Abstract
Conventional particulate additives fail to control drilling fluid losses into large-aperture fractures. The separate injections of a bentonite-cement-oil suspension and water can cause rapid hydration, swelling and hardening to effectively plug fractures. This experimental study investigates underlying processes and implications in view of optimal fluid flow control in fractures. Results identify several concurrent hydro-chemo-mechanical coupled processes: capillarity-driven water invasion; cement hydration and the release of Ca2+ and OH− ions; bentonite contractive aggregation and increased hydrophilicity; enlarged inter-aggregate pores that facilitate fluid flow; oil pressurization leading to the formation of oil-filled opening mode discontinuities that facilitate oil escape towards free draining boundaries, and calcium silicate hydrate formation and growth resulting in hardening. The hydration of bentonite-cement-oil suspensions proceeds several times faster than in bentonite-oil suspensions. The optimal mixture should balance competing requirements between flowability, water invasion speed, swelling pressure and plug strength.
•Bentonite-cement-oil suspensions experience surprisingly fast hydration.•Bentonite aggregation due to cement hydration leaves pathways for fluid transport.•Bentonite and bentonite-cement plugs are self-healing.•Optimal bentonite-cement-oil mixtures balance hydration rate and plug strength.