Effect of low-concentration HCl on the mineralogy, physical and mechanical properties, and recovery factors of some shales

Oil and gas-bearing shale formations have received a great deal of interest in recent years because they could make a significant contribution to global hydrocarbon production. However, their development has been hindered by the complexity of drilling and completion strategies, which must be adapted in response to shale's mineralogy and physical properties. Matrix acidizing is commonly used as a pre-flush 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 some plays it is used as the main stimulation technique (e.g. Monterey shale, California). The mineralogical, mechanical, and physical responses to matrix acidizing of several important North American shale formations have been evaluated, and the effect on their recovery factors are described. Samples of Eagle Ford, Mancos, Barnett, and Marcellus shale formations were exposed to 1, 2 and 3 wt% HCl. Mass loss, compositional analysis, and X-ray diffraction based mineral assemblage characterization and quantification, show samples lost as much as 16 wt% by mass when treated with 3 wt% HCl for 3 h. The majority of the mass loss was attributed to carbonate dissolution. Analysis of post-acid treated samples show increases in porosity relative to the starting materials, but the increases in porosity are not necessarily correlated with acid strength. Images of post-acid samples demonstrate the development of cracks and fractures in Mancos, Barnett, and Marcellus samples. In contrast, the Eagle Ford samples show a homogenously distributed decrease in density, which based on mineralogical and compositional characterization, is attributed to spatially near uniform dissolution of calcite. Eagle Ford samples showed the largest increase in oil recovery factors ranging from 38% to 71% with a significant reduction in Young's modulus ranging from 25% to 82% when exposed to HCl solutions at 93 °C.