Development of Chemical EOR Formulations for a High Temperature and High Salinity Carbonate Reservoir
Authors: Han Ming , AlSofi Abdulkareem , Fuseni Alhasan , Zhou Xianmin , Hassan Saleh 
Published: May 2016

Abstract
Although over 60% of the world-wide hydrocarbon reserves are held in carbonate reservoirs, chemical flooding application and research endeavors have been focused on sandstones rather than carbonates. Challenges to develop chemical EOR formulations for carbonate reservoirs are significant and unique because of the complexity of rock mineral compositions, matrix pore structures, rock surface properties, fracture density, aperture and orientation, as well as oil types. The fact that some chemical EOR technologies have been successfully used in sandstone reservoirs cannot be simply extrapolated to carbonate reservoirs. Carbonate reservoirs are usually characterized as low permeability matrix with fractures, while a considerable portion present high permeability matrix with fractures as shown in the Middle East. This paper presents the development of chemical formulations addressing the challenges of chemical EOR in a representative Middle East carbonate reservoir, which has high reservoir temperature and high brine salinity.
In the screening process, more than 50 surfactants and 30 polymers were studied at both ambient and reservoir conditions. Few surfactant-polymer (SP) formulations were optimized in terms of good compatibility with field brines, low interfacial tensions between the crude oil and chemical solution, and low adsorption of chemicals on the carbonate core samples. The results of the compatibility tests showed that an effective SP slug can be prepared in regular Arabian Gulf seawater without the requirement of water softening. Amphoteric surfactants showed a promising performance for the given reservoir conditions. Oil displacement tests using selected SP formulations demonstrated significant recovery potential in tertiary mode. This paper presents an overview of the chemical EOR research for the carbonate reservoir and provides insights of chemical enhanced oil recovery from carbonate rather than sandstone reservoirs.

Introduction
It has been well documented that about 60% of the world's oil and gas resources are contained in carbonate reservoirs. Fig. 1 shows the worldwide distribution of basins that produce hydrocarbons from carbonate reservoirs. It indicates that the number of basins that produce hydrocarbons from the carbonate rocks is lowest for South America, Africa, and Australia and highest for North America and Eurasia. Tertiary carbonate reservoir rocks are found mainly in Southeast Asia whereas the rest of the world's production is primarily from Paleozoic and Mesozoic carbonate reservoirs, which includes the giant and supergiant Jurassic and Cretaceous fields of the Middle East (Jordan Jr. and Wilson 1994 and Ping 2006). Carbonate reservoirs are usually characterized as low permeability matrix with fractures (Ehrenberg and Nadeau 2005), whereas a considerable portion present high permeability matrix with fractures as shown in the Middle East (Clerke 2007). In recent years, much effort directed towards chemical enhanced oil recovery (EOR) methods has focused on increasing oil production particularly from reservoirs with such lithology.
Chemical EOR uses chemical formulations (polymer, surfactant, and alkaline) as a displacing slug in a waterflooding process. The chemicals promote a decrease in mobility ratio and an increase in the capillary number. Many pilot tests were conducted in 1960's and 70's. In the US, commercial projects were in operation in the 1980's. Many chemical EOR activities took off in China since the late 1980's (Clark et al., 1988; Wang et al., 1998; Vargo et al., 1999). Current global chemical EOR production is about 300,000 bpd, most of which is from polymer flooding. Some alkaline-surfactant-polymer flooding projects are ongoing and some new projects in surfactant, surfactant-polymer, and alkaline flooding are being tested.