Acid-induced mineral alteration and its influence on the permeability and compressibility of coal

Abstract

The natural fracture system in coal serves as the primary conduit for water and gas flow in coal seam gas fields. For low permeability coal with highly mineralised fracture systems, the dissolution and/or modification of mineral occlusions could potentially enhance permeability and improve stress resilience.This study investigated the effect of mineral alteration by hydrochloric and hydrofluoric acid (HCl -HF) on fracture compressibility and coal permeability. Coal core immersion in 15 % HF-4% KCl solution has enhanced coal permeability to brine from 0.10 to 0.45 mD and reduced fracture compressibility from 0.020 to 0.006 bar -1 . Enhanced permeability and improved stress resilience were attributed to kaolinite (Al2Si2O5(OH)4) dissolution and hieratite (K2SiF6) precipitation, respectively. Geochemical speciation, simulating HF interactions with coal fracture minerals, predicted the occurrence and prevalence of both dissolution and precipitation reactions. Scanning electron microscopy-energy dispersive spectroscopy confirmed the mineral alteration phenomena. Identification of resultant structural changes and the differentiation of chemical from physical effects were elucidated using X-ray computed tomography. The overall findings show that mineral alteration by HF yielded relatively large, crystalline minerals that appeared to provide structural support to fractures, resulting in enhanced fluid flow and improved resistance to compression.

Experimental Study of CO2-Water-Mineral Interactions and Their Influence on the Permeability of Coking Coal and Implications for CO2-ECBM

“…HCO 3 -combined with positive ion in water solution as shown in Equations (7) and (8). Subsequently, bicarbonate and carbonate ions collided with divalent cation, they formed carbonate (Equations (9)(10)(11)(12) …”

“…Dissolution of mineral occlusions in coking coal seams with highly mineralized fractures is expected to considerably influence fluid flow capacities, since high-ash coal reservoirs with low permeability often contain considerable amounts of clay minerals, carbonate minerals, pyrite, and quartz, which tend to fill up the pores and cracks of the coal bed. Fluid seepage mainly occurs through natural cleats or fractures, the interaction of minerals with CO 2 and groundwater significantly affects the flow characteristics of coal reservoirs [10]. Various studies on the mechanism of CO 2 -water-rock interactions have been conducted [11][12][13][14], the role of acidification to rock minerals through CO 2 injection, with the huge potential for CO 2 storage, was identified in saline aquifers and depleted oil and gas reservoirs.…”

Repeated Low‐Intensity Shock Wave Simulation Experiment for Fracture Development and Propagation of Coal

“…However, due to its own characteristics, the fracturing effect is severely constrained by the conditions of the coal seam. Many injuries caused by fracturing to coal seams will affect subsequent mining, reduce production efficiency, and increase production costs [7][8][9][10]. It is an effective method to improve the efficiency of gas extraction by adopting explosions increasing permeability of coal seam [11][12][13].…”

Experimental Study of the Pore Structure and Permeability of Coal by Acidizing

“…Balucan et al presented an acidizing means consisting of a pre-flush (HCl), main flush (HCl-HF), and an overflush (HCl) to improve production in the oil and gas industry. Turner et al launched for the first time an acid-induced mineral alteration of the microstructure of coal using scanning electron microscopy and X-ray [24,25]. It is conceivable that cleats demineralization using hydrochloric acid and hydrofluoric acid can be an effective means to overcome low permeability provided cleat connectivity and permeability are characterized.…”