Shale Reservoir Damage Evaluation Method Based On Gas Diffusion Capability

Shale gas reservoir belongs to hydrocarbon source rock reservoir, and quantities of gas is stored as absorbed gas in the formation. The pores in shale matrix spans a large size scale, and the organic pores, inorganic pores and micofractures constitute a extremely complex gas storage and flow space. Thus the output of shale gas experiences a multi-mass-transport process which contains gas desorption, diffusion and seepage. Exploring gas diffusion, the middle process, is of great significance to understand the shale gas production mechanism. Meanwhile, nearly all shale wells need hydraulic fractruing stimulation, whereas the fractruing fluid is heavily detianed in the strata and the flowback rate is low, so it is vital to analyze gas diffusion ability damages caused by fracturing fluid invasion.Taking the Longmaxi underground shale as sunject, this paper mainly focuses on the experimental desription of gas diffusion capacity in shale rock, and a new characterization model that accords with the gas output process. Also, based on the designed testing method, the gas diffusion coefficient before and after fracturing fluid imbibition damages is evauated. Then, the relative damage evaulation index as well as control measurements are given out.Analysis shows that Longmaxi underground shale is mainly composed of quartz, feldspar and clay minerals, and the average porosity and permeability is3.45%and0.056mD. The organic nano-pores is developed in shale matrix, and the dominating pore size ranges from3nm to100nm, in which pores less than10nm occupies50%～80%of the total pore space.Gas diffusion type and stage are determined in shale matrix. Accordig to the classic kn number flow pattern classification standards, Knudsen diffusion is the primary gas diffusion type. Moreover, combining the shale pore structure and storage characteristics, the gas diffusion in shale matrix can be divides into three stages, includig the Knudsen diffusion in the slip flow, diffusion in nano-pore networks(stage I) and concentration diffusion in kerogen(stage II).Confirm the contribution of Knudsen diffusion in the slip flow. This paper formulates the equivalent permeability representation model, and verifies this model through methane low speed flow experiments. By introducing the permeability correction factor F, the contribution of Knudsen diffusion is quantified, which indicates that correction factor F increases with the growth of pore diameter and the decrease of gas pressure, and the bigger the value of F is, the higher the contribution rate.Estabolish gas diffusion coefficient test and damage assessment method based on the pressure decay experiments. The test results show that the diffusion coefficient of stage I is 3.78×10-13～7.04×l0-13cm2/s, and that of stage II is2.89×10-17～3.29×10-16cm2/s. After the fracturing fluid imbibition damages, the diffusion coefficient decreases remarkably. The damage index of stage I is0.18～0.65, and the damage degree is weak to moderate damages, while that of stage II is0.81-0.98, and strong to extremely strong damage, respectively.Put forward the preventing and controlling measurements for the fracturing fluid imbibition damages. Fracturing fluid formula optimization and nitrogen injection can depress the fluid imbibition and enhance the flowback, which can reduce the damages to gas diffusion ability. Non-aqueous fracturing and high temperature heat treatment technologies can eliminate the damages brought by conventional fracturing fluid radically, and morevoer, these technologies can improve the gas diffusion ability in shale matrix on a smaller scale, which have wide application potential.