页岩气水平井压裂过程中水泥环完整性分析
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摘要
页岩气井工程实践表明套管压裂易导致水泥环完整性破坏,进而引发环空带压。统计数据表明:中国涪陵页岩气田投产井166口,出现环空带压井占比达79.52%。进一步分析研究表明:一级套管头(生产套管和技术套管之间)压裂前后带压比例从14.85%提升至50.05%;二级套管头(技术套管和表层套管之间)压裂前后带压井比例从15.84%提升至53.01%,充分说明页岩气井压裂对环空带压影响较大。针对该问题,基于页岩气井压裂工程实际,在考虑压裂液摩擦生热和排量对换热系数影响的基础上,建立了压裂过程中井筒温度场模型和套管—水泥环—地层组合体有限元模型,采用解析法和数值法相结合的方式,计算了页岩气井压裂过程中瞬态温度—压力耦合作用下水泥环径向、切向应力变化规律,开展了井筒内压、压裂液排量、初始温度、弹性模量、泊松比对水泥环应力影响的敏感性分析,并基于Mohr-Coulomb准则对水泥环是否失效进行了分析。研究结果表明:(1)页岩气井压裂过程中,水泥环温度随时间发生剧烈变化,且内外壁之间存在显著温差,该温差随压裂作业的进行先增大、后减小。(2)页岩气井压裂过程中,水泥环径向、切向应力随时间不断变化,径向应力先减小、后增大,切向应力先降低、后升高,然后再降低。依照Mohr-Coulomb准则分析可知,压裂过程中水泥环易发生拉伸破坏,压裂初期为水泥环失效的"风险段"。(3)降低井筒内压,可以显著降低水泥环径向、切向应力;降低压裂液排量,水泥环径向、切向应力有所降低,但降低幅度不明显;提升压裂液初始温度,将会提高水泥环径向应力、降低水泥环切向应力;水泥石弹性模量降低至4 GPa时,水泥环径向、切向应力低于水泥石抗拉、抗压强度,在压裂过程中有利于保持水泥环的完整性;提高水泥石泊松比,对径向应力影响极小,可忽略不计,但可以有效降低水泥环切向应力。依据计算结果设计了一种新型水泥浆体系,降低了水泥石弹性模量。应用该水泥桨的工程实践表明,压裂后并未出现环空带压情况,保证了分段压裂后页岩气井的安全生产。研究结果可为页岩气井压裂过程中水泥浆设计以及井筒完整性控制提供参考。
Shale gas well engineering practice demonstrates that volume fracturing easily leads to the integrity failure of cement sheaths, thus causing sustained casing pressure(SCP). According to statistics, the SCP wells account for as high as 79.52%among 166 brought in wells in China's Fuling shale gas field. The analysis has shown that the ratio of SCP is very low before fracturing, but it increases substantially after fracturing. The ratio of the first-grade casing head(between surface casing and intermediate casing) increases from 14.85% to 50.05%, meanwhile, that of the second-grade casing head(between intermediate casing and production casing) increases from 15.84% to 53.01%. This fully indicates that the influence of casing fracturing on sustained casing pressure of shale gas wells is relatively greater.To address this problem, a wellbore temperature field model is established so as to obtain the required input parameters of the dynamic temperature boundary, considering the friction heat of liquid and the influence of fracturing fluid on heat exchange coefficient, and at the same time, a numerical model of casing-cement sheath-formation is proposed and analyzed. Sensitivity analysis is conducted on the influences of internal wellbore pressure, fracturing fluid displacement, initial temperature, Young's modulus as well as Poisson's ratio on the cement sheath stress. Furthermore, the cement sheath integrity is evaluated by using Mohr-Coulomb failure criteria.The results show that:(1) during the fracturing of shale gas wells, the temperature of the cement sheaths change drastically with time. Besides, there exists a significant temperature difference between inner and outer walls, which increases at first and then decreases throughout the fracturing operation.(2) During the fracturing of shale gas wells, the radial and tangential stresses of the cement sheath continuously change over time. The radial stress decreases first and then increases. The tangential stress decreases first, then rises and finally decreases again. Judging from the Mohr-Coulomb failure criterion, cement sheaths are prone to tensile failure, and the initial stage of fracturing is the "risky phase" for the failure of cement sheaths.(3) Reducing internal wellbore pressure can effectively decrease the radial and tangential stresses of cement sheath. Besides, the reduction of fracturing fluid displacement decreases the radial and tangential stresses of the cement sheath, however, the decreases are not obvious. Elevation of initial fracturing fluid temperature can increase the radial stress of the cement sheath and decrease the tangential stress. In addition, reducing the Young's modulus significantly decreases the radial and tangential stresses of the cement sheath. When the Young's modulus reaches a certain value, the tangential stress is lower than the tensile strength. As well, the radial stress is lower than compressive strength. Elevation of the Poisson's ratio of the cement sheath can decrease the tangential stress.Based on the calculated results, a new cement slurry system was designed, which can decrease the Young's modulus of the cement sheath. Engineering practice demonstrates that there is no occurrence of SCP after multistage hydraulic fracturing, thus ensuring the safe production of shale gas wells. Besides, the results of this study can provide a reference for cement slurry design and wellbore integrity control during fracturing of shale gas wells.
Shale gas well engineering practice demonstrates that volume fracturing easily leads to the integrity failure of cement sheaths, thus causing sustained casing pressure(SCP). According to statistics, the SCP wells account for as high as 79.52%among 166 brought in wells in China's Fuling shale gas field. The analysis has shown that the ratio of SCP is very low before fracturing, but it increases substantially after fracturing. The ratio of the first-grade casing head(between surface casing and intermediate casing) increases from 14.85% to 50.05%, meanwhile, that of the second-grade casing head(between intermediate casing and production casing) increases from 15.84% to 53.01%. This fully indicates that the influence of casing fracturing on sustained casing pressure of shale gas wells is relatively greater.To address this problem, a wellbore temperature field model is established so as to obtain the required input parameters of the dynamic temperature boundary, considering the friction heat of liquid and the influence of fracturing fluid on heat exchange coefficient, and at the same time, a numerical model of casing-cement sheath-formation is proposed and analyzed. Sensitivity analysis is conducted on the influences of internal wellbore pressure, fracturing fluid displacement, initial temperature, Young's modulus as well as Poisson's ratio on the cement sheath stress. Furthermore, the cement sheath integrity is evaluated by using Mohr-Coulomb failure criteria.The results show that:(1) during the fracturing of shale gas wells, the temperature of the cement sheaths change drastically with time. Besides, there exists a significant temperature difference between inner and outer walls, which increases at first and then decreases throughout the fracturing operation.(2) During the fracturing of shale gas wells, the radial and tangential stresses of the cement sheath continuously change over time. The radial stress decreases first and then increases. The tangential stress decreases first, then rises and finally decreases again. Judging from the Mohr-Coulomb failure criterion, cement sheaths are prone to tensile failure, and the initial stage of fracturing is the "risky phase" for the failure of cement sheaths.(3) Reducing internal wellbore pressure can effectively decrease the radial and tangential stresses of cement sheath. Besides, the reduction of fracturing fluid displacement decreases the radial and tangential stresses of the cement sheath, however, the decreases are not obvious. Elevation of initial fracturing fluid temperature can increase the radial stress of the cement sheath and decrease the tangential stress. In addition, reducing the Young's modulus significantly decreases the radial and tangential stresses of the cement sheath. When the Young's modulus reaches a certain value, the tangential stress is lower than the tensile strength. As well, the radial stress is lower than compressive strength. Elevation of the Poisson's ratio of the cement sheath can decrease the tangential stress.Based on the calculated results, a new cement slurry system was designed, which can decrease the Young's modulus of the cement sheath. Engineering practice demonstrates that there is no occurrence of SCP after multistage hydraulic fracturing, thus ensuring the safe production of shale gas wells. Besides, the results of this study can provide a reference for cement slurry design and wellbore integrity control during fracturing of shale gas wells.
引文
[1]SHEN Z,LING K G.Maintaining horizontal well stability during shale gas development[C].Presented at the SPE Middle East Unconventional Gas Conference and Exhibition held in Muscat,Oman,28-30 January,2013.SPE 164037.
[2]NIGEL L,PHILLIP P,GARY K.Evaluation,impact,and management of casing deformation caused by tectonic forces in the Andean Foothills,Colombia[C].Presented at the IADC/SPE Drilling Conference held in Dallas,USA,26-28 February 2002.SPE 74560.
[3]刘硕琼,李德旗,袁进平,等.页岩气井水泥环完整性研究策[J].天然气工业,2017,37(7):76-82.[LIU S Q,LI D Q,YUAN J P,et al.Casing sheath integrity of shale gas wells:A case study from the Sichuan Basin[J].Natural Gas Industry,2017,37(7):76-82.]
[4]刘奎,高德利,曾静,等.温度与压裂作用下页岩气井环空带压力学分析[J].石油钻探技术,2017,45(3):8-14.[LIU K,GAO D L,ZENG J,et al.Annulus pressure analysis of a shale gas well under varied temperature and pressures[J].Petroleum Drilling Techniques,2017,45(3):8-14].
[5]沈吉云,石林,李勇,等.大压差条件下水泥环密封完整性分析及展望[J].天然气工业,2017,37(4):98-108.[SHEN J Y,SHI L,LIY,et al.Analysis and perspective of cement sheath integrity under a high differential pressure[J].Natural Gas Industry,2017,37(4):98-108.]
[6]刘洋,严海兵,余鑫,等.井内压力变化对水泥环密封完整的影响及对策[J].天然气工业,2014,34(4):95-98.[LIU Y,YAN H B,YU X,et al.Negative impacts of borehole pressure change on cement sheath sealing integrity and countermeasures[J].Natural Gas Industry,2014,34(4):95-98.]
[7]LANDRY G,WELTY R D,THOMAS M,et al.Bridging the gap:An integrated approach to solving sustained casing pressure in the Cana Woodford shale[C].Presented at the SPE Well Integrity Symposium held in Galveston,Texas,USA,2-3 June 2015,SPE174525.
[8]XU R,WOJTANOWICZ A K.Pressure buildup test analysis in wells with sustained casing pressure[J].Journal of Natural Gas Science and Engineering,2016,38:608-620.
[9]VALADE R T,HASAN A R,MANNAN M S,et al.Assessing wellbore integrity in sustained casing pressure annulus[J].SPE Drilling&Completion,2014,29(1):131-138.SPE169814.
[10]VALADE R T,MENTZER R A,HASAN A R,et al.Inherently safer sustained casing pressure testing for well integrity evaluation[J].Journal of Loss Prevention in the Process Industries,2014,29(1):209-215.
[11]VIGNES B,AADNO B.Well-integrity issues offshore Norway[J].SPE Productions&Operations,2010,25(2):145-150.
[12]WASTON T L,BACHU S.Evaluation of the potential for gas and CO2 leakage along wellbores[J].SPE Drilling&Completion,2009,24(1):115-126.
[13]ZHU H,LIN Y,ZENG D,et al.Calculation analysis of sustained casing pressure in gas wells[J].Journal of Petroleum Science,2012,9:66-74.
[14]殷有泉,蔡永恩,陈朝伟,等.非均匀地应力场中套管载荷的理论解[J].石油学报,2006,27(4):133-138.[YIN Y Q,CAI Y E,CHEN C W,et al.Theoretical solution of casing loading in non-uniform ground stress feld[J].Acta Petrolei Sinica,2006,27(4):133-138.]
[15]李军,陈勉,柳贡慧,等.套管、水泥环及井壁围岩组合体的弹塑性分析[J].石油学报,2005,26(6):99-103.[LI J,CHEN M,LIUG H,et al.Elastic-plastic analysis of casing-concrete sheath-rock combination[J].Acta Petrolei Sinica,2005,26(6):99-103.]
[16]陈朝伟,蔡永恩.套管-地层系统套管载荷的弹塑性理论分析[J].石油勘探与开发,2009,36(2):242-246.[CHEN C W,CAI Y E.Study on casing load in a casing-stratum system by elastoplastic theory[J].Petroleum Exploration and Development,2009,36(2):242-246.]
[17]XU H L,ZHANG Z,SHI T H,et al.Influence of the WHCP on cement sheath stress and integrity in HTHP gas well[J].Journal of Petroleum Science and Engineering,2015,126:174-180.
[18]ZHANG Z,WANG H.Effect of thermal expansion annulus pressure on cement sheath mechanical integrity in HPHT gas wells[J].Applied Thermal Engineering,2017,118:600-611.
[19]RAOOF G,BRENT A,NIKOO F.A thermo-poroelastic analytical approach to evaluate cement sheath integrity in deep vertical wells[J].Journal of Petroleum Science and Engineering,2016,147:536-546.
[20]JESUS D A,SIGBJ?RN S.Cement sheath failure mechanisms:numerical estimates to design for long-term well integrity[J].Journal of Petroleum Science and Engineering,2016,147:682-698.
[21]GOODWIN K J,CROOK R J.Cement sheath stress failure[J].SPE Drilling Engineering,1992,7(4):291-296.
[22]JELENA T,KAMILA G,ALEXANDRE L,et al.Integrity of downscaled well models subject to cooling[C].Presented at the SPEBergen One Day Seminar held in Bergen,Norway,20 April 2016.SPE180052.
[23]田中兰,石林,乔磊.页岩气水平井井筒完整性问题及对策[J].天然气工业,2015,35(9):70-77.[TIAN Z L,SHI L,QIAO L.Research of and countermeasure for wellbore integrity of shale gas horizontal well[J].Natural Gas Industry,2015,35(9):70-77.]
[24]尹虎,张韵洋.温度作用影响套管抗挤强度的定量评价方法-以页岩水平井大型压裂施工为例[J].天然气工业,2016,36(4):73-79.[YIN H,ZHANG Y Y.A quantitative evaluation method for the effect of temperature on casing collapsing strength:A case study of large-scale hydraulic fracturing in shale gas horizontal wells[J].Natural Gas Industry,2016,36(4):73-79.]
[25]董文涛,申瑞臣,梁奇敏,等.体积压裂套管温度应力计算分析[J].断块油气田,2016,23(5):673-675.[DONG W T,SHEN R C,LIANG Q M,et al.Calculation and analysis of casing thermal stress during stimulated reservoir volume fracturing[J].Fault-Block Oil and Gas Field,2016,23(5):673-675.]
[26]XU H L,ZHANG Z,SHI T H,et al.Influence of the WHCP on cement sheath stress and integrity in HTHP gas well[J].Journal of Petroleum Science and Engineering,2015,126:174-180.
[27]张智,许红林,刘志伟,等.气井环空带压对水泥环力学完整性的影响[J].西南石油大学学报(自然科学版),2016,38(2):155-161.[ZHANG Z,XU H L,LIU Z W,et al.The effect of sustained casing pressure on the mechanical integrity of cement sheath in gas wells[J].Journal of Southwest Petroleum University(Science&Technology Edition),2016,38(2):155-161.]
[28]初纬,沈吉云,杨云飞,等.连续变化内压下套管-水泥环-围岩组合体微环隙计算[J].石油勘探与开发,2015,42(3):379-385.[CHU W,SHEN J Y,YANG Y F,et al.Calculation of micro-annulus size in casing-cement sheath-formation system under continuous internal casing pressure change[J].Petroleum Exploration and Development,2015,42(3):379-385.]
[29]施士昇.混凝土的抗剪强度、剪切模量和弹性模量[J].土木工程学报,1999,32(2):47-52.[SHI S S.Shear strength,modulus of rigidity and Young’s modulus of concrete[J].China Civil Engineering Journal,1999,32(2):47-52.]
[30]SUGDEN C,RING G A,CHAMBERS M R,et al.Special considerations in the design optimization of high rate,multistage fractured shale wells[C].Presented at the IADC/SPE Drilling Conference and Exhibition held in San Diego,California,USA,6-8 March 2012.
[2]NIGEL L,PHILLIP P,GARY K.Evaluation,impact,and management of casing deformation caused by tectonic forces in the Andean Foothills,Colombia[C].Presented at the IADC/SPE Drilling Conference held in Dallas,USA,26-28 February 2002.SPE 74560.
[3]刘硕琼,李德旗,袁进平,等.页岩气井水泥环完整性研究策[J].天然气工业,2017,37(7):76-82.[LIU S Q,LI D Q,YUAN J P,et al.Casing sheath integrity of shale gas wells:A case study from the Sichuan Basin[J].Natural Gas Industry,2017,37(7):76-82.]
[4]刘奎,高德利,曾静,等.温度与压裂作用下页岩气井环空带压力学分析[J].石油钻探技术,2017,45(3):8-14.[LIU K,GAO D L,ZENG J,et al.Annulus pressure analysis of a shale gas well under varied temperature and pressures[J].Petroleum Drilling Techniques,2017,45(3):8-14].
[5]沈吉云,石林,李勇,等.大压差条件下水泥环密封完整性分析及展望[J].天然气工业,2017,37(4):98-108.[SHEN J Y,SHI L,LIY,et al.Analysis and perspective of cement sheath integrity under a high differential pressure[J].Natural Gas Industry,2017,37(4):98-108.]
[6]刘洋,严海兵,余鑫,等.井内压力变化对水泥环密封完整的影响及对策[J].天然气工业,2014,34(4):95-98.[LIU Y,YAN H B,YU X,et al.Negative impacts of borehole pressure change on cement sheath sealing integrity and countermeasures[J].Natural Gas Industry,2014,34(4):95-98.]
[7]LANDRY G,WELTY R D,THOMAS M,et al.Bridging the gap:An integrated approach to solving sustained casing pressure in the Cana Woodford shale[C].Presented at the SPE Well Integrity Symposium held in Galveston,Texas,USA,2-3 June 2015,SPE174525.
[8]XU R,WOJTANOWICZ A K.Pressure buildup test analysis in wells with sustained casing pressure[J].Journal of Natural Gas Science and Engineering,2016,38:608-620.
[9]VALADE R T,HASAN A R,MANNAN M S,et al.Assessing wellbore integrity in sustained casing pressure annulus[J].SPE Drilling&Completion,2014,29(1):131-138.SPE169814.
[10]VALADE R T,MENTZER R A,HASAN A R,et al.Inherently safer sustained casing pressure testing for well integrity evaluation[J].Journal of Loss Prevention in the Process Industries,2014,29(1):209-215.
[11]VIGNES B,AADNO B.Well-integrity issues offshore Norway[J].SPE Productions&Operations,2010,25(2):145-150.
[12]WASTON T L,BACHU S.Evaluation of the potential for gas and CO2 leakage along wellbores[J].SPE Drilling&Completion,2009,24(1):115-126.
[13]ZHU H,LIN Y,ZENG D,et al.Calculation analysis of sustained casing pressure in gas wells[J].Journal of Petroleum Science,2012,9:66-74.
[14]殷有泉,蔡永恩,陈朝伟,等.非均匀地应力场中套管载荷的理论解[J].石油学报,2006,27(4):133-138.[YIN Y Q,CAI Y E,CHEN C W,et al.Theoretical solution of casing loading in non-uniform ground stress feld[J].Acta Petrolei Sinica,2006,27(4):133-138.]
[15]李军,陈勉,柳贡慧,等.套管、水泥环及井壁围岩组合体的弹塑性分析[J].石油学报,2005,26(6):99-103.[LI J,CHEN M,LIUG H,et al.Elastic-plastic analysis of casing-concrete sheath-rock combination[J].Acta Petrolei Sinica,2005,26(6):99-103.]
[16]陈朝伟,蔡永恩.套管-地层系统套管载荷的弹塑性理论分析[J].石油勘探与开发,2009,36(2):242-246.[CHEN C W,CAI Y E.Study on casing load in a casing-stratum system by elastoplastic theory[J].Petroleum Exploration and Development,2009,36(2):242-246.]
[17]XU H L,ZHANG Z,SHI T H,et al.Influence of the WHCP on cement sheath stress and integrity in HTHP gas well[J].Journal of Petroleum Science and Engineering,2015,126:174-180.
[18]ZHANG Z,WANG H.Effect of thermal expansion annulus pressure on cement sheath mechanical integrity in HPHT gas wells[J].Applied Thermal Engineering,2017,118:600-611.
[19]RAOOF G,BRENT A,NIKOO F.A thermo-poroelastic analytical approach to evaluate cement sheath integrity in deep vertical wells[J].Journal of Petroleum Science and Engineering,2016,147:536-546.
[20]JESUS D A,SIGBJ?RN S.Cement sheath failure mechanisms:numerical estimates to design for long-term well integrity[J].Journal of Petroleum Science and Engineering,2016,147:682-698.
[21]GOODWIN K J,CROOK R J.Cement sheath stress failure[J].SPE Drilling Engineering,1992,7(4):291-296.
[22]JELENA T,KAMILA G,ALEXANDRE L,et al.Integrity of downscaled well models subject to cooling[C].Presented at the SPEBergen One Day Seminar held in Bergen,Norway,20 April 2016.SPE180052.
[23]田中兰,石林,乔磊.页岩气水平井井筒完整性问题及对策[J].天然气工业,2015,35(9):70-77.[TIAN Z L,SHI L,QIAO L.Research of and countermeasure for wellbore integrity of shale gas horizontal well[J].Natural Gas Industry,2015,35(9):70-77.]
[24]尹虎,张韵洋.温度作用影响套管抗挤强度的定量评价方法-以页岩水平井大型压裂施工为例[J].天然气工业,2016,36(4):73-79.[YIN H,ZHANG Y Y.A quantitative evaluation method for the effect of temperature on casing collapsing strength:A case study of large-scale hydraulic fracturing in shale gas horizontal wells[J].Natural Gas Industry,2016,36(4):73-79.]
[25]董文涛,申瑞臣,梁奇敏,等.体积压裂套管温度应力计算分析[J].断块油气田,2016,23(5):673-675.[DONG W T,SHEN R C,LIANG Q M,et al.Calculation and analysis of casing thermal stress during stimulated reservoir volume fracturing[J].Fault-Block Oil and Gas Field,2016,23(5):673-675.]
[26]XU H L,ZHANG Z,SHI T H,et al.Influence of the WHCP on cement sheath stress and integrity in HTHP gas well[J].Journal of Petroleum Science and Engineering,2015,126:174-180.
[27]张智,许红林,刘志伟,等.气井环空带压对水泥环力学完整性的影响[J].西南石油大学学报(自然科学版),2016,38(2):155-161.[ZHANG Z,XU H L,LIU Z W,et al.The effect of sustained casing pressure on the mechanical integrity of cement sheath in gas wells[J].Journal of Southwest Petroleum University(Science&Technology Edition),2016,38(2):155-161.]
[28]初纬,沈吉云,杨云飞,等.连续变化内压下套管-水泥环-围岩组合体微环隙计算[J].石油勘探与开发,2015,42(3):379-385.[CHU W,SHEN J Y,YANG Y F,et al.Calculation of micro-annulus size in casing-cement sheath-formation system under continuous internal casing pressure change[J].Petroleum Exploration and Development,2015,42(3):379-385.]
[29]施士昇.混凝土的抗剪强度、剪切模量和弹性模量[J].土木工程学报,1999,32(2):47-52.[SHI S S.Shear strength,modulus of rigidity and Young’s modulus of concrete[J].China Civil Engineering Journal,1999,32(2):47-52.]
[30]SUGDEN C,RING G A,CHAMBERS M R,et al.Special considerations in the design optimization of high rate,multistage fractured shale wells[C].Presented at the IADC/SPE Drilling Conference and Exhibition held in San Diego,California,USA,6-8 March 2012.