煤岩裂缝渗流能力实验评价及近井地层等效渗透率分析

煤岩裂缝的渗流能力是影响煤层气产能的重要因素.采用API导流仪及岩心驱替装置,模拟煤层气排采过程中煤层有效应力的连续变化,评价裂缝闭合压力逐渐升高、地层压力连续下降以及频繁开关井(间歇性排采)等工况条件下煤岩压裂裂缝的动态导流能力和含天然裂缝煤心的渗透率变化.实验结果表明,提高裂缝闭合压力、降低孔隙流体压力以及频繁开关井等都会降低煤岩裂缝的渗流能力,尤其对压裂裂缝的导流能力影响更为显著.基于此,利用实验数据分别回归了煤岩压裂裂缝导流能力和含天然裂缝煤岩渗透率与有效应力的经验关系式,建立近井煤层等效渗透率计算模型.利用该模型可以根据现场测算的等效渗透率,初步判断煤层压裂裂缝的有效导流能力或缝长,可以为煤层气压裂效果评价、分析排采过程中煤岩裂缝参数的动态变化提供一种新方法.

Experimental evaluation on seepage capability of coal rock fractures and Near-well formation equivalent permeability analysis

Seepage capability of coal rock fracture is an important parameter which can influence coalbed methane production significantly. In this paper, the dynamic conductivities of hydraulic fractures and the permeability change of coal rock with natural fractures are evaluated using the API fracture conductivity equipment and core-flooding equipment, respectively. The experimental results show that increasing the closure pressure of fracture, decreasing the porous fluid pressure, and switching well frequently can reduce the seepage capability of fracture in coal rock, and the conductivity of hydraulic fracture is more easily affected by these factors. Based on this, according to the experimental results, the empirical formula of the relationship between the hydraulic fracture conductivity and permeability of coal rock with natural fracture and the effective stress are regressed, and a calculation model of the equivalent permeability of the coal layer near well is established. The model can be used to estimate the effective conductivity or the length of the hydraulic fracture, which can provide a new method for evaluating the hydraulic fracturing performance of coalbed methane and analyzing the dynamic changes of coal fracture parameters in the process of coalbed methane production.