WorldWide Drilling Resource

New Model May Lead to More Efficient Hydraulic Fracturing Adapted from Information by Los Alamos National Laboratory and the U.S. Department of Energy The United States has been hydraulic fracturing for decades, making the country a pioneer in refining the process of extracting gas and oil through fractures. The next step would be to learn more about how fractures work and move, which is exactly what researchers at Northwestern University and LosAlamos National Laboratory are working on. Its latest computational model is capable of predicting previously hidden fractures and more accurately interprets the amount of gas being released during the process. “Our model is far more realistic than current models and software used in the industry,” said Zdeněk Bažant, McCormick Institute professor and Walter P. Murphy professor of civil and environmental engineering, mechanical engineering, and materials science and engineering at Northwestern’s McCormick School of Engineering. “This model could help the industry increase efficiency, decrease cost, and become more profitable.” Although the industry has seen a great deal of growth, a great deal of the fracturing process remains a mystery. With the fractures occurring deep underground, it’s impossible for researchers to see how fractures move and release the gas from shale. “This work offers improved predictive capability that enables better con- trol of production while reducing the environmental footprint by using less fracturing fluid,” said Hari Viswanathan, computational geoscientist at Los Alamos National Laboratory. “It should make it possible to optimize various parameters such as pumping rates and cycles, changes of fracturing fluid properties such as viscosity, etc. This could lead to a greater percentage of gas extraction from the deep shale strata, which currently stands at about 5% and rarely exceeds 15%.” The new model used information gathered by studying the closure of existing fractures caused by ancient tectonic events, and considering water seepage forces not previously taken into account. Researchers were able to develop a new mathematical and computational model showing how branches form off vertical cracks during the fracturing process, allowing more natural gas to be released. The model is the first to predict this branching while being consistent with the known amount of gas released from the shale during this process. Classic fracture mechanics research predicted cracks which run verti- cally from the horizontal bore, with no branches. However, these cracks alone cannot account for the quantity of gas released during the fracturing process. In fact, the gas production rate is about 10,000 times higher than calculated from the permeability measured on extracted shale cores in the laboratory. Other researchers previously hypothesized the hydraulic cracks connected with preexisting cracks in the shale, making it more permeable. However, Bažant and his fellow researchers found the tectonically-produced cracks, which are about 100 million years old, must have been closed by the viscous f low of shale under stress. This led the team to hypothesize the shale layer had weak layers of microcracks along the closed cracks, and i t must have been these layers which caused the branches to form off the main crack. Unlike previous studies, they also took into account the seepage forces during diffusion of water into porous shale. When they ran a simula- tion of the process using this new idea of weak layers, along with the calculation of all the seepage forces, they discov- ered their results matched those found in reality. “We show, for the first time, that cracks can branch out laterally, which would not be possible if the shale were not porous,” Bažant said. After establishing these basic principles, researchers hope to model this process on a larger scale. Branching into densely spaced hydraulic cracks is an essential process for gas and oil extraction from shale. Although it is suspected to occur, existing mathematical models and current software aren’t able to predict it. A newmodel from Northwestern University and Los Alamos National Laboratory presents a method to not only predict when the branching occurs, but also how to control it. G&O 17 WorldWide Drilling Resource ® AUGUST 2019