Structural Interaction between Rock Formation and Wellbore Components in a Highly Depleted Reservoir
Authors: Vasquez Auristela , Søreide Ole Kristian , Nasvik Håvard 
Published: March 2015

Production of hydrocarbons from the Wilcox reservoirs in the Gulf of Mexico is disadvantaged by low permeability and hence low well productivity. By introducing artificial lift methods, the production and drainage of hydrocarbons can be increased and prolonged, improving the project economics. However, as a consequence of the well depletion, the reservoir rock experiences geomechanical changes that leads to forces and possible failures on the well completions components. In particular, the production casing can be subjected to high stresses, resulting from the large differential pressures across it. This work was performed with the purpose to estimate these stresses, and investigate the structural interaction between the reservoir and well completions during large depletion (&gt;15ksi) of a typical Gulf of Mexico reservoir.

During depletion, the large pressure difference between the wellbore and the intra-reservoir impermeable layers induces stresses in the completion. In addition, the pore pressure difference between permeable and impermeable layers may induce large bending moment on the casing. To investigate this phenomenon, a two-dimensional axisymmetric finite element model conformed by a 9 5/8″ casing, 6-5/8″ screen, gravel, cement and borehole was built in a Finite Element Analysis Software (Plaxis). The target reservoir model consisted of turbidities sandstones with low permeability, with an impermeable shale layer inserted in between the sandstones.

The input parameters of the numerical model were based on experimental data from laboratory testing on Wilcox cores and sand prediction studies. The results from simulations showed that the casing installed in the reservoir will be subjected to large loads from the formations as the reservoir is being depleted. It was demonstrated how the presence of an impermeable layer (shale) in the sandstones increases the hoop and radial stresses. Sensitivity studies were carried out with the aim to evaluate how different elastic properties of the formations and well components, including cement and gravel, affect the casing loads. As the shale and gravel elastic modulus are increased, reduced loads are transferred to the well completion during depletion, reducing hoop stresses on the casing. In contrast, the increment of the cement Young's modulus increases hoop stresses with minor influence compared with the gravel and shale elasticity. The hoop stresses have shown to be less sensitive to the change in the modulus of elasticity of the sandstones.

This study allows understanding the mechanical interaction between reservoir and completion components, which may aid to ensure well integrity and achieve a long service life and cost-effective application of artificial lift methods. The challenge for the industry is to design casings that can sustain the large depletions experienced in deep wells. Hence, through this work some key parameters with significant impact on the casings loads are identified, which should be considered when optimizing the casing design of wells under high pressure draw down for ultimate high oil recovery from the reservoirs.