Date of Award

2006

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

A. Ghassemi

Abstract

Wellbore instability is a widespread problem, especially when drilling in deep, low strength shale formations at high temperature and pressure. The instability can be caused by the high compressive effective stress or tensile stress due to the stress concentration and pore pressure increase while drilling. In addition to the in-situ stress and shale strength, shale instability is affected by drilling mud properties including mud weight, temperature and salinity. To assess wellbore stability in these situations, Diek and Ghassemi (2004) developed a non-linear coupled chemo-poro-thermoelasticity theory. The linearized version of the theory allows one to analytically investigate the coupled impacts of mud properties including mud weight, temperature and salinity.

The theory views shale as an isotropic and homogeneous imperfect membrane, and considers the flow of matter across it. A difference of chemical potential between the drilling mud and shale will cause water flow (chemical osmosis), and the difference of solute concentration causes solute flux by diffusion. Heat flow also exists when there is a temperature difference between drilling mud and shale. Due to the low permeability of shale, the conductive fluxes are dominant in the system space, so the convective fluxes are neglected. The three fluxes are driven directly by the gradient of pore pressure, solute concentration and temperature. The three fluxes directly and indirectly impact on the stress distribution around the wellbore and the shale stability.

The field equations of the coupled chemo-poro-thermoelasticity were solved using Laplace transform and implemented in wellbore stability model to analyze the impacts of mud properties on the wellbore stability and optimize mud properties including mud weight, salinity and temperature to maintain wellbore stable. The results suggest that cooling tends to prevent shear failure, radial spalling and hydraulic fracturing, while heating enhances them. The thermal impact is different from that predicted for chemically inert rock by porothermoelasticity which indicates cooling increases hydraulic fracturing and heating prevents it. Drilling with a higher salinity mud reduces the swelling pressure, thereby enhancing the wellbore stability, and drilling with lower salinity reduces the wellbore stability. Furthermore, the interaction between thermal and chemical phenomena can be used to maintain a wellbore stable while drilling namely, lowering salinity when the mud is cooler than the formation and increasing salinity if the mud is warmer.

The coupled chemo-poro-thermoelasticity solutions were also implemented to analyze the chemical and thermal impacts on the mud weight window. The results suggest that cooling reduces the critical low mud pressure and increases the critical high mud pressure, thereby expanding mud weight window. On the other hand, heating increases the critical low mud pressure and reduces the critical high mud pressure, thereby contracting mud weight window. Drilling with a higher salinity mud tends to reduce the critical low mud pressure and increase the critical high mud pressure, therefore expanding the mud weight window. Drilling with a lower salinity mud contracts the mud weight window by increasing the critical low mud pressure and reducing the critical high mud pressure.

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