Oil-based mud (OBM) has strong inhibitive ability, which can prevent shale formations from hydration and swelling, and thus is often used in the development of shale gas reservoirs in China. Due to the low formation temperature (under 90 °C), high pressure (with pressure coefficient up to 2.2), long barefoot interval (up to 2138 m), borehole instability problem is easily encountered in dealing with the drilling of horizontal intervals of shale gas wells, which brings great challenges to drilling fluid technology.

Maintaining wellbore stability is the most critical aspect of drilling operations of horizontal well intervals. However, most of OBM system can not prevent filtrate invasion into nano-sized pores and micro-fractures on shale surface, and subsequent cause borehole instability problems. The main reason is that conventional drilling fluid particles are too large to seal the nano-sized pore throats or micro-fractures in shales, and thus can not stop fluid invasion. A new type of composite particles is developed, and the mean particle diameter of composite particles is about 105.6 nm. These micro-nano particles (MNP) are added to OBM, and their influence on shale stability is evaluated.

The core displacement experiment indicates that these micro-nano particles can reduce the shale permeability by a significant amount, and thus stop fluid invasion and inprove wellbore stability. The reason is that these particles are small enough to penetrate and seal the pore throats in shale, and built an effective internal mud cake possessing low permeability on shale surface, resulting in the reduction of filtrate penetration into the shale. The experimental results show that these particles can prevent pressure transmission and improve wellbore strength greatly. In addition, MNP have no effect on rheological properties of drilling fluids, and they are environmentally friendly, too.

As a result, MNP can reduce fluid losses to the formation significantly, and consequently provide an excellent sealing effect, so MNP are an effective plugging agent for strengthening wellbore stability. Reduction of fluid losses and enhancement of wellbore strength can be achieved by addition of these micro-nano particles to oil-based drilling fluids. MNP-based drilling fluids have been successfully run for dozens of wells in Weiyuan-Changning and Shaotong shale block. The field application indicates that no borehole instability problems are encountered in the drilling process of horizontal well intervals. Using these MNP-based fluids containing micro-nano particles to seal the shale is a very powerful and economical approach to address borehole instability problem in troublesome shale formations. These MNP-based fluids are suitable for the drilling of long sections of horizontal laterals. In the future, the MNP-based drilling fluids might hold great promise to resolve shale instability problem.

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