Experimental Investigation of the Impact of Coal Fines Migration on Coal Core Water Flooding

Abstract

Coalbed Methane (CBM) has become an important gas resource in recent decades. The brittle property of coal matrix and overactive operation make the migration of coal fines inevitable. Blockage by coal fines that plugs flow paths is a non-negligible issue that results in a significant decline in gas production. By setting different experimental conditions with the following factors—coal fines concentration of the mixture displacing fluids, constant flow pump rate, inlet pressure, outlet pressure and confining pressure—six experimental schemes were designed to investigate the two-phase water and coal fines flow in natural core samples. When the differential pressure and flooding flow reach a pseudo-steady status, the equivalent permeability of coal samples can be approximately calculated considering coal fines migration. Furthermore, the influences of coal fines migration on the cleat opening and permeability variation are analyzed in the porous coal medium. The study will benefit CBM development and save pump maintenance costs. In this work, we found that maintaining the differential pressure for a longer period may result in new cleat openings and severe coal rock damage during the single-phase water flooding process. While coal fines may plug some natural cleats and pores, especially in the core samples with micro-cleats during the two-phase flooding stage, coal fines migration significantly reduces the equivalent permeability and dewatering ability of the coal rock in the earlier flooding. While enlarging the differential pressure in two-phase water and fines flooding, breakthrough of coal fines from the samples contributes to widened cleats. While coal fines are difficult to flood into the core pores for low-permeability core samples, coal fines gather in the inlet, and it is also difficult to reach the pseudo-steady status even under higher differential pressure. The damage to permeability mainly occurs in the early stage of coal fines migration, and an abrupt increase in the flow velocity can damage reservoirs and induce substantial coal fines generation. Thus, maintaining a stable effective strength and a controlled depressurization rate during drainage can effectively constrain coal fines output and decrease permeability damage within coal reservoirs.

An Experimental Study on Coal Fines Migration during Single Phase Water Flow

“…or attached to the edge of fractures with water flow; thus, the degree of blockage and the characteristic of coal fines decide the permeability variation. Larger particles (less than 120 mesh) are difficult to invade into the fractures, and most of them were only attached on the edge of fractures [42,43]. And for large particle size, the pores and throats between the particles are relatively bigger; thus, the degree of permeability damage is relatively weak.…”

Investigation on Pulverized Coal Control Using Calcium Sulfoaluminate Cementitious Proppants in Coalbed Methane Fracturing

“…For drainage control, in accordance with the pulverized coal production, water production, and reservoir pressure of coalbed methane wells, different drainage control measures are applied at different drainage stages to achieve refined drainage. 14,15 Meanwhile, different well-washing measures are adopted for process management, and a powder-proof tail pipe or a wire-wound screen is connected to the pump suction port. 16,17 Although different pulverized coal control measures have been proposed, effective prevention and control of the generation and migration of pulverized coal in reservoirs have not been achieved yet.…”

Coal Fines Migration, Deposition, and Output Simulation during Drainage Stage in Coalbed Methane Production

“…16−19 velocity and confining pressure, the effect of coal fines output within a fracture was examined. 20,21 It was found that the production of coal fines was positively correlated with displacement velocity and negatively correlated with confining pressure. According to the stress state of coal fine particles in the fracture and wellbore, the mathematical model of coal fines deposition was established, and the critical velocity for different sizes of coal fines was calculated.…”

“…Characterization of coal fines migration and production is mainly carried out by field and laboratory monitoring of the drainage process and by detecting the concentration of coal fines. , Coal fines generation and production are closely associated with coal petrography properties. , Numerical calculation methods, including Risnes, Morita, Weingarten, Bratli, and other algorithms, were employed to simulate coal fines production. − However, due to the complex geological conditions of coal seams, the results of these methods could not describe the real coal fines migration. In this regard, many physical experiments with the help of fracture conductivity instruments were conducted. − By changing the flow velocity and confining pressure, the effect of coal fines output within a fracture was examined. , It was found that the production of coal fines was positively correlated with displacement velocity and negatively correlated with confining pressure. According to the stress state of coal fine particles in the fracture and wellbore, the mathematical model of coal fines deposition was established, and the critical velocity for different sizes of coal fines was calculated. − Under the conditions of different particle sizes, flow velocities, and pore characteristics, the relationship between coal fines migration and permeability was investigated by physical experiments. , It was found that the shedding of coal fines increased the porosity and permeability, while the deposition could reduce the porosity and permeability.…”