A New Algorithm for Automated ISIP Interpretation

Abstract In multi-stage unconventional horizontal wells instantaneous shut-in pressure (ISIP) is useful for the analysis of hydraulic fracturing and its interaction with the stress state in the reservoir. ISIP can be estimated from the leak-off pressure response after the end of the treatment. However, this value is normally eclipsed by pressure oscillations that occur immediately after well shut-in, known as water hammer, which complicate its interpretation. We developed an automated workflow to estimate ISIP values from fracture stages that allows for the utilization of various shut-in physical models. The workflow is based on guidelines proposed by Roussel et al. (2021) and incorporates the Trust Region Reflective algorithm to optimize all parameters in the model, which allows for model convergence in a wider range of cases and is validated using field data. Comparison of fitting model parameters between stages and the addition of a ‘goodness of fit’ metric allow for quick identification of outliers and poorly matched model predictions. ISIP estimations at different step-down rate managements were then compared for validation using a commercial hydraulic fracture simulator capable of reproducing water hammer. Simulations showed that ISIP interpretation is consistent at varying step-down rates. Additionally, water hammer frequency in each stage of the well was converted into depth to test its potential as a diagnostic tool for fracture stage isolation by comparing results to observations made through fiber optic measurements.