Compressibility and Productivity Performance of Deep Coal Seam under the Effect of Pore–Fracture Structure

The pore–fracture system and compressibility of deep coal reservoirs affect the productivity of deep coalbed methane (CBM). The compressibility of deep coal seam 8 in the eastern edge of the Ordos Basin, under the effect of pore and fracture structure, needs to be studied. In this paper, deep coal seam 8 of the Benxi Formation, collected from the Ordos Basin, was selected as the target sample, and the coal rock quality of all the selected samples was determined. The pore and fracture structure types of the selected samples were determined by using the saturated T 2 spectrum. The overburden pore-permeability test was used to study the dynamic variation of permeability in coal samples with different types of pore and fracture structures, and the factors of compressibility were determined. Moreover, the productivity simulation equation is used to study the productivity of deep coal reservoirs. The results are as follows: (1) all the samples can be divided into fracture-developed type (A), seepage pore-developed type (B), and adsorption pore-developed type (C). The type A sample has developed fractures, and there is good connectivity among the pores-fracture system. T 2 spectrum of the type B sample shows a bimodal state, with lower percentages of seepage pores and fractures. The T 2 spectrum of type C samples shows a single-peak shape, where the adsorption pore volume percentage can exceed 90%, and pore connectivity is poor. The compressibility of type A samples is 0.0693 MPa –1, which is higher than that of type B (0.018 MPa –1 ) and type C samples (0.0413 MPa –1 ). The compression space of type A samples and the other two types of samples is provided by fractures and pores, respectively. The permeability is affected by pore structure parameters and mechanical parameters, while the permeability damage rate is controlled by pore structure parameters. The simulation results show that the average gas and water production of type C samples are 2066.1m 3 /d and 3.2m 3 /d, respectively, which are much lower than those of type B reservoirs. Since type A reservoirs provide a higher initial permeability and compression space, this results in an increase in CBM production and migration rate within the same reservoir pressure drop range.