Experimental Study and Percolation Analysis on Seepage Characteristics of Fractured Coal and Sandstone Based on Real-Time Micro-CT

Abstract

Sandstone and coal are the two most common types of reservoirs in nature. The permeability of sandstone in oil-bearing formations controls its oil and gas production; the permeability of the coal seam containing gas has a crucial influence on the gas drainage efficiency. One of the main factors affecting rock permeability is the spatial distribution and connectivity of pores and fissures in the rock. In this paper, a small-sized sample with a diameter of 5 mm and a height of 10 mm was used for the test. The rock samples under different stress states were scanned in real-time during the seepage testing. Based on 2D images, a 3D digital sample was reconstructed. We extracted the pores and fissures from the 3D digital sample, studied the size and distribution of the largest cluster in the sample, and revealed the influence of confining pressure and seepage pressure on the percolation probability and permeability of the sample. The research results show that brittle sandstone and plastic coal, two types of rocks with completely different properties of mechanics, have obvious differences in the spatial distribution of the largest clusters. Under the same stress state, in brittle sandstone-like rocks, the connectivity of the fissures is the primary factor affecting permeability, and the pores are the auxiliary factor; for plastic rocks such as coal, the situation is just the opposite, pores are the primary factor affecting permeability, and fissures are the auxiliary factor. The research results answer the question: Hydraulic fracturing technology can increase the oil and gas production of sandstone reservoirs but cannot increase the drainage efficiency of coalbed methane.