Environmental-friendly salt water mud with nano-SiO2 in horizontal drilling for shale gas

Oil-based muds (OBMs) have been widely used in horizontal drilling for shale gas, but environmental pressure is increasingly intensified. Using silica nanoparticles and environmental-friendly materials, we present a kind of salt water mud with high-temperature resistance for shale gas horizontal drilling. Thermal stability tests on rheology, filtration, water activity and lubricity at ambient temperature to 130 °C, along with pressure transmission tests and rheology model analyses are performed to evaluate the comprehensive properties of the nano-silica and Pure-bore additives based brine mud (NPBM). Using inductively coupled plasma mass spectrometer (ICP-MS) and microplate reader (GloMax-Multi), we investigated the heavy metal content and biological toxicity of NPBM. Results showed that the NPBM embodies excellent thermal resistance in salt water and the rheological parameters did not suffer significant fluctuation. The NPBM-1 containing 4% NaCl had a filtration of 9 mL at 100 °C while the NPBM-2 containing 4% KCl had a filtration of 9.5 mL at 120 °C. Meanwhile, rather low water activity and good lubrication performance of the NPBM improved its capability of wellbore stabilization and circulation friction resistance. The total content of heavy metals did not exceed 5 mg/L, the cadmium content (Cd) of NPBM was 0.00006 ppm, lower than 1 ppm, and the mercury content (Hg) was 0.00212 ppm below 3 ppm. Accordingly, NPBM is nontoxic, and the emission standard is achieved (EC50 > 30000 ppm). Chemical composition analysis showed that environmental Pure-bore additives did not contain any metal elements, while inhibition performances of shale swelling increased by 61.93% for NPBM-1 and 62.44% for NPBM-2 compared to fresh water. Therefore Pure-bore proved to be environmental-friendly and shale inhibitive. From the macro and micro perspective, the pore pressure transmission test, atomic force microscope (AFM) test and scanning electron microscope (SEM) test showed that silica nanoparticles could block the shale pores that average pore size was reduced by 50.18%, mitigate the pore pressure transmission that reducing rate of permeability was 97.56%, and thus enhanced wellbore stability. Additionally, the optimal rheological models for the NPBMs was based on Herschel-Bulkley model. Therefore, the NPBM has a promising future in horizontal drilling for shale gas.