A New Computational Model to Predict Breakdown Pressures in Cased and Perforated Wells in Unconventional Reservoirs

Abstract

Abstract Unconventional shale reservoirs are characterised by their extreme low permeabilities and their high in-situ stresses. Multi-stage hydraulic fracturing therefore plays a key role in developing such reservoirs. However, depending on the in-situ stress magnitude and/or regime, breakdown pressures can be too extreme to achieve, given the available surface horsepower and downhole completion capabilities. This paper presents a newly developed model to predict the breakdown pressures in cased and perforated wells. As the in-situ stresses are perturbed near the wellbore wall due to the drilled wellbore cavity, the local stresses near the wellbore wall need to be deduced. Using MATLAB software program, a transformation from the in-situ stresses to the wellbore local stresses is performed. As perforation notches also perturb the in-situ stresses, another transformation to the local stresses around the perforation notches is required. Increasing breakdown pressure causes the stresses surrounding the perforation notches to exceed the rock tensile strength. The value at which the tensile strength is exceeded dictates the required breakdown pressure. As these transformation calculations depend on the wellbore inclination/azimuth angles, in-situ stress magnitude/regime and perforation phasing angle, a number of recommendations are presented. The results from the model recommend deep perforation penetration depths to expose the perforation tunnels to far field in-situ stresses. This eliminates the effects of local stresses and decreases the breakdown pressures. For normal faulting stress regimes, the results recommend steering the drilling bit as close as possible to Sh azimuth as any deviation results in an increase of breakdown pressure magnitude. It is also recommended to use oriented perforations rather than a typical phased perforation technique to minimize the formation breakdown pressures. The recommended oriented perforation angles depend on the in-situ stress magnitudes/regime and the wellbore trajectory. The model aims to advance the current understanding of fracture initiation in highly deviated wells in shale reservoirs. It can also assist engineers to better select sweet spots for well and cluster placement to avoid excessive breakdown pressures and/or potential early proppant screenout.