层理性页岩水平井井壁稳定性分析

Analysis of wellbore stability for horizontal wells in stratification shale

基于孔隙弹性力学和单结构面强度理论,建立层理性页岩水平井井壁稳定力学分析模型,分析层理面产状（走向、倾角）、井眼轨迹（井眼方位）和层理面强度弱化对水平井井壁稳定的影响。研究结果表明：页岩层理面产状和层理面强度弱化是导致水平井井壁坍塌失稳的主要因素;层理面倾角为0°-30°时,沿着任何方位钻水平井的井壁稳定性都比较差,而层理面倾角为30°-90°时,沿特定方位钻水平井的井壁稳定性却比较好的,这便为井眼钻进方位的优化设计提供重要依据;层理面强度弱化的影响可以比层理面产状的影响更大,不同产状下坍塌压力差异可达0.45 g/cm^3,而层理面强度弱化导致的井壁坍塌压力可增加约0.54 g/cm^3;钻井液在渗透作用下沿层理面侵入地层,导致层理面黏聚力和内摩擦角降低,使井壁岩石更容易沿层理产生滑移,从而加剧井壁坍塌失稳的风险。最后,采用该模型在四川东南部威远构造第一口页岩气水平井W201-H1井进行了验证,井壁坍塌压力分析结果与实际情况吻合良好。

Based on the poroelastic mechanics and the single weak-plane strength theory, the wellbore stability mechanics model of horizontal wells for stratification shale was established, so as to the influence of the bedding occurrence（strike, dip）, hole trajectory（hole azimuth） and weakened strength of bedding planes on wellbore stability of horizontal well was analyzed. The results show that the occurrence of stratification and weakened strength of stratification are major factors causing wellbore collapse. When the bedding dip is 0°-30°, the wellbore stability is relatively poor, whatever azimuth of horizontal well is drilled. When the bedding dip is 30°-90°, the wellbore stability is better when horizontal well is drilled along a particular azimuth. These provide an important foundation for the azimuth optimization of horizontal wells. The effect of the weakened strength of bedding may be greater than that of bedding occurrence, the collapse pressure difference under different bedding occurrences is up to 0.45 g/cm^3, but the collapse pressure amplification caused by the weaken strength of bedding is about 0.54 g/cm^3. Drilling fluid invades formation with osmosis along the bedding planes, leading to the cohesion and internal friction angle of bedding plane decrease, which makes wellbore rock along bedding slip easily, causing the increase of the risk of the wellbore collapse. At last, this model is verified in the first horizontal well（W201-H1） for shale gas reservoir of Weiyuan structure, southeastern region of the Szechwan Basin, and the analysis results of collapse pressure are consistent with the actual situation.