Abstract
As a bottleneck problem, borehole instabilities have severely restricted further development and application of gas drilling technology. Here, the mechanisms of borehole instabilities in gas drilling are studied and four instability mechanisms are presented, including mechanical instability of weak rocks or crumbliness rocks because of their low mechanical strength, mechanics-chemical coupling instability caused by production or injection of fluid, rock burst of payzone and dynamically instability of overlying mudstone with pressure during hi-speed production. Because gas drilling has special operating conditions compared with conventional drilling, the experiment simulating field operating conditions is crucial to evaluation of borehole stability in gas drilling and need relate to more evaluating parameters, including formation temperature, pore pressure, overlying pressure, confining pressure, borehole pressure, types and production of formation fluid, velocity of slide wave as well as drilling operation, including dry gas drilling, foam or liquid drilling and conversion process of gas-liquid drilling operation. Meanwhile, deepening study on the mechanisms and evaluation methods of borehole instabilities, improving treating agents and treating process and advancing forecasting technique and sealing technique of formation fluid are thought of as effective approaches of enhancing borehole stability in gas drilling. This will be helpful for understanding the mechanisms of borehole instabilities in gas drilling and improving evaluation methods.
