裂缝性地层水力裂缝张性起裂压力分析
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摘要
室内实验和矿场压裂实践表明,裂缝性地层水力裂缝在近井区域可能扩展为复杂的径向缝网,这与均质地层水力压裂产生的平面对称双翼裂缝具有巨大差异。由于水力裂缝延伸形态受起裂和延伸2个过程的控制,为此,研究裂缝性地层水力裂缝起裂机制是认识径向缝网扩展的前提。基于弹性力学和岩石力学理论,考虑天然裂缝与射孔孔眼相交,结合张性起裂准则,建立裂缝性地层水力裂缝沿天然裂缝张性起裂的起裂压力计算模型。计算结果表明:天然裂缝与孔眼相交点越靠近孔眼指端,张性起裂压力越小;天然裂缝与孔眼相交点越靠近孔眼顶部,张性起裂压力越小;受天然裂缝和水平地应力方位的影响,井眼周向上不同方位孔眼的张性起裂压力差可能急剧减小,这种作用效应将导致水力裂缝从井眼周向上不同方位的孔眼同时起裂延伸,产生径向缝网。实例计算表明,文中建立的起裂压力模型计算精度高,计算结果可靠,可用于裂缝性地层水力裂缝起裂压力计算和径向缝网扩展分析。
Laboratory experiments and field fracturing practices have shown that complex radial fracture-network extended system is easily formed in the area around borehole due to the effect of natural fractures during fracturing of fractured reservoirs,which greatly differs from the single,bi-wing,symmetric planar fracture generated in the homogeneous reservoirs.As the forming of radial fracture-network is mainly controlled by the initiation and propagation of hydraulic fracture at the beginning stage of hydraulic fracturing,studying the initiation mechanism of hydraulic fracture for fractured formations is the prerequisite for understanding the forming of radial fracture-network.Based on the theories of elastic mechanics and rock mechanics,taking account of the spatial relation of natural fractures and perforations under intersection,and integrating with the criterion of tensile initiation,the calculating model of initiation pressure in which the hydraulic fracture is tensile initiation along natural fractures is established.The calculation results show that the deeper position of natural fractures intersecting with perforation is,the lower the tensile initiation pressure is.Tensile initiation pressure in the position where natural fractures intersect the top of perforation is the minimum value.The difference of tensile initiation pressure may sharply decreases from perforations in different azimuths around borehole due to the effect of natural fractures and geostress azimuth,which leads to the extending of hydraulic fracture simultaneously from perforations in different azimuths and forms radial fracture-network.The example calculation results indicate that this calculating model can obtain high calculating accuracy and reliable calculating results,and can be used to calculate initiation pressure of fractured formations and analyze the probability of radial fracture-network propagation during fracture-network fracturing.
Laboratory experiments and field fracturing practices have shown that complex radial fracture-network extended system is easily formed in the area around borehole due to the effect of natural fractures during fracturing of fractured reservoirs,which greatly differs from the single,bi-wing,symmetric planar fracture generated in the homogeneous reservoirs.As the forming of radial fracture-network is mainly controlled by the initiation and propagation of hydraulic fracture at the beginning stage of hydraulic fracturing,studying the initiation mechanism of hydraulic fracture for fractured formations is the prerequisite for understanding the forming of radial fracture-network.Based on the theories of elastic mechanics and rock mechanics,taking account of the spatial relation of natural fractures and perforations under intersection,and integrating with the criterion of tensile initiation,the calculating model of initiation pressure in which the hydraulic fracture is tensile initiation along natural fractures is established.The calculation results show that the deeper position of natural fractures intersecting with perforation is,the lower the tensile initiation pressure is.Tensile initiation pressure in the position where natural fractures intersect the top of perforation is the minimum value.The difference of tensile initiation pressure may sharply decreases from perforations in different azimuths around borehole due to the effect of natural fractures and geostress azimuth,which leads to the extending of hydraulic fracture simultaneously from perforations in different azimuths and forms radial fracture-network.The example calculation results indicate that this calculating model can obtain high calculating accuracy and reliable calculating results,and can be used to calculate initiation pressure of fractured formations and analyze the probability of radial fracture-network propagation during fracture-network fracturing.
引文
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[18]金衍,张旭东,陈勉.天然裂缝地层中垂直井水力裂缝起裂压力模型研究[J].石油学报,2005,26(6):113–114.(JIN Yan,ZHANG Xudong,CHEN Mian.Initiation pressure models for hydraulicfracturing of vertical wells in naturally fractured formation[J].ActaPetrolei Sinica,2005,26(6):113–114.(in Chinese))
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[20]陈勉,金衍,张广清.石油工程岩石力学[M].北京:科学出版社,2008:168–169.(CHEN Mian,JIN Yan,ZHANG Guangqing.Rock mechanics of petroleum engineering[M].Beijing:Science Press,2008:168–169.(in Chinese))
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[22]黄宣国.空间解析几何[M].上海:复旦大学出版社,2004:41–44.(HUANG Xuanguo.Spatial analytic geometry[M].Shanghai:FudanUniversity Press,2004:41–44.(in Chinese))
[2]MCDANIEL B W,MCMECHAN D E,STEGENT N A.Proper use ofproppant slugs and viscous gel slugs can improve proppant placementduring hydraulic fracturing applications[C]//SPE Annual TechnicalConference and Exhibition.[S.l.]:Society of Petroleum EngineersInc.,2001:SPE 71661.
[3]陈勉,周健,金衍,等.随机裂缝性储层压裂特征实验研究[J].石油学报,2008,29(3):431–434.(CHEN Mian,ZHOU Jian,JINYan,et al.Experimental study on fracturing features in naturallyfractured reservoir[J].Acta Petrolei Sinica,2008,29(3):431–434.(in Chinese))
[4]FISHER M K,WRIGHT C A,DAVIDSON B M,et al.Integratingfracture mapping technologies to optimize stimulations in the Barnettshale[C]//SPE Annual Technical Conference and Exhibition.[S.l.]:Society of Petroleum Engineers Inc.,2002:SPE 77441.
[5]FISHER M K,HEINZE J R,HARRIS C D,et al.Optimizinghorizontal completion techniques in the Barnett shale usingmicroseismic fracture mapping[C]//SPE Annual Technical Conferenceand Exhibition.[S.l.]:Society of Petroleum Engineers Inc.,2004:SPE90051.
[6]HUBBERT M K,WILLIS D G.Mechanics of hydraulic fracturing[J].Transaction of the American Institute of Mining,Metallurgical andPetroleum Engineers,1957,210:153–163.
[7]EATON B A.Fracture gradient prediction and its application in oilfieldoperation[J].Journal of Petroleum Technology,1969,21(10):1 353–1 360.
[8]HAIMSON B,FAIRHURST C.Initiation and extension of hydraulicfractures in rocks[J].SPE Journal,1967,7(3):310–318.
[9]黄荣樽.地层破裂压力预测模式的探讨[J].华东石油学院学报,1984,(4):335–346.(HUANG Rongzun.A model for predictingformation fracture pressure[J].Journal of East China PetroleumInstitute,1984,(4):335–346.(in Chinese))
[10]李传亮,孔详言.油井压裂过程中岩石破裂压力计算公式的理论研究[J].石油钻采工艺,2000,22(2):54–56.(LI Chuanliang,KONGXiangyan.A theoretical study on rock breakdown pressure calculationequations of fracturing process[J].Oil Drilling and ProductionTechnology,2000,22(2):54–56.(in Chinese))
[11]刘翔.垂直射孔井地应力及破裂压力研究[J].钻采工艺,2008,31(2):36–38.(LIU Xiang.Study on earth stress and fracturingpressure in vertical perforated wells[J].Drilling and ProductionTechnology,2008,31(2):36–38.(in Chinese))
[12]FALLAHZADEH S H,SHADIZADEH S R,POURAFSHARY P.Dealing with the challenges of hydraulic fracture initiation indeviated-cased perforated boreholes[C]//Trinidad and Tobago EnergyResources Conference.[S.l.]:Society of Petroleum Engineers Inc.,2010:SPE 132797.
[13]YEW C H,LI Y.Fracturing of a deviated well[J].SPE ProductionEngineering,1988,3(4):429–437.
[14]HOSSAIN M M,RAHMAN M K,RAHMAN S S.A comprehensivemonograph for hydraulic fracture initiation from deviated wellboresunder arbitrary stress regimes[C]//SPE Asia Pacific Oil and GasConference and Exihibition.[S.l.]:Society of Petroleum EngineersInc.,1999:SPE 54360.
[15]HOSSAIN M M,RAHMAN M K,RAHMAN S S.Hydraulic fractureinitiation and propagation:roles of wellbore trajectory,perforation andstress regimes[J].Journal of Petroleum Science and Engineering,2000,27(3):129–149.
[16]陈勉,陈治喜,黄荣樽.大斜度井水压裂缝起裂研究[J].石油大学学报:自然科学版,1995,19(2):30–35.(CHEN Mian,CHENZhixi,HUANG Rongzun.Hydraulic fracturing of highly deviatedwells[J].Journal of University of Petroleum:Natural Science,1995,19(2):30–35.(in Chinese))
[17]罗天雨,郭建春,赵金洲,等.斜井套管射孔破裂压力及起裂位置研究[J].石油学报,2007,28(1):139–142.(LUO Tianyu,GUOJianchun,ZHAO Jinzhou,et al.Study of fracture initiation pressureand fracture starting point in deviated wellbore with perforations[J].Acta Petrolei Sinica,2007,28(1):139–142.(in Chinese))
[18]金衍,张旭东,陈勉.天然裂缝地层中垂直井水力裂缝起裂压力模型研究[J].石油学报,2005,26(6):113–114.(JIN Yan,ZHANG Xudong,CHEN Mian.Initiation pressure models for hydraulicfracturing of vertical wells in naturally fractured formation[J].ActaPetrolei Sinica,2005,26(6):113–114.(in Chinese))
[19]金衍,陈勉,张旭东.天然裂缝地层斜井水力裂缝起裂压力模型研究[J].石油学报,2006,27(5):124–126.(JIN Yan,CHENMian,ZHANG Xudong.Hydraulic fracturing initiation pressuremodels for directional wells in naturally fractured formation[J].ActaPetrolei Sinica,2006,27(5):124–126.(in Chinese))
[20]陈勉,金衍,张广清.石油工程岩石力学[M].北京:科学出版社,2008:168–169.(CHEN Mian,JIN Yan,ZHANG Guangqing.Rock mechanics of petroleum engineering[M].Beijing:Science Press,2008:168–169.(in Chinese))
[21]郭建春,辛军,赵金洲,等.酸处理降低地层破裂压力的计算分析[J].西南石油大学学报:自然科学版,2008,30(2):83–86.(GUOJianchun,XIN Jun,ZHAO Jinzhou,et al.The calculation analysis ofdecreasing formation fracturing pressure by acidizing pretreatment[J].Journal of Southwest Petroleum University:Science and Technology,2008,30(2):83–86.(in Chinese))
[22]黄宣国.空间解析几何[M].上海:复旦大学出版社,2004:41–44.(HUANG Xuanguo.Spatial analytic geometry[M].Shanghai:FudanUniversity Press,2004:41–44.(in Chinese))