致密灰岩储层压裂裂缝扩展形态试验研究

开发低孔、低渗的致密灰岩储层需要进行大规模的水力压裂。致密灰岩的成岩过程、矿物组成以及岩石力学性质与致密砂岩等储层差异很大,在不同应力状态以及施工参数条件下水力裂缝扩展形态有待研究。采用真三轴水力压裂试验系统对致密灰岩露头展开压裂物模试验,研究地应力差、压裂液黏度、变排量、酸处理等多种因素对水平井压裂裂缝扩展规律的影响。试验结果表明:当水平地应力差在2~8 MPa之间时,水力裂缝易于沟通天然裂缝形成复杂裂缝网络;压裂液黏度升高,会降低剪切滑移和滤失膨胀的可能性,从而降低裂缝的复杂程度;在走滑断层的应力状态下,即σ_Hσ_Vσ_h,容易形成水平缝,特别是当井眼方向沿着层理面时极易沿着层理起裂;变排量压裂可以激活更多的天然裂缝,有助于形成复杂的裂缝网络;酸液预处理裸眼井段能够显著降低破裂压力,随泡酸时间的增加,破裂压力下降幅度逐渐增大。研究成果为现场压裂施工提供参考。

Experimental investigation on propagation geometry of hydraulic fracture in compact limestone reservoirs

Large-scale hydraulic fracturing technology is significant in the development of compact limestone reservoirs with low porosity and low permeability. The diagenesis, mineral composition and mechanical properties of the compact limestone reservoirs are greatly different from those of the compact sandstone ones. Hydraulic fracture geometry is complicated under different stress states and construction conditions. Based on the hydraulic fracturing simulation experiments on compact limestone outcrops carried out by using a true triaxial test system, the effects of multiple factors on fracture propagating laws of horizontal well in compact limestone are studied. The results show that hydraulic fractures likely interact with natural ones when the horizontal stress difference ranges from 2 to 8 MPa. A higher fracturing fluid viscosity reduces the possibility of shearing slip and filtration expansion of the natural fractures, forming a relatively less complex fracture network. Under the strike-slip fault stress state (), the horizontal hydraulic fracture can be easily generated and propagates along the bedding plane, especially when the direction of borehole is parallel to the bedding plane. The fracturing with variable pump rates can reactivate more natural fractures, forming a more complicated fracture network. Acidizing treatment for openhole section can significantly reduce the fracture pressure, and the decrease range of fracture pressure gradually increases with the increase of acidizing treatment time. This study may provide a reference for field fracturing treatment.