裂缝性储层天然缝与水力缝开启条件及扩展规律研究

Opening Conditions and Extension Law of Natural and Hydraulic Fractures in Fractured Reservoirs

为了揭示裂缝性储层天然裂缝与水力裂缝开启条件及扩展规律,降低水窜风险,采用地应力测试、天然裂缝观察、水力裂缝扩展试验及模拟等方法,系统研究了天然裂缝与水力裂缝开启条件及扩展规律、水平应力差对水力裂缝扩展的影响及天然裂缝与水力裂缝相沟通的净压力。研究结果显示,研究区的垂向应力σ_(v)>最大水平主应力σ_(H)>最小水平主应力σ_(h),垂向应力、最大水平主应力和最小水平主应力的梯度分别为0.025,0.020和0.017 MPa/m;研究区致密砂岩中裂缝滑动的临界压力梯度在0.018~0.020 MPa/m,平均为0.019 MPa/m;当天然裂缝内临界压力梯度超过0.020 MPa/m时,无效缝均转变为有效缝。水平应力差从0增至10 MPa过程中,水力裂缝长度逐渐增加;10 MPa应力差条件下所形成水力裂缝的长度约为0及5 MPa应力差条件下水力裂缝的1.52倍。压裂试验结果显示,当水力裂缝遇到局部发育的砾石颗粒时会发生一定程度偏转,之后会继续沿着原来方向延伸。研究区目的层天然缝张开形成分支裂缝条件下的裂缝沟通净压力梯度为0.003 MPa/m,该裂缝沟通净压力在4.5~9.0 MPa。研究认为,构建裂缝性储层中水力裂缝的非稳态扩展模式时,应综合考虑水平应力差、天然裂缝发育情况、岩性特征及压裂施工条件。

In order to reveal the opening conditions and extension law of natural and hydraulic fractures in fractured reservoirs and reduce the risk of water channeling,in-situ stress test,natural fracture observation,hydraulic fracture extension test,and simulation were used.In addition,the opening conditions and extension law of natural and hydraulic fractures,the effect of horizontal stress difference on the extension of hydraulic fractures,and the net pressure of communication between natural and hydraulic fractures were systematically studied.The results show that the vertical stressσ_(v) in the study area is higher than the maximum horizontal principal stressσ_(H),which is higher than the minimum horizontal principal stressσ_(h).The gradients of vertical stress,maximum horizontal principal stress,and minimum horizontal principal stress are 0.025,0.020,and 0.017 MPa/m,respectively.The critical pressure gradient of fracture sliding in the tight sandstone of the study area ranges from 0.018 to 0.020 MPa/m,with an average value of 0.019 MPa/m.When the critical pressure gradient in the natural fracture exceeds 0.020 MPa/m,all the ineffective fractures are transformed into effective fractures.As the horizontal stress difference increases from 0 to 10 MPa,the length of hydraulic fractures increases gradually.The length of hydraulic fractures formed under the stress difference of 10 MPa is about 1.52 times that of hydraulic fractures formed under the stress difference of 0 MPa and 5 MPa.The fracturing experiment results show that the hydraulic fracture will be deflected to a certain extent when encountering locally developed gravel particles and then continue to extend along the original direction.In the study area,the net pressure gradient of fracture communication under the condition that natural fractures in the target layer open to form branch fractures is 0.003 MPa/m,and the net pressure of fracture communication is 4.5–9.0 MPa.It is concluded that the construction of an unsteady extension mode of hydraulic fractures in fractured reservoirs should consider the horizontal stress difference,natural fracture development,lithology characteristics,and fracturing operation conditions.