As is well known, hydraulically fractured horizontal wells have been extremely successful in the development of low permeability reservoirs throughout the world. The vast majority of these completions employ cased and cemented wellbores drilled approximately in the direction of minimum horizontal stress. Multiple, relatively short perforation clusters are included within each frac stage along the lateral. This efficiently creates many hydraulic fractures propagating orthogonal to the well, but it does not insure that each perf cluster is effectively stimulated.
Many efforts have been made to improve the effectiveness of horizontal completions. This has mainly focused on using lateral measurements to place perforation clusters in rock of similar stress so they are more likely to be successfully stimulated. But this ignores the impact of formation initiation pressure and tectonics on fracture initiation. In addition, the number, dimensions and orientation of the perforations in each cluster can greatly influence the effectiveness of the stimulation at each initiation point.
To address these issues a near-wellbore fracture initiation calculator has been developed that predicts whether a fracture will initiate at a perforation, the minimum initiation pressure, the fracture initiation location and orientation at each perforation, and the injection rate into each perforation. These parameters are a function of the casing size and orientation, the mechanical properties of the rock and cement, the principle effective stresses, and properties of the perforations.
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. The sensitivities demonstrate that fractures may not initiate at many clusters and that within an active cluster some perforations may not be accepting fluid. Incorporating the results from this model enables engineers to design completions that insure all perforation clusters are effectively stimulated and near-well fracture complexity is minimized.
This methodology does not just look at a single perforation, or cluster. Instead, it accounts for the stress variation between multiple perforation clusters within a frac stage, in addition to perforation orientation, dimensions and eccentricity, to predict the likelihood that each perforation cluster will be stimulated. By employing this methodology one can better design a perforating system and optimize perforation placement within a lateral to insure hydraulic fractures are created at all perforation clusters.