Drilling fluid constitutes an important part of the drilling operations. Drilling mud is circulated into the wellbore to minimize the formation damage, transport cuttings from the bottom to the top of the well, cool and lubricate the bit, and maintain the stability of shale formations. Gel strength property of drilling fluids plays a key role in drilling multilateral and long horizontal reservoir sections. Losing the gel strength will accumulate drilled cuttings and as a result, sticking of the drill string. Solving this issue takes a long time and increase the total cost of the drilling operation. It was observed that, by increasing the temperature to 200°F, the gel strength of calcium carbonate water-based drilling fluid reached zero lb/100 ft2.
The objectives of this paper are to: (1) determine the rheological properties of calcium carbonate water- based drilling fluid over a wide range of temperature, (2) assess the effect of using clays (bentonite) to solve the gel strength problem associated with the current field formulation of calcium carbonate water- based drilling fluids, and (3) optimize the weight percent of bentonite. Rheological properties such as the apparent viscosity, plastic viscosity, yield point and gel strength were measured by using a high-pressure high-temperature rheometer.
At low temperatures (85°F), the calcium carbonate water-based drilling fluid showed no change in the gel strength with time. However, at high temperatures, the gel strength starts decreasing with time. Rheological properties confirmed that the gel strength of the calcium carbonate water-based drilling fluid reached zero lb/100 ft2 by increasing the temperature to 200°F. This issue was solved by adding different concentrations of bentonite. The bentonite concentration was varied from 3 wt.% to 10 wt.%. At low bentonite concentrations (3.33 wt.%), the gel strength still reduced with time. At high bentonite concentration (10 wt.%), the gel strength increased with time. The optimum concentration of bentonite was 6.66 wt.%, which yielded a flat rheology profile of the gel strength. These results confirmed that the rheological properties of the water-based drilling fluid were optimized by using clays (bentonite).
The novelty of this research is the complete investigation of the drilling fluid properties especially the gel strength with time over a wide range of temperature. The results obtained can leads to the development of a cheap solution without any side effect.