致密煤岩气藏压裂液损害实验评价

Experimental evaluation on damage induced by fracturing fluids in tight coal rock gas reservoirs

致密煤岩气藏必须采取增产改造措施才能获得经济产量。部分煤层气井在压裂施工后,未达到理想产量,主要原因之一是压裂液对储层的损害。国内外开展压裂液对煤岩储层损害评价时,多采用煤粉压制的人造岩心,不能真实反映煤岩储层天然结构特性。文中以宁武盆地致密煤岩为研究对象,钻取柱状煤岩样品,开展了盐敏和压裂液损害评价实验。盐敏实验确定实验用KCl溶液的质量分数为1%。压裂液损害实验表明,活性水压裂液、清洁压裂液A、离子平衡压裂液的平均损害程度分别为46.61%,39.12%,85.42%,煤样与这3种压裂液作用后接触角减小、亲水性增强、压裂液返排困难;而弱酸性清洁压裂液B使煤样渗透率平均提高了46.57%,亲水性减弱。综合分析认为,损害煤岩储层的主要机理是亲水性压裂液吸附导致的煤岩基质膨胀。该研究方法及获得的认识对致密煤岩气藏压裂液设计具有借鉴意义。

Stimulation is essential for obtaining economic productivity in tight coal rock gas reservoirs. However, the stimulation effect of some coalbed methane wells is far from expected goal because of reservoir damage induced by fracturing fluids after fracturing operation. Domestic and overseas researchers mostly use artificial coal samples suppressed by coal fines to evaluate the formation damage induced by fracturing fluid, which cannot reflect the natural structure characteristics of coal rock reservoirs authenticly. Salt sensitivity and experimental evaluation of fracturing fluids damage were carried out for the tight columnar coal rock in Ningwu Basin. Based on salt sensitivity experiment, 1% concentration was ascertained for KC1. The experiment results of fracturing fluids damage show that for the average damage degree, surfactant solution is 46.61%, clear fracturing fluid A is 39.12% and ion balance fracturing fluids is 85.42%. The contact angle decreased after coal samples being exposed to those three kinds of fracturing fluids, and hydrophilism enhanced, as a consequences, fracturing fluids return back difficultly. Whereas coal samples permeability increased by 46.57% and hydrophilism was poorer because of clear fracturing fluid B which shows weak acidity. Through a comprehensive analysis, it is considered that the adsorption of hydrophilic fracturing fluids is the main mechanism of coal rock gas reservoirs damage. Method and awareness have been gained from research, which can provide some references for fracturing fluids design of tight coal rock gas reservoirs.