Acid Fracturing Carbonate-Rich Shale: A Feasibility Investigation of Eagle Ford Formation

Abstract

Due to the extensive technically recoverable shale natural gas resources and an increase in production from shale plays over the last 10 years, it is essential to focus on stimulation techniques to enhance productivity in shale resources. One such stimulation technique is acid fracturing. Similar to hydraulic fracturing, acid fracturing injects acid into a formation at a pressure greater than the fracture pressure, creating fractures and etching the created fracture face. The high carbonate content of the Eagle Ford Shale makes the formation a possible acid fracturing candidate. Minimal research has been completed on the effects of acid fracturing high carbonate-containing shale formations, where various conditions can limit the permeability of petroleum fluids. In this study, eight Eagle Ford shale samples, four from Zone C and four from Zone D, are used to determine the effect of surface etching pattern, acid concentration, calcite content, and Brinell Hardness Number on created acid fracture conductivity in the Eagle Ford shale. The resultant surface etching volume is determined by a pre-and post-surface Profilometer scan to calculate the change in surface profile. Calcite content is determined by X-Ray Diffraction analyses. Conductivity tests are performed on the eight samples up to a closure stress of 4000 psi, sample integrity permitting. Based on the results determined in this study, acid fracturing the Eagle Ford shale could provide optimistic increases in production. Acid fracturing treatments in high calcite containing zones of the Eagle Ford shale are expected to result in an increase in productivity. For the Eagle Ford Shale, the surface etching patterns that result in iii highly sustained conductivity values are channeling and roughness. A direct correlation is shown between calcite content of the shale and resultant surface etching volume. Furthermore, the higher the calcite content of the samples, the more likely the etching pattern will result in channeling and surface roughness. Brinell Hardness tests are completed on Eagle Ford samples determining that, on average, the higher the Brinell Hardness number, the better resulting surface etching pattern and sustained acid fracture conductivity.iv

Evaluation of Acid Fracturing Treatments in Shale Formation

“…act as proppant to prop the fractures under closure pressure. The experiment of acidizing in shale by Cash et al 15 showed that acidizing with 28% HCL produces a high initial conductivity, which reduces sharply in the latter period, and treatment with 15% HCL produces a constantly high conductivity. It is thought that a higher content of carbonatite in shale results in a larger dissolution space, and high conductivity is likely to generate.…”

Chemical and Reactive Transport Processes Associated with Hydraulic Fracturing of Unconventional Oil/Gas Shales

“…Acid fracturing for stimulating carbonate-rich shale, such as calcareous mudstone, has been examined as a method for improving conductivity and matrix permeability by etching the microfractures. , Experimental studies on different shale samples concluded that dilute acid concentrations that are optimized for specific materials are necessary to balance hydrocarbon recovery and unfavorable rock weakening by dissolving carbonate minerals from the rock matrix. − These studies have provided an understanding of changing permeability, porosity, and constitutive properties that are utilized in continuum-scale models for simulating macroscopic fracture dynamics. , …”

Data-Driven Acid Fracture Conductivity Correlations Honoring Different Mineralogy and Etching Patterns

“…Thus, tracking the effect of changing one condition on the resulting conductivity will be a challenge. An extensive literature review was applied to gather published data on the API acid fracture conductivity experiments. ,,,− It is worth mentioning that the data collected are consistent as they are obtained from the same laboratory using the same experimental procedure. Table illustrates the physical properties, meanings, and units of different features.…”

“…A recent study has also shown that acid-fracture fluid type could alter the pattern of rock removal which has a significant effect on the conductivity, even more so than the volume of the dissolved rock . For example, conductivity is higher when the acid fracture treatment creates channels rather than rough surfaces, given that they could withstand closure stress. − The leaking of acid from the fracture into the surrounding rock matrix can also result in more heterogeneous fracture surfaces that enhance conductivity if the rock’s mechanical properties are unharmed …”