页岩气储层可压裂性评价技术

从页岩气储层岩石的脆性指数、断裂韧性、岩石力学特性3个方面,对中国页岩气储层可压裂性评价技术进行了研究。根据弹性模量与泊松比的分布范围,建立了适用于四川盆地下志留统龙马溪组页岩的脆性指数预测模型;总结了室内实验测定和利用测井数据计算页岩储层Ⅰ型、Ⅱ型断裂韧性的计算方法;初步建立了以弹性模量E、泊松比μ、单轴抗拉强度S_t三项岩石力学参数为自变量,可压裂指数F_rac为因变量的可压裂性评价方法;绘制了可压裂指数立体图版,据此评价页岩气储层压裂的难易程度,弹性模量值越高,泊松比和抗拉强度值越低,则F_rac值越高,反映储层越容易被压裂。根据地震测井和垂直地震剖面(VSP)等资料,结合室内实验校正,获取整个储层的岩石力学参数后,可以计算任意处的可压裂指数,建立储层可压裂性立体分布图。

Fracability evaluation of shale-gas reservoirs

The fracability of shalegas reservoir rocks was evaluated according to three rock mechanics paramete which including brittleness, fracture toughness and mechanical property. Based on the elasticity modulus and Poisson’s ratio scopes, a brittleness index prediction model was specially designed for the Lower Silurian Longmaxi Formation shale in the Sichuan Basin. Calculation methods for the mode I and II fracture toughness were established through lab experiments and logging data. A method to evaluate the fracability index was preliminarily developed by employing the elasticity modulus(E), Poisson's ratio(μ) and uniaxial tensile strength(S_t) as independent variables, and the fracability index(F_rac) as a dependent variable. A 3D chart of the fracability index was compiled to assess the difficult degree of multi-stage hydraulic fracturing of gas-shale reservoirs, from which one can see that when the elasticity modulus increases and the Poisson’s ratio and uniaxial tensile strength decrease accordingly, the fracability index will definitely increase to indicate the hydraulic fracturing of a reservoir is easier. Therefore, when the whole rock mechanics parameters of a reservoir are available, the fracability index at any part of the reservoir can be calculated according to the seismic logging and vertical seismic profile(VSP) data with lab calibration, moreover, a 3D fracability distribution of shale-gas reservoirs can be achieved as well.