Wellbore stability in drilling transversely isotropic and chemically active shale formations

Instability issues often occur when water-based muds are applied to shale formations. Failure to accurately predict pore pressure has been widely accepted as one of the major causes of shale instability. Water activity is a key factor to control the pore fluid moving in and out of the formation due to osmotic effects. Ideal and single-solute models have been developed in the past to predict the pore pressure by considering chemical osmotic effects. However, due to their complex nature, pore fluids/drilling fluids rarely behave as ideal solutions. A new mathematical model was developed to predict pore pressure in shale formations taking into account the non-ideality and multiple solutes of both drilling fluid and pore fluid. Study results explain the fact that the salinity of drilling fluids has to be controlled in order to achieve shale stability. Wellbore stability models can provide more accurate results by including the proposed pore pressure propagation model, which considers drilling fluid induced chemical osmotic in-situ stress.;A Shale-Fluid Interaction Testing Cell (SFITC) was designed and completed in order to test the interaction between drilling fluid and pore fluid. Parameters such as hydraulic conductivity, membrane efficiency, and ion diffusion rate can be obtained through experimental data curve fitting. Water activity is found to be crucial in order to control pore pressure in shale formations.;The new concept of a safe salinity window is introduced to help drilling engineers determine the proper salinity level in drilling fluids. The safe salinity window can be obtained from a chemo-poro-elastic wellbore stability model, which incorporates drilling fluid induced chemical osmotic in-situ stress. A multiple solute, non-ideal solution pore pressure transmission model is adopted for accurately predicting the pore pressure term in a wellbore stability model. A safe salinity window can be determined using the proposed procedure in this study. The results of this study can benefit drilling fluid design. Shale instability can be reduced by adjusting the water activity of drilling fluid. This can be achieved through changing the type of solutes and their concentrations based on information from the initial water activity of the shale. The costs of drilling fluid and the much greater expense associated with wellbore instability can be reduced by understanding the safe salinity window.;A chemo-poro-elastic wellbore stability model considering the shale transverse isotropic effect is also developed. Analysis from this model shows that wellbore inclination angle, hydraulic conductivity, and water activity of drilling fluid play the most important roles in determining the mud weight window.