Simulation of multiple hydraulically driven fractures

Abstract

Hydraulic fracturing is a process that is used to release and extract natural gas from the pores of shale rocks. The process involves drilling vertical and horizontal wells through shale formation beds. After which, a mixture of fluid and sand is pumped into the rock formation through the horizontal well, pressurizing the shale around the well, causing multiple permeable cracks to form. Studying hydraulic fracturing helps Oil and Gas companies to improve the efficiency and productivity of this process. Because the hydraulic fracturing process takes place hundreds of meters below the ground’s surface, its behavior is difficult to physically assessed. Computer modeling is an efficient and economical way to study and analyze the behavior of the process. Finite element modeling, as a numerical tool, can be used to solve such non-linear fracture problems. In this thesis, finite element modeling is used to study two-dimensional, single, and multiple crack propagation problems that occur during fluid injection. The single fracture problem is compared with a well known analytic model (KGD model) in order to verify the efficiency of the numerical finite element model. The effects that rock and fluid material properties have on the fracture propagation, crack width, and fluid pressure is studied. As a result, the finite element numerical model is found to be in good agreement with the KGD analytical solution. Moreover, the analysis revealed that small changes in the material properties (e.g., rock elasticity modulus, permeability, and fluid viscosity) have significant effects on fracture propagation. Multiple crack problems, using three parallel cracks, are also investigated. The effects of the fracture spacing and type of fluid control (flow rate or pressure control) are studied. Stress shadowing (induced stresses from the adjacent fracture) between multiple fractures is evaluated. For the edge cracks, it is found that as the fracture spacing decreases, the crack length increases. While, for the middle crack, as the fracture spacing decreases, the crack length decreases. It is shown that fluid flow controlled injection leads to stable crack injection, while pressure control injection leads to unstable crack propagation. In summary, this thesis finds that an optimal spacing for three crack hydraulic fracturing is between equal fracture spacing and two-third the distance between the middle and any of the edge cracks. It is recommended that future engineers extend this research to simulate a three-dimensional problem with randomly oriented fractures.