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如何确定深部煤层的地质力学参数是气体钻井井壁稳定评价的关键。基于量化的地质强度指标GSI和Hoek-Brown强度准则分析了反演深部煤层地质力学参数的方法,结合DB1井氮气钻井,以弹塑性方法分析了侏罗系炭质泥岩和煤层的井壁稳定性。研究表明,塔里木盆地依奇克里克区块侏罗系煤GSI介于45~55,变形模量约在1 560~2 850 MPa之间。井眼钻开后炭质泥岩和煤层井周存在塑性区,炭质泥岩层井周塑性区约为1.37倍井眼半径,煤层井周塑性区约为2.07~2.59倍井眼半径。塑性区应力松弛造成围压减小,导致节理岩体抵抗工程扰动的能力减弱。以工程允许塑性半径等于1.5倍井眼半径为临界稳定条件,井眼钻开后煤层会出现大面积坍塌失稳,不宜采用气体钻井;工程扰动造成炭质泥岩井眼扩径,扩径后井眼仍能保持稳定。
How to determine the geomechanical parameters of deep coal seams is the key to the wellbore stability evaluation of gas drilling. Based on the GSI and Hoek-Brown strength criterion of quantitative geologic strength index, the method of inversion of geomechanical parameters of deep coal seam is analyzed. Combined with nitrogen drilling in DB1 well, the wellbore stability of Jurassic carbonaceous mudstone and coal seam is analyzed by elastic-plastic method . The study shows that the GSI of Jurassic coal in the Ikekiliki area of the Tarim Basin ranges from 45 to 55 and the deformation modulus is about 1 560 to 2 850 MPa. There are plastic zones around the carbonaceous mudstone and the coal seam after the borehole is drilled. The well-plastic zone in the carbonaceous mudstone is about 1.37 times the borehole radius, and the plastic zone in the periphery of the coal seam is about 2.07-2.59 times the borehole radius. Plastic zone stress relaxation caused by confining pressure decreases, resulting in joint rock mass ability to resist engineering disturbance weakened. With the allowable plastic radius equal to 1.5 times the borehole radius as the critical stability condition, large-area collapses and instability will occur in the coal seam after the drilling of the borehole, so it is not suitable to use gas drilling. The engineering disturbance causes the diameter expansion of the carbonaceous mudstone borehole, Eyes can still be stable.