The Effects of Pore Pressure and Temperature Difference Variation on Borehole Stability

Abstract

Considering that drilling fluid filtration and the temperature difference between borehole wall rock and drilling fluid can cause the stress variation of the borehole wall. The stress distribution model was derived under the effect of thermal-flow-solid coupling. The safe mud density window calculating model considering pore pressure and temperature difference variation was established according to Moore-Coulomb criterion and borehole wall rock tensile failure criterion. The result calculated by the model can be expressed as follow. (1) When the temperature difference between borehole rock and drilling fluid is constant, with the enhancement of fluid filtration, borehole rock pore pressure increasing, the collapse pressure increasing, breakdown pressure decreasing, the stability of the borehole becomes deteriorating. (2) When the borehole wall rock pore pressure is constant, if drilling fluid makes wall rock temperature decreasing, with the temperature difference increasing, both the collapse pressure and breakdown pressure decreasing, the stability of the borehole becoming deteriorating, it is not conducive to drilling safely. If drilling fluid make wall rock temperature increasing, with the temperature difference increasing, both the collapse pressure and breakdown pressure increasing, the borehole tending to stabilize, it is conducive to drilling safely.
Key words: Pore pressure; Temperature difference; Borehole stability; Safe mud density window