Experimental Studies on the Enhancement of Permeability of Anthracite by Acidizing: A Case Study in the Daning Block, Southern Qinshui Basin

Abstract

As the most active and top producing area of coalbed methane (CBM) in China, the southern Qinshui Basin (SQB) is dominated by anthracite. Due to the low permeability of coals, plenty of non-gas-producing and low production CBM wells exist in the SQB. The permeability enhancement through some technological means is the key to increasing the CBM production of this area. In this paper, some typical anthracites were selected from the Daning block of the SQB to assess the effect of acidification treatments on permeability enhancement. The maceral composition determination shows that approximately 15% of minerals exist in the collected coal samples, and the X-ray diffractometer (XRD) results reveal that the minerals consist primarily of clay minerals, along with a little amount of quartz, calcite, and dolomite. Two types of acidizing fluids were used to conduct acidification treatments on the anthracites for different lengths of time. The N2 permeability of the anthracites before and after acidification was measured and compared. The results show that the original samples exhibit low permeability. As the acidification time increases, the permeability of all of the samples shows an increasing trend, and the acid sensitivity index I a increases rapidly first and then levels off, and finally approaches 1. After 48 h of acidification, the samples show an increase ranging from 8.75 to 22.67 times (avg. 14.3 times) the original permeability. The permeability enhancement of the SQB anthracites is mainly attributed to the dissolution of acid-soluble minerals in the cleat system of coal. The minerals in the cleats are completely or partially dissolved by the acids, generating some soluble and insoluble substances; when the fluid flows through, the cleat space is reallocated. Overall, the cleat demineralization by acids frees up a lot of cleat spaces, leading to an increase in cleat connectivity. As a result, the fluid movement becomes smooth and the permeability of coal improves.

Geological Controls on the Gas Content and Permeability of Coal Reservoirs in the Daning Block, Southern Qinshui Basin

“… (a) Composite stratigraphic column showing Carboniferous–Permian coal-bearing strata in the Daning block (modified from ref ( 1 ), Copyright (2021), with permission from ACS Omega) and (b) column diagram of the No. 3 coal seam.…”

“… 33 In this area, nearly all of the CBM wells require hydro-fracturing stimulation technology to enhance the coal permeability. 1 Compared with granulitic and mylonitic structure coals, the primary and cataclastic structure coals have a complete coal-body structure and high strength, leading to a good hydro-fracturing potential. 81 In this area, the field experiences indicate that the greater proportion of the primary structure coal layer (C 1 ) and cataclastic structure coal layer (C 32 ) in the total coal thickness ( Figure 2 b), the higher the permeability of the coal seam through hydro-fracturing.…”

“…3 coal seam in the Daning block. (modified from ref , Copyright (2021), with permission from ACS Omega).…”

“…3 coal seam floor contour map with the major structures marked in the Daning block. (modified from ref ( 1 ), Copyright (2021), with permission from ACS Omega).…”

“…The global resources of coalbed methane (CBM) are very rich and amount to 263.8 trillion m 3 . , As a clean energy and a significant supplement to conventional natural gas, the CBM has received extensive attention in many countries worldwide. − The United States, Canada, and Australia are currently the most successful countries in terms of CBM development in the world . The CBM in China is very abundant, with resources up to 30.05 trillion m 3 , ranking the third in the world .…”