Designing Drilling Fluids Rheological Properties with a Numerical Geomechanics Model for the Purpose of Improving Wellbore Stability

Abstract

Abstract Wellbore instability remains one of the major causes of non-productive drilling time and a source of risk that can jeopardize the well objectives. Traditionally, the mud weight has been used as the main parameter through which instability is constrained. This approach fails to consider the dynamic nature of operations. While the mud weight is used to estimate the necessary downhole hydrostatic pressure for preventing breakouts, there are several events and considerations related to dynamic wellbore hydraulics and rig equipment operation than can substantially negate the influence of mud weight. This deficiency can be overcome by integrating the design of the drilling fluid rheological properties with a suitable geomechanics model. The integrated approach relies on estimating the downhole pressure fluctuations in dynamic settings and assessing their influence on wellbore stability. The integration process is achieved through the use of a finite element geomechanics model with different wellbore hydraulics models that are applicable to a variety of operational scenarios. The first step in the analysis is defining the operational scenarios that can lead to downhole pressure fluctuations. These include well control incidents, circulating versus static conditions, and surge and swab effects while running in and out of hole, respectively. Once the scenario and the corresponding pressure fluctuations are defined and determined, the finite element geomechanics model is used to assess the influence of the pressure fluctuations on the wellbore stability. The final step in this analysis is proposing adjustments to the drilling fluid rheological properties. This will constrain downhole pressure changes to a range that can minimize instabilities. This step is supported with lab experiments to ensure that the proposed drilling fluid formulation can fit the design criterion. The analysis shows that for each operational scenario, apart from the mud weight, the drilling fluid rheological properties have a tangible effect on wellbore breakouts. It also shows that different operational scenarios can dictate completely different adjustments to the drilling fluid formulation. For example, the addition of a viscosifier to the fluid formulation can be beneficial in a scenario involving a well that is experiencing a gas kick. This aids in maintaining a high equivalent circulating density that counters the downhole pressure reduction caused by the gas intrusion. In another scenario with a well exhibiting surge and swab effects, the reduction of a viscosifier or the addition of a thinner can also be beneficial for constraining the downhole pressure fluctuations, which causes wellbore instabilities and other failures. The design approach proposed in this work provides a unique perspective that joins geomechanics modeling with drilling fluids formulation and operational considerations.