深部煤层力学参数反演和气体钻井井壁稳定性

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。

Estimation of Geomechanical Properties of Deep Coal Bed and Wellbore Stability Analysis for Gas Drilling

How to assess the geomechanical properties of deep coal bed is a key issue for wellbore stability analysis of gas drilling.Geomechanical properties inversion method of deep coal bed is proposed in this work based on the quantitative geological strength index (GSI) and Hock-Brown strength criterion.The strength and deformation parameters of Jurassic coal were estimated.Taking nitrogen drilling well DB1 in Yiqikelike region as an example,wellbore stabilities of Jurassic carbonaceous mudstone and coal bed were investigated with FLAC.Analysis indicates that GSI of coal bed at Yiqikelike region in Tarim Basin ranges from 45 to 55,and deformation modulus is from 1 560 MPa to 2 850 MPa.The results also reveal that carbonaceous mudstone and coal bed exhibit elastoplastic behavior during gas drilling.The plastic zone radius of carbonaceous mudstone is about 1.37 times of wellbore radius,while that of coal bed is 2.07 to 2.59 times of wellbore radius.Stress relaxation in the plastic zone is captured,thereby leading to a decrease in confining stress,and reducing the anti-disturbance ability of jointed rock mass with respect to excavation.The plastic zone radius of carbonaceous mudstone that results from drilling is less than 1.5 times of wellbore radius,which shows a stable hole.While that of coal bed is far more than the plastic radius permitted by project,thus predicting borehole collapse,therefore gas drilling should not be used.Borehole enlargement of carbonaceous mudstone may be encountered as a result of engineering disturbance,and the borehole can remain stable during gas drilling.