逆流自吸效应对页岩油储层坍塌压力的影响研究

页岩油储层在采用水基钻井液欠平衡钻进时，由于毛细管力作用，钻井液滤液仍会进入地层，降低地层稳定性。为此，进行了逆流自吸效应对页岩油储层坍塌压力的影响规律研究。基于两相渗流理论，建立了逆流自吸作用下的水侵模型，分析发现页岩油储层井眼附近的含水饱和度随欠压差值增大而降低；在考虑水化作用对地层岩石强度的影响的基础上，建立了井周应力模型，对页岩油储层的坍塌压力当量密度变化规律进行了分析。研究发现:逆流自吸作用下，钻井时间越长，欠压差值越小，页岩油储层坍塌压力越大，越不利于井眼稳定；当只改变欠压差值时，页岩油储层最大井径扩大率存在最小值。研究认为，建立的井周应力模型可为页岩油储层欠平衡钻井设置合理欠压差值以及调整钻井液密度提供理论依据。 

The Influence of Countercurrent Spontaneous Imbibition on the Collapse Pressure of Shale Oil Reservoirs

During under-balanced drilling in a shale reservoir, water based drilling fluid can enter the formation due to capillary force when the under-pressure difference is small.Thus, it causes pore pressure change around the wellbore and reduces the stability of the borehole.Becuase of the potential for blowouts, there is a significant need to to carry out the research on the influence law of countercurrent spontaneous imbibition on wellbore collapse pressure in under-balanced drilling.In this paper we discuss how we established a water invasion model of shale reservoir considering the countercurrent spontaneous imbibition in under-balanced drilling which we based on two-phase seepage theory.Our work demonstrates that water saturation near the wellbore will decrease as the under-pressure difference increases.Considering rock strength changes with hydration we set up a stress analysis model near wellbore and studied the equivalent mud density of wellbore collapse pressure of shale.It can be determined from the model that longer drilling times will generate smaller under-pressure differences, and a alarger variation range of water saturation near the borehole will cause lower collapse pressure considering the influence of countercurrent spontaneous imbibition, which is more unfavorable for borehole stability.When other parameters remain constant except for under-pressure difference, a minimum borehole diameter enlargement rate is achieved in the shale.This borehole stability model can provide a theoretical basis for setting a reasonable under-pressure difference and adjusting the drilling fluid density in under-balanced drilling of horizontal shale gas well.