Natural gas is one of the cleaner sources of energy, and our challenge is to produce it safely and economically. The extreme operating conditions that occur in gas-storage and gas-producing wells could cause the cement sheath to fail, resulting in fluid migration through the annulus. Designing cement sheaths that can withstand the stresses induced by the various operations and maintain integrity during the life of the well will help minimize the risk of cement failure.
A design procedure has been developed to estimate the risk of cement failure as a function of cement sheath and formation properties and well-stress loading. A few examples of the loading are cement hydration, pressure testing, gas injection, and gas production. The design procedure simulates the sequence of events from drilling to cement hydration, hydraulic fracturing, injection, production, etc. Cement failure modes simulated are de-bonding, cracking, and plastic deformation. An appropriate nonlinear material model, including cracking and plasticity, is used in the analysis.
The sustained casing pressure observed on a number of wells after they have been put on production emphasizes the need to design a cement sheath that will maintain integrity during the life of the well. Field cases and applications are discussed. From the process discussed in this paper, one can estimate the risk of failure of different cement systems and select the system that can help minimize the overall cost. The process should improve the economics of constructing and producing gas and oil wells and also improve the safety due to the reduced risk of zonal isolation failure.
The main purpose of the annular cement is to provide effective zonal isolation for the life of the well so that oil and gas can be produced safely and economically. To achieve this objective, the drilling fluid should be removed from both the wide and narrow annulus and the entire annulus should be filled with competent cement. The cement should meet both the short-term and long-term requirements imposed by the operational regime of the well. Typical short- and long-term properties that are required from the cement are listed in Table 1.
Traditionally, the industry has concentrated on the short-term properties that are applicable when the cement is still in slurry form. This effort is necessary and important for effective cement-slurry mixing and placement. However, the long-term integrity of cement depends on the material/mechanical properties of the cement sheath, such as Young's modulus, tensile strength, and resistance to downhole chemical attack. Considering properties of the cement sheath for long-term integrity is critical if the well is subjected to large changes in stress levels.
After placing cement in the annulus, if no fluid immediately migrates to the surface, short-term properties such as density, rate of static gel strength development, and fluid loss of the cement may have been designed satisfactorily. However, recent experience has shown that after well operations such as completing, pressure testing, injecting, stimulating, and producing, the cement sheath could lose its ability to provide zonal isolation. This failure can create a path for formation fluids to enter the annulus, which pressurizes the well and renders the well unsafe to operate. The failure can also result in premature water production that can limit the economic life of the well. Remedial jobs can then be performed (if feasible) before the well can continue oil and gas production.
Hence, if the cement sheath fails during its active life, it defeats the objective of producing hydrocarbons safely and economically. Failure of the cement sheath is most often caused by pressure- or temperature-induced stresses inherent in well operations during the well's economic life.