Modelling of Hydraulic Fracturing

This thesis employs numerical modelling of hydraulic fractures on a large time- and length scale. The aim is to analyze the Fracture Aligned Sweep Technology-technique, first used in the extraction of oil and gas in the Halfdan field in the North Sea. Different macroscopic effects of the technique have been investigated, with the intent of deepening the comprehension of the technique’s process. In order to perform thenecessary analysis, a Finite Element Model of the reservoir was designed. Fluidstructure interactions are simulated with a coupled poroelastic model. The potential of hydraulic fracturing is proven by numerically measuring the increased oil extraction, as well as the reach (sweep volume) of the injected fluid for constant fracture heights. It was discovered that any increase in fracture height greatly increased production.Comprehension of the fracturing process and the flow in the reservoir was achieved in a detailed, realistic model, where material depth dependency and changes to the fluid composition during production are accounted for. It became apparent that numerous factors affect the flow, fracture initiation and fracture growth. The depth dependency of the material data proved essential to include, since it provided considerable changes to the flow in the reservoir. Furthermore, it also affects both the fracture initiation and growth by changing the stiffness of the material. Changes to the fluid composition are included by establishing and tracking the frontline of the injected fluid during production, which also ended up greatly impacting the results. Investigations of how to best control the fractures by adjusting the operation parameters, injector- and producer pressure, was performed. If the difference between the two was great, it lead to larger fracture growth, while too little difference let to no fracture initiation at all. Finally, the influence of so-called ”skin”-effects, also known as caking effects, is investigated for different degrees of fluid entry resistance from the fracture into the poroelastic material. For this consideration, the flow in the fracture is also  modelled by incorporating existing models for fracture flow and fluid entry which allow for large time scale simulations. It is shown that the fluid entry resistance leads to earlier fracture initiation, along with changing the width of allowable operational pressure for the injector.