Determining Effective Mixture Viscosities in Oil-Water Flows for Downhole Oilfield Operations

Abstract

Abstract In downhole oilfield operations that involve pumping oil and water, information on mixture viscosity is important to further understand the performance of the downhole production equipment. This study presents a method for determining the effective absolute viscosity of oil-water mixtures by applying fluid mechanics first principles to data representative of downhole production measurements. Knowledge of effective absolute viscosity in upstream and downstream oilfield operations is essential to extend equipment life and optimize production operations. A 7-inch, 26 lb/ft pipe and a 3.5-in, 9.20 lb/ft pipe, typically used as casing and tubing, respectively, in field operations were installed in an oil-water flow loop. The volume flow rates during the tests varied from 2000 barrels per day (bpd) to 12,000 bpd, representative of target field production rates. The tests were performed at different inclinations to represent varying well deviation angles. For each oil and water flow rate, the corresponding pressure drop measurements across a fully developed section of the flow were measured. Using fundamental hydraulic relationships, the effective absolute viscosities of the oil-water mixtures were determined. The results showed that as water cut varied, the effective mixture absolute viscosity also changed, and in some cases substantially. The highest effective absolute viscosity of 500 cP occurred at the mixture flow rate corresponding to 2000 bpd and 40% water cut. In general, mixture flow rates of 2000 bpd and water cuts between 40 and 60% consistently showed the highest effective absolute mixture viscosities, which varied between 160 cP to 500 cP. For water cuts outside the 40% to 60% range, the effective absolute viscosities were about 120 cP and less. The above results indicate that the effective mixture viscosity is about two and close to three orders of magnitude of pure oil and pure water viscosities, respectively. The results also suggest that certain flow rates and water cuts significantly affect the viscosity of oil-water flows. Other results showed that for a given mixture flow rate the effective viscosities were about the same order of magnitude for the different inclinations. This indicates that well deviation angle has minimal or negligible influence on the effective mixture absolute viscosity. From the combined results the technique used in this study provides additional insight to ascertain the effective viscosity of oil-water mixtures during field operations.This study highlights the importance of having the capability to determine the effective absolute viscosity of oil-water mixtures during oilfield production operations. The methodology is simple and has the advantage of easy integration into a flow measurement system. Such systems are beneficial tools for reservoir engineers, production engineers and field operators in general for making appropriate production modifications during operations, to increase asset life and maximize hydrocarbon recovery.