固井水泥浆侵入对煤储层压裂裂缝延展的影响

目前针对于煤层气井近井部位固井水泥浆侵入的方式和形态规模、固井水泥浆侵入对煤储层压裂裂缝延展的影响机制及其与煤层气井开采效果的内在关联方面的研究较少。为了深化煤储层压裂裂缝延展理论并给煤层气井水力压裂施工方案优化提供支撑,选取沁水盆地煤储层中基质—裂隙发育组合迥异的区块,对部署在不同部位气井的固井水泥浆侵入方式和水泥环形态规模进行了系统刻画,分析了不同固井水泥浆侵入方式下的压裂力学判据,针对深部气井难于开挖解剖其固井水泥浆侵入特征的实际问题,提出了破裂压力当量的定义,在此基础上,对郑庄区块39口煤层气井的压裂排采数据进行了分析,总结了固井水泥浆不同侵入方式对煤层气井压裂、排采的影响。研究结果表明:①煤层气井固井水泥浆的侵入方式包含固井水泥浆正常充注型(硬煤基质)、加厚型(构造煤)及煤岩—水泥胶结界面型(硬煤裂隙带)等3种;②由破裂压力当量(pt)小于1.50 MPa,固井水泥浆均匀充注在气井井眼—套管环空,可以判断气井位于硬煤基质;由pt介于1.50～9.00 MPa,固井水泥浆沿井壁构造节理缝挤侵入储层内形成胶结滤饼,可以判断气井位于硬煤裂隙带;由pt大于9.00 MPa,固井水泥在井眼环空垮塌空间加厚形成纺锤体,可以判断气井位于构造煤;③位于硬煤基质的气井在排采期间气井产气量缓慢增加到峰值,并可在峰值部位稳产较长时间,而后产气量缓慢下降,位于构造煤的气井在排采初期很快见产,随后产气量迅速衰减,硬煤裂隙带气井在排采初期产气量快速上升并达到峰值,但稳产时间较短,而后产气量缓慢下降。

Impact of cement slurry invasion on the propagation of hydraulic fractures in coal reservoirs

At present, invasion mode, morphology and scale of cement slurry in the immediate vicinity of a coalbed methane (CBM) well and their impacts on the propagation of hydraulic fractures in coal reservoirs and their intrinsic relationships with the production effects of the CBM well were less researched. In order to deepen the theories on the propagation of hydraulic fractures in coal reservoirs and provide support for the optimization of CBM well hydraulic fracturing scheme, this paper selected the blocks in the Qinshui Basin whose matrix–fracture development combinations in coal reservoirs are totally different as the research objects. The invasion modes of cement slurry and the morphologies and scales of cement sheath in the wells deployed at different locations were characterized systematically, and the fracturing mechanics criteria in different invasion modes of cement slurry were analyzed. Then, in view of the drilling difficulties of deep gas wells, the actual problems related to the invasion characteristics of cement slurry were dissected, and the definition of fracture pressure equivalent was proposed. Finally, the fracturing and production data of 39 CBM wells in the Zhengzhuang Block were analyzed, and the influences of different invasion modes of cement slurry on the fracturing and production of CBM wells were summarized. And the following research results were obtained. First, the invasion modes of cement slurry in CBM wells include three types, i.e., normal cementing type [primary structural coal matrix (PCM)], thickening type [tectonic coal (TC)] and coal–cement cementing interface type [primary structural coal fracture (PCF)]. Second, when the fracture pressure equivalent (pt) is less than 1.50 MPa, cement slurry is evenly filled in borehole–casing annulus of gas well, which indicates that the gas well is located at PCM; when pt is in the range of 1.50–9.00 MPa, cement slurry invades into the reservoir through the structural joint fissures to form cemented filter cakes, which indicates that the gas well is located at PCF; and when pt is greater than 9.00 MPa, cement slurry thickens to form spindles in the collapse space of hole annulus, which indicates that the gas well is located at TC. Third, the gas production rate of the CBM well deployed at PCM remains stable for a long time after increasing slowly to the peak, and then declines slowly. That of CBM well deployed at TC reaches its peak value rapidly at the initial stage of the production and then decreases quickly. It's the gas production rate of the CBM well deployed at PCF increases rapidly to the peak value at the initial stage of the production, but its stable production lasts short, and then it declines slowly.