Enhancing Depth of Penetration to Improve Capillary Number by Application of EK Low-Concentration Acid IOR in Abu Dhabi Carbonate Reservoirs
Authors: Ansari Arsalan A. , Haroun Mohammed , Rahman Mohamed Motiur , Sarma Hemanta , Chilingar George V. 
Published: May 2015

Abstract
Most of the World's oil reserves have declined by an average of 3.8% per year since 1970s according to a U.S.A general report [Behrens and Glover, 2012]. Oil reserves can be increased by developing more efficient methods for recovering oil. EOR (Enhanced Oil Recovery) has been on the forefront of oil and gas R&D for the past 4 decades as it helps in the improvement of the sweep efficiency by causing an increase in recovery factor and a reduction in residual oil saturation. EK Low-concentration acid IOR (EK LCA-IOR) is one of the emerging EOR technologies which involves the application of the Low-concentration acidizing integrated with electrically enhanced oil recovery (EK-EOR) conducted on Abu Dhabi carbonate core-plugs. This approach is gaining increased popularity due to a number of economic and reservoir-related advantages such as reduction in fluid viscosity, reduced water-cut, increased reservoir permeability, reduced HSE concerns, increased targeting the unswept oil and enhanced depth of penetration.
Core flood tests were performed on core plugs retrieved from Abu Dhabi producing oilfields with medium crude oil in a specially designed EK core-flood setup. After the waterflooding stage, EK was applied using acids of varying voltage gradient and varying concentrations from 0.125 to 1.2% HCl injected at the anode at 0.25 ml/min (consistent with average reservoir flow rate of 1 ft/day) and transported by EK to the target producer (cathode). Experiments were also repeated with conventional LCA-IOR. This was compared with sequential and simultaneous application of EK LCA-IOR in water-wet rocks at reservoir conditions. EK LCA-IOR process was compared with other Electrokinetic recovery processes to evaluate the environomic profitability of each project. Moreover, the capillary number change for the LCA-IOR process was also modelled in order to develop an optimum process that allows us to shift from conventional to simultaneous LCA-IOR process.
Several correlations related to acid concentration, displacement efficiency, permeability enhancement and power consumption are presented here. The experimental results have shown that the application of waterflooding on the carbonate cores yield an average oil recovery of 60%. An additional 17-28% oil recovery was enhanced by the application of EK-assisted low-concentration acid IOR (EK LCA-IOR) recording a maximum oil displacement of 88% at reservoir conditions which is higher than sequential EK-LCA-IOR (77%) and Conventional LCA-IOR (62%). In addition, Simultaneous EK LCA-IOR was shown to enhance the reservoir's permeability by 53% on average at reservoir conditions which is higher than 32% (Sequential EK-LCA-IOR) and 23% (Conventional LCA-IOR). EK LCA-IOR also causes an increase in the capillary number (Nc) by 120% to 200 % due to an increase in the fluid velocity and viscosity along with a decrease in interfacial tension.
Finally, LCA-IOR also allows us to save on the OPEX by optimizing the process and maintaining a low current density with reduced power consumption while reducing the acid/water requirement upto 10 times as compared to conventional EOR/IOR. This study takes one step forward towards the development of Low-concentration acid IOR method feasible for Abu Dhabi oil fields.