Oxidation and rheological aspects of oil and synthetic-based drilling fluids at elevated pressures and temperatures

In conventional or overbalance drilling, oil and synthetic-based drilling fluid systems are the drilling muds of choice for high pressure and high temperature wells. The hot drilling fluid is returned to the surface tanks and is exposed to air in mud tanks. Since the base liquids of these drilling fluid systems are expensive, they are used in many wells before disposal. The long exposure of the oil with air can result in substantial oxidation of the oil, especially in hot reservoirs in summer temperatures. Changes in rheological properties and the probability of explosions could be serious issues in drilling operations. In underbalanced drilling employing aerated muds (gasified liquids), in some situations oil is used as the base liquid and de-oxygenated or vitiated air with almost five percent oxygen as the gas phase. The presence of oxygen in the de-oxygenated or vitiated air causes the oil in the aerated mud to be oxidized. The oil is circulated and reused in subsequent wells leading to ongoing oxidation. Due to the resulting compositional modification, changes in rheological properties of the liquid phase and safety considerations with regard to explosions may be of concern.; An experimental study was designed to examine the oxidation and rheological aspects of some oil and synthetic base liquids together with some drilling fluid systems using batch reactors, rheometers, and viscometers. In oil and synthetic base liquids, the effect of temperature (from 100 to 195°C) at a nominal initial pressure of 14 MPa, the effect of total pressure (from 7.86 to 43.63 MPa) at a nominal temperature of 150°C, the effects of oxygen partial pressure and multiple injections at a nominal temperature of 150°C and nominal initial pressure of 14 MPa, and the behaviors of clear and aged base liquids at high pressures (from 0.7 to 103.4 MPa) and high temperatures (from 35 to 152°C) were investigated. In oil and synthetic-based drilling fluid systems, at a nominal temperature of 150°C and nominal initial pressure of 14 MPa, the effects of presence of drilling cuttings, water scavengers, viscosifiers, wetting agents, fluid loss controllers, and weighting materials in the base liquids were studied.; Simulated distillation results revealed that the carbon number distributions of aged and clear samples were almost the same. Gas chromatography results for Drillsol showed that no CO and CO2 was produced at temperatures below 120°C. Total production of carbon oxides (CO + CO2) increased from 120 to 150°C and then plateaued at higher temperatures. The variation of viscosity ratio with temperature was found to be similar to those of solids precipitation and CO2 formation.; It was shown that for Drillsol, while the amount of formation of CO did not change with pressure, the formation of CO2 increased and reached a maximum at about 25 MPa and then stayed almost constant. Near linear relations existed between the amounts of solids precipitated and viscosity ratio with the initial pressure of the reactor.; It was found that viscosity ratio and the amount of solids precipitated increased in a linear fashion with the total mole fractions of CO and CO 2 in the post-test gases. Moreover, viscosity ratio, amount of solids precipitated, and cumulative concentration of CO plus CO2 were correlated to oxygen uptake.; An induction delay time was observed prior to the onset of significant oxygen uptake. This behavior was reflected as two distinct regions in the pressure decline curves. The induction delay time was not reached within the seven day duration for the tests at 120°C and a test at 135°C with an oxygen mole fraction of 0.1019 in the initial gas. Production of CO and CO2 could be considered as indications that the induction delay time has been exceeded. (Abstract shortened by UMI.)...