Borehole Wall Strengthening with Drilling Fluids in Shale Gas Drilling

Formations drilled in shale gas drilling are full of micro fractures and are brittle, and are therefore easy to hydrate and slough. Oil base drilling fluids and synthetic base drilling fluids have been commonly used in shale gas drilling, and are effective in preventing borehole wall sloughing and pipe sticking. However, when drilling into a broken belt or into formations with abnormally developed fractures, large amount of sloughing and severe borehole wall collapse still happen even oil base drilling fluids are used. To address borehole wall collapse and mud losses when drilling broken formations, the borehole wall should be strengthened promptly and effectively. A drilling fluid treated with two borehole wall strengthening agents, YH11 and BT100, was tested for its capacity to strengthening borehole wall. The test results showed that this drilling fluid formulation is suitable for shale gas drilling. The density of this drilling fluid is adjustable in a range of 1.14-1.50 g/cm³. It has strong inhibitive capacity, and is capable of preventing mud losses by drilling at low densities. Good filming and sealing performance of the drilling fluid helps resolve the problem of stabilizing formations with low pressure bearing capacity. Minimizing of downhole troubles and downhole safety insurance help enhance rate of penetration. Both laboratory experiment and field application showed that YH11 and BT100 are able to strengthen borehole wall by promptly cementing the broken formations and plugging the fractures. Cementing and plugging of the formations with YH11 and BT100 greatly reduce the porosity and permeability of the formations, effectively restraining the transfer of hydraulic pressure of mud column into the pores of formation and the invasion of mud filtrates into the depth of formation, hence minimizing loss of pressure supporting the borehole wall. The use of YH11 and BT100 has double effects of preventing borehole wall collapse and mud losses.