钻井液损害煤层评价方法探讨

针对国内尚缺少统一有效的钻井液损害煤层评价方法的现状，基于煤层气产出及损害机理，提出了解吸量比值法和渗透率恢复率法联合评价钻井液对煤层损害效果的评价方法和指标，并对煤样解吸量比和渗透率恢复率的概念进行了定义。在此基础上，重点阐述实验评价方法以及煤层气自降解钻井液（CBMD）和常规无固相聚合物钻井液（WGJH）对煤层的保护和损害能力。实验结果表明：①CBMD降解后，渗透率恢复率较高，可达100%，对煤储层基本不伤害，而WGJH钻井液渗透率恢复率最高不超过30%；②CBMD污染过的煤样解吸量比达95.89%与清水接近，WGJH钻井液污染过的煤样解吸量比为20.11%。实验结果同时也解释了现场应用WGJH后煤层产气量低甚至不产气的原因，即煤层渗流通道恢复能力变差、煤层气解吸能力降低。结论认为，该方法实用、有效，可作为钻井液损害及保护煤层效果的评价方法。

Discussion on the evaluation method for coalbed damage caused by drilling fluids

Currently, there is no unified and effective evaluation methods for coalbed damages induced by drilling fluids in China. With consideration to CBM production and damage mechanisms, combination of desorption ratio and permeability recovery rate and relevant indexes were proposed to evaluate coalbed damages. In addition, concepts of desorption ratio and permeability recovery rate were defined. On this basis, the experimental evaluation techniques, together with protection and damage potentials of coalbed methane degradable drilling fluids (CBMD) and conventional solid-free polymer drilling fluids (WGJH), were described in detail. Research results show that: （1） upon degradation, the permeability recovery rate of CBMD reaches as high as 100%, indicating that the fluid may basically produce no damage to coalbed, whereas the permeability recovery rate of WGJH is no higher than 30%; （2） the CBMD contaminated samples have a desorption ratio up to 95.89%, which is similar with those of freshwater, while the WGJH contaminated samples have a desorption ratio of 20.11%. Moreover, the test results clarify the reasons for low CBM productivity or no production upon application of WGJH on site, namely, a poor recovery capacity of flow channels in coal beds and a low desorption capacity of CBM gas. This practical and effective method can be deployed for evaluating damages caused by drilling fluids and performances of coalbed protection.