Effect of fracturing fluids on the mechanical response of shale

Shale is a fine-grained sedimentary rock composed of mud, which is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is a variable which changes with location from which shale rock is excavated. The predominance of clays influences mechanical properties of shale and typically imparts a strong elastic anisotropy. Shales are often rich in organic material called Kerogen. Kerogen rich shales are known as oil shales. Due to the presence of organic material, often liquid hydrocarbons called shale oil and natural gas can be produced from oil shale. Shale oil can be a substitute for conventional crude oil, but extraction is economically less viable than the production of conventional crude oil. Recent development in extraction technology has shown potential in reducing the cost of extraction. This new technique is called hydraulic fracturing or fracking. In fracking the shale rock is fractured by high pressure jets of fracturing liquid and thereby the trapped gas and oil can be recovered from the rock. It is therefore imperative that the fracture behavior and effect of hydraulic fracturing fluids on oil shale is studied in detail. This knowledge will help to predict the environmental impact as well as extraction potential of a particular formation of oil shale.

Shales are heterogeneous by nature. Understanding the chemical composition and morphology of different constituent phases are essential for the understanding of the fracture process. Shale samples are therefore also characterized with electron microscope and XRD to find out the constituent minerals. Finally, to assess the effect of hydraulic fracturing fluid on the shale rock, shale rock samples are treated in different hydraulic fracturing fluid solutions and characterized with electron microscope, XRD, Raman and nano-indentation.

Mancos shale samples obtained from Kocurek Industries, Inc. were ground, polished and then characterized with XRD and electron microscopy. The main constituents of Mancos shale are shown to be clay rich matrix and silica, feldspar, calcite and dolomite particles embedded in the matrix. The fracture toughness of the Mancos shale measured by semicircular beam method, 0.60 0.15 MPa m0:5. Chemical treatment of Mancos shale with different hydraulic fracturing fluids provided by Halliburton company and simulated fracturing fluid i.e. 7 wt.% KCl solution and DI water were carried out for upto 7 days. Treatment of KCl solution showed that penetration of salt into the shale occurs to a depth up to nearly 500 micron. Dissolution of calcite, dolomite and clay phases were observed by XRD of the sample and ICP analysis of the solutions. Dissolution of these phases were also confirmed by SEM and EDS analysis. Nano indentation carried out on the shale sample showed gradual reduction in hardness (10 to 8 GPa)and modulus (80 to 60 GPa) values with chemical treatment in KCl solution and DI water. This reduction in modulus and hardness was confirmed to be the results of the dissolution of carbonates and clay. Hydraulic fracturing fluids provided by Halliburton showed lower dissolution and correspondingly better retention of modulus and hardness after the treatment. In summary, this study shows that the shale rock mechanical properties are influenced by the presence of hydraulic fracturing fluids.