A new low-cost drilling fluid for drilling in natural gas hydrate-bearing sediments

• A new Nano SiO2 drilling fluid for drilling in natural gas hydrate-bearing sediments is developed.
• The bit life-span is prolonged and the drilling efficiency is highly improved in low temperature.
• Shale hydration expansion and the formation of hydrates are efficiently inhibited.
• The cost is 15%–20% lower than existing drilling fluids for drilling in hydrate deposit in the South China Sea.

With increasing concerns relating to nature gas hydrate exploration and development worldwide, increasing attention has been paid to drilling technology in gas hydrate-bearing sediment (GHBS). According to the drilling characteristics of hydrate formation and the existing nanomaterials, a new low-cost drilling fluid has been established through more than 120 groups of experiments for drilling in GHBS: seawater + 2% Nano SiO2 + 3% bentonite + 1% Na-CMC (sodium carboxymethyl cellulose) + 3% SMP-2 (sulfomethylated phenolic resin) + 1% PVP (K90) (polyvinylpyrrolidone) + 2% KCl. Additionally, the traditional low-temperature performance and inhibition of hydrate formation of this drilling fluid have been evaluated. The experimental results show that this drilling fluid has optimal density, good low-temperature rheology, and sufficient shale hydration inhibition and is able to effectively inhibit free gas from undergoing hydrate dissociation near boreholes to reform the hydrate in the drilling fluid circulation system. In addition, the cost of per cubic meter of this drilling fluid is only $400–450, which is 15%–20% less expensive than existing drilling fluids for drilling in GHBS in the South China Sea. Therefore, this drilling fluid will contribute to safe and efficient borehole drilling operation in GHBS.