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Coupled geomechanics, thermal and fluid flow model for wellbore integrity analysis, A
Orynbassar, Akezhan
Orynbassar, Akezhan
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2021
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Abstract
Wellbore instability often results in nonproductive time and is costly for drilling operations as well as long term integrity of the wellbore. The most important factor that affects the stability of the bore wall is the drilling fluid density. High density drilling fluid can cause a fracture in the wellbore because the hydrostatic pressure in the wellbore causes tensile failure in the bore wall. Using low density fluid often results in the collapse of the wellbore because the hydrostatic pressure is not sufficient to balance the in-situ stresses around wellbore. Hence, determining a proper mud weight is important to safely drill the wellbore minimizing the cost and risk associated with drilling operations. Currently, analytical models are used to evaluate the stress state around the wellbore. These models provide a quick determination of the stress field, but they have many limitations coming from the assumptions used to obtain the analytical solutions. One of the most important limitations of the analytical models is that they do not often account for the effect of the fluid and rock interaction because only the conservation of momentum equation is considered in these models. Fluid-rock interaction is an important aspect to consider when determining stresses around the wellbore. Hence, these models cannot be used to solve many challenging problems that arise in modern wellbore stability studies.
To address these limitations, a coupled model is formulated from equations of motion, heat transfer, and mass transport. The model accounts for the transport of two-phase water and oil as well as the heat transfer near the wellbore. The coupled model is solved numerically to obtain the stress change around the wellbore and the invasion of the fluid into the formation as well as the change of formation temperature. Using the developed model, case studies are conducted to evaluate the effect of thermal, fluid and rock interaction, and formation transport properties, permeability and wettability, on the time-dependent stability of the wellbore. The numerical results from this study show that the interaction between fluid and rock is the most considerable factor affecting the long term stability of the wellbore. Hence, changing the wettability of the drilling fluids can be considered as a method to improve the integrity of the wellbore. We observe that the minimum and maximum mud weights decrease with the decrease of temperature. Therefore, controlling the circulation temperature can also be effective to maintain the wellbore integrity. Permeability and wettability also play an important role in the determination of the mud window. Changing the wettability toward oil-wet can improve the stability of the wellbore, especially when the rock mechanical properties are very sensitive to water saturation.
The model developed in this study helps us to study the complexity of the mechanical interaction between drilling fluid and rocks and determine more accurate mud weights for drilling operations. This model can also be used for evaluating the integrity of the cement sheath during hydraulic fracturing operations and other problems related to the wellbore. Hence, this research provides the industry with a new practical tool to solve more challenging problems in wellbore integrity analysis.
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