A study on the interaction of mud acid with rock for chemical stimulation in an enhanced geothermal system

Abstract

In order to explore the thermal energy from the low-permeability hot dry rocks, both thermal and shear stimulations were employed to create connective fracture networks that allow fluid flow in the reservoir, which is the concept of enhanced geothermal system (EGS). However, strong hydraulic stimulation may lead to preferential shortcircuiting flow paths where the fluid transports too fast to take in sufficient heat from the reservoir. To solve this problem, the chemical stimulation was proposed recently, which injects aqueous chemical agents such as acids into reservoir to enlarge the fractures by dissolving the minerals in the rocks. This study examines in the laboratory the effects of using mud acid to enhance the permeability of the rock cores that were recovered from Yingcheng Formation in Songliao Basin, China. The results suggest that the permeability in the core increases with the injection rate of the mud acid. The core scanning electron microscope analysis shows that the increase in permeability is mainly induced by the dissolution of K-feldspar and albite. Secondary amorphous silica was formed due to the acidrock interaction. The chemical stimulation in the laboratory can be reproduced by reactive transport simulation. The experimental and numerical studies confirmed that the injection of mud acid allows the permeability enhancement during field chemical stimulation for an EGS reservoir.

Experimental Study on the Effect and Mechanism of Chemical Stimulation on Deep High-Temperature Granite

“…NaOH and mud acid (HCl + HF) were selected as reference stimulants, and their relative concentration was set according to relevant literature (Na et al, 2017;Guo et al, 2020). In order to avoid excessive secondary precipitation in the reaction systems when alkaline chemical stimulants are used, two stable and efficient chelators, ATMP and PESA, were added in schemes D1 and D2, respectively, to chelate the metal ions produced by chemical stimulation and inhibit secondary precipitation.…”

Impact of phase saturation on wormhole formation in rock matrix acidizing

“…While HF acid is used to dissolve the silicate minerals (clay, feldspar, and quartz), HCl 89 enhances the dissolving power of HF with minerals by maintaining a low pH value through removal 90 of carbonate minerals in the formation (Na et al, 2016). Nevertheless, unlike carbonates, the 91 5 precipitation of fluorides, fluorosilicates, hydroxides, and amorphous silica caused by fluid-mineral 92 interactions can potentially reduce the reservoir permeability (Shafiq and Mahmud, 2017).…”

Reactive Transport Modeling of Chemical Stimulation Processes for an Enhanced Geothermal System (EGS)

“…The performance of EGS mainly depends on the development of reservoir fracture networks and the hydraulic connection between injection/production wells [4,5]. The commonly used methods for reservoir stimulation currently include hydraulic shearing, thermal stimulation [6], and chemical stimulation [7]. Chemical stimulation refers to the injection of chemical stimulants into the reservoir at a relatively low injection pressure, relying on the dissolution effect of the stimulants on minerals to improve the permeability of reservoir fractures [8].…”