Effect of Low-Concentration Hydrochloric Acid on Properties of Shale Rocks
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 165689, ’Effect of Low-Concentration Hydrochloric Acid on the Mineralogical, Mechanical, and Physical Properties of Shale Rocks,’ by Samiha Morsy, SPE, C.J. Hetherington, and J.J. Sheng, Texas Tech University, prepared for the 2013 SPE Eastern Regional Meeting, Pittsburgh, Pennsylvania, USA, 20-22 August. The paper has not been peer reviewed.
Matrix acidizing is commonly used as a preflush to the hydraulic-fracturing stimulation of shale formations. The process dissolves sediments and mud solids that inhibit the permeability of the rock, enlarging the natural pores of the reservoir and stimulating flow of hydrocarbons. In this paper, the mineralogical and physical responses to matrix acidizing of several important North American shale formations are evaluated.
Introduction
A few studies have quantified the effect of hydrochloric acid (HCl) matrix acidizing on mineralogical and physical properties of shale formations. However, less is known about the development of conductivity and the acid concentrations necessary to optimize conductivity and, by extension, the impact on production and rock stability. The mechanical properties of shale formations vary as a function of mineralogy, with shales rich in brittle minerals such as quartz and carbonates having higher Young’s- modulus values. Increases in the clay content, total organic carbon (TOC) content, and porosity may lower the Young’s- modulus values. Therefore, dissolution of brittle minerals, while improving short-term conductivity, may lower long-term shale rock stability. HCl reacts rapidly with calcite [Ca2 (CO3)2] and, to a lesser extent, with dolomite [CaMg(CO3)2], and it is predicted that the main impact of HCl on shale will vary as a function of how much calcite is dissolving, which, in turn, is controlled by acid strength, temperature, and pressure.
Shale Samples
Eagle Ford shale reservoir core samples and outcrop samples from the Mancos, Barnett, and Marcellus shale formations with contrasting assemblages—ranging from the calcite-/clay-rich, quartz-poor Eagle Ford to the quartz-/ illite-rich, carbonate-poor Mancos—were used in this study.
The Eagle Ford shale contains the greatest proportion of carbonate, but the absolute volumes of carbonate vary systematically. At deeper structural levels, such as those exploited in south Texas, there is upward of 70% carbonate by volume. With progression toward the northwest, the clay content increases, and the formation is exploitable at shallower depths. The high percentage of carbonate makes it more brittle and fracturable.
The Mancos is predominately steel-gray sandy shale but includes stringers of earthy coal, impure limestones, and many thin beds of fine-grained yellow and brown sandstone that are chiefly composed of subangular and angular quartz grains cemented by lime.
The Barnett is a very brittle gasbearing siltstone. Most Barnett shales are siliceous mudstones, rich in quartz, and may be considered argillaceous siltstones. Some of the Barnett lithofacies are insensitive to acid because of low volumes of carbonate, but their clay- mineral assemblage may be moderately sensitive to fresh water. Other litho facies have higher abundances of carbonate and are therefore more reactive to matrix acidizing.
The Marcellus formation is dominated by black shale with some interspersed limestone beds. Bedding is well developed and often splits along bedding planes. Pyrite is also relatively common in this shale.