Massive fines detachment induced by moving gas-water interfaces during early stage two-phase flow in coalbed methane reservoirs

Field observations have shown that the concentration of fines produced during the early two-phase flow stage in coalbed methane (CBM) reservoirs is much higher than single-phase water flow, leading to more frequent workover activities. The typical flow pattern in early two-phase flow is bubble-water flow, in which the liquid phase is mostly contained in the form of liquid plugs, separated by a series of moving bubbles/gas-water interfaces (GWIs). Here, we quantify the impact of moving bubbles/GWIs on fines detachment, aiming at better understanding of fines mobilization mechanisms in bubble-water flow towards their effective control. First, fines migration tests in the absence and presence of moving bubbles in water flow, including effluent and permeability measurements, were conducted on fractured coal plugs. The experimental flow velocity was less than the critical flow velocity (CFV) for massive fines release. Experimental results showed that both peak effluent concentration and permeability impairment in the presence of moving bubbles were much greater than when absent, indicating that the quantity of fines detached by moving bubbles/GIWs was much larger than simply by water flow. Second, forces and torques acting on coal fines were analyzed to determine the combined effects of extended Derjaguin-Landau-Verwey-Overlook (DLVO), hydrodynamic, surface tension, and frictional forces on fines mobilization. Theoretical results showed that all in-situ fines could be mobilized by moving GWIs, whereas no fines could be released by water flow when the flow velocity is smaller than the CFV, as the surface tension force arising from passing GWIs greatly dominates hydrodynamic forces by 2-4 orders of magnitude. Theoretical results support experimental data, in spite of some errors due to model assumptions, i.e., spherical fines and homogenous surface.