煤层气排采中的“灰堵”问题应对技术——以沁水盆地多分支水平井为例

煤层的性质决定了煤层气在排水采气过程中"出灰"(产出煤粉)是一种必然现象,在煤层气低能量开采及关井后再次开井条件下,煤粉的产出对产能的负面影响是不可逆转的。为此,通过分析山西沁水盆地煤层气排采特征以及该区多分支水平井实钻井眼轨迹特征,找出了多分支水平井产气量达不到预期目标,以及关井实施维护性作业后难以恢复到关井前产量的2个原因:煤粉沉积堵塞了储层的运移通道,水平井段的波谷处形成的地层水和煤粉沉积加大了气体流向井底的阻力。进而提出了应对"灰堵"问题的关键技术:①煤层气水平井的井眼轨迹应在保证一定煤层钻遇率的前提下,以井眼光滑、总体上倾为原则,避免出现"波浪状"井眼;②排采井洞穴具有"沉沙"功能,便于气、液、固三相分离,是煤层气多分支水平井不可或缺的重要组成部分;③主支在稳定的煤层顶板或底板的仿树形水平井为疏灰提供了稳定的洗井通道。该研究成果为解决煤层气排水采气过程中的排灰问题提供了有益的尝试。

Key technology for treating slack coal blockage in CBM recovery: A case study from multi-lateral horizontal wells in the Qinshui Basin

Due to the natures of coal beds, slack coal production is inevitable in gas recovery by water drainage. The negative effect of slack coal production on production capacity of coalbed methane (CBM) is irreversible, when CBM wells are reentered after low-energy exploitation and shut-in. In this paper, therefore, the CBM production characteristics and multi-lateral horizontal well trajectory in the Qinshui Basin, Shanxi Province, were analyzed. In the multi-lateral horizontal wells, the expected gas production rate could not be reached and the production rate after shut-in maintenance could not recover to the level before shut-in. The reason for these issues is that migration pathways in the reservoirs are blocked by slack coal deposits and formation water and slack coal deposit accumulate at the troughs of horizontal sections. Furthermore, the three key technologies to deal with slack coal blockage were proposed. First, CBM horizontal well trajectory should follow the principle of keeping the wellbores smooth and up-dip instead of being "wavy", on the premise of guaranteeing coalbed drilling rate. Second, the caves of production wells, as an important part of multi-lateral horizontal wells, are capable of settling sand, and can be used for gas-liquid-solid separation. And third, a tree-like horizontal well with its main laterals set at the stable roof or floor, provides a stable well flushing passage for coal powder. This research provides a useful attempt in solving the problem of slack coal production in gas recovery by water drainage.