Properties of Bentonite Slurry Drilling Fluid in Shallow Formations of Deepwater Wells and the Optimization of Its Wellbore Strengthening Ability While Drilling

[Image: see text] Offshore oil- and gas-field development is shifting from shallow water to deepwater on a large scale. Deepwater shallow bentonite slurry drilling fluid has a single composition and a simple structure. Therefore, the bentonite slurry drilling fluid has been neglected for the shallow wellbore strengthening ability. Based on the shallow geological characteristics and bentonite hydration mechanism, considering the economy and application effect, the optimization of bentonite slurry drilling fluid from four aspects of viscosity enhancement, adsorption, trapping, and physical plugging to carry out deepwater shallow wellbore strengthening research has been undertaken. For an indoor simulation of bentonite slurry and its drilling slurry-making process using a 2–10% mass concentration of bentonite slurry drilling fluid, laser particle size analysis found an interesting phenomenon different from the traditional understanding: for every 5% increase in particle size accumulation in the range of 0.1–100 μm, the bentonite slurry particle size increases linearly. Based on this interesting phenomenon, the basic performance of drilling fluids with different concentrations of bentonite slurry was evaluated. Experiments were conducted to introduce cationic emulsified asphalt as a deformation filler and to explore a new inexpensive drilling and wellbore strengthening material, AEH-P. The effectiveness of deepwater shallow strengthening was evaluated for AEH-P and cationic emulsified asphalt from both mechanistic and experimental aspects. It is obvious that the wellbore strengthening effect is the result of both particle settling and particle size matching. By exploring the relationship among bentonite slurry hydration dispersion, the charged nature, particle concentration, and the wellbore strengthening effect, a set of low-cost deepwater shallow bentonite slurry drilling fluids with a good wellbore strengthening effect are constructed. The research results provide a method to strengthen the wellbore for the subsequent fast and efficient drilling of deepwater shallow wells, further improving the drilling efficiency.