An Experimental Investigation of Carbonated Water Flooding

Abstract

Abstract It has been proven that it is possible to improve the performance of water flooding (WF hereafter) by chemically altering the brine/water composition. There are a number of compounds that can be added for this purpose and CO2 is one of them. Carbonated water flooding (CWF hereafter) is to inject CO2 saturated (or nearly saturated) water into reservoirs as the displacing fluid. CO2 stays in the water phase first and migrates into the oil phase afterwards, without forming an individual CO2 rich phase. Such mass transfer of CO2 from water into oil is substantial due to the fact that under the same pressure and temperature conditions, CO2 is more soluble in oil than in water. With the dissolution of CO2, oil viscosity is reduced, which makes mobility ratio between water and oil more favorable in the contacted zone, and oil volume expands (swelling effects), which increases the relative permeability of oil. Both effects result in improved ultimate recovery over conventional WF. Since miscibility is not sought during the CWF process, it has less restrictive requirements of reservoir conditions and oil types. In addition, ease of separation of CO2 water mixture at the production wells and less gas handling make CWF relatively easier to implement in fields with ongoing WF and/or in future candidates. Moreover, at the end of CWF cycle, it is still possible to implement other tertiary flooding techniques as in the case of conventional WF. CWF is an improved oil recovery (IOR hereafter) method that has been tested in the field [6][15][16][26][35]. Pilot programs and field trials were conducted in 1950's. Lab scale experiments were also active between 60's and 80's. During the past a few decades, however, little work was done on the topic. Literature data in this area are inconclusive in several aspects, for example the large variations of the incremental recovery and experimental conditions that can be represented in terms of dimensionless scaling factors (that control the displacement performance). This paper summarizes the results of a series of CWF experiments conducted using a specifically designed core flooding apparatus at our labs. These experiments consist of the Phase I study of a project designed to obtain a comprehensive understanding of CWF's displacement mechanism as well as the impact of several pre-identified sensitivity parameters, such as injection rate, salinity, etc. Eight sand pack experiments (divided into four sub groups) were completed under the same pressure, temperature, and CO2 saturation level, yet with different injection rates. The main contributions of this study are: Overall, CWF results in better oil recovery than its WF counterpart with improvement ranges from 35% (high speed, 15 PV/day) to 3% (low speed, 1 PV/day) of STOIIP, using a medium viscosity crude oil. Cross-comparison and interpretation of current and historical experimental results in terms of identification of flow regimes utilizing dimensionless numbers, such as gravity number, capillary number, etc. Identification of the optimal flow rate and the range of incremental recovery over WF. Providing new data for the literature.