Hydrothermal Carbon Spheres as Multifunctional Agents in Water-Based Drilling Fluids

Abstract With the increase of well depth, more and more complex problems such as polymers high temperature degradation, wellbore instability, and high friction are encountered during the drilling operation. Developing novel eco-friendly and multifunctional materials to prepare high performance water-based drilling fluids is desirable. The main objective of this research is to systematically evaluate the properties of hydrothermal carbon spheres (HCSs) in water-based drilling fluids. HCSs were synthesized using xylose as precursors by the hydrothermal carbonization method. The thermal stability of typical polymers including xanthan gum and acrylamide (AM)/2-acrylamido-2-methylpropanesulfonic acid (AMPS)/ vinylpyrrolidone (NVP) copolymer with and without HCSs were examined by comparing the solution viscosity after thermal aging to that of before aging. The rheological and filtration properties of simplified polymer-based drilling fluid and bentonite-free water-based drilling fluids containing various concentrations of HCSs were measured before and after dynamically thermal aging at different temperatures. Shale plugging properties of HCSs were evaluated by measuring the permeability variation of filter cake for a bentonite-based mud and the filtration loss of drilling fluid flowing through an artificial stainless steel microfracture. Furthermore, the lubricity improvement by HCSs was measured by extreme pressure lubrication instrument and four-ball friction and wear testing machine. The results indicated HCSs exhibit magical multiple functions in water-based drilling fluids. The polymer degradation under high temperature can be greatly inhibited by CHSs. The addition of 0.5 w/v% HCSs improve the thermal stability of XC solution and AM/AMPS/NVP copolymer solution from 90 °C to 140°C and from 120 to 200 °C, respectively. For the simplified polymer-based drilling fluid, the addition of 0.3 w/v% HCSs achieved apparent viscosity retention of 90% and filtration loss decrease of 54% after thermal aging at 240 °C. For bentonite-free water-based drilling fluid, the incorporation of 0.5 w/v% HCSs resulted in quite stable AV and filtration loss decrease of 76% after thermal aging at 150 °C. HCSs can remove the dissolved oxygen and free radicals from the drilling fluids, preventing the thermal oxidative degradation of polymers. Regarding to sealing and lubrication properties, the presence of 1 w/v% HCSs reduces the permeability of filter cake formed by calcium bentonite slurry by 22% and the extreme pressure lubricity coefficient by 29%, respectively. The environmental-friendly and sustainable HCSs exhibit multiple functions including preventing polymers degradation under elevated temperatures, decreasing filtration loss, sealing micropores and fractures, and improving the lubricity. These properties make HCSs highly applicable in water-based drilling fluids.