钻井完井液浸泡弱化页岩脆性机制

BRITTLENESS WEAKENING MECHANISMS OF SHALE SOAKED BY DRILLING &COMPLETION FLUID

页岩脆性是页岩地层钻井、水力压裂设计的关键参数之一,目前针对钻井过程中工作液浸泡对页岩脆性的影响还未引起关注.通过开展钻井完井液浸泡前后页岩三轴力学实验,利用脆性评价模型分析了页岩脆性变化特征.结果表明,延长组页岩脆性强于龙马溪组页岩;油基和水基钻井完井液浸泡均能导致页岩脆性降低,且油基钻井完井液浸泡后的页岩脆性降低幅度更大;龙马溪组页岩浸泡后脆性减弱幅度较延长组页岩大.页岩脆性弱化机制包括:(1)由层理面胶结强度不同引起的脆性强弱差异;(2)由毛管自吸作用导致的高孔压、高应力强度因子、低临界断裂韧性;(3)由碱液侵蚀导致的页岩溶蚀孔形成及矿物颗粒碎裂;(4)由黏土矿物水化膨胀产生的膨胀应力;(5)由钻井完井液滤液润滑导致的页岩破裂面摩擦系数降低.延长组页岩层理面强度较龙马溪组页岩低,导致延长组页岩脆性强于龙马溪组页岩.其次,和水基钻井完井液相比,油基钻井完井液具有更大的自吸量、更高的p H值、更强的润滑性,因此,油基钻井完井液浸泡降低页岩脆性幅度更大.另外,由于龙马溪组页岩具有更小的润湿角、更强的毛管自吸和碱液侵蚀作用,相同浸泡条件下,龙马溪组页岩脆性降低幅度更大.本研究可为页岩地层钻井液性能优化、井壁稳定控制、水力压裂设计等提供理论指导.

Brittleness is one key parameter during drilling and fracturing in shale formations, however, influencing mechanism of soaking by drilling completion fluid on shale brittleness has not been focused. In this paper, triaxial mechanical tests of shales soaked by fluid were performed; and optimized brittleness evaluation model was employed to evaluate brittleness of shale. Results indicate that the brittleness of Yanchang shale is larger than that of Longmaxi shale,and two kinds of drilling completion fluid can decrease shale brittleness, in which oil-based drilling completion fluid reduces shale brittleness more evidently. The reduction of brittleness Longmaxi shale is greater than that of Yanchang shale. It is demonstrated that five factors can weaken the brittleness of shale as follows：（1） Higher strength of bedding plane can lead to lower brittleness;（2） Spontaneous imbibition causes high pore pressure and great stress intensity factor of fractures;（3） Alkali erosion can cause mineral grains crack and dissolution pore;（4） Hydrate swelling of shale results in swelling stress;（5） Lubrication of drilling completion filtrate results in lower friction coefficient. Yanchang shalehas lower bedding plane strength. So the brittleness of Yanchang shale is larger than that of Longmaxi shale; Oil-based drilling completion fluid has greater imbibition amount, higher p H and stronger lubrication; therefore, brittleness of shales soaked by oil-based drilling completion fluid decreases more evidently. Besides, Longmaxi shale has smaller contact angle, greater imbibition amount, and stronger alkali erosion, the reduction of brittleness is greater than that of Yanchang shale under the same condition. This work can provide some evidence for drilling and hydraulic fracturing in shale formation.