Abstract
Both overbalanced drilling and underbalanced drilling will lead to the change of pore pressure around wellbore. Existing research is generally based on hydraulic-mechanical (HM) coupling and assumes that pore pressure near the wellbore is initial formation pressure, which has great limitations. According to the coupled theory of mixtures for rock medium, a coupled thermal-hydraulic-mechanical (THM) model is proposed and derived, which is coded with MATLAB language and ABAQUS software as the solver. Then the wellbore stability is simulated with the proposed model by considering the drilling unloading, fluid flow, and thermal effects between the borehole and the formation. The effect of field coupling on pore pressure, stress redistribution, and temperature around a wellbore has been analyzed in detail. Through the study of wellbore stability in different conditions, it is found that (1) for overbalanced drilling, borehole with impermeable wall is more stable than that of ones with permeable wall and its stability can be improved by reducing the permeable ability of the wellbore wall; (2) for underbalanced drilling, the stability condition of permeable wellbore is much higher than that of impermeable wellbore; (3) the temperature has important influence on wellbore stability due to the variation of pore pressure and thermal stress; the wellbore stability can be improved with cooling drilling fluid for deep well. The present method can provide references for coupled thermal-hydraulic-mechanical-chemical (THMC) process analysis for wellbore.
Highlights
Boreholes, as the access for the development of oil, gas, and geothermal energy, as well as deep geological storage of CO2, experience instability phenomena such as sloughing and borehole wall fracturing during drilling process, which arise from removal of the original supporting rock and the interaction between the drilling fluid and formation [1,2,3]
Based on continuum mechanics and the theory of mixtures, a THM coupling model of rock medium is established, and it is coded with MATLAB language as platform and ABAQUS software as the solver
The results show that the stress, pore pressure, and temperature around wellbore obtained by full-coupling and partial-coupling are quite different, which prove that the fully coupled model is reasonable for wellbore stability analysis
Summary
As the access for the development of oil, gas, and geothermal energy, as well as deep geological storage of CO2, experience instability phenomena such as sloughing and borehole wall fracturing during drilling process, which arise from removal of the original supporting rock and the interaction between the drilling fluid and formation [1,2,3]. Mechanical behavior, pore pressure, and heat flux all Mathematical Problems in Engineering influence wellbore stability, while instances of instability are a result of a combination of coupled effects. Analysis involves studying the interactions among changes of related effective stress, temperature, and pore pressure during drilling process. For air or foam drilling (underbalanced), the drilling fluid column pressure can be lower than formation pressure, which may cause formation fluid flow into wellbore, resulting in the change of pore pressure and stress state near wellbore. For numerical analysis of wellbore stability, most studies have focused on HM coupling, elastic behavior, and overbalanced drilling, without considering the influence of drilling unloading process, stress redistribution, filter cake quality, and permeability change of the borehole wall [16, 17], which is not consistent with the real case. The drilling unloading, fluid flow, and thermal effect of wellbore are analyzed in overbalanced drilling and under balanced drilling, and the influence of drilling fluid and thermal effect on pore pressure and stress redistribution around borehole with time are all discussed
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