The Maximum Permissible Pressure in Well Stimulation Operations
Authors: Guo Boyun , Li Ben , Wei Na 
Published: March 2015

Abstract
Mechanical failure of wellbores may result in loss of zonal isolation, which is the cause of many problems, such as sustainable casing pressure, crossflows between formation zones, and leakage of fluid to surface. Pressure- and temperature-induced stresses in the cement sheath have been numerically analyzed by a number of previous investigators with sophisticated computer models. However, these investigations do not provide a practical method for predicting the maximum permissible pressure limited by the mechanical failure of cement sheath.
An analytical model was derived in this work to predict the Maximum Permissible Pressure (MaxPP) in fluid injection and well stimulation. Results given by the model were verified with data from a CO2 sequestration operation. Case studies with the model suggest that if the cement placement efficiency in the annulus is 100% as detected by CBL, formation in-situ stress should be utilized to make a conservative prediction of the MaxPP. If the cement placement efficiency in the annulus is less than 100% detected by CBL, the outer radius of the cement sheath should be considered and formation pore pressure in the cap rock should be employed to make a conservative prediction of the MaxPP. Improving cement strength is not an effective means of increasing MaxPP. The MaxPP should be enhanced by maximizing the outer radius of the cement sheath through improving the cement placement efficiency. The Maximum Permissible Net Pressure is lower than the burst-pressure rating of casing, suggesting that cement sheath failure will occur before casing failure occurs in the burst condition. Therefore burst design should be carried out for cement sheath, not production casing. This paper provides engineers a practical guideline to design and apply fluid injection pressure in well stimulation and operations.

Introduction
Cement sheath is the most important well element for maintaining integrity of fluid-injection wells. Failure of cement sheath can cause loss of zonal isolation, resulting in not only severe operational difficulties, possibly leading to the loss of the well, but also detrimental impacts to personnel, equipment, and environments.
The malfunction of cement sheath may be because of improper cementing practice involving poor hole calibration, poor casing centralization, poor selection of chemical agents for mud removal, poor matching between volume/rheologies/displacement rate of the cement train, and inappropriate cement properties. All these wrong-doing items can result in isolation defect of the cement sheath due to insufficient filling of the well annulus with cement, leaving ways for fluids to pass the cement barrier, often referred as flow-behind-pipe. The loss of zonal isolation may also be the result of secondary damage of cement sheath by the improper well operating practice including over-pressure injection, dynamic loading, thermal loading, and corrosion. This paper focuses on the issue of over-pressure injection to determine the maximum permissible pressure based on the mechanical failure of cement sheath.