Numerical modeling for identification of closure pressure from diagnostic fracture injection tests
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Diagnostic fracture injection tests (DFIT) are used to estimate the magnitude of the minimum principal stress, which is assumed to be equal to fluid pressure at the moment of fracture closure. A small volume of water is injected into formation to create a fracture, the well is shut in, and eventually the fracture closes. The pressure during shut-in can be analyzed by several pressure transient methods to pick the time of fracture closure. Pressure at that time is taken to be the fracture closure pressure. In this study, DFIT simulations are performed with a fully numerical hydraulic fracturing simulator. Sensitivity analysis is done to investigate how reservoir parameters such as fracture toughness, permeability, fracture stiffness, and the magnitude of the minimum principal stress impact the DFIT pressure transient. Based on these insights, we use the simulator to match a DFIT pressure transient from a low permeability formation. We analyze the field data with conventional methods for picking closure. The simulation matches suggest that the conventional methods can underestimate the closure pressure in low permeability formations. Based on our results, we propose a new method for picking fracture closure based on the evolution of fracture compliance during closure. Our simulations provide insight into how the fracture compliance impacts to pressure transient. Assuming the closure pressure from our simulations matches to the data are correct, our proposed method picks the correct closure point. This study includes simulation matches to the field data with simulations that use complicated fracture geometry, which may be realistic in some formations. The multiple fractures cases have similar pressure transients and similar reservoir parameters as the single hydraulic fracture simulations, indicating that network complexity will not necessarily be evident from the pressure transient. In the future, DFIT simulations with more complex fracture geometries will be conducted to understand how fracture geometry affects the DFIT pressure transient.