| With the development of drilling technologies and the expansion of its applications, underbanlced drilling (UBD) of horizontal wells has been widely used in Chinese drilling markets and the drilling markets abroad. However, wellbore instability is always one of the problems that should be solved during the expansion and promotion of this technology. And in the process of UBD of horizontal wells, the effective column pressure in the wellbore is lower than original pore pressure, then formation fluid in different rock pore structures flows into wellbore, which leads to the change of pore pressures and stress state on rocks; and in previous studies of wellbore stability in UBD of horizontal wells, the effects of formation fluid seepage are usually ignored, which add errors to the results. Then, it is quite necessary to do certain basic theoretic researches about wellbore stability during UBD of horizontal wells, which can provide theoretical support for a better, safer and faster drilling with higher efficiencies.With the support from two projects of National Science Foundation of China and based on theoretical studies, numerical analysis and filed applications, major researches conducted are as follows:(1) On basis of the circumferential stresses from in-situ sresses and the circumferential stresses from both fluid column pressure and pore pressure, circumferential stress model during UBD of horizontal wells for considering the effects of fluid seepage and not considering the effects of fluid seepage are set up by combining the change pore pressures in both porous medium and fissured medium; meanwhile, the variation trends of circumferential stresses under the condition of effective stresses are fully analyzed.(2) Based on the circumferential stress model during UBD of horizontal wells, for porous medium, principal stresses on the wellbore are firstly analyzed, after which collapse pressure models of wellbore and failure index models around wellbore are established by using Mohr-Coulomb criterion for the conditions of considering fluid seepage and not considering fluid seepage; for fissured medium, on basis of principal stresses analysis, the relation between the direction of maximum principal stress and the normal direction of micro fracture plane is fully analyzed, after which collapse pressure models of wellbore and failure index models around wellbore are established by using the theory of weak planes. Besides, the acquisition methods for obtaining parameters in the analysis of wellbore stability are widely surveyed.(3) Based on the wellbore stability models, the solving procedures for analyzing influencing factors of wellbore stability during UBD of horizontal wells are listed, after which the effects of well trajectory (inclination anlges and azimuthal angles), in-situ stresses (in-situ stress regimes and anistropic coefficient of horizontal in-situ stresses), rock mechanical parameters, mechanical parameters and attitude of fissured planes (dip angles and dip directions), well radius, etc. are analyzed.(4) By referring to the basic data in actual wells of Estreito field, Pedernales field and Gulfask field, checking analysis are done for both conditions of considering fluid seepage and not considering fluid seepage in both rock pore structures, which can be explanations for wellbore instabilities in both rock pore structures during UBD of horizontal wells.Results of the studies show that the analysis of the models can show the actual field conditions to some extent, and the effects of fluid seepage are harmful for wellbore stability, so the effects of fluid seepage should be considered during UBD of horizontal wells; meanwhile, the existence of fissured planes affects the design of well trajectories and wellbore stability.Research findings of the thesis enrich basic theories of wellbore stability and UBD, which can be helpful for promoting the application of UBD and have reference values for related projects. |
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