J. Xu, Z. Qiu, G. Chen, H. Zhong, X. Zhao
China University of Petroleum,
Keywords: nano-sized polymer microsphere, water-based drilling fluids, shale formations, shale hydration, pressure transmission
Summary:Borehole instability caused by pore pressure transmission and shale hydration gives rise to a number of problems, which has been a challenge during oil and gas drilling process for many years . Recent years, the exploration and development of shale gas resources has become a new bright spot, but wellbore instability in the drilling process was still the major problem to be solved . The conventional drilling fluid agents are too large to retard the pore pressure transmission as shale has a significant pore volume in the nano-pore range, therefore the application of nano materials in drilling fluids has gained much attention . Some researchers have introduced silica nanoparticles as a shale stabilizer, but only marginal success has been achieved because they are easily aggregated and precipitated in the water-based drilling fluids . In this paper, a novel nano-sized polymer microsphere as a potential shale stabilizer in water-based drilling fluids for drilling troublesome shale formations was successfully synthesized and characterized. The spherical polymer microsphere was prepared by emulsion polymerization of styrene, butyl acrylate and acrylic acid. And it was characterized in detail include Flourier transformation infrared spectroscopy (FT-IR), particle size distribution analyzer (PSD), transmission electron microscopy (TEM) and thermo gravimetric analysis (TGA). It possessed uni-modal distribution from 50 nm to 190 nm with the D50 value of 88 nm and had good dispersion in aqueous solution. Its shale inhibition characteristic was investigated with hot-rolling cuttings dispersion test. Under the concentration of 2.0% prepared polymer microsphere, the shale recovery rate was as high as 93.7%, which increased 2.3 times than that of samples in tap water, obviously better than KCl and JHC as commonly used shale inhibitors at the same concentration. The results indicated that it had good performance to prevent shale hydration. In addition, pressure transmission test was carried out to evaluate the sealing performance of prepared polymer microsphere. The nano-sized polymer microsphere could effectively retard pore pressure transmission and reduce the permeability of shale samples. Further, the pore throats of shale samples before and after testing were characterized through scanning electron microscope analysis. The nano-sized polymer microsphere could bridge and seal the micro-cracks and nano-pores of shale samples through the compression and deformation, which could help form a dense plugging film on shale surface. Hence, the prepared novel nano-sized polymer microsphere may be a potential stabilizer in the water-based drilling fluids for drilling troublesome shale formations.