The development study on gas recycling injection in Yaha Gas Condensate Field, Tarim Basin, China

Vol. 46 No. 6SCIENCE IN CHINA(Series D)June 2003The development study on gas recycling injection in Y ahaGas Condensate Field. Tarim Basin. ChinaSUN Longde(孙龙德), SONG Wenjie(宋文杰)& JIANG Tongwen(江同文)Tarim Oil Field Company, PetroChina Company Limited, Korla 841000, ChinaCorrespondenceshouldbeaddressedtoSunLongde(email:sunld@tz.cnpc.com.cn)Received December 19, 2002Abstract The Yaha Gas Condensate Field is the largest one discovered in China so far. In order toenhance the condensate recovery, gas recycling at 50 MPa has been implemented in Yaha fieldSuch high-pressure injection is very rare throughout the world. This paper offers the description ofthe geology and fluid phase behavior of Yaha field. The reservoir productivity and injectivity equations, the optimal development and well patterns are presented. The follow-up investigation of thedesign implementation leads to the reasonable adjustment of the original development plan. Theperforation principle and program of gas recycling for Yaha field are set up. The numerical simulation is used to predict the future production performance of gas recycling. Finally, a complete set ofhigh-pressure gas recycling technology suite has been established. The field has come on-streamfor one and a half year with condensate throughput of 590000 t in 2001, and fat profit returned. Thesuccess of high-pressure gas recycling in the Yaha field is of considerable importance from thetheoretical and practical points of view and will considerably benefit the future development of othergas condensate fields in ChinaKeywords: Tarim Basin, gas condensate, phase behavior, development pattern, gas recycling, numerical simul ation.Yaha gas Condensate Field is located in Kuche County, Xinjiang, China, and within the Yahafault zone of Luntai fault-block in the middle Tabei Uplift in Tarim Basin(fig. 1). The original8500Oil fielduche deAnAkesu/-E Tabei upliftYaha Condensate tas Field Beibu depressionter upliftHetian中国煤化工CNMHGFig 1. Schematic location map of Yaha Gas Condensate FieldSCIENCE IN CHINA (Series D)gas-equivalence in place in Yaha field is estimated to be 25232X 10m with the original gasplace(OGIP)of 226.51X10m and the original condensate in place(OCIP) of 1573.8X"t. Itranks as the largest gas condensate reservoir delineated in China to date, and characterized by thereservoir conditions of high-temperature, high-pressure, high content wax and condensate, smalldifference between the original reservoir pressure and dew-point pressure. The oil-and gasbearing formations contain three reservoirs, the bottom Neogene sandstone, the Bottom Eogenesandstone and the Top Cretaceous sandstone. The Bottom Neogene sandstone (Ni) is a layeredgas condensate reservoir with edge water. The Bottom Eogene sandstone and Top Cretaceoussandstone(E+K) are a massive gas pool with bottom water(fig. 2)Fig. 2. Well pattern of Yaha2-3 unit on the top structure map of E+K formation. I, Injector; 2, producer1 Development study of gas recycling1. 1 Structure and reservoir of yaha fieldThe study of gas recycling concentrates on Yaha 2-3 unit of considerable interest, the majorreservoir in Yaha field. The trend of the Yaha structural belt is NEe-sww!l with the structuraltop depth shallowing from west to east (fig 3). The depth of pay zone averages about 5000 m. Theratio of long vs short axis of the anticline is about 9: 1. The structural dip angles of the formationin the south and north are 4.5 and 2.7 degrees, respectively. The south side of the structure isYH3OIYH30Z397541254200中国煤化工CNMHGFig 3. The W-E section of Yaha 2-3 gas reservGAS RECYCLING INJECTION IN YAHA GAS CONDENSATE FIELDbound by a normal fault. The gas-bearing formation is dominated by silty, medium and fine sandstones deposited in the fluvial and lacustrine settings 21.stone, Nli, is characterized as a gas-bearing reservoir with an average po-rosity of 15.6%(moderate-low), air permeability of 51.1X10i m"(moderate), and good homogeneity and connectivity. The single sand net pay typically ranges from 9 to 20 m thick.The Eogene and Cretaceous (E+K) formations are also gas-bearing. The Bottom Eogenesandstone has good reservoir quality with the average moderate porosity of 15.4%, high air permeability of 2286X 10 i m, and good homogeneity and connectivity. The Top Cretaceoussandstone is characterized by low porosity and low-moderate permeability(average porosity of14%0, air permeability of 47.0X10i m)and high heterogeneity and poor connectivity. The netpay of a single sand bed is typically 25--40 m1. 2 Phase behavior of reservoir fluid1. 2.1 Fluid compositions. The reservoir fluid composition is as follows: C1+N2: 77.7181.95mol%;CO2+C11. 81-16.75 mol%; C7+: 5.48--7.7 mol%. The initial reservoirpressure and temperature are 56MPa and 138C, respectively. The initial producing gas-oil ratiosrange from 925 to 1565 m/m, implying that the reservoir fluids are rich gas condensate2.2 Depletion. The dew-point pressure of the reservoir fluid is above 50MPa with 2-5MPa lower than the initial reservoir pressure. The maximum liquid dropout percentage by volumeabove 20%o during constant volume depletion process and the corresponding pressures are in therange from 20 MPa to 32 MPa. The Ci mole fraction gradually increases with decreasing pressurehowever, the C2 and C3 mole fractions vary slightly, and the C4+ mole fraction decreases. Theheavier the components, the faster the compositions decline, indicating that most of the heaycomponents are lost in the reservoir with continuous pressure drawdown1. 2.3 Phase diagram. Fig. 4 clearly depicts the phase behavior of reservoir fluid as follows1)The gas condensate fluid is nearly-saturated at reservoir condition. The dew-point is about4MPa lower than the initial reservoir pressure2)The dew-point pressure is close to the cricondenbar, the reservoir temperature is betweenthe critical temperature and the cricondentherm. Considerable retrograde condensation occursduring pressure depletion and often is responsible for low condensate recovery3)Maximum liquid dropout of more than 20vol. during constant composition expansionshows that the subsurface fluid is a rich gas condensate1.2.4 The influence of porous media on phase behavi n\中国煤化工ure: The resultsof both the laboratory experiments and the phase equilTHCNMHGn show that thedifference between the dew-point pressures determined in the presence of porous media and thosedetermined in the conventional PVT cell is very small. As a result, the dew-point pressure in PVTSCIENCE IN CHINA (Series D)VoL 46Pm=52.47 MPaP=49.87 MPaT=113℃010000300400T℃Fig 4. Phase diagram of the fluid taken from well YH301(5109--5117 m), and the Eogene formation. Pe, Critcal pressure, T critical temperature; Pm, cricondenbar; Tm, cricondenthermcell can be directly used for the optimalchoice of gas recycling pressure25YH6condensate reservoir withoutfurther consideration of the influence of po-rous media3.(ii)Retrograde liquid saturationDuring constant volume depletion, the liquidsaturation in porous media is obviously largerthan that in pvt cell because of retrograde102030P/MPacondensation(tablel, fig. 5). This is due tothe phenomena that the desorption of heavy vg. 5. The effect of porous media on liquid dropout,in wellsYH6 and YH301components from the porous surfaces withdeclining pressure leads to the relative content increase of the heavy components existing in thequilibrium gas phase system, resulting in more liquid dropout.These results also show that the loss problem of the valuable heavy components is much m-re serious than expected during gas reservoir depletion. Therefore, the gas recycling scheme isinitiated to maintain the reservoir pressure above the dew point in order to enhance oil recovery 4.Table 1 The maximum percentage liquid volume and liquid saturation of fluids in wells YH6 and YH301YH6YH301Parameterwith media中国煤化工16.32154CNMHGRetrograde liquidsaturation(%0)18323.8165214829.7GAS RECYCLING INJECTION IN YAHA GAS CONDENSATE FIELD1.3 Establishment and analysis of productivity and injectivity1.3. 1 Productivity analysis. (i)Open-flow capacity of a single well: On the basis of the welltest data, the single-point formula, eq. (1), is used to predict the open-flow capacity in Yaha reser√1+48Pb-1dimensionless pressure defined by PDPe-P) bixtuwhere OAOF, absolute open-flow potential, 10 m/d; Q, gas equivalence of the mixture of conden-sate and gas, 10m/d; Pp,(ii) Single wellproductivity equations of the formations, N E and K: The relationship of open-flow potentialand flow capacity, and the overall average productivity equation of individual reservoirs are com-bined to derive the following average equations of a single wellNF:Pk-P=142y+0.780702,QAoF=54.68×10m3/dE:PR2-P2=5.45730+01189QAoF=146.39×104m/dK:P2-P7=6747530+109670(4)QAOF=1.84×10m/d,where niis lower sandstone reservoir of the Neogene-age formation; E, lower sandstone reservoirof the Eogene-age formation; K, top sandstone reservoir of the Cretaceous-age formation; LAOF,absolute open-flow potential, 10 m/d; 2, gas-equivalent production rate of the mixture of con-densate and gas, 10/d; PR, average reservoir pressure, MPa; Pwf, bottom-hole flowing pressurMPa(iii) Analysis of reasonable production pressure difference: Because of no sand productionduring the well tests, three key aspects should be concerned in the reasonable determination ofproduction pressure difference, i.e. no considerable retrograde condensate near the well; @2avoiding accumulation of liquid in the wellbore; no hydrate formation at the wellhead. Theabove-mentioned three considerations lead to the reasonable production pressure difference of 2MPa.(iv) Productivity prediction: As shown in table 2, the single well average productivity inach formation is estimated at the condition of production pressure drop of 2 MPaTable 2 The single well average productivityOpen-tlow pYH中国煤化工 ctivity/lo'm'.d10CNMHG26KSCIENCE IN CHINA (Series D)1.3.2 Injectivity analysis. Because the produced gas and the injected gas are mutually dissolu-ble remaining as the single gas phase, and no effect of relative permeability and capillary pressurebetween them occurs, the above productivity equations can be used alternatively to predict injec-tivity. (i) Injectivity equation: For the Nii dlower sandstone reservoir of the Neogene-age formation)and e (lower sandstone reservoir of the Eogene-age formation) formation sandstones, the equationP2-P2=0.03923P2(kh)03g+001847×105P2(kh)03g2For K(top sandstone reservoir of the Cretaceous-age formation) formation sandstone, the equationbecomesP2-P=9.1575/2(Kh)10292+100631970×105P2(Kh)2022(6)where Q injection rate, 10,m/d; PR, average reservoir pressure, MPa; Pwf.i, bottom-hole flowinpressure of injection well, MPa; Kh, flow capacity, 10'imm. m.(ii) The level of reservoir pressure maintenance: For the purpose of retarding liquid dropout, keeping the average bottomholeflowing pressure(BHFP) around the dew point, and avoiding a too high reservoir pressure, itseems reasonable that the gas recycling operation should maintain the reservoir pressure at 54The calculated maximum bottom-hole flowing pressure of injectors is 63 MPa, the corre-sponding maximum wellhead pressure 50 MPa, and the single well injection rate 20--30X104m/d. When the producers and compressors are under stable operation, in order to achieve theproduction-injection balance, the injection BHFP should be kept at 0.95 of the maximum BHFP,i.e. 59.85 MPa. Thus the injection pressure drop is 5.85 MPa. (iii) Analysis of maximum injectpressure drop: When the average reservoir pressure remains 54 MPa, the calculated maximuminjection pressure drop is 9.093 MPa for Nii formation, and 9.393 MPa for E+K formation. (iv)Average injection rate of single injector: When the reservoir pressure is kept at 54 MPa, the aver-age injection rate of single injector is 30x10 m/d for N,i formation, and 75x104 m/d for E+KformationIf the pressure maintenance level is the same as above and the injection flowing pressure is95% of maximum BHFP, the average injection rate of single injector is 22. 6x10- m/d for Ni for-mation and 56.9x104 m/d for E+K formation1.4 Development scheme, well pattern and project optimization 51The Ni and E+k formations belong to two different pressure systems. They can be devel-oped independently due to their reserves abundance and productivity, so they are divided into twosets of production zone中国煤化工according to the numerical simulation, the econCNMHproduction is about 900 m for N and 725 m for E+K, respedne upper nmit well spacing ofthe injectors and producers is 1100 m. Finally the reasonable well spacing is determined at 800-1100m.GAS RECYCLING INJECTION IN YAHA GAS CONDENSATE FIELDThe conclusive results of the optimization of well pattern and development scheme of E+Kormation, are that the best well pattern is to inject gas along the axis of Yaha2-3 unit, and locateproduction wells on the axis and edge of the structurelo. The best development scheme is to firstdeplete the initial reservoir pressure to some lower pressure at which gas is then recycledThe counterpart results for Ni formation are that the best way is to inject gas at the axis orsaddle, produce gas at axis or top of the structure, and partially maintain the reservoir pressure bygas recycling.The project optimization shows that 17 new wells are to be drilled (fig. 2). The total wellnumber is 22, including 2 injectors and 4 producers at N formation, 6 injectors and 9 producers atE+k formation, and l monitoring well1.5 The designed development index and implementation results 71On the whole, the production of Yaha gas condensate field shows that each actual productionindex is in agreement with the project development design level (table 3). In the light of the predicted production index of Yaha gas condensate field by numerical simulation, the gas injectionstarts at the time of field production, the whole life of injection is 9 years. The annual condensateield is 500000 to 600000 t. The stable production life is 6 years. During gas injection, reservoirpressure will decrease gradually from 5524MPa to 51.79MPa. The average gas production rate is4.59% of oGip per year. At the end of gas injection, the condensate recovery percentage is about30.85% of OGIP and natural gas about 3. 06%of OGIP. The predicted ultimate condensate and gasrecovery are estimated to be about 48. 14% and 64.1%, respectively, after 25-year production. Thisstudy indicates that the condensate recovery by gas recycling in Yaha gas condensate field is 25%higher than that of depletionGas recycling in Yaha field started in October 2000. Condensate of 600000 t and liquefiedpetroleum gas of 30000 t were produced in 2001. The gas injection rate is 2.9 million m' per day,the designed maximum injection pressure is 50 MPa, however, the actual pressure is 43 to 46 MPaThe total investment of this project is 0.838 billion Yuan(RMB) with investment payback time of2.5 years. The total construction investment of l million t condensate productivity is only 1.676billion Yuan(RMb)which is even much less than the average 2. 1 billion Yuan(RMB) investmentof domestic construction of l million t oil productivitTable 3 Comparison of the actual production and predicted data in June 2001Average single well Average single wellProductionoil production gas production rate GOR/m2.tIGas productivity Open-flow potenFormationrate/t i d-l/10+m3dlindex/m: MPa 2 tial/10-m. dlactual predic. actual predic. actual中国煤化工pedc.aual75.810.1513.3115571533CNMHG 55E+K138.7165423.2631.321677172522.151137155014183SCIENCE IN CHINA (Series D)2 Conclusion(1) Yaha Gas Condensate Field is a relatively simple elongated axial anticline with pay zonebeing clastic rocks dominated by sandstone. The Ni formation sandstone is a gas condensate reservoir with low-moderate porosity, medium permeability, good homogeneity and connectivityThe Eogene and Cretaceous formations are separated by a thin interbed. The Eogene formationsandstone is a gas condensate reservoir with moderate porosity, good homogeneity and connec-tivity. The Cretaceous gas reservoir has the characteristics of low permeability and severe hetero-geneity and moderate connectivit(2) The Yaha reservoir fluid is rich in gas condensate with dew-point pressure near the cricondenbar, Retrograde liquid is built up rapidly with decreasing pressure isothermally. Theheavier components condensed earlier from the initial gas phase. Considerable loss of condensatein the reservoir leads to very low condensate recovery(3)The study shows that the open hole potential is 12x10 m/d to 146x10 m /d, and the reasonable pressure drop is 2 MPa. The gas recycling development with reservoir pressure kept at64MPa can reduce the retrograde liquid dropout. The ultimate condensate recovery due to gasrecycling and well pattern optimization is estimated to be 25% higher than that of depletion(4)The gas recycling development scheme has been carried out for one and a half year. Allthe predicted technical parameters are consistent with the actual production index, conforming theonclusions in this study. Furthermore, the theoretical achievements and the technology obtainedin this study, together with the successful example of Yaha gas recycling at high pressure, are of agreat theoretical and practical significance for the high efficient development of other gas conden-sate fields in ChinaAcknowledgements This work was supported by the Tenth Five-year Key Technologies R&D Programme( Grant No.2001BA605A02)References1. 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