煤层气欠平衡水平井的井眼清洁监测方法

煤层气欠平衡水平井钻井作业时，由于水平井具有水平位移大的特点，极易造成钻井液携岩不畅等问题，易导致井壁不稳定，增加了井壁坍塌及井漏的风险。为此，基于井眼清洁程度与井筒环空循环压力的变化具有相关性这一特点，把PWD测量的井底压力用于井眼清洁程度监测。采用C++语言开发出一套煤层气欠平衡水平钻井井眼清洁监测系统，该系统通过实时采集数据监测井底的环空压力数据，与理论计算的环空压力数据进行对比，寻找井眼清洁程度与环空压力变化规律及组合特征，用专家系统隶属度脉冲曲线判断井眼清洁程度，从而实时诊断井眼清洁度并对井眼不清洁发出预警，实现了对煤层气欠平衡水平井井眼清洁度的实时评价。实际监测应用结果表明，在井下出现煤层垮塌或钻井液性能变化而表现出井眼重度不清洁预警情况下，需要及时改变注气量、钻井液性能等工程措施，进行钻井液循环排除岩屑在环空的滞留，以消除沉砂带来的安全风险。

Monitoring methods of borehole cleanliness for underbalanced horizontal CBM gas wells

Since horizontal wells are characterized by large horizontal displacement, many problems easily occur in drilling underbalanced horizontal CBM wells, such as a poor carrying performance of drilling fluid. For this reason, the wellbore is liable to become unstable, increasing the likelihood of caving and lost circulation. In view of this, based on the correlation between borehole cleanliness and circulating pressure variance in the wellbore annulus, the bottomhole pressure measured with PWD was used for monitoring the borehole cleanliness. With the C⁺⁺Language used, a borehole cleaning monitoring system was developed for underbalanced horizontal CBM drilling. This system monitors the bottomhole annulus pressure by collecting real time data. By comparing the measured data with the theoretically calculated data of annulus pressure, we analyzed the change law and combination characteristics of borehole cleanliness and annulus pressure. The borehole cleanliness was then judged according to the membership pulse curve of an expert system. On this basis, the borehole cleanliness was diagnosed in real time for sending out an alarm of borehole uncleanness. In this way, the real time assessment was realized for borehole cleanliness of underbalanced horizontal CBM wells. Based on the actual monitoring, if an uncleanness alarm is sent out due to the caving or any change in drilling fluid performance down the hole, relevant engineering measures should be taken, including timely adjusting the gas injection volume and drilling fluid, so as to circulate the drilling fluid for cleaning up the debris retained in the annulus, and thereby avoiding the risk of sand setting.