Optimization of the Low Salinity Water Injection Process in Carbonate Reservoirs
Authors: Al-Shalabi Emad Waleed , Sepehrnoori Kamy , Delshad Mojdeh 
Published: March 2015

Abstract
The low salinity water injection (LSWI) is one of the emerging improved oil recovery techniques. Minimizing risk and uncertainty is a major part of any proposed improved oil recovery method by highlighting the most significant design parameters and optimizing the entire process. This paper investigates the optimization of the LSWI process at field scale for carbonate reservoirs by considering the most influential design parameters. Seven uncertain and decision design parameters were selected. 5-spot LSWI pilot models were simulated using UTCHEM reservoir simulator with an empirical LSWI model. The Design of Experiment (DoE) method was used for sensitivity analysis and screening out insignificant parameters. The Response Surface Methodology (RSM) was implemented to optimize the LSWI cumulative oil recovery where a response surface was built. The performed sensitivity analysis showed that the three most important design parameters are LSWI slug size, reservoir heterogeneity (VDP), and injected water salinity. An optimum LSWI design was suggested and the results were validated using the UTCHEM simulator. Moreover, two scenarios (best and worst) were created to highlight the individual and combined effects of the seven tested design parameters on cumulative oil recovery by LSWI. By understanding the most influential LSWI design parameters, the field scale development can be conducted with more certainty and lower risk.

Introduction
The low salinity water injection (LSWI) is gaining popularity as an improved oil recovery technique because of its simplicity compared to other techniques. The LSWI effect on oil recovery has been shown at laboratory scale and to a limited extent at field scale for both carbonate and sandstone rocks. For full field scale development, optimizing the LSWI process by minimizing the related risk and uncertainty, and identifying the most significant design parameters is still a concern, which is addressed in this paper. Reviews of LSWI effect on oil recovery at field scale, Design of Experiment (DoE), Response Surface Methodology (RSM), and optimization are presented.
The field scale studies started in sandstone rocks to investigate the effect of LSWI on oil recovery. The first field pilot was reported by Webb et al. (2004) as a single well chemical tracer test (SWCTT) and then McGuire et al. (2005) in sandstone reservoirs. Both tests showed a positive response by reducing the remaining oil saturation in both secondary and tertiary modes. The reported remaining oil saturation for these studies ranged from 30 to 50%, which is in match with the conducted laboratory studies. Seccombe et al. (2008) investigated the benefits of tertiary LSWI at the Endicott Field located in the North Slope of Alaska. They observed a constant water relative permeability at residual oil saturation for both low and high salinity water injections, which was consistent with their corefloods, numerical matching of the data, and the constant productivity of the wells from the SWCTTs. Later in the same field, the first comprehensive inter-well application was reported by Seccombe et al. (2010) involving an injector and a producer 1040 feet apart. The results were in agreement with the corefloods and SWCTTs as 10% incremental oil was recovered after the injection of 1.6 pore volumes of LSWI.