Study On Two Phase Flow In Oil Based Drilling Fluid For Deepwater Dual Gradient Drilling

Gas-lift dual gradient drilling(DGD)is an important solution for complex problem caused by narrow drilling window in deepwater drilling.The researches on gas-lift dual gradient drilling for the water based drilling fluids have been carried out more deeply,but the research on oil-based drilling fluid is rarely reported.The new idea in this dissertation is using oil based as drilling fluid in this technology,and study about gas-lift dual gradient drilling in the case of using oil based drilling fluid and Nitrogen as injecting gas into riser.Before using oil based drilling fluid(OBDF)in gas-lift DGD,the PVT behavior of gas/OBDF mixture must be understood.One of the main objectives of this study is figuring out the PVT behavior of Nitrogen,as injected gas,and white mineral oil(#5)as liquid phase,by experimental determination of such thermodynamic properties as the bubble pressure and solubility.The new models are introduced for the solubility and bubble pressure for white mineral oil/Nitrogen mixture.In comparing with water based drilling fluid,gas is more soluble in oil based drilling fluids,therefore these novel models could be used in steady state and unsteady state(transition)hydraulic model to improve the accuracy of the bottom-hole pressure prediction.The PVT measurements were conducted in window cell PVT apparatus by using Constant Composition Expansion(CCE)method.The conditions of experiment were planned for common riser condition,pressure up to 3600 psi,and temperature in the range of 68 to 176 ~oF.In practice,the most common representation of the solubility of a gas in a liquid is by the bubble point pressure of the gas/liquid.The bubble pressure measurements were conducted in a window PVT cell.The fluids inside the cell could be observed through a sapphire window on the cell.The visual observation was assisted to interpret P-V graph for finding the bubble pressure,then the solubility was determined by the bubble point pressure of the gas/liquid mixture.In addition,the steady state hydraulic simulator is designed by MATLAB software.This simulator can determine PVT parameter in any position in the marine riser at any pressure and temperature.In the calculation of the PVT behavior of oil/water/Nitrogen mixture,the simulator considers solubility and bubble pressure in the Nitrogen/white mineral oil mixture.This dissertation introduces the new models for the solubility and the bubble point pressure of nitrogen/white mineral oil.In addition,this dissertation compares these new models with The Standing correlation which are one of the conventional Black oil models.These comparisons show that the Standing correlation overestimates the solubility of Nitrogen in white mineral oil and underestimates the bubble pressure point for this mixture.Moreover,the Standing correlation predicts temperature has inverse relation with solubility and direct relation with bubble point pressure.Nevertheless,the results of the current experimental investigation and the new models indicate that temperature has direct relation with solubility of nitrogen in white mineral oil and inverse relation with bubble pressure point of this mixture.This issue is important in the case of deepwater drilling in high temperature area.Also the PVT behaviors of drilling fluid(oil/water/nitrogen)in gas-lift dual gradient drilling operation is evaluated in the case of using the new models and the Standing correlations.Eventually,pressure profile and optimum injected gas graph for gas-lift dual gradient drilling in the marine riser for both of models(the new models and the Standing models)is presented.The pressure profile in the riser indicates that the calculated pressure in the case of using the Standing solubility and the Standing bubble pressure is higher than calculated pressure which obtained by the new models.In constant GOR(gas oil ratio),when the liquid flow rate increases,the differences between calculated pressure profile as a function of the Standing and the new models will increase.The reason for this pressure discrepancy between the two models is the Standing model overestimate the solubility and underestimate bubble pressure,thus it assumes more gas is dissolved in oil and consequently the Standing model underestimate gas holdup.This lower gas holdup prediction leads to overestimate pressure in the riser.Therefore,compared with the new models,the Black oil model are not recommended for calculating the two phase flow pressure of Nitrogen and white mineral oil mixture.Moreover,according to gas optimum design,in hydrostatic domain area,the discrepancy between pressures(as a function of the Standing and the new models)is higher for higher liquid flow rates because higher liquid rates leads to higher the solubility in the Standing model.In frictional dominated zone,the differences between pressures due to the new models and the Standing model is not too big.The reason for these behaviors is the effect of solubility variations in hydrostatic domain(lower gas holdup)is more considerable than effect of solubility variation in the higher gas hold up(frictional domain).Finally,the real conditions of the whole well is simulated for deep water drilling which has two phase flow above BOP(in the riser)and one phase flow below the BOP.The bottom-hole pressure and pressure profile in the well could be control by adjusting injecting gas(Nitrogen)flow rate.The casing design was simulated on real data and it indicated that employing gas-lift dual gradient drilling decreases casing size number dramatically.In addition,the casing cost and rig time cost estimations indicated that in deep water drilling,in compare with conventional drilling,dual gradient drilling decrease costs of casing and rig time dramatically and it can improve the economic benefits of drilling operation.