Fluid flow characterisation of naturally fractured coals in Jharia basin using cleat aperture scaling for enhanced CBM and CO2 storage potential
摘要
The Jharia coalfield, one of India’s major coal-bearing regions, holds substantial CBM potential, and offers prospects for geological carbon storage in un-mineable coal seams. In coal reservoirs, fluid flow of CO2 and CH4 is predominantly governed by fracture (cleat) permeability, as the matrix permeability is extremely low compared to that of conventional oil and gas reservoirs. This study investigates cleat (natural fractures present in coal) parameters from Jharia coalfield, especially length and aperture, their mutual relationships and their impacts on fluid flow. Cleat parameters were measured in several samples collected from coal mines, both underground and opencast, representing varied depth, rank and structural domains. The dimension scaling between cleat length and aperture resulted in a modified fluid flow equation. The traditional cubic law (Q = –ρgb3δhL/(12μ)), which equates fluid flow volume with the cube of aperture and length was modified to empirical relationships with different exponents (Q = –c′ρgbnδh/μ; n = 5.08, 4.38, 4.88; c′ = –0.15, –0.83, –2.15) which are particularly applicable for Jharia coalfield. The modified equations are also independent of cleat length, which is a difficult attribute to measure, especially in smaller drill cores due to the risk of under-sampling. The correlation coefficient between length and aperture was further compared with a global data base of different fracture types around the world, viz., Ethiopia dikes, Lodeve veins, Moros joints, Raniganj coal cleats. The coefficient applicable to Jharia coalfield cleats seems to be aligning well with the global average. This paper also explains observed deviations in detail. Cleats being formed as mode I desiccation cracks formed by the coalification process, the impact of deformation on modifying cleat parameters was thoroughly documented and explained. The work presented in this article would certainly make an impact in producing more precise dynamic fluid flow models, which are essential for CBM field development and understanding the CO2 plume migration in coal seams. The workflow presented in this paper can be applied to any other coalfields or fractured reservoirs around the world to obtain a field-specific fluid flow equation rather than relying on a standard cubic law.
Research highlightsExamine the scaling behaviour of coal fracture attributes across structurally diverse zones. Structural influence on permeability with larger cleat apertures near fault zones. Regional variation of cleat network formation between east and west. Jharia coal cleats show higher permeability potential than Raniganj, likely due to greater structural deformation. Refines CBM recovery models by providing empirical permeability relationships.