水力压裂体积张开度-应力扰动本构关系及压裂效果调控

THE CONSTITUTIVE RELATIONSHIP BETWEEN STRESS DISTURBANCES AND HYDRAULIC FRACTURING VOLUME OPENINGS AND EFFECTS CONTROLLING

储层体积张开和应力扰动是水力压裂过程中相互伴生的力学行为.前者创造了储层水力裂隙网络,包括主裂缝和分支裂缝网络,它们为游离气提供运移通道.后者是储层基质骨架上的压应力增量,包括主裂缝应力扰动和静水应力扰动,它们通过在脆性储层产生剪胀扩容或在塑性储层产生塑性变形,控制骨架基质吸附气的解吸和扩散通道.二者共同决定水力压裂的工程效果.为了进行压裂效果调控,文章建立体积张开度-应力扰动本构关系,包括体积张开度-应力扰动细观机理、数学表达式以及施加流体载荷的方法,从而将水力压裂众多影响因素融入一个模型中.水平井压裂实例表明,该本构关系揭示了水力压裂和效果评价的各主要力学特征、机制和影响因素,可以反映断裂传播机制(黏性-储存主导机制和黏性-滤失主导机制)、压裂液(水、液态CO_(2)、超临界CO_(2)和N2)以及流量对体积张开度和应力扰动的影响,为建立压裂效果调控机制:流体载荷-体积张开度-应力扰动-长效导流机制,奠定了关键理论基础.

Hydraulic fracturing volumetric openings and stress disturbances are concomitant mechanical behaviors in hydraulic fracturing process.The former create hydraulically crack net-works including the main fracture and branching crack net-works,which provide passages for free gas transportation.The latters are increments of compressive stresses on the skeleton matrix,including the main fracture stress disturbance and hydrostatic stress disturbance,which are resulted from main fracture volumetric opening and crack net-work volumetric opening,respectively.Stress disturbances control the passages for gas desorption and diffusion by producing shear dilation in brittle formations or irreversible compression deformation in plastic formations.Therefore,the both mechanical behaviors are jointed together to control the hydraulic fracturing effects.In this paper,in order to establish a controlling mechanisms of gas production,the constitutive relationship between the two mechanical behaviors are established,which includes the meso-mechanisms of stress disturbances stimulated by hydraulic fracturing volumetric openings,the mathematical expressions between stress disturbances and hydraulic volumetric openings,and the method of applying fluid load.By this model,the multitudinous influence factors on the HF effects are integrated into a unified model.As examples,two limiting hydraulic fracturing cases,which respectively belong to the viscosity-storage dominated regime M and viscosity-leak-off dominated regime M,are employed to demonstrate the constitutive behaviors under varied injection flow rates and fracturing liquids such as water,liquid CO_(2),supercritical CO_(2),and nitrogen gas N2.It shows that with the total hydraulic volumetric opening increasing,the hydrostatic stress disturbance firstly goes up to a peak value linearly,then falls to a low level,while the main fracture stress disturbance goes up monotonously following a power law with the exponent larger than 1,and that in M regime,the change paths of stress disturbances are coincident in a line under varied injection flow rates,but in regime,these paths are divergent,and the values in M regime are far smaller than in M regime,and that with the fracturing medium change from liquid to supercritical state and to gas,the hydraulic volumetric openings and stress disturbances become smaller and the final stress disturbance from fracture stress dominated changes to hydrostatic stress dominated.This constitutive relationship establish the base for a controlling mechanism:injection fluid load-hydraulic volume openings-stress disturbances-long term gas production.