To maximize the formation contact in unconventional reservoirs, multi-stage horizontal well completion with multiple short perforation clusters is widely adopted to facilitate simultaneously initiating multiple fractures from the perforation clusters in each stage. Many previous field observations have shown that a significant percentage of perforation clusters are often not effectively stimulated to contribute to the production. Efforts have been made to improve the effectiveness of horizontal completions by focusing on using lateral measurements to place perforation clusters in rock of similar stress so they are more likely to be successfully stimulated. While this has significantly improved the completion efficiency, the fundamental understanding of the fracture initiation process in a completion configuration with multiple clusters of spiral perforations is still lacking and hence unable to provide a quantitative means to predict and assess the completion efficiency.
To address these issues a near-wellbore fracture initiation calculator based on analytical elastic solution for the cased and perforated completion configuration has been developed. It predicts fracture initiation pressure from a perforation of any phase angle, the initiation location and fracture orientation relative to the wellbore. For a multiple clustered perforated completion, with many perforations oriented in different angles and possibly subjected to different local stresses, fractures may initiate only from some of the perforations. The model predicts a breakdown pressure for a given pump rate and a subset of perforations that are broken down. In this presentation, the fracture initiation model will be described, along with some examples and comparison with 3D numerical simulations and experimental results. A series of sensitivities have been performed to quantify the impact of injection rate, tectonic setting, stress variation between clusters, and perforation properties on hydraulic fracture creation, orientation and complexity at each perf cluster. They demonstrate that fractures may not initiate from some clusters and that within an active cluster some perforations may not be accepting fluid. Incorporating the results from the model can help engineers design completions that maximize effective stimulation of perforation clusters and reduce near-well fracture complexity.