Mechanisms of Wellbore Strengthening and New Advances in Lost Circulation Control with Dense Pressure Bearing Zone

Wellbore strengthening has become one of the important drilling fluid technologies for enhancing the pressure bearing capacity of formations. The micro-mechanisms, simulation test methods and development of new materials presently in China all require extensive study. Methods of controlling mud losses into fractured formations with dense pressure bearing zone are investigated based on the basic principles of force chain network structure analysis of micro particle mechanics. Fine characterization parameters for the characteristics of wellbore strengthening materials are presented in the paper. A dense pressure bearing plugging zone with "strong force chain network structure" can be formed with rigid particles, elastic particles and fibrous materials through sizing of particles (type and size) and optimization of particle concentration. Using self-developed simulation test device, wellbore strengthening with drilling fluid plugging materials was studied through simulation test. The experimental results have proved that the new wellbore strengthening material developed is not only able to effectively plug micro-fractures to form a dense pressure bearing zone to seal off the fractures, it also enhances pressure bearing capacity of the formation when the widths of the fractures are opened to the designed width by the wellbore strengthening material.