大庆致密油井页岩井壁稳定性实验研究

Experimental Study on Shale Borehole Wall Stability of Tight Oil Wells in Daqing Oilfield

大庆致密油区块青山口组、泉头组页岩容易发生井壁失稳情况。目前用于评价页岩井壁稳定性的常规方法为热滚分散和线性膨胀实验。此类实验通常存在人为缺陷,会误导对页岩膨胀活性的认识和配伍性流体的选择。采用更接近井下地层条件的3种实验方法(多级三轴应力实验、压力传递实验(PTT)、厚壁圆筒实验(TWC)),对大庆致密油页岩进行研究。通过多级三轴应力实验,绘制摩尔-库伦破坏包络线,确定了维持井壁稳定性所需钻井液密度。压力传递实验反映了特定流体系统中所预期的钻井液压力侵入速率和孔隙压力升高的延迟情况。厚壁圆筒实验研究了过平衡压力下暴露在钻井液中的岩心样品的破坏特性。这3种实验方法,模拟了在钻井过程中钻井液对井下页岩地层应力的影响,对研究井壁稳定性的影响因素更具指导意义。

Borehole wall instability has long been a problem in drilling the Qingshankou Formation and the Quantou Formation in the tight oil blocks in Daqing.Common methods of evaluating the stability of shale formations presently in use are hot rolling test and linear expansion test.These methods however,have defects caused by those who do the experiments,and the experimental results are therefore misleading in the understanding of the activity of the swelling shales and the selection of compatible drilling fluid.Multistage triaxial stress experiment,pressure transfer test(PTT)and thick-walled cylinder test(TWC),which can simulate more closely the downhole conditions,were used to study the stability of shale formations in tight oil wells in Daqing.Using the multi-stage triaxial stress test,the Mohr-Coulomb failure envelop of the formation rocks was drawn to determine the mud density enough to stabilize the borehole wall.PTT was conducted to understand the anticipated invasion rate of pressure caused by drilling fluid column in the hole and the delay in pore pressure increase for a specific fluid system.TWC experiment was used to study the failure characteristics of core samples exposed to drilling fluid under overbalanced pressure.These three test methods simulate the effects of drilling fluid on the stresses of downhole shale formation,and are instructive to the study on the factors affecting borehole wall stabilization.