CO2-水-岩作用对致密砂岩性质与裂缝扩展的影响

基于CO2水溶液浸泡致密砂岩实验和室内压裂模拟实验,研究了CO2-水-岩作用对致密砂岩性质和裂缝扩展的影响,并通过CO2水溶液浸泡试样裸眼段考察CO2在压裂过程中的化学作用。研究结果表明:CO2水溶液浸泡后,方解石和白云石含量显著降低,钾长石和斜长石被溶蚀生成高岭石;石英和黏土矿物含量升高,但伊利石和绿泥石含量降低;溶蚀孔隙数量增多,孔径变大,孔隙度和渗透率增大;抗张强度降低,且平行层理降低幅度大于垂直层理;相比于滑溜水压裂,超临界CO2压裂的破裂压力降低14.98%,形成的水力裂缝数量增多,裂缝形态更复杂;CO2水溶液浸泡裸眼段后,超临界CO2压裂的破裂压力相比于未浸泡情况降低21.61%,且水力裂缝多点起裂,裂缝复杂程度进一步提高。实验证明CO2的物理和化学特性能有效提高CO2压裂裂缝的复杂性。

Experimental Investigation on the Influence of CO2-Brine-Rock Interaction on Tight Sandstone Properties and Fracture Propagation

Based on the experiments of static tight sandstone soaked in CO2-saturated brine and indoor fracturing simulation, the influence of CO2-brine-rock interaction on tight sandstone properties and hydraulic fracture propagation were studied, and the special chemical effect of CO2 was simulated by soaking the open-hole section of fracturing sample in CO2-saturated brine. The study results show that after soaking in CO2-saturated brine, the contents of calcite and dolomite in the rock sample significantly decrease, kaolinite occurs after the corrosion of potassium feldspar and anorthose;the contents of quartz and clay minerals increase and those of illite and chlorite decrease;the amount of dissolved pores rises, the pore diameter, porosity and permeability become larger;the tensile strength of the tight sandstone decreases, and the decrement extent of the tensile strength in parallel bedding is larger than that in vertical bedding;compared with slickwater fracturing, the formation fracture pressure of supercritical CO2 fracturing reduces by 14.98% and the number of hydraulic fractures increases. After the openhole section soaking in CO2-saturated brine, the formation fracture pressure of supercritical CO2 fracturing decreases by 21.61% compared with that of supercritical CO2 fracturing without the soaking treatment and the fracture complexity is greatly enhanced. The experiment result proves that the physical and chemical properties of CO2 can effectively improve the fracture complexity during CO2 fracturing.