海相页岩压裂液低返排率成因

为了研究压裂液与页岩作用以及滞留机理,以中国南方海相页岩昭通、长宁和威远区块的岩心为样品,从吸附、膨胀、溶出孔、溶出液矿化度及离子组成等实验研究入手,揭示水在页岩中的赋存状态及高矿化度来源,模拟计算了水的滞留及矿化度增加机理。扫描电镜显示页岩中含有黏土矿物质形成的大量微裂缝,超声浸泡页岩矿物溶出率介于0.5%~0.7%,离子组成与地层水一致,取样岩心黏土矿物含量介于18%~20%,主要为绿泥石、伊利石和少量伊/蒙混层,没有蒙脱石(SS)层,XRD及红外光谱验证压裂液不会进入黏土矿物层间,不会引起页岩膨胀,压裂液滞留主要为压裂新形成的页岩微裂缝表面吸附作用的结果,吸附饱和速度与孔径成正比,为逐步渗透。结论认为,压裂液对页岩的逐级萃取及纳米缝隙对离子排斥是返排后期矿化度急剧增加的主要原因,气藏形成时甲烷携液作用也是可能的原因。结合实验及现场数据,分析了9口井压裂液返排率与产气量数据,结果表明相同区块总体趋势为返排率越低,微裂缝越发达、体积改造效果越好、产气量越大。

Reasons for the low flowback rates of fracturing fluids in marine shale

In this paper, marine shale cores taken from Zhaotong, Changning and Weiyuan Blocks in South China were used as samples to investigate the interaction between fracturing fluids and shale and the retention mechanisms. Firstly, adsorption, swelling, dissolution pore, dissolution fluid salinity and ionic composition were experimentally studied to reveal the occurrence of water in shale and the reasonfor high salinity. Then, the mechanisms of water retention and salinity increase were simulated and calculated. The scanning electron microscopy (SEM) analysis shows that there are a large number of micro fractures originated from clay minerals in the shale. Mineral dissolution rates of shale immersed in ultrasonic is around 0.5–0.7%. The ionic composition is in accordance with that of formation water. The clay minerals in core samples is mainly composed of chlorites and illites with a small amount of illites/smectites, but no montmorillonites(SS), and its content is between 18% and 20%. It is verified by XRD and infrared spectroscopy that the fracturing fluid doesn't flow into the space between clay mineral layers, so it can't lead to shale swelling. Thus, the retention of fracturing fluids is mainly caused by the adsorption at the surface of the newly fractured micro fractures in shale in a mode of successive permeation, and its adsorptive saturation rates is proportional to the pore diameters. It is concluded that the step-by-step extraction of fracturing fluids to shale and the repulsion of nano-cracks to ion are the main reasons for the abrupt increase of salinity in the late stage of flowing back. In addition, the liquid carrying effect of methane during the formation of a gas reservoir is also a possible reason. Based on the experimental and field data, fracturing fluid flowback rates and gas production rates of 9 wells were analyzed. It is indicated that the same block follows an overall trend, namely, the lower the flowback rates, the more developed the micro fractures, the better the volume stimulation effect and the higher the gas production rates.