Computational Model Predicts Breakdown Pressures in Unconventional Plays
Authors: Carpenter Chris 
Published: May 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 194038 STU, “A New Computational Model To Predict Breakdown Pressures in Cased and Perforated Wells in Unconventional Reservoirs,” by Mohammed Kurdi, University of New South Wales, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed.

Unconventional shale reservoirs are characterized by extreme low permeability and high in-situ stress. Multistage hydraulic fracturing plays a key role in developing such reservoirs. However, depending on the in-situ stress magnitude or regime, breakdown pressures can be too extreme to achieve, given available surface horsepower and downhole completion capabilities. This paper presents a newly developed model to predict the breakdown pressures in cased and perforated wells.

Introduction

In unconventional shale reservoirs, multi stage hydraulic fracturing often is applied to stimulate a number of clusters across the target zone. Once the clusters are perforated and hydraulic fracturing is applied, these clusters initiate transverse fractures, creating a fracture network that is required for reservoir depletion. At each cluster, the pressure delivered from the surface pumping units is exerted on the rock until the breakdown pressure is achieved. If the breakdown pressure is not achieved at a given cluster, there will be no induced fractures and, consequently, no production.

The main objective of the author’s study is to investigate the physics behind fracture initiation of cased and perforated wells in extremely tight reservoirs. This involves building a computational model to calculate the breakdown pressure for an inclined well at given azimuth and inclination angles. The developed model considers the in-situ stress magnitudes and faulting stress regimes to analyze local stresses around the perforations to predict the fracture-initiation pressure and orientation. Fracture propagation is outside of the scope of study.

Development of the Proposed Model

Stress Distribution Around the Well-bore. The stresses acting on the rock de-posited at a given depth include the vertical (overburden) stress, the maximum horizontal stress (SH), and the minimum horizontal stress (Sh). These are called in-situ stresses, or far-field stresses, and, depending on their magnitudes, they define the faulting stress regimes. When a wellbore is drilled into the rock and a mud pressure is exerted, the wellbore becomes a free surface and stresses are perturbed near the wellbore wall. Assuming the rock is elastic and isotropic, the local stresses converge near the wellbore wall at the azimuth of the minimum horizontal stress magnitude, creating a high compressive area that is prone to drilling breakouts. The stresses diverge in the azimuth of the maximum horizontal stress orientation, creating a low compressive area that is prone to drilling tensile failure. Therefore, these local stresses need to be deduced from the in-situ stresses to analyze the near-wellbore area. To take the effective stress into account in the calculation, the pore pressure needs to be subtracted from the in-situ stress. Previous formulas from the literature are modified to take the effective stress concept into account. A new coordinate system is introduced in the complete paper. Stresses at the wellbore wall (transformation of Cartesian stresses to cylindrical stresses) also are analyzed.