Experimental analysis on the effect of tectonically deformed coal types on fines generation characteristics

Highlights

• Coal structure types can affect the yield, particle size and morphology of fines.

• Tectonically deformed coal types have no direct effect on fines compositions.

• Granulated coal can generate more fines with flow rates and stress increases.

• Stress sensitivity of vitrinite can strengthen fines production in coal reservoir.

• Water sensitivity of clay minerals can aggravate the generating intensity of fines.

Abstract

Solid fines generation in coalbed methane (CBM) development can cause serious formation damage and production breakdown. The anisotropy of coal reservoirs makes fines issue more complicated. In this study, the experimental analysis of the correlation between tectonically deformed coal types and fines generation characteristics was implemented. Two samples with different coal structure types, undeformed coal and granulated coal, were collected from the same coal seam. Under single-phase fluid flow, two sets of core flooding experiments were conducted to generate fines from these samples. The yields of fines produced at varying experimental conditions were analyzed quantitatively. The characteristics of these fines were microscopically observed through the use of a laser particle size analyzer, a scanning electron microscope, and a polarizing microscope. The results indicated that tectonically deformed coal types significantly influenced the generating intensities, particle sizes and morphological features of fines. Because of the varying degrees of structural destruction, compared with undeformed coal, granulated coal contained more original tiny fines in the fractures and was more sensitive to variations of fluid flow rates and reservoir effective stress, which could intensify the generation of fines. Undeformed coal could generate fines with wider particle-size distribution ranges and larger mean particle sizes than granulated coal could. The micromorphology of fines produced from undeformed coal was mainly angular. However, for granulated coal, most of the fines were subangular and even subrounded. No appreciable impact of the development of tectonically deformed coal on the compositions of fines was found. Because of the water sensitivity of clay minerals and the stress sensitivity of vitrinite, the produced fines contained more clay minerals and vitrinite with increases of displacement velocities and confining pressures under laboratory conditions. Through well logging interpretation, the development intensity of tectonically deformed coal in three CBM wells was identified. After analysis of the concentration and mean particle size of fines collected from these wells, it was concluded that the more developed the tectonically deformed coal, the higher the fines concentrations and the smaller the fines mean particle sizes, which was consistent with the experimental results.