THE PREDICTION AND UNDERSTANDING OF WAXY CRUDE BEHAVIOUR IN SUB-SEA FLOW LINES

Transcription

1 THE PREDICTION AND UNDERSTANDING OF WAXY CRUDE BEHAVIOUR IN SUB-SEA FLOW LINES AUTHOR: David Brankling Managing Director Oilfield Chemical Technology Ltd. Aberdeen, Scotland

2 SUMMARY The behaviour of waxy crude during flow through sub-sea or low ambient temperature flow lines is a complex, dynamic situation in which liquid properties and composition undergo continuous change. In order to define flow characteristics it is important that well planned and pertinent testing is undertaken in the laboratory in advance of field production. Typically this involves the use of test rigs of ml capacity which are capable of operating at high pressure and are tailored to measure discrete properties of the sample under defined test conditions. When the elements of such a study are combined a coherent picture of fluid behaviour in a flow line is obtained allowing prediction of wax deposition rates, rheology and gelation. However it is important that the complexity of a waxy crude below the wax appearance temperature where non-newtonian behaviour dominates flow is fully appreciated and reproduced in the laboratory to obtain quantitative data. This may then be used to improve field performance by considering necessary preventative measures to handle wax related problems or reducing outlay on unnecessary facilities installed to handle difficulties which do not actually occur during the field life. Accurate fluid behaviour predictions allow the fit for purpose production system to be designed and optimised to reduce operating costs and capital expenditure appropriate to the development scenario. This paper summarises waxy crude behaviour and the critical properties which require accurate measurement to predict the field performance. Page 1 of 14

3 INTRODUCTION The minimum cost development of offshore crude oil reserves is leading to the exploitation of small accumulations tied back to an existing process facility through sub-sea flow lines. Furthermore as the search for new reserves continues to deeper, colder waters or to more remote areas both onshore and offshore, and as heavier crudes are developed the problems which can accrue due to wax crystallisation and modification of flow behaviour become more critical to successful field development. The presence of crystallised wax in a crude oil (or condensate) has a number of major impacts on the behaviour of the hydrocarbon phase in any flow line or process system: Crude behaviour changes from a predictable Newtonian to complex non-newtonian fluid where flow properties which can affect pipeline operations are difficult to measure and predict. Wax may deposit within the pipeline leading to a reduction in diameter and achievable throughput. Deposition can be localised and in extreme circumstances lead to complete line blockage. Sufficient wax may crystallise in a specific orientation to lead to gelation of the crude should flow be terminated for any reason. The gel structure may be too strong to allow flow to re-start in the line without a physical intervention and the field production can be lost. Wax crystals can lead to the formation of stabilised emulsions within the flow line and in co-precipitation with asphaltenes produce complex emulsified hydrates which can result in flow line blockage. It is important to realise that at some critical temperature any hydrocarbon containing high molecular weight (+C 20 ) structures will exhibit the above properties. The crucial questions to field operations are will the above occur under actual operating conditions and if so what is the magnitude of the effect? If these questions are not answered prior to start up of production then the productivity on which field economics have been based is likely to be compromised and in severe circumstances the well may be lost. However if crude properties are fully defined and considered during the design and planning of the development measures can be taken to minimise problems in the most cost effective manner and ensure a predictable production profile. This paper summarises techniques which may be used to define crude behaviour and highlights the importance of testing which is responsive to potential development design in order that accurate data can be provided. Page 2 of 14

4 DEFINITION OF REQUIREMENTS One of the major flaws in work which has been conducted to predict the properties of crude flow in pipelines has been a deficiency in the test procedure whereby scaling of the field system has been simplified to a re-circulating loop with measurements made until a steady state situation is achieved. Obtained values are then assumed to apply to the entire field transport system. Crude produced through a flow line does not behave in this fashion. An immediate observation is that the field crude experiences a once through passage along the line and is not subject to re-circulation. Properties are also continuously changing along the length of the line as fluid cools and wax crystals appear in the system. For example if we consider a defined unit of crude flowing through a cold line it will experience a number of distinct although related changes which affect the properties of the element shown in Figure 1. Initially the hot fluid remains above the wax appearance temperature and predictable Newtonian behaviour occurs. Fig 1. Crude Changes Flow of element along pipeline Temperature Wax content Wall deposit Viscosity At some point along the line the Distance wax appearance temperature will be encountered at the pipe wall and wax deposition may occur. However the bulk fluid will still remain hot and deviation from Newtonian flow will be limited. Once the bulk fluid cools below the wax appearance temperature crystals will form in the crude and non-newtonian behaviour will develop. The viscosity will be modified by loss of some waxes to the pipe wall and shearing effects in the system which lead to wax crystal distortions. Further along the line more wax will precipitate to the pipe until the crude element reaches effective equilibrium with the surrounding temperature and wax related effects on rheology are now at a steady state. This complete situation cannot be easily reproduced in a laboratory test rig and it is therefore important to consider and evaluate the crude property changes as discrete elements in a test programme. Page 3 of 14

5 DEVELOPMENT OF EXPERIMENTAL TECHNIQUES The primary purpose of fluid characterisation covered in this paper is to define and predict the behaviour of hydrocarbon fluids under critical field conditions which can be used to assess the feasibility of a field development option and engineering design or chemical treatments proposed to handle wax related production problems. This data should be generated at an early stage in the proposed development and ideally at a similar time to compositional PVT studies to gauge field potential. Dissolved gas content has a significant effect on the behaviour of waxy crude and the test protocol must enable accurate measurements to be made on live fluids as well as provide reference data and scope for inhibitor screening tests on stabilised samples. As a consequence of current practices where design plans may be based upon a single well test the quantity of fluid available for such studies can be extremely limited. The methods developed to cope with this restriction by Oilfield Chemical Technology Limited therefore all address the need to provide accurate performance data from very small quantities of available crude. Comparison of the developed techniques against large scale test loops demonstrates significant advantages in small scale testing but requires novel procedures to quantify behaviour. Based upon previous statements crude testing must be able to determine the following: Temperature at which wax appears as a discrete crystalline phase (wax appearance temperature - WAT). Rheology of a fluid as a function of shear rate (i.e. flow) to enable prediction of flowing viscosity. Quantities of wax which may deposit on a pipe wall or cold surface under flowing conditions and predict growth along a pipeline. The tendency for a crude to gel under a shut in situation and pressures required to re-initiate flow. All of the above at operating temperatures and pressures appropriate to the development. It should be noted that these are performance related tests and do not provide compositional or basic analytical data without further evaluation. The intent is to actually measure fluid properties and not introduce doubt using predictive models. Page 4 of 14

6 Wax Appearance Temperature (WAT) WAT measurements are the most critical and absolutely essential data point required to define the subsequent behaviour of a crude oil. If the WAT is below the expected operating temperature at all points in the development then the crude will remain as a simple Newtonian fluid (in the absence of emulsification) and wax related properties are not an issue in the field development. Identifying the WAT as a function of pressure then immediately defines operating regimes where wax will not be present and eliminate unnecessary testing. There are many proposed and laboratory utilised techniques for the measurement of wax appearance temperatures including visual (cloud point) techniques, thermal analysis (DSC), viscosity variation and deviation from Newtonian behaviour, wax deposition on cold surfaces and volumetric method (1). In our opinion and supported by a number of other studies these techniques have a limited accuracy, especially in low wax content crudes and are difficult to undertake at pressure. The method developed by Oilfield Chemical Technology Limited some six years ago and finding increasing industry use relies on the measurement of pressure drop across a fine filter (less than 1 micron pore size) during crude flow and determination of the wax appearance temperature by filter plugging. The technique has some limitation when applied to high viscosity fluids due to the difficulty in actually flowing the crude through such a fine filter but in general gives a well defined wax appearance value. The major advantage of the method is that using suitable apparatus the WAT can be measured readily at any line pressure and within Oilfield Chemical Technology Limited is routinely operated at up to +9,000 psi. Figure 2 gives typical WAT plots as a function of system pressure and it is not unusual for wax crystallisation to be depressed by 10(C or more as dissolved gas content increases with pressure in a crude. Such information can dramatically reduce heating or insulation requirements in field lines. Fig 2. Wax Appearance Typical Black Oil Plot psig 500 psig 1000 psig 5000 psig Temperature / C Page 5 of 14

7 Fig 3. Wax Appearance Waxy Condensate Plot Fluid Rheology 0 psig 500 psig 1000 psig 5000 psig Temperature / C 5 In addition condensate streams, although low in wax content can exhibit severe wax deposition properties due to anomalous behaviour whereby heavy molecules initially precipitate below the dew point. Wax contents and WAT's within the liquid phase can therefore be much higher at elevated pressure than in low pressure or stabilised fluid samples where diluent effects come in to play (Figure 3). WAT tests require close control of fluid temperatures and flow in specific test rigs to define these values. A further critical aspect of fluid behaviour subsequent to the WAT is the rheology of the crude sample under operating conditions. Viscosity has an impact on calculated hydraulics, throughput, pumpability and wax deposition and must be carefully considered in all tests in the non-newtonian region. Unfortunately the measured viscosity of a crude oil sample is highly sensitive to the shear history experienced by the sample and hence the technique used to reach the point of measurement influences the measured viscosity (2). There is therefore no "standard" which can be used to define the viscosity of a sample at a defined temperature. Figure 4 gives rheology profiles achieved on a North Sea crude Shear rate / sec sample following three different cooling and mixing profiles reflecting changes in shear history which could occur in the field Fig 4. Crude rheology Crude oil with varied shear history Cooled under turbulent flow Cooled with low shear Static cooled The changes in viscosity are purely a function of the altered morphology of the wax crystals formed under the differing cooling and shearing regimes. Page 6 of 14

8 Figure 5 introduces the third variable in such behaviour which is dissolved gas content which again has a major impact upon the measured rheology. It should be noted that further complications caused by incorporation of a third discrete phase such as brine, asphaltene or other particulate material have not been considered. In order to fully define crude properties and to predict fluid behaviour under a number of likely field conditions such as start up, full production or reduced production rates it is therefore important that a number of viscosity curves considering the varying shear history which a fluid may experience in the field are generated Fig 5. Crude rheology Effect of gas content 0 0 psig 100 psig 500 psig 1000 psig Shear rate / sec It must also be appreciated that all measuring techniques using commercial viscometers have a degree of error (2) which is difficult to quantify but includes the following: In order to cool the sample to test conditions a thermal gradient must exist. This gradient will lead to some loss of wax to the test vessel cold surface and change in viscosity. Potential bridging effects, especially in narrow gap rotational cylinders due to wax crystal growth which increases the apparent viscosity. Surface effects in rotational and capillary viscometers arising from flow pattern distortions at the edges or ends of surfaces. In general a capillary viscometer technique or "model pipeline" assembly has the advantage over rotational viscometers in that rheologies can be readily measured to high pressure over a very wide viscosity and shear range whereas rotational viscometers tend to be low pressure (if any) and of limited viscosity range. Within Oilfield Chemical Technology Limited however both types are used dependent upon specific test requirements and crude properties. In order to predict crude behaviour the test sample must therefore be subjected to a series of controlled shear and thermal history cycles which reflect expected conditions in the field development and will provide relevant fluid data. It is far to simplistic to expect that a single measurement of fluid viscosity at a defined temperature gives a value for the real crude rheology. Page 7 of 14

9 Wax Deposition Wax deposition will occur in most pipelines where wall temperatures fall below the WAT and may require intervention by pigging operations to maintain line capacity. Knowing the rate at which deposition will occur allows an estimate of the pigging frequency to be made and a judgement on the economics of installation of pigging facilities versus additional pipeline insulation to be made. A poorly known fact based upon experimental studies is that the quantity of wax deposited is critically effected by viscosity of the flowing fluid such that at low temperature deposited waxes may be scoured from the pipe wall by a viscous fluid which removes and suspends wax crystals (high carrying capacity) whereas at a higher temperature deposited wax quantities may increase. Conditioning of crude to reflect the expected shear history, subsequent viscosity and measurement of deposition rates over a small thermal gradient is therefore important to achieve accurate data. Attempting to measure wax deposition using relatively large flow loops which remove large quantities of wax over a wide thermal gradient and usually re-circulate the crude sample can lead to erroneous results. Best results are therefore achieved by conditioning crude to a defined temperature and pumping this through a short section of tubing at a lower temperature and in which deposition will occur. Although not always possible to achieve the volume of crude used and line dimensions should be sufficient to allow a once through passage of crude and measurement of wax deposition to be made. However if re-circulation is required measurements should be made after a minimum number of circulations in order that compositional changes do not influence results to any significant extent. By undertaking this study at a 6 number of differing flow rates and thermal gradients it is possible to develop a picture of how wax will deposit along the length of a pipeline and based upon the overall heat flux scale the results to define the rate at which a layer will grow in the field (Figure 6). Fig 6. Wax deposition Bulk fluid / C % deposit in lab loop Pipe ID / inches Page 8 of 14

10 This area is still open to considerable improvement given a fundamental lack of knowledge on the shear stresses which apply at a pipe wall under turbulent flow, relationship of viscosity, velocity and shear rate to wax transportation and even the mechanism by which wax actually adheres to a pipe wall and deposition continues. Final interpretation of the laboratory results to give a meaningful prediction of field performance is therefore needed and a degree of experience and common sense required to extrapolate to the field condition. Pipeline Re-Start Values The measurement of the yield stress of a crude oil and thereby pressure required to initiate flow in a pipeline on start of operations after a line shut down is one of the most widely measured crude properties and many techniques have been developed to define this value. The vast majority measure re-start values using model pipelines in which the pressure drop required to initiate flow either visually or by a volumetric measurement is directly recorded and scaled to a field value (3). The re-start pressure is directly influenced by the cooling and shearing regime (shear history) experienced by the crude sample and this must be carefully considered in relation to field conditions. Cooling a hot crude sample in a flow line from above the WAT whilst shut in (no flow) will generally result in a high re-start pressure if sufficient wax is present to form a structural gel. Cooling from below the WAT will generally result in a lower re-start pressure. This is further modified by cooling and shearing to simulate flow through the line when the lowest restart pressure is normally seen (Figure 7). Within the field line elements of the crude will on shut down normally experience the above transitions along the length of a line and hence the re-start value of a crude cannot be fully described by a single re-start value although the high temperature cooling profile will result in the most conservative value. Fig 7. Restart Pressures Crude oil with varied shear history Cooled under turbulent flow Cooled with low shear Static cooled Time since pressure applied Page 9 of 14

11 Furthermore the field line is unlikely to consist of a complete length of coherent gelled fluid since under multiphase flow the contents will consist of oil continuous blocks, gas pockets and brine which may be distributed as layers within the line. However given that these points must be borne in mind the pipeline re-start measurement is extremely valuable in identifying a need to apply chemicals to obviate production loss by line blockage. It should also be noted that the pipeline re-start test measures the shear stress needed to start flow, not the shear stress required to maintain flow at operating rates and it is therefore important to follow the change in pressure drop and hence fluid viscosity after flow commences in such a test. Circumstances can arise in that although the field pressure drop required to start flow can be achieved the fluid viscosity is so high that normal production cannot be achieved because mobility is limited and pipe contents do not re-warm (4). Evaluation of Chemical Inhibitors Given the differences in crude properties apparent from the shear history conditioning detailed above it should be appreciated that the selection of chemical inhibitors to mitigate wax problems should address specific operating problems. If gelation occurs at ambient temperature leading to an excessive re-start pressure then a chemical treatment with pour point depressant (crystal modifier) may be an economic alternative to insulation or displacement of line contents to maintain fluidity. At the same time the calculated wax deposition rates from the laboratory tests may define a requirement to apply a wax dispersant to reduce the tendency for wax crystals to adhere at the wall surface. Again proper evaluation can accurately define the economic arguments for chemical treatment, line insulation, pigging facilities or a combination approach. It should be appreciated that crystal modifiers may act as both pour point depressants and wax dispersants but dependent upon the crude and chemical properties one behaviour may dominate. It is therefore unwise to evaluate chemicals purely on the basis of a simple pour point depression test when wax deposition may be the real field problem. In general chemical inhibitors may be screened in stabilised fluid samples using techniques such as model pipeline re-circulating loops, cold finger static and coaxial cell tests and rheological measurements. In our experience products selected on the basis of stabilised fluid studies always show a similar performance in live samples but only the ultimate live fluid studies can accurately quantify behaviour. A final live fluid study is therefore essential to fully evaluate the feasibility and economics of any chemical treatment which may be suggested as part of the development plan. Page 10 of 14

12 FIELD RESULTS Feedback from the field related to waxy crude behaviour is difficult to gain. In general if a field is producing well there is little incentive to spend money to qualify predictions made from a waxy crude study. However Oilfield Chemical Technology Limited has now been involved in over 30 producing fields where the predicted behaviour of the crude in sub-sea flow lines appears to have matched observed results. Correlations which have proved accurate and have been observed include: Light crude produced to a floating production vessel which was initially treated at the wellhead with chemical inhibitor as a "safety factor" but which was subsequently reduced and eliminated matching predicted behaviour from the laboratory study. 200,000 bpd export pipeline, accurate rheologies required to gauge pressure drop and hence capacity of throughput. As production developed operator estimated a 98% correlation between laboratory predictions and observed pressure drop after allowing for pipe roughness. Predicted line blockage by development of a stable wax deposit in an extended subsea flow line. From the estimates of rate of wax growth and evaluation of inhibitors applied to the field the operator was advised that loss of production could occur unless alternative chemicals were sourced. 4 months later before alternative chemical treatment could be sourced the line totally blocked. Field now abandoned. UKCS waxy crude development was predicted to deposit wax in process equipment and export flow line at high rates. Initial chemicals screened were felt to be ineffective and additional studies were recommended to ensure start up of production was not compromised. This was not initially undertaken leading to an estimated line blockage with 400 tonnes of wax reducing throughput by over 50%. Oilfield Chemical Technology Limited is currently undertaking studies to remediate and prevent further damage. The last two of the above illustrate major problems experienced by the operators during field production but are regarded as successfully validating the techniques used in the laboratory to predict behaviour. Unfortunately in both cases the operators chose to ignore laboratory results as overly pessimistic and relied upon ineffective chemical treatment. More cases like this can only serve to illustrate the need to pay attention to testing laboratory advice as crude streams and development options move away from established norms. The production of difficult crudes with high wax appearance temperatures, high viscosities and wax contents will also see a need to introduce more effective chemical or other treatments. Page 11 of 14

13 This may include dispersant molecules which act upon nucleating species such as asphaltenes to reduce growth, surfactant polymer dispersants which prevent crystals from adhering to each other and line surfaces, selective solvent species and black box methods such as magnetic or ultrasonic treatments. In order to validate the use of any of these as part of an integrated development plan it is essential that accurate laboratory data is available. SAMPLING AND GENERAL CONSIDERATIONS Laboratory tests can be developed to give sufficient point data on crude elements which may be combined to give an overall prediction of how a fluid will behave in the field. However none of this information will be field accurate unless the sample used for test purposes has been gathered in the correct fashion. This is the most crucial element in any wax related work and experience has shown that in many instances sampling techniques are only poorly considered and controlled during well site operations. Sampling points which must be addressed are as follows: 1. If a process system/separator sample is to be taken the well must be producing for some time under steady state conditions. If samples are gathered too soon after flow commences wax may have been lost to the cold flow lines followed by a period of wax elevation as any deposits re-dissolve in hot oil. 2. Samples must be taken as hot as possible and never below the wax appearance temperature. In general a value of at least 40(C is required. 3. Never collect crude in to large containers and subsequently sub-sample to smaller vessels unless this is undertaken by an experienced laboratory which can restore the bulk sample to a homogeneous state. 4. Ensure the well has cleaned up and samples are not contaminated with oil mud filtrate. Taking downhole samples after well clean up will give the most representative reservoir sample but it is very important to realise that this has been taken from only a small interval in the potential field development and it is unwise to make global field predictions based upon this single sample. Segregation with depth in reservoirs frequently leads to viscous waxy deposits overlain by much lighter crude and it is rare that multiple samples have identical wax related properties. Be prepared to undertake a number of reference measurements on differing samples to quantify variability in the field crude. Page 12 of 14

14 Figure 8 gives a general decision tree which can be used to logically characterise crude behaviour identifying areas which require further study and elements which will not present any field problem. Fig.8 WAXY CRUDE STUDY FLOW CHART Destroy sample thermal and shear history by relevant heat soak at pressure if required Fresh sample required No Valid sample for further work? Analyse for wax, asphaltene and resin contents True field conditions obtained using pressurised crude samples at relevant temp and pressure Yes Worst case conditions experienced in dead crude samples - simple screening Measure bubble point if required Problems may be alleviated in live crude Measure wax appearance temperature Above potential operating conditions? No No production problems Yes Measure rheologies as a function of temperature shear rate Measure pour point according to ASTM / IP standards Measure rate of wax deposition under flowing conditions temperatures No Measure pipeline restart pressures Gel indications? Yes No Measure live crude pour point to OCTL methods Are results indicative of production problems? Yes Remedial treatment, production or process control required Page 13 of 14

15 A comprehensive evaluation following this chart and a pro-active dialogue with the engineering design teams will ensure that all options for cost effective development can be considered. However be prepared to spend money on laboratory tests to obtain good data. Too many studies have been compromised by limiting the number of elements measured to characterise fluid behaviour. Considering that pipeline costs are in the tens of millions of pounds and the benefits which could be achieved by efficient design limitations on process and chemical requirements a good laboratory study, as has been proven from field results, is a very low cost investment which can only aid the field development and maximise profitability. REFERENCES (1) V.R. Kruka, E.R. Cadena, T.E. Long; Cloud point determination for crude oils; J. Inst. Pet. March (2) L.T. Wardhaugh, D.V. Boger; Flow characteristics of waxy crude oils; AIChE Journal June (3) S. Misra, S. Baruah, K. Singh; Paraffin problems in crude oil production and transport; SPE PF February (4) A. Uhde, G. Kopp; Pipeline problems resulting from handling of waxy crudes; J. Inst. Pet. March Page 14 of 14