A study to optimize drilling fluids to improve borehole stability in natural gas hydrate frozen ground

The rising price of natural gas and long term energy supply problems are a global stimulus to find alternative energy sources to mitigate an impending crisis. Natural gas hydrate (NGH) is a new and promising research area in modern earth sciences and the energy industry. An important issue for the development of potential nature gas hydrate exploitation is successful drilling and core sampling. As many of the world deposits of NGH occur in permafrost or frozen soil regions, studying low temperature drilling mud and borehole stability in frozen soil is very important. This paper describes a study into the effects on borehole stability by low temperature drilling fluids, and presents a potential NGH drilling fluid formula that has been optimized by orthogonal testing. The experiment tested 4 different additives and each at 3 different concentrations. The main drill fluid ingredients investigated to influence borehole stability under low temperature conditions were HEC, PEG, NaCl, and KCl. Low temperature rheology behavior and API filtration tests show that the optimum fluid formula is: HEC (0.5%) + PEG(5%) + NaCl (20%) + KCl (5%). In the soil/borehole stability experiments, it was found that the fluid can play an important role in preventing drilling borehole collapse. The study demonstrates that NGH drilling requirements under frozen conditions could be met by choosing appropriate treating-agent additives to adjust the performance of drilling fluids and borehole/fluid interaction behavior.