Rock-Fluid Chemistry Impacts on Shale Hydraulic Fracture and Microfracture Growth

摘要

The role of surface chemical effects in hydraulic fracturing of shale is studied using the results of unconfined compression tests and Brazilian tests on Mancos shale- cored at depths of 20-60 ft. The rock mineralogy, total organic carbon and cation exchange capacity were determined in order to characterize the shale. Adsorption tests to study the interaction of the shale and aqueous fluid mixture were also carried out using surface tension measurements.The uniaxial compressive strengths and tensile strengths of individual shale samples after four hours exposure to water, 2.85 x 10^-3 M cationic surfactant (dodecyltrimethylammonium bromide-DTAB) and 2.81 x 10^-3 M anionic surfactant (sodium dodecylbenzenesulfonate-SDBS) were analyzed using ANOVA and Bonferroni tests. These mechanical strengths were largely reduced on exposure to the aqueous environments studied, despite the relatively low clay and low swelling clay content of the Mancos shale. Further comparison of the uniaxial compressive strengths and tensile strengths of the shale on exposure to water, to the strengths when exposed to the surfactant solutions showed that their difference was not statistically significant indicating that exposure to water had the greatest effect on strength loss.The surface tension measurement of 2.85 x 10^-4 M DTAB and 2.81 x 10^-4 M SDBS solutions before and after equilibration with shale showed about 80% increase in surface tension in the DTAB solution and 10% increase in surface tension in the SDBS solution. The probable sorption mechanism is electrostatic attraction with negatively charged sites of the shale as shown by significant loss of the cationic surfactant (DTAB) to the shale surface, and the relatively minor adsorption capacity of the anionic surfactant (SDBS). Although these adsorption tests indicate interaction between the shale and surfactant solutions, within the number of tests carried out and the surfactant concentration used, the interaction does not translate into a significant statistical difference for impacts of surfactants on mechanical strength of this shale compared to the impact of water alone. The relevance of this work is to facilitate the understanding of how the strength of rock can be reduced by the composition of hydraulic fracturing fluids, to achieve improved fracture performance and higher recovery of natural gas from shale reservoirs.