寿阳和柿庄区块煤储层压裂效果及其影响机理分析

通过分析寿阳和柿庄区块携砂液阶段的压力曲线类型,对压裂效果进行了评价,创新性地对携砂液阶段的砂比曲线类型进行了划分,揭示出压力曲线类型与砂比曲线类型之间普遍存在的统计对应关系,以此为基础,考虑地应力类型、煤岩与围岩力学性质及应力差异多因素,分析了煤储层压裂效果影响机理。研究表明:①压力曲线可划分为下降、稳定、上升和波动4种类型,是评价压裂效果的直接依据;②砂比曲线可划分为连续和断续两种类型,发现压力曲线类型与砂比曲线类型之间存在普遍的统计对应关系,其成因机制为:在弱挤压应力井层(正常应力场),在携砂液阶段早期垂直压裂缝扩展路径选择过程中,易出现砂堵,多形成断续型砂比曲线,随后,若满足垂直压裂缝压开顶底板临界条件,则垂直压裂缝垂向扩展压开顶底板,即逐层压开顶底板不同岩性层,易出现波动型压力曲线。若不满足垂直压裂缝压开顶底板临界条件,则垂直压裂缝被限制在煤层内侧向扩展,易出现稳定型、下降型和上升型不同类型的压力曲线;③与柿庄区块相比,寿阳区块出现断续型砂比曲线和波动型压力曲线占比的双高现象,整体压裂效果较差,其根本的地质因素是该区块的应力状态决定多数井层发育压开顶底板的垂直压裂缝,为无效造缝,使得压裂效果不佳;④砂比曲线可作为预警曲线,若砂比曲线为断续型,且压力曲线开始出现上升或波动,预期压裂效果差,可考虑提前终止压裂。

Evaluation of fracturing effect of coal reservoirs in Shouyang and Shizhuang CBM blocks and analysis of influencing mechanism

By analyzing the types of pressure curve,this paper evaluates the fracturing effect of the Shouyang block and Shizhuang block,and divides the proppant concentration curve types for the first time,revealing the statistical correspondence between the pressure and the proppant concentration curve types.Considering the types of in situ stress,the difference of mechanical properties and stress between coal and surrounding rock,the influence mechanism of coal reservoir fracturing effect is analyzed.The study shows that:① The pressure curve can be divided into four types:descending,stable,rising and fluctuating,which is the direct basis for evaluating the effect of fracturing.② The proppant concentration curve is divided into two types:continuous and discontinuous.It is found that there is a certain statistical correspondence between the pressure and the proppant concentration curve types.Its genetic mechanism is that in the weakly squeezed stress well layer (normal stress field),during the initial stage of proppant injection,the extension direction of the fracture is selected,sand plugging is easy to occur,more discontinuous proppant concentration curves are formed.And if the critical conditions of the vertical fracture to cut through the roof and bottom of the coal seam are satisfied,the vertical fracture will be vertically expanded to the surrounding rock,the different lithologic layers of the roof and bottom are cracked,the fluctuating pressure curve appears.If the critical conditions of the vertical fracture to cut through the roof and bottom of the coal seam are not satisfied,the vertical fracture will be limited to the inner side of the coal seam,and it is prone to different types of pressure curves of stable,descending and rising types.③ Compared with the Shizhuang block,the Shouyang block has a double high phenomenon of the discontinuous proppant concentration curve and the fluctuating pressure curve types,and the overall fracturing effect is poor.The fundamental geological factor is that the stress state of the block determines that more wells develop vertical fractures that cut through the roof and bottom of the coal seam.④ The proppant concentration curve can be used as an early warning curve for the fracturing effect.If the proppant concentration curve is discontinuous and the pressure curve begins to rise or fluctuate,the fracturing effect will be poor.And it may be considered to terminate the fracturing in advance.