The Influence of Countercurrent Spontaneous Imbibition on the Collapse Pressure of Shale Oil Reservoirs
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摘要: 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。Abstract: 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.
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图 1 毛管力作用下井眼附近含水饱和度随时间和无因次径向距离的变化规律
Figure 1. Variation law of water saturation near borehole with time and dimensionless radial distance under capillary force
图 2 不同欠压差值下钻井10 d后井眼附近含水饱和度随无因次径向距离的变化规律
Figure 2. Variation law of water saturation near borehole with dimensionless radial distance after 10 days of drilling under different under-pressure
图 3 欠压差为0.5 MPa时页岩吸水后井眼附近岩石力学特性随无因次径向距离的变化规律
Figure 3. Variation law of rock mechanical properties near borehole with dimensionless radial distance after shale water absorption under-pressure difference 0.5 MPa
图 5 井眼周围地层坍塌压力随无因次径向距离和井周角的变化规律
Figure 5. Variation law of formation collapse pressure equivalent drilling fluid density around borehole with dimensionless well radius and well round angle
图 6 钻井10 d后不同欠压差下井眼附近地层坍塌压力随无因次径向距离的变化规律
Figure 6. Variation law of formation collapse density with radial distance near borehole under different under-pressure difference after 10 days of drilling
图 7 不同欠压差下的最大井径扩大率
Figure 7. The maximum hole diameter enlargement rate under different under-pressure difference
图 8 欠压差2 MPa条件下钻井10 d后渗透率对井眼附近地层坍塌压力的影响
Figure 8. Influence of permeability on formation collapse density near borehole after 10 days of drilling at under-pressure difference of 2 MPa
图 9 欠压差2 MPa条件下钻井10 d后渗透率对最大井径扩大率的影响
Figure 9. Influence of permeability on maximum hole diameter enlargement rate after 10 days of drilling at under-pressure difference of 2 MPa
图 10 欠压差为2 MPa下钻井10 d后孔隙度对坍塌压力的影响
Figure 10. Influence of porosity on collapse pressure after 10 days of drilling at under-pressure difference of 2 MPa
图 11 不同孔隙度下的最大井径扩大率
Figure 11. The maximum hole diameter enlargement rate at different porosity
图 12 欠压差2 MPa条件下钻井10 d后界面张力对坍塌压力的影响
Figure 12. Influence of surface tension on collapse pressure after 10 days of drilling at under-pressure difference of 2 MPa
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