The Effects of Fracturing Fluids on Shale Rock Mechanical Properties and Proppant Embedment

The development of shale reservoirs has grown significantly in the past few decades, spurred by evolving technologies in horizontal drilling and hydraulic fracturing. The productivity of shale reservoirs is highly dependent on the design of the hydraulic fracturing treatment. In order to successfully design the treatment, a good understanding of the shale mechanical properties is necessary. Some mechanical properties, such as Young’s modulus, can change after the rock has been exposed to the hydraulic fracturing fluids, causing weakening of the rock frame. The weakening of the rock has the potential to increase proppant embedment into the fracture face, resulting in reduced conductivity. This reduction in conductivity can, in turn, determine whether or not production of the reservoir will be economically feasible, as shale rocks are characterized by their ultra-low permeability, and conductivity between the reservoir and wellbore is critical. Thus, shale reservoirs are associated with economic risk; careful engineering practices; and a better understanding of how the mechanical properties of these rocks can change are crucial to reduce this risk. This paper discusses various laboratory tests conducted on shale samples from the Bakken, Barnett, Eagle Ford, and Haynesville formations in order to understand the changes in shale mechanical properties, as they are exposed to fracturing fluids, and how these changes can affect the proppant embedment process. Nanoindentation technology was used to determine changes of Young's modulus with the application of fracturing fluid over time and under high temperature (300 °F) as well as room temperature. Mineralogy, porosity, and total organic content were determined for the various samples to correlate them to any changes of mechanical properties. The last part of the experiments consisted of applying proppants to the shale samples under uniaxial stress and observing embedment using scanning acoustic microscope. The results of this study show that maximum reduction of Young’s modulus occurs under high temperature and in samples containing high carbonate contents. This reduction in Young’s modulus occurs in “soft” minerals as well as the “hard” rock-forming minerals. This reduction of modulus can cause the effective fracture conductivity to decrease significantly. Introduction In geology, shale has traditionally been defined as a sedimentary rock containing high percentages (more than 50%) of clays and lower percentages of silica or carbonate minerals (Britt and Schoeffler, 2009). However, many of the shale prospects that are currently being developed in the petroleum industry are not shales, as defined in geology. They are, instead, “prospective shale” reservoirs, which are fine-grained clastics that are characterized by their ultra low permeability and usually composed of silica and carbonate with a small amount of clay minerals (Britt and Schoeffler, 2009). Generally, shale rocks have been considered as source rocks for conventional oil and gas reservoirs. However, with technological evolution in the petroleum industry, such as hydraulic fracturing, the rising oil and gas prices, as well as the escalating demand for fossil fuels, these rocks are increasingly regarded as the source, the seal, and the reservoir. Subsequently, development of such reservoirs is becoming more and more technically and economically feasible. The reasons behind the success of these shale systems are largely dependent on excellent hydraulic fracturing designs that require a good understanding of the mechanical properties of the subject and confining formations. In hydraulic fracturing design, Young's modulus is one criterion used to define the most appropriate fracturing fluid and other design considerations. Young's modulus provides an indication of how much fracture conductivity, kfw, can be expected due to width and embedment considerations. Without adequate fracture conductivity, production from the hydraulic fracture will be minimized if not completely eliminated. This paper discusses conditions where Young’s modulus is shown to decrease