Chapter 4 Saturation. S o. S w (4.1) where: (4.2) 4.1

Transcription

1 4.1 aturatins The pre spaces in undergrund rcks that frm il and gas reservirs are always cmpletely saturated with fluid. In the pres f the reservir, there is never an ccasin r lcatin where nthing exists (i.e., truly "vid space ). The pres are cmpletely filled with sme cmbinatin f the fllwing fluids: (1) il and its assciated impurities in the liquid phase; (2) natural gas and its assciated impurities in the vapr phase; (3) water--either cnnate water r water that flwed r was injected int the reservir. During depsitin, when sediments were being depsited (usually in an aqueus envirnment), the pres were cmpletely saturated with water (i.e., water saturatin was 100% f the pre space). Later, during deep burial, cmpactin, and partial cementatin, the water may have changed in cmpsitin, but the saturatin remained 100% unless hydrcarbns entered the pres and frced the water ut. If the water-saturated pres happen t be near an active hydrcarbn surce rck, such as rganic-rich shale, and the pres are in pressure cmmunicatin with the surce rck, hydrcarbns can enter the pres and ccupy space. Nrmally, the hydrcarbns are less dense than the water, and the resulting buyant frce causes the il r gas t migrate thrugh the prus, permeable rck until it escapes at the surface r is stpped by an impermeable layer that frms a seal. If there is sufficient clsure the hydrcarbn accumulatin may result in a cmmercial il r gas reservir. In the pres f il r gas reservirs, there always remains sme water that was there befre the hydrcarbn entrapment. At any time during the life f an il r gas reservir, the fllwing relatinship must hld true. where: w g 1.0 w g (4.1) il vlume pre vlume water vlume pre vlume gas vlume pre vlume p g p w p (4.2) 4.1

2 It is cmmn fr il r gas saturatin t be zer, but water saturatin is always greater than zer. aturatin is a direct measure f the fluid cntent f the prus rck. It therefre directly influences the hydrcarbn strage capacity f the reservir. Other uses are the identificatin f gas/il r il/water cntacts by changes f residual saturatin with depth, and indirectly it is used as a crrelatin variable t estimate the prductivity f reservir rcks aturatin distributin in reservirs During hydrcarbn accumulatin in the reservir, water saturatin can be reduced t sme small value, typically 5-40%, after which n mre water can escape frm the pres. This ccurs when water saturatin becmes immbile, at the irreducible water saturatin. Petrleum literature cntains several symbls fr water saturatin; wi, wc, wir. Care must be taken t ensure crrect interpretatin f the symbl. The fllwing definitins shuld help. 1) wir -irreducible water saturatin, belw which water cannt flw. 2) wc -cnnate water saturatin existing n discvery f the reservir. It may r may nt be irreducible. Be careful! 3) wi -may mean irreducible, cnnate, r interstitial, which means saturatin amng the interstices, r pres. Interstitial may r may nt signify irreducible. It may be the value n discvery f the reservir, r the value at any time thereafter. wi may als mean initial r riginal, which truly means the water saturatin n discvery, but it may r may nt be irreducible. Density differences between gas and il as well as between il and water result in nrmal reservir situatins in which il flats n water. If there is a free gas phase, the gas flats n the il. Keep in mind that there will be sme water saturatin (at least the irreducible water saturatin) thrughut the reservir, even in the pres at the very tp. Figure 4.1 shws a typical crss sectin f a reservir where all three fluid phases are at mbile saturatins. If a cntainer were filled with il, water, and gas with n prus medium in the cntainer (prsity = 100%), the fluid interfaces wuld be distinct and fluid saturatins wuld be: gas cap g = 100%; 4.2

3 il zne - = 100%; aquifer, water zne w = 100%. Figure 4.1 Crss sectin f reservir shwing vertical segregatin f fluids Hwever, in actual reservirs cmpsed f prus rck, the fluid interfaces are nt s distinct. Nt nly des water exist thrughut the il and gas znes at a saturatin f at least irreducible water saturatin, but the fluid cntacts are generally spread ver a distance f a few feet t tens f feet, depending n the density difference between the fluids and the permeability f the rck. Figure 4.2 shws the spreading f fluid cntacts and the nrmal distributin f fluids in a reservir. Figure 4.2 Nrmal initial fluid distributin in a reservir f unifrm permeability and static equilibrium 4.3

4 In Figure 4.2, nte the fllwing imprtant pints: (1) OWC at 4245 ft.; (2) il-water transitin zne, t 4245 ft.; (3) GOC at ft.; (4) gas-il transitin zne, t ft.; (5) thickness f il-water transitin zne, 6.5 ft; (6) thickness f gas-il transitin zne, 1.0 ft; (7) irreducible water saturatin, 20%; (8) free water level at 4248 ft. and the free il level at 4234 ft. (the levels at which the OWC and GOC wuld ccur in the wellbre in the absence f a prus medium, r in the reservir if it were an pen cntainer with 100% prsity). The spreading f transitin znes is a micrscpic phenmenn which will be discussed mre in Chapter 5. Figure 4.3 is a clse-up f the saturatin distributin acrss the OWC and thrugh the il-water transitin zne. Figure 4.3 Micrscpic crss sectin f OWC and transitin zne Factrs Affecting Fluid aturatins A cmmn methd f btaining fluid saturatins is frm measurements taken n cre samples. Unfrtunately, the fluid cntent in the cre is altered by tw prcesses: 1. the flushing f mud and mud filtrate int the adjacent frmatin, and 2. the release f cnfining pressure as the cre is pulled t surface. 4.4

5 Figure 4.4 illustrates n a micrscpic level the invasin prcess f a water-based mud int an il-bearing frmatin. The tp diagram is prir t being penetrated by the bit, therefre the saturatins present are the cnnate water and il. The middle diagram is after the bit has penetrated the frmatin and fluid invasin has flushed the riginal reservir fluids. Nte the increase in water saturatin during this time. The final diagram shws the gas expansin as the cre is brught t the surface. Figure 4.4 aturatins in Characteristic sand during cring and recvery [CreLab, 1983] 4.5

6 An example f saturatin changes ccurring in the cre frm insitu t surface cnditins is shwn in Figure 4.5. Nte the significant decrease in il saturatin due t the invasin prcess. Als, nte the gas expansin as the cre is brught t surface, subsequently expelling the fluids in the cre. In this illustratin, primarily water is expelled. Finally, as the pressure and temperature are reduced, the il will shrink in vlume, therefre als reducing the saturatin. Figure 4.5 example f saturatin changes ccurring in the cre frm insitu t surface cnditins [Helander,1983] 4.6

7 everal slutins have been prpsed t address these prblems. T minimize the invasin prblem, it is suggested t use il-based muds (OBM) as the cring fluid. Figure 4.6 shws a cmparisn between tw different examples, ne using water-based mud and the secnd using il-based mud. A significant imprvement in btaining riginal reservir saturatins ccurs using il-based mud. Oil 67.6% Wtr 32.4% Oil 53.4% Wtr 46.6% Gas 34.8% Oil 26.7% 67.6% Wtr 38.5% Original After flushing At surface Water-based Muds Oil 50.9% Wtr 49.1% Oil 32.9% Filtrate18% Wtr 49.1% Gas 25.6% Oil 26.7% Wtr 47.7% Original After flushing Oil-based Muds At surface Figure 4.6 Cmparisn f water- and il-based muds n the saturatin distributin Other research has lead t using empirical factrs t crrect measured cre saturatins t riginal cnditins [Amyx, et al.,1963]. Furthermre, as an alternative t cre analysis, gephysical well lgs prvide accurate and cntinuus measurements fr the calculatin f insitu saturatins. Als, capillary pressure measurements n samples prvide saturatin results. electin f the prper cring fluid is essential t btaining meaningful results. The bjectives and desired tests n the cre shwn in Table 4.1 belw prvide a guide fr using suitable cring fluids. 4.7

8 Table 4.1 Cring bjectives and suitable mud types [CreLab, 1983] Measurement f Fluid aturatins In determining the fluid saturatins frm a cre sample, tw techniques are cmmnly emplyed; evapratin f the fluids in the pre space, knwn as the retrt methd, and the leaching f fluids in the pre space, knwn as the Dean-tark extractin methd. In the retrt technique the sample is sealed inside an aluminum cell and then heated in stages frm 400 F t 1100 F. Figure 4.7 is an illustratin f cnventinal retrt apparatus. 4.8

9 Figure 4.7 Picture f a cnventinal retrt [CreLab,1983] The advantages t this methd is the time fr the experiment is shrt, typically less than 24 hurs, and multiple samples can be run simultaneusly. The disadvantages are heating prcess burns il t the pre surfaces. This is knwn as the cking effect and thus results in il recvery less than the initial amunt in the sample. A crrectin factr (Figure 4.8) has been empirically develped t vercme this prblem. Figure 4.8 Retrt il crrectin curve [CreLab, 1983] 4.9

10 A secnd disadvantage f the heating prcess is the remval f bth pre water and water f crystallizatin. The later is the bund water in clays and ther hydrates. ubsequently, the water recvery is t high. Figure 4.9 presents an example f water recvery vs. time and temperature. The first plateau represents the vlume f water in the pres. The secnd plateau is the additinal water due t the vaprizing f the crystallized water. In this way, the retrt can be calibrated fr the given samples. A final disadvantage f this methd is it destrys the sample, therefre n further testing can be achieved. Figure 4.9 Retrt water calibratin curves [CreLab,1983] Example 4.1 The crrected vlumes f il and water recvered frm the retrt methd were 4.32 and 1.91 ml, respectively. Prir t this experiment, the bulk vlume was measured t be ml and the grain vlume was ml. Determine the saturatins f this sample. 4.10

11 lutin The fllwing stepwise prcedure is presented. a. The pre vlume f the sample is, p = b g = 8.64 ml. b. The prsity f the sample is 24.7%. c. Applying Eq. (4.2), the il and water saturatins are: w r p wr p % % 8.64 d. The gas saturatin cannt be measured and therefre is determined by the vlume balance (Eq. 4.1), g 1 w 28% The Dean-tark extractin methd uses the vapr f a slvent t rise thrugh the cre and leach ut the il and water. The water cndenses and is cllected in a graduated cylinder. The slvent and il cntinuusly cycle thrugh the extractin prcess. A typical slvent is tluene, miscible with the il but nt the water. Figure 4.10 is an illustratin f the apparatus. Figure 4.10 Dean-tark Apparatus [CreLab,1983] 4.11

12 The vlume f the water cllected relative t the pre vlume prvides an estimate f the water saturatin. The il saturatin is determined by, W wet W dry W wtr (4.3) p* that is, by the weight lss nt accunted fr by the water. Equatin (4.3) requires: Example 4.2 a. the weight f the cre prir t the test (nt cleaned!) b. the weight f the cre after the test, cleaned and dried c. the pre vlume frm ther methds d. an estimatin f the il density The fllwing prcedure illustrates the usefulness f the extractin methd. a. Obtain the mass f the saturated sample = 57 gms. b. Determine the bulk vlume by nndestructive means= 25 cc c. Determine the il density = 0.88 gm/cc d. Place the sample in the extractin apparatus and heat the slvent. Recrd the vlume f water cllected and when the reading becmes cnstant stp. w = 1.4 ml e. After cling, remve the cre and dry, btain dry weight = 53 gms. f. Using the saturatin methd, resaturate the sample with fresh water ( = 1.00 gm/cc) and weigh. 58 gms. g. Calculate the pre vlume and prsity, p cc % h. Calculate the water saturatin (Eq. 4.2), w % 5 i. Calculate the il saturatin (Eq. 4.3), * % 5*

13 j. Calculate the gas saturatin (Eq. 4.1), g % The advantages f the Dean-tark methd are the accuracy, the il and water measurement are n the same sample and the cre can be used fr further analysis. The primary disadvantage f this methd is the lng time it takes t cmplete the measurement; smetimes weeks. Als, it has been suggested il in small pre thrats and channels are bypassed. 4.13