Performance Evaluation of Synthetic-Based Drilling Fluid with Flat Rheology

In offshore deepwater drilling, because of the low temperature aroud seafloor, the conventional synthetic-based drilling fluid becomes very thick, resulting in severe loss of drilling fluid during drilling through the deepwater formations with a narrow safe-density window. Therefore, it is important to improve the rheological properties of drilling fluid at deepwater drilling conditions. In this work, by investigating the effects of the key components including the base oil, emulsifier, wetting agent and rheology modifier on the rheological properties of synthetic-based drilling fluid at 4°C to 65°C, the optimal additives were selected, and a synthetic-based drilling fluid with flat-rheology characteristics was developed. It was found that the gas-to-liquid oil had low viscosity at a low temperature, and was suitable for preparing deepwater drilling fluids. The evaluation experiments show that at 4°C to 65°C, the drilling fluid could maintain stable yield point, 10-min Gel and 6 r/min reading value at a density of 0.9-1.5 g/cm3 even after high temperature aging, while the rheological properties of the conventional synthetic-based drilling fluid were significantly influence by the low temperature. Moreover, it showed excellent performance in filtration reduction and shale inhibition, and could maintain good properties even being contaminated by seawater. Therefore, its stable rheological properties at the deepwater drilling temperature range can effectively control the equivalent circulating density, thus reducing the risk of drilling fluid loss during deepwater drilling.