wells Introducton Short-Term Optmzaton Full Horzon Optmzaton Computatonal Results Conclusons Further Work Conclusons References v

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11 1 Introducton In ths chapter, the work n ths thess s restrcted, motvated, and the contrbutons are placed n a wder perspectve. The thess s based on fve papers. One paper s accepted to a journal, four papers have been presented on conferences and prnted n the proceedngs of the conferences, and one paper s currently unpublshed. A summary of each paper s gven. Furthermore, the major contrbutons of the ndvdual papers are outlned. 1.1 Offshore Ol Producton System In ths secton, a bref ntroducton to offshore ol producton wll be gven. Most of the components and termnology used wthn the thess wll be defned. Ol producton s the extracton of ol and gas from the reservor to the refnery [1]. Several dscplnes are nvolved n the producton and plannng Reservor A reservor s a porous rock contanng producble hydrocarbons such as ol and gas. The reservor wll typcally contan a mxture of hydrocarbon components, water, and varous contamnatons. The reservor pressure s typcally between the hydrostatc pressure (approxmately 10,000 Pa/m) and the rock pressure (approxmately 20,000 Pa/m) [1]. The reservor pressure s reduced when the fluds are extracted. The reservor temperature ncreases wth the depth of the reservor, typcally 0.03 K/m [1]. The hydrocarbon components and water wll separate naturally n the reservor because of dfferent flud denstes. At the top of the reservor, 1

12 there wll be a gas cap. The water wll be on the bottom and the ol n the mddle. The rato between the gas and lqud n the reservor does however depend on pressure, and much of the ol n the reservor wll be gas at surface pressure condtons Well The hydrocarbons n the reservor are produced by a well nto the reservor. Frst, a wellbore s drlled nto the reservor by removng parts of the rock along a path from the surface to the reservor. The wellbore s stablzed by a casng, whch s a large-dameter ppe lowered and mounted usng cement nto the wellbore. The hydrocarbons from the reservors do not flow n the casng, but n the tubng nstalled wthn the casng. The space between the tubng and the casng s the annulus. A packer solates the annulus from the reservor. The casng s perforated n the reservor allowng the fluds to be extracted. Ths part of the wellbore s descrbed as bottom-hole. The part of the subsurface wellbore s descrbed as downhole. Some wells may have downhole or bottom-hole pressure or temperature measurement devces, whch allow measurng the temperatures or pressure at those locatons. Some smart horzontal wells even have downhole valves for controllng the nflow from multple reservor zones. The most mportant components of a well are shown n Fgure 1.2. The wellhead s the surface termnaton of the wellbore. It ncludes facltes such as chokes for controllng the flow from the well. The choke s smlar to a valve. Typcally, pressure and temperature measurement devces are located both upstream and downstream the choke. The extracton of the reservor s drven by the pressure dfference between the reservor pressure and the pressure located upstream the choke. As the reservor s depleted, the producton rates may declne be- 2

13 cause of a reduced reservor pressure. Varous artfcal lft methods are used to ncrease the ol producton rates from the wells. These artfcal lft methods nclude pumps and gas lft. Gas lft s an artfcal lft method n whch gas s njected nto the tubng to reduce the hydrostatc pressure drop of the well by decreasng the average flud densty. The reduced hydrostatc pressure drop decreases the downhole pressure of the well allows the reservor lquds to enter the wellbore at a hgher flow rate. The tubng-casng annulus s typcally used to transport the njecton gas down to the lower part of the wellbore at whch there s a gas lft valve connectng the tubng-casng annulus and the wellbore. There are two types of gas lft: ntermttent and contnuous gas lft. The contnuous gas lft method njects gas at a contnuous bass. The ntermttent gas lft method njects gas at a cyclcal bass to enable the buldup of lquds n the wellbore. The ntermttent gas lft method s used n relatvely low productvty wells. The extracton fluds from the reservor wll make the reservor pressure reduce, and the reduced reservor pressure wll reduce the producton rates from the wells. Gas or water njectors may be used to replace the extracted flud volumes by njectng gas or water at convenent locatons n the reservor n order to support the pressure. In gas njecton, separated gas from the producton wells or gas mported from other producton systems are njected nto the reservor. In fact, other gases such as CO 2 have also been tred. Water njecton s popular n offshore ol producton because of good avalablty of seawater, whch may be fltered and treated nexpensvely. Conng s the change n ol-water or gas-ol nterface profles because of drawdown pressures. The result of conng may be hgher gas ol ratos or water cuts because of perforatons on the water or gas sdes of the nterface levels. 3

14 1.1.3 Gatherng Network The producton from the wells has to be gathered from the wells and transported to the processng facltes of the producton system. For ths, a set of flow lnes s used. In offshore ol producton systems the chokes are typcally located at surface. The rser, a specal type of flow lne, s a part of the well. However, subsea wells have become more popular as the technology has evolved. In subsea wells, the chokes are located n a subsea faclty and they share a flow lne to the processng facltes. More recent subsea facltes may even host some processng facltes separatng gas and lquds to prevent sluggng. The producton manfold s downstream to the chokes. The manfold s a mxng pont at whch the well stream of each well s mxed. Typcally, a producton system has one producton manfold and one test manfold. The producton manfold mxes the well streams from the wells producng to the producton separator. The test manfold mxes the well streams from the wells, typcally one, producng to the test separator. The nstrumentaton of subsea facltes may vary slghtly some may nclude pressure and temperature measurement devces that communcate wth the rest of the producton system. Some chokes of subsea facltes are remotely controlled and some are not remotely controlled. The chokes of subsea facltes that are not remotely controlled may requre remotely operated vehcles to adjust choke settngs makng changes very expensve Processng Facltes The overall objectve of the processng facltes of an offshore ol and gas producton system s to make the ol and gas from the reservor transportable. The ol s typcally transported usng tankers, whch requre that the ol s stable at stock tank condtons. Furthermore, most of the water s removed from the lqud to reduce the cost of transportaton. 4

15 Gas s often exported usng flow lnes to remote gas termnals. The gas must be dred to prevent lqud slugs to buld up n the flow lne, and compressed to the requred pressure. In order to stablze the ol, separators are used. The well streams enter the separator, whch s a horzontal tank, horzontally and ht a seres of perpendcular plates, causng the lquds to drop to the bottom and the gas to rse to the top. Gravty separates the lqud of the well streams, whch s a mxture of ol and water, nto ol and water layers at the bottom of the separator. An abeam-vertcal plate prevents water to enter the part of the tank farther from the nlet, allowng the ol to be tapped here. Water s tapped on the other sde of the abeam-vertcal plate. An outlet s also located at the top of the tank for tappng of gas. A separator s llustrated n Fgure 1.1. The separators are typcally seral-coupled to mprove the qualty of separaton, and a stage number dstngushes them. A scrubber s a vertcal separator desgned to remove drt, water, foregn matter, or undesred lquds from a gas stream. The ol-water and gas-ol nterfaces have to be controlled to prevent ol to enter the gas outlet, water to enter the ol outlet, or ol to enter the water outlet. The nterfaces are controlled usng a control valve at each outlet. An automatc feedback controller s typcally used to mantan each of the nterfaces at ther desred set ponts usng a measurement of the nterface level. A smlar controller s typcally used to control the separator pressure by adjustng the choke at the gas outlet and a separator pressure measurement. A test separator, possbly equpped wth specal measurement devces, s used to measure propertes of the flow stream of a sngle well at the tme. The test separator enables the measurement of the gas ol rato and the water cut of each well usng the flow rate measurements of the separator. 5

16 The separator may be effectve, but the water from the separator wll stll nclude ol after leavng the separator. A hydro cyclone can be used for separatng the remanng ol from the water. A hydro cyclone works by hurlng the oly water n the hydro cyclone wth a large force (about 10,000 m/s 2 ). Because of the dfferent denstes between ol and water, the water wll be hurled to the cyclone wall, whle the ol wll be n the mddle. The ol and water can then be tapped. For each stage of separaton, the pressure s dropped untl the ol reaches stock tank condtons. The gas, however, s exported or renjected at a hgher pressure, and compresson usng gas compressors s requred. Because compresson ncreases the temperature of the gas, coolng s requred. Compresson of the gas ncreases the gas temperature, demandng a cooler of the gas downstream to the compressor. In offshore ol producton systems, seawater s used as a coolng medum for the heat exchangers. Storage cells are used for storage of the produced ol untl a tanker s ready to pck t up. 1.2 Motvaton The world s experencng an ncreased demand for petroleum n the begnnng of the 21 st century, and many of the reservors of the exstng ol producton systems are maturng reducng the ol producton rates from these systems. The ncreasng demand and reducng supply s materalzng n rasng ol prces are motvatng development of new technologes ncreasng the ol producton. The new technologes are gven many names ncludng the dgtal ol feld, ol feld of the future, and ntegrated operaton. Although the names and the content are dfferent, the goal s the same to ncrease the ol producton from the exstng ol producton 6

17 systems. The potental net present value of ntegrated operatons on the Norwegan Shelf s estmated to 250 bllon NOK (approxmately bllon USD) n a study for The Norwegan Ol Industry Assocaton [2]. Most of the net present value s due to ncreased and accelerated producton owng to producton optmzaton. By changng the work flow of the decson-makng n the operaton to allow more nteractons between dscplnes, better decsons are supposed to be taken. The slo thnkng n operatons s reduced by buldng collaboraton rooms rooms located onshore where professonals from multple dscplnes are supposed to collaborate both wthn the room and wth the operators located offshore. The collaboraton rooms are equpped wth large screens enablng teleconferences wth the operators offshore. The screens are also used for showng process measurements and calculatons based on these measurements targetng the professonals on the shared objectve. Many of the companes operatng the ol producton systems are nvestng n nformaton systems makng the process measurements avalable onshore to allow remote operatons. The decson-makng n the collaboraton rooms s related to the daly operatons of the ol producton system. The goal s to maxmze some knd of performance measure, whch typcally s the total ol producton rate of the ol producton system adjusted for the varable cost of operaton. Many of the decsons are made usng numercal smulatons and trends of process measurements, but mathematcal optmzaton s rarely used. The demand for smarter operatons makes mathematcal programmng more of a topc. The ncreased avalablty of real-tme process measurements onshore s an enabler. The models used by the mathematcal pro- 7

18 grams can be updated to ft the nput-output behavor of the ol producton system. Many professonals study varous applcatons of mathematcal optmzaton n ol producton optmzaton. Ths ncluded the optmzaton of well placement, drllng operatons, reservor dranage, and daly operatons. New gas and water njecton strateges are currently beng developed where the goal s to maxmze the recovery of the reservor. By usng more process measurements ncludng pressures, temperatures, and sesmc, the current state of the reservor can be more accurately observed. The more accurate nformaton on the reservor allows njectng gas and water wth reduced rsk of a water breakthrough. Because of the delayed water breakthrough, the processng equpments can produce more ol wth the same water treatment capacty. Smart wells, whch are wells equpped wth downhole measurement devces and valves controllng the flow from a multtude of reservor zones, ncrease the degrees of freedom avalable to enable more control of the extracton of ol and gas. Gas and water njecton strateges to the reservor wll however not be the focus of the thess. Improved methods for operaton of wells are also a topc currently developed. The methods nclude fndng the optmal mxture of wells n order to maxmze the ol producton rate wthout volatng any constrants n the processng equpments. Such constrants are typcally related to capacty, qualty or safety. The ol produced must not nclude more than a specfed amount of water n order to be accepted by the purchasers. The water produced s often dsposed or renjected nto the reservor. If dsposed, local envronmental regulatons restrct the amount of ol and chemcals that t may nclude. Renjectng oly water may also be a problem because t may clog the njecton well. Sand producton may also be an ssue because of eroson n bends and chokes. Sand takng up space n 8

19 the separator, thus reducng the separator capacty, may also be an ssue. H 2 S may cause corroson n the flow lne, and the amount may be restrcted to prevent ths. If exported or sent through flow lnes, the gas has requrements on the dryness to avod lqud slugs. Furthermore, gas typcally has qualty specfcaton related to the gas composton, such as the amount of H 2 S. Safety requrements may be related to desgn pressures or temperatures of the processng equpments, or the ppng or valves connectng them. In order fully to utlze the lmted capacty gven by the processng equpments, the well mxture must be optmzed to consder the flud composton from the wells. The focus of the thess wll be to develop methods that can be used to fnd such optmal well mxtures usng real-tme data n day-to-day operaton. The reservor s a dynamcal system where ol s extracted from the reservor through the well to the processng equpments. The extracton affects the reservor states by reducng the ol, water, and gas n the reservor. Accordngly, the pressure s also reduced. The reducton of pressure may be partly compensated by the njecton of gas and water; however, the flud compostons of the reservor are changed. The focus of ths thess wll not consder effects on the reservor, and the proposed solutons wll only try to maxmze the current total ol producton rates rather than total recovery over the lfe cycle. Ol producton systems nclude many measurement devces, but the numbers of states or values that are desred are even hgher. In order to fnd the optmal well mxture, accurate nformaton about the flud composton s requred. The thess wll also focus on developng method for optmally obtanng the desred nformaton or measurements. 9

20 1.3 Summary and Contrbutons of Papers In ths secton, a bref summary of each of the papers wll be gven. The major contrbutons of each paper wll be stated. The papers are connected because they all propose producton optmzaton schemes for maxmzng the total ol producton rate. Two of the methods explctly handle the uncertantes by ncludng t nto the model Paper I: Real-Tme Optmzaton of Ol and Gas Producton Systems: A Technology Survey Ths paper s a non-crtcal survey of key lterature n the feld of realtme optmzaton of offshore ol and gas producton. The goal s to gve an overvew of technologes that may be appled n a real-tme producton optmzaton applcaton. The concept of real-tme producton optmzaton s also dscussed. It s ncluded as the frst paper to functon as an ntroducton chapter n ths thess. The paper ncludes an nformaton flow descrpton of the operaton of an offshore ol and gas producton system. The elements n ths descrpton nclude data acquston, data storage, processng faclty model updatng, well model updatng, reservor model updatng, producton plannng, reservor plannng, and strategc plannng. Methods for well prortzaton, gas lft optmzaton, gas or water njecton optmzaton, and model updatng are revewed n the vew of the nformaton flow descrbed. Challenges of real-tme producton optmzaton are also dscussed. Ths paper contrbutes an overvew and organzaton of exstng technologes that may be used for real-tme optmzaton applcatons n offshore ol and gas producton. 10

21 1.3.2 Paper II: Global Optmzaton of Multphase Flow Networks n Ol and Gas Producton Systems A mathematcal program for fndng the optmal ol producton rates of the wells n an ol producton system s developed. Each well may be manpulated by njectng lft gas and adjustng a producton choke. The ol producton from the wells may be restrcted by multple constrants n the maxmum ol flow rate, water flow rate, lqud flow rate, and gas flow rate. The wells may also be restrcted wth a maxmum total lft gas rate. In ol producton systems wth subsea wells, flow lnes are often shared between two or more wells. The pressures n the producton manfold n such ol producton systems are affected by the flow rates from the wells. The commonly used models based on gas lft performance curves (GLPC) no longer apply drectly to these problems due to changng pressure condtons n the producton manfold. Because of ths, a model of the flow lne s also requred to get results that are more accurate. Ths work ncorporates such a model. A pecewse lnear approxmaton s proposed. Ths makes t possble to fnd a proven global optmum, wthn the approxmaton, for the optmzaton problem. The problem s formulated as a mxed nteger lnear program, and t s solved usng a commercal branch and cut solver. Ths paper contrbutes a novel model of pressure drops n flow lnes for producton optmzaton. A contrbuton of the paper s the use of pecewse lnear models of the pressure drop n the common flow lne. Further, t s a contrbuton to solve ths as a mxed nteger program, whch allows for easy global optmzaton (of the approxmate model). The method s a refnement of the master thess [3] of the author. The method s mproved by calculatng the pressure drop from the source to the snk nstead of the opposte way. Ths elmnated the use of a numer- 11

22 cal solver to fnd the pressure drop, speedng up the calculaton. Furthermore, the method s modfed to use specal ordered sets of type two, allowng faster convergence of the numercal solver. A further contrbuton s the use of a branch and bound method for solvng the producton optmzaton problem of multphase flow networks. A case study s conducted usng feld data from a Norwegan offshore ol producton system comprsng four subsea wells. The case study focuses on the computatonal load of the proposed method. The method s able to solve the optmzaton problem wthn ten seconds Paper III: Optmal Well-Testng Strategy for Producton Optmzaton: A Monte Carlo Smulaton Approach Well testng may be performed to support many decsons ncludng ones related to producton optmzaton of an ol producton system. The nformaton flow used for optmzaton of the system s descrbed. In producton optmzaton, nformaton such as the gas ol rato and water cut s used to decde, for example, on the wells to prortze for chokng back or openng to avod over-utlzaton or under-utlzaton of the producton capacty. Snce the reservor propertes change wth tme, the uncertantes of ther estmated values ncrease wth tme, and eventually a new well test wll be requred. The rsk of prortzng the wrong wells, gvng a lower total ol producton rate than what s possble, ncreases as the uncertantes n the estmates ncrease. A computer program s developed to choose the well to test at a gven tme based on hstorcal well test data. The program uses a Monte Carlo approach for dentfyng the well test beng more lkely to lead to the hghest ncrease n the total ol producton rate when the well test nformaton s utlzed to optmze the ol 12

23 producton. The computer program s appled to feld data quantfyng the benefts when appled to ths specfc feld. The current mplementaton s lmted to producton systems where the pressure nteracton among wells may be neglected. Furthermore, the current mplementaton assumes that a sngle treatment constrant s actve. Ths paper contrbutes a novel method for choosng wells for routng to test separators. A contrbuton of the paper s the calculaton of the expected total ol producton rate usng the measurements obtaned n a possble well test for choosng a well for testng. Furthermore, a contrbuton of the method s to use Monte Carlo smulatons to calculate an expected total ol producton rate usng the possble outcomes of the measurements n the well test. It s a further contrbuton to test the well gvng the maxmal expected total ol producton rate. The ndustry practce s to do well testng based on equal frequency for all wells, and to do ad hoc testng when the measurements from a well look suspcous. The paper contrbutes a method for fndng a stochastc dstrbuton of the gas ol rato or the water cut of a well usng hstorcal well test data Paper IV: Well Management under Uncertan Gas or Water Ol Ratos In the daly operaton of an ol producton system, t s often requred to choke back some of the ol producton wells to ensure that the processng capacty s not over-utlzed. When the capacty of some processng resource s over-utlzed, wells havng large ratos between the consumpton of the resource and the ol producton rate are choked back. When there s free processng capacty, the chokes of the wells havng small ratos are opened. Often, the gas or water ol ratos (derved from the water cuts) are used as such ratos. These ratos are uncertan. Ths paper proposes to use nformaton about the uncertantes of the gas or water ol ratos 13

24 to fnd the order of openng and closng the wells to maxmze the expected total ol producton rate from the wells. In a computatonal study based on feld data, the order was found to be dfferent from the order found usng the expected value of the gas or water ol ratos. Ths paper contrbutes a novel method for prortzng wells producng to a shared processng faclty havng a sngle processng constrant. A contrbuton of the method s the use of stochastc dstrbutons of the gas ol ratos or water cuts to fnd the optmal wells to choke back or open maxmzng the expected total ol producton rate and not volatng the processng constrants. The ndustry practce s to regard the gas ol ratos or water cuts as parameters wthout uncertanty, and to do prortzaton usng these presumably accurate values. The method further handles uncertantes n the processng capactes and ol potentals explctly. It s a contrbuton that mxed nteger lnear programmng s used for fndng such an order. Further, a contrbuton, n ths context, s to use values drawn from the stochastc dstrbuton to approxmate the stochastc dstrbutons themselves Paper V: Optmal Start-up Schedulng of Producton Wells A lnear program for fndng the order to open wells after a shutdown s proposed. The ol producton over a horzon s maxmzed, thus mnmzng the total losses durng a start-up. The method s able to handle multple constrants such as ol, gas, water, and lqud treatment capactes as well as qualty constrants on the gas. The method s shown to ncrease cumulatve producton compared to a method usng short-term optmzaton only. Ths paper contrbutes a novel method for handlng the uncertantes n the treatment capactes. A lnear model of the ol producton system s 14

25 optmzed gvng suggestons for changes to the chokes the model s updated usng measurements related to the excess treatment capacty. In a closed loop, the operatng pont wll therefore typcally approach the physcal lmtatons of the system, and t wll not just be on the constrant mposed by the uncertan model. 15

26 PC PT Gas Well streams Seres of perpendcular plates LT LT Abeam-vertcal plate LC Ol Water LC Fgure 1.1: A separator typcally comprses two level control loops and a pressure control loop. 16

27 Well head Seawater Seabed Annulus Tubng Casng Downhole Packer Perforaton Rock Gas layer Ol layer Bottom-hole Water layer Fgure 1.2: A well extracts fluds from a reservor through tubng to the surface. 17

28 18

29 2 Real-Tme Optmzaton of Ol and Gas Producton Systems: A Technology Survey Based on H.P. Beker, O. Slupphaug, and T.A. Johansen, accepted for SPE Producton & Operatons Journal, presented at 2006 SPE Intellgent Energy Conference and Exhbton Amsterdam, The Netherlands, Aprl Introducton In the daly operaton of an ol and gas producton system, many decsons (an element of a soluton) have to be taken affectng the volumes produced and the cost of producton. These decsons are taken at dfferent levels n the organzaton, but eventually they wll reach the producton system layout. Fgure 2.1 gves an overvew of a physcal producton system. For such producton systems, the decsons are typcally related to the choke or valve openngs, compressor, and pump settngs at every nstance of tme. An objectve functon s a sngle-valued and well-defned mathematcal functon mappng the values of the decson varables nto a performance measure. Examples of such performances measures are the total ol producton rate, net present value (proft), or the recovery of the reservor. In the efforts towards better performance of the producton system, a queston to be answered s whch decsons are better n order to optmze 19

30 the objectve functon. In the process of makng good decsons, nformaton about the producton system s used. Ths nformaton may be the physcal propertes such as ppe dameters and lengths, or t may be measurements from the producton system. The envronment n whch the producton of ol and gas s obtaned s contnuously changng. Ths wll, therefore, affect the value of the performance measure of the decsons beng used. For example, f the coolng capacty of the producton system s an operatonal bottleneck at some gven tme, ths may no longer be the case f the seawater temperature drops or another pump n the coolng system s started. Incdents n the producton system may also affect the value of the performance measure of the decsons. A partal shutdown of the producton system due to mantenance may also affect system bottlenecks. Real-Tme Optmzaton (RTO) s a method for complete, or partal, automaton of the process of makng good or optmal decsons. The term optmal s defned below. By contnuously collectng and analyzng data from the producton system, optmal decsons may be found. Ether these settngs are then mplemented drectly n the producton system or they are presented to an operator or engneer for consderaton. If the settngs are mplemented drectly, the RTO s sad to be n a closed loop. RTO defned by Saputell et al. [4] reads: a process of measure-calculatecontrol cycles at a frequency, whch mantans the system's optmal operatng condtons wthn the tme-constant constrants of the system. The man am of RTO s to mprove the utlzaton of the capacty of a producton system to obtan hgher throughput or net present value. The dea s to operate the producton system, at every nstant of tme, as near to the desred optmum as possble [5]. To acheve ths, a model of the producton system s optmzed to furnsh an optmal soluton. The model s contnuously beng updated by measurements from the producton sys- 20

31 tem to ft the actual nput-output behavor of the processng facltes, wells or network, and reservor better. A general RTO system used n, for example, downstream petrochemcal producton systems conssts of the followng four components [6] shown n Fgure 2.2: Data valdaton: The valdated nput and output data are valdated usng data reconclaton and sgnal processng technques (for nstance usng materal and energy balances). Model updatng: The processng faclty models, well models, flow lne network models, and reservor models are updated to ft the valdated nput and output data avalable the best. Model-based optmzaton: An optmzaton problem based on the updated models, objectve functon, and constrants s set up and solved to obtan an optmal soluton. Optmzer command condtonng: A post-optmalty analyss s performed to check the valdty of the computed soluton before t s mplemented. Although the defnton of Saputell et al. [4] was wrtten wth ol and gas producton systems n mnd, t s general n the sense that t s not restrctve to some specfc type of producton system or method. The defnton can be related to Fgure 2.2. Recently, SPE started a techncal nterest group that focuses on RTO for ol and gas producton systems. The drver behnd ths development s, as n any ndustry, the demand for more proftable producton systems. Ths survey wll help to organze prevous work related to RTO. The focus wll 21

32 be on offshore ol and gas producton systems; however, relevant references from other ndustres are also ncluded. A prevous survey [7] was recently publshed. It focused on the organzatonal ssues of usng RTO. Because a survey on the organzatonal ssues s already gven, ths survey wll focus on the exstng software, tools, methods, and approaches that can be appled for RTO. However, the survey wll not focus on the processng facltes. Furthermore, ths s a noncrtcal survey of key lterature n the feld. Ths paper s organzed as follows. A descrpton of the nformaton flow assocated wth the optmzaton of offshore ol and gas producton systems s gven to relate the general RTO technology to ths specfc applcaton area. Technologes for optmzaton and model updatng of such producton systems are revewed, and reference cases wll be presented. Fnally, key challenges are addressed and conclusons are stated. 2.2 Informaton Flow n Producton Optmzaton The operaton of an ol and gas producton system may be llustrated accordng to Fgure 2.3. The man components of the operaton are descrbed below Data Acquston Modern producton systems usually have good nstrumentaton. Level (the heght of ol-water or gas-ol nterface n a separator), pressure, and temperature transmtters are most common. In addton to requred fscal meters, there are often also a few flow transmtters to measure flow rates n gas, water, and ol ppes. Flow transmtters for multphase flow may also be avalable, but they are rare. Varous off-lne analyzers of parameters ncludng ol-n-water and other product qualtes may also be avalable. The nstrumentaton vares consderably between dfferent produc- 22

33 ton systems, typcally wth the age of the system and the country or regon t s stuated Control A typcal ol and gas producton system has many (automatc) feedback control loops to support an effcent producton and meet the producton targets. A feedback control loop generates decsons, such as valve openngs, based on measurements from the producton system. The smplest form of such control s used to control levels and pressures n the separators. Centrfugal compressors are always protected by ant-surge control loops. The control loops ensure that the compressors do not surge, and prevent damage. Control s also used to balance the load among parallelcoupled and seral-coupled processng unts. A phenomenon that may be observed n an ol and gas producton system s severe sluggng. The pressure and flow rate n a well or flow lne start oscllatng, and the effectve producton capacty s reduced. Ths can sometmes be stablzed by feedback control[8] Producton Plannng A typcal ol and gas producton system s operated by perodcally generatng a producton and njecton plan. Ths producton and njecton plan lsts the target producton of ol, gas, and water for a specfc perod for each ndvdual well. Smlarly, the njecton of gas or water s stated for the njecton wells. The cycle tme of the producton and njecton plan depends on the polcy of the feld operator, but t wll typcally be between a week and a month. The models and constrants of the processng facltes and wells or networks are used together wth constrants from the reservor plannng as nputs to the plannng. Poltcs or constrants from the strategc plannng may also be enforced here. 23

34 2.2.4 Operator Above all, the operators are responsble for ensurng safe operaton. Furthermore, they are responsble for mplementng the recommendaton from the producton and njecton plan. When mplementng the producton and njecton plan, the operators have to meet the operatonal targets whle obeyng mnmum and maxmum lmts on varables such as pressures, temperatures, and rates Strategc Plannng The producton and njecton plan s somehow connected to the market and the strategc consderatons or polcy of the company Reservor Plannng The long-term reservor dranage s planned here. Ths ncludes plannng of gas and water njecton. The updated reservor model s used for fndng proper dranng strateges. Poltcs from the strategc plannng may also be enforced here Well Model Updatng To support makng good decsons, models may be used to develop the producton plans. Typcally, well tests are performed to determne the gas ol rato, water cut, and producton rates of each ndvdual well. Well tests are performed by routng a well to a dedcated separator. Ths separator wll separate the three phases, and a flow transmtter s connected to the outlet of each phase. The well model s then updated usng the measurements taken durng the test. Flud samplng may be used to obtan the flud composton ncludng the water cut. 24

35 2.2.8 Processng Faclty Model Updatng Typcally, the processng facltes are modeled as constrants on ol, gas, and water processng capactes. Ths means that the model s updated whenever the capacty changes Reservor Model Updatng To be able to conduct the reservor plannng, a reservor smulator may be used to evaluate dfferent dranage strateges for the reservor. The ntal state and parameters of the reservor model must be updated by measurements from the producton system. The volumes produced, volumes njected, and pressures are mportant measurements used n ths updatng process. To ensure good accuracy of the model, parameters and the ntal state are ftted to longer seres of hstorcal producton data. The method s typcally called hstory matchng. 2.3 Technology and Reference Cases Fgure 2.3 shows an example of how decsons n producton optmzaton may be taken. Most or all the decsons are made wth support by some form of technology. Ths secton wll provde an outlne of relevant technologes and reference cases from the ndustry that may be extended and used as a part of an RTO system. More specfcally, the technologes belongng nsde the large rectangle of Fgure 2.3 wll be dscussed here Global Versus Local Optmzaton For all the plannng actvtes, numercal optmzaton may be used to fnd good or optmal feasble solutons (or decsons). Ths works by defnng an objectve functon to be mnmzed, or maxmzed, as a functon of decson varables. The feasble set of these decson varables s defned by a set of equalty and nequalty constrants on the decson varables. 25

36 The objectve functon and the constrants defne the optmzaton problem, or mathematcal program. A solver s used to fnd an optmal soluton, and the solver should be chosen usng nformaton about the problem structure. In partcular, lnear and convex quadratc programs [9] are often preferred because of ther convexty and the exstence of mature algorthms for solvng them. In some cases, nonlnear constrant and objectve functons may be reformulated to lnear equvalents [10], and these mature algorthms may be used. A local optmal soluton s defned as a feasble soluton havng a neghborhood where no strctly superor feasble soluton exsts (n terms of the objectve functon). A global optmal soluton s defned as a feasble soluton not havng a strctly superor feasble soluton (n terms of the objectve functon) n the feasble set, and hence a global optmal soluton s also a local optmal soluton. The dfference between a local and global optmal soluton are llustrated n Fgure 2.4. Convex optmzaton problems are preferred because they guarantee that a local optmal soluton s also a global optmal soluton (however a unque global optmal soluton s not necessarly guaranteed). Unfortunately, the term optmal soluton s ambguous, and t s used for both local and global optmal solutons Local Solvers Many local solvers use local nformaton about the neghborhood of a current soluton to fnd a step that mproves the objectve functon and mantans the feasblty of the current soluton. If a step s found, t s used to update the current soluton. If not found, the algorthm termnates. Typcally, there s a threshold on the mprovement n the objectve functon that should be satsfed for the current soluton to be updated. 26

37 Examples of the local nformaton used are the dervatve of the objectve functon and the constrants wth respect to the decson varables evaluated at the current soluton. Examples of solvers usng dervatve nformaton are the Actve Set Quadratc Programmng, Successve Quadratc Programmng (SQP), and Successve Lnear Programmng (SLP) methods [9]. The algorthms such as SQP and SLP may requre an nfnte number of teratons to fnd a local optmum (.e., they only converge); however a termnaton crteron s used to termnate n fnte tme Global Solvers Global solvers are desgned to handle multple local optma [11, 12]. Examples of such solvers nclude the genetc algorthms and the branch and bound method. Genetc algorthms mmc the survval of the fttest [13]. A populaton of solutons s mantaned. The solutons are evaluated for ftness, meanng for feasblty and the objectve functon value. Pars of solutons are chosen randomly from the populaton and recombned. The hgher the ftness, the hgher the chance for reproducton. The recombnaton process s done by combnng random parts of each decson value. Mutatons are ncluded n order to ensure a suffcent large varaton n the populaton for convergence to the optmum. The genetc algorthms do not use any structural nformaton on the optmzaton problem, and any black box models may easly be appled. However, ths s also the man drawback as the method gves a bound of nether the global optmum nor the local optmum on termnaton, and the computatonal load s usually large. A general framework for global optmzaton s the branch and bound method. The method termnates wth an upper and lower bound of the objectve functon. By teratvely dvdng the optmzaton problem n 27

38 properly posed sub-problems, the upper and lower bounds converge. The bounds are calculated usng structural nformaton on the optmzaton problem, and the bounds become more accurate as the sub-problems are dvded. The branch and bound framework has shown to be partcularly useful for solvng (mxed) nteger lnear programs because an upper bound for maxmzaton (or lower bound for mnmzaton) may easly be calculated by solvng a lnear program where the nteger constrants are dropped. For mxed nteger lnear programs wth lower and upper bounds for the nteger varables, the number of sub-problems to be solved s fnte and bounded. Each sub-problem s a lnear program that s solved n fnte and bounded tme, and the complete mxed nteger lnear program s solved n fnte and bounded tme; however, the bounded tme grows generally exponentally wth the sze of the problem [14, 15]. The branch and bound framework may also be used for general nonlnear programs; however, much work s typcally requred fndng good and vald boundng functons. Some solvers are able to fnd boundng functons by analyzng the constrants and the objectve functon automatcally. However, ths requres that the constrants and the objectve functon are avalable n analytcal, or symbolc, form to the solver. In practce, the optmzaton problem often ncludes a black box model (for nstance a reservor smulator), and such boundng functons may not be calculated nether by hand nor automatcally because the structure of the model s unknown to both the user and the solver Hybrd Solvers Global solvers such as genetc algorthms may termnate far from a local optmum. By passng the soluton as an ntal value to a local solver, the soluton s mproved to brng t close to a local optmum. 28

39 Proxy Models Proxy models are smplfed models that are used because they are faster to evaluate [16] or have better numercal propertes [17], and both are propertes that are very mportant n RTO systems. Artfcal neural networks have been successfully used as proxy models to reduce the computatonal load n hstory matchng of reservors [16, 18, 19]. The proxy model s frst typcally ftted to a set of model evaluatons, and then used as the model n the optmzaton. The soluton found by the solver usng the proxy model may be valdated or used as an ntal value for the orgnal model to mprove the soluton further. The success of proxy models was llustrated by Cullck et al. [16] where the number of reservor model evaluatons requred was reduced by 25 % usng such a model n hstory matchng. Each evaluaton of the reservor model took sx hours, and hundreds of evaluatons were requred for the hstory matchng Producton Plannng The goal of ths plan s typcally to maxmze the daly producton rate, for example of ol, and to nject gas and water accordng to some gven rules provded by the reservor plannng Well Prortzaton If the goal s to maxmze the ol producton rate, some method s requred to fnd an optmal way to prortze the wells to produce and the rate to produce. It s often requred to prortze because the avalable processng capacty s less than the combned flowng capacty of the wells. The processng capacty constrants are related to satsfacton of product qualty specfcatons, safe operaton, processng faclty capactes, utltes capactes, etc. 29

40 When a processng capacty constrant s met, t s often related to the processng of gas, water, or lqud. The operator wll typcally choke back the well havng the largest rato of the consumpton of the assocated processng capacty to the ol produced. Examples of such ratos are the gas ol rato, water-ol-rato, and lqud-ol-rato. By successvely chokng back and openng the wells based on the ratos, the capacty s fully utlzed and the producton system s assumed to gve the maxmum total ol producton rate. When the total ol producton rate s maxmal, one well wll be partly opened and the rest ether fully closed or opened. The method above has proven successful because t s unaffected by the uncertantes n the flowng potentals, whch are the maxmal ol producton rates of wells, and processng capactes. The man drawbacks of the method are ts nablty to handle multple actve processng constrants and the assumpton of the flowng potentals of the wells to be ndependent. The flowng capactes of a well may be regarded as ndependent when changng the ol producton rate from the well does not change the flowng capactes from the other wells and the gas ol rato and the water-olrato are nvarant wth respect to the ol producton rate. An example of such wells s platform wells wth wellheads at the processng platform and a short common large dameter flow lne to the nlet separators. Lo and Holden [20] used a lnear program for fndng whch wells that should be opened, partally opened, or closed. They assumed each well could produce any ol producton rate between zero and the flowng potental, and that the water cut and gas ol rato were the same for all rates (.e. not conng gas or water). The method s able to handle multple constrants on ol, water, lqud, and gas producton for groups, or all, of the wells. However, uncertantes n the model are not handled. 30

41 A way of handlng a gas compresson-constraned producton system under gas conng condtons was proposed by Barnes et al. [21]. The method s able to handle wells where the ncremental gas ol rato (IGOR) s monotoncally ncreasng wth the ol producton rate. A smlar method was proposed by Urbanczyk [22]. The dea s to ncrease the producton from the well wth the lowest IGOR wth unused capacty, and reduce the producton wells wth the hghest IGOR. At the optmum, all the wells have the same IGOR or they are on a mnmum or maxmum ol producton rate constrant. Naus et al. [23] nvestgated the use of a combnaton of a reservor smulator and real-tme data could be used to maxmze the daly producton of ol. The parameters of the reservor smulator were contnuously updated to ft measurements from the producton system as they became avalable, and the reservor smulator was used to fnd dervatve nformaton. The cases conssted of a reservor and a horzontal well wth four contnuous nflow control valves to control the segments of the well. The total water and gas processng capactes were constraned. An SLP algorthm was used to solve the problem Gas Lft Gas lft may be used to ncrease the productvty of wells havng low gas ol rato. By njectng gas nto the tubng, the densty of the well bore flud s reduced and thus the pressure drop component resultng from gravty s reduced. However, the gas lft also gves a larger pressure drop component resultng from frcton, gvng some optmum lft gas rate for the well. Because of frcton, the optmum lft gas rate may be 0 Sm 3 /D. Usually, the avalable lft gas s less than the sum of the ndvdual optmum lft gas rates. The gas lft optmzaton problem s to fnd the lft gas rates for each well gvng the maxmum total ol producton rate subject to a gas lft processng capacty constrant, and possbly other opera- 31