Modeling Transient Thermo-Poroelastic Effects on 3D Wellbore Stability

Abstract

Abstract The temperature difference between the wellbore drilling mud and the formation, especially in deep wells, causes volumetric expansion of pore fluid and rock matrix. Most existing models ignore the effect of convective heat transfer, which is a valid assumption for low permeability formations such as shales. However, convection plays an important role in controlling wellbore stability in high permeability formations such as sandstones. A 3-D thermo-poroelastic model that accounts for the effect of convective heat transfer is developed in this study. Transient coupled pore pressure and temperature equations for non-isothermal conditions are developed based on conservation laws. Thermal effects are generated by the temperature imbalance between the drilling fluid and drilled formations, and increase as the temperature imbalance increases. Cooling the formation is found to be helpful in lowering collapse pressure, resulting in a more stable borehole. However, it is also found that a formation is more vulnerable to fracture because cooling also lowers the breakdown pressure. A higher mud weight is required to fracture the formation when hot drilling fluid is used because hotter fluids increase the breakdown pressure. Also, a higher mud density is needed to prevent a wellbore from collapsing when a hotter fluid is being circulated through it. The model presented in this study is useful for wellbore stability analysis for deep wells and deep water applications where mud temperature varies significantly along the well path. Introduction The technology of drilling deeper, deviated and horizontal wells has been growing rapidly in recent years. Some of the benefits of drilling such wells are lower development costs and faster production rates. However, these wells demand operating under high pressure and temperature conditions and as a result generate increased wellbore stability problems. In order to control the instability one needs to understand the behavior of rock around the wellbore. The behavior of the rock is defined by the state of stresses and material properties. Material properties are usually considered to be constants of the rock character for the purpose of wellbore stability analysis. It is the state of the stresses that is responsible for extreme variance in rock behavior. Pressure and temperature contribute to the state of the stresses. There are three major mechanisms that affect the state of the stresses in the rock formation around the wellbore: mechanical, thermal and chemical effects. Drilling a well causes a stress concentration at the wall of the wellbore as a result of replacing the drilled-out rock with a drilling fluid of different density from that of the original rock. Inclination from vertical and deviation from the direction of the maximum horizontal stress result in additional stresses around the wellbore. The redistributed stresses are the tangential stress, the radial stress, the axial stress, and shear components generated in deviated wells. The formation around a wellbore is considered to be a porous medium saturated with pore fluid. The effect of the pores should also be considered in analyzing the behavior of the rock around the wellbore. For this purpose, the concept of effective stress was introduced1 and is defined as the overall effects of normal stresses and pore pressure. Pore pressure changes will contribute to the state of the effective stresses by two mechanisms:redistribution of the pressure profile; andadditional stresses on the matrix. Pore pressure changes occur as a result of hydraulic diffusion of the drilling fluid into the formation, chemical effects and thermal effects. In shale formations there is additional pore pressure changes caused by a chemical potential difference between the drilling fluid and the shale pore fluid. The chemical potential difference will cause fluid flow into or out of the formation and results in pore pressure changes that are coupled with hydraulic fluid flow. Chemical effects have been studied by Dokhami2 and Yu et al. 3, 4