An experimental study of coal-fines migration in Coalbed-methane production wells

Abstract

Coalbed Methane (CBM) has become an important gas resource in the last several decades. The brittle property of coal matrix and overflushing operation make the migration of coal fines inevitable. As the liquid flows below a critical velocity, the coal-fines accumulate in the bottomhole. This leads to a significant production decline causes the problems of pump failure/underperformance, severe liquid/solid loading etc. It is important to study the apparent transport mechanisms of coal fines in the wellbore, which is crucial in terms of optimization design, avoiding coal fines setting and burying pump in some extreme cases. By innovative experiment under flowing conditions, this paper presents a piece-wise regressed correlation to calculate the critical velocity to lift the coal fines at different sizes, and consequently identify the onset of coal fine loadings in wells. Operators can then maintain the de-watering rate above the calculated critical velocity, which indeed lowers the maintenance frequency and related operational costs.The origin of mobile coal fines and/or particles falls into categories: 1) large coal fines created during drilling and completion, 2) and small coal fine generated in production and migrate through cleats, cracks, and fractures into bottomhole. The former is large in particle size but has a small volume in total. The latter is small in particle size while has a large cumulative volume. Because large particles required a high liquid velocity to be removed to the surface and are major cause of equipment failure. Small coal fines, although are the majority of migrating coal during back-flow after completion and during production, are easier to be lifted to surface and require a low lifting velocity. The velocity that can remove large coal fines will not have difficulty to lift small coal fines and will prevent them accumulating at bottomehole. Therefore, we focus on the removal of large solids because it is more important. The purposes of this study are to estimate statics settling velocity and critical velocity, which are critical to the selection of appropriate operation parameters.