Permeability Loss of Bituminous Coal Induced by Water and Salinity Sensitivities: Implications of Minerals’ Occurrence and Pore Structure Complexity

Abstract

Water sensitivity (WS) and salinity sensitivity (SS) are key issues to be investigated for instructing coalbed methane (CBM) production. This work studied the influences of minerals and pores on WS and SS of medium-volatile bituminous coal (MVBC) and highly volatile bituminous coal (HVBC) deposited in northwestern China by detecting and observing minerals using the TESCAN Integrated Mineral Analyzer, simulating WS and SS, and characterizing pore structural complexities using rate-controlled mercury penetration. The results show that (1) kaolinite is mainly distributed as irregular particles or fragile aggregates attaching on the bedding surface or filling in meso-pores or transition pores, showing a high potential for detachment; (2) MVBC and HVBC in this study are characterized as medium to weak WS and weak SS, respectively; (3) for HVBC during the WS or SS process, kaolinite distributed in meso-pores or transition pores first detaches and then migrates to the narrow throat of macropores and super macro-pores, leading to volume decreases of macro-pores and super macro-pores and loss of permeability; and (4) kaolinite filling in macro-pores of MVBC detaches, then migrates, and finally deposits in super macro-pores after WS and SS, leading to losses of super macro-pore volume and permeability. Results of this study can enhance the scientific knowledge on WS and SS of coal during CBM development.

Dewatering Optimization of Coal Seam Gas Production Wells: A Review and Future Perspectives

“…It has been reported that the reservoir permeability and production rate decrease with the stress sensitivity coefficient for both coal seams and shale reservoirs, and the stress sensitivity coefficient reduces with the increasing effective stress. ,,, In other words, the increase of effective stress results in compaction of cleats, making cleats more resistant to compaction force, and the stress sensitivity coefficient drops, as proven by a number of experimental data (e.g., Figure ). ,− Studies have also revealed that (i) temperature and moisture content can enhance the stress sensitivity, (ii) the stress sensitivity decreases with the pore/fracture aperture and can be affected by fracture morphology, ,− and (iii) the stress sensitivity coefficient show strong anisotropy with a higher value typically in the horizontal direction. ,, Zhang et al proposed that the new fractures generated from shear and tensile failure in coal exert an evident impact on stress sensitivity, which matched with the results obtained by Tan and Konietzky . It is clear that the stress sensitivity coefficient constantly varies during production, so it is vital to quantify its variation to reliably estimate coal permeability evolution during the dewatering as well as gas production stages.…”

Review of the Effect of Diagenetic Evolution of Shale Reservoir on the Pore Structure and Adsorption Capacity of Clay Minerals

“…Additionally, the pores and structure of clay minerals are damaged by water swelling, collapsibility, and the water lock effect caused by water sensitivity. Specifically, the hydration and expansion of clay minerals may produce particle migration, which lead to the decline of reservoir porosity and permeability, and irreversible damage to the pore system of reservoir. − Collapsibility may lead to pore throat blockage, and poor porosity and permeability. − Also, the water lock effect caused by the increase of reservoir water saturation reduces the reservoir permeability. ,− In addition, it is not conducive to the formation and distribution of three-dimensional fracture networks in the reservoir fracturing process. − The interaction mechanism between water and clay minerals is extremely complex. So far, the damage mechanism of clay pore structure and the control mechanism of competitive adsorption of water and CH 4 have not been determined.…”

Experimental Study on the Influence of Hydrochemical Properties on the Migration of Coal Fines Containing Kaolinite in Propped Fractures

“…Initially, kaolinite particles detach from the mesopores or transition pores and migrate toward the narrow throats of macropores and super-macropores. This migration pattern results in volume reduction of macropores and super-macropores, ultimately leading to a loss of permeability . Although numerous research results on the influence of hydrochemical properties on the migration of coal fines have been reported, there are few studies on the migration law of coal fines containing kaolinite in propped fractures under different hydrochemical properties.…”