Fines migration in coalbed methane (CBM) fields comprises a serious environmental and gas-production challenge. The literature widely reports two kinds of fines: potential coal fines, which are a part of the coal body and can be detached by breakage under a significant drag force exerted from the inflowing water, and detrital coal fines, which are attached to the coal body by electrostatic forces. The theory for detrital coal fines migration is well developed. A theory for potential coal fines, where the drag deforms the coal asperities and detaches fines by rock failure, is not available. The objectives of this study are (1) to derive the governing equations for fines generation by breakage using failure criteria, and (2) to predict well productivity during dewatering and gas production using laboratory-based modelling. The micro-model developed is based on beam theory and comprises static rock deformation by the flow-through water and calculating failure criteria by tensile and shear stresses. The failure condition determines the number of fines that detach after the application of each flow rate, allowing determining the maximum retention function of potential coal fines. The breakage micro-model is incorporated into filtration equations that account for fines mobilisation, migration, straining and consequent permeability decline. Eight series of lab flooding data with coal cores have been treated. The close match between the lab and model validates the model developed. The model allows predicting productivity decline due to permeability reduction by fines breakage and straining.

Fines migration is one of the most drastic causes for formation damage - the detached clays migrate and impairs well productivity. Two types of damaging clays are encountered in petroleum reservoirs: authigenic clays that grew on the grain surfaces during geological times, and detrital clays that have been broken off the grains by local stresses. Detailed laboratory and mathematical modelling have been carried out for detrital-clay formation damage. The theory for formation damage by authigenic clays is not available. The aim of this work is the development of a laboratory procedure to estimate formation damage by authigenic clays and the derivation of a mathematical model for core scale. We performed two test of corefloods using Castlegate core samples. In the first test, injection rate increased in a stepwise manner up to 100 mL/min and in the second one up to a 200mL/min to make sure both detrital and authigenic particles are detached. The pressure drop across the overall core and the concentration of the fine in the produced fluid have been measured. We have derived equations for authigenic-fines detachment using the beam theory and the von Mises failure criteria to obtain analytical solutions for linear system of equations. Matching the laboratory data by the analytical model allows determining the percentage of authigenic and detrital clays in the cores. The laboratory data exhibit a good match with the mathematical model for the two coreflood tests. The non-monotonic change of the concentration of the detached fine, with the initial and final risings, determines the type curve that evidence the mobilization of both, authigenic and detrital clays. The treatment of the measured data in test#2 shows that 82% of the initial attached particles are authigenic. The model parameters in order of decrease of their sensitivity are contact-bond radius, pore radius, particle size, lever-arm ratio, tensile strength and aspect ratio. A novel experimental procedure to determine fines-migration formation damage by authigenic and detrital clays was developed. A newly derived mathematical model allows determining the model coefficients from the laboratory tests and predict future detachment rate of authigenic and detrital particles.

Fines detachment is an important component of methane production from Coal Bed Methane reservoirs. Production of coal fines is widely observed during dewatering and simultaneous gas-water production. The theory for fines detachment by drag against electrostatic attraction, model of the transport of those detrital fines, and their validation by laboratory test is widely used for planning and design of Coal Seam Gas developments. However, clay particles that naturally grow on coal grains and asperous parts of coal surfaces (authigenic and potential coal fines) are detached by breakage. To the best of our knowledge, the analytical theory for detachment of authigenic and potential coal fines is not available. The present paper fills the gap. Based on Timoshenko's beam theory, we derive failure conditions for breakage of authigenic and potential coal fines of the rock surface. It allows defining maximum retention function for fines breakage. The maximum retention is incorporated into transport equation of mobilized fines, allowing developing analytical models for linear flow of core flooding and radial flow of well inflow performance. Matching of laboratory coreflood data from four laboratory studies show high agreement. The model coefficients obtained by treatment of laboratory data allow predicting skin growth in production wells under fines migration.