Incorporating injection stage into DFIT analysis for permeability estimation, and its significance

Abstract

Diagnostic fracture injection tests (DFIT) have been broadly used in unconventional reservoirs to derive properties such as initial formation pressure, formation permeability, and closure pressure. DFIT involve pumping a small volume of untreated fluid into the formation to form a small crack. Formation permeability is usually obtained by modeling fluid leakoff during well shut-in. Prevailing studies assume constant fluid pressure boundary conditions on the fracture walls, constant leakoff coefficient, or both. However, the results deduced from these assumptions can introduce tremendous errors because the fluid pressure inside the fracture dissipates fast as fluid leaks off. Therefore, this study proposes a material balance approach to analyze DFIT data and obtain formation permeability. The proposed analysis incorporates fluid leakoff during both injection and well shut-in periods. According to the material balance relationship, volume of water injected during hydraulic fracturing should be equal to the summation of leakoff volume during hydraulic fracturing, leakoff volume during shut-in period and residual fracture volume. Thus, we propose leakoff models for both fracture propagation and shut-in periods. These models are then validated through a few synthetic cases. The model accounting for leakoff during fracture propagation is modified to more accurately calculate the leakoff volume. The model for the shut-in period is solved using the Fourier transform and the eigenfunction expansion method. The results suggest that the proposed DFIT analysis approach can provide a good estimation of the formation permeability. The proposed solution can be used in the field even when pressure data contain significant levels of noise. We showed that more accuracy can be achieved using this method compared to available solutions in the literature.