Waterflooding of carbonate reservoirs : EOR by wettability alteration

Abstract

Oil production is generally divided into three stages, primary recovery, secondary recovery and tertiary recovery. In primary recovery, which is usually (but not necessarily) the first production stage, the natural energy of the reservoir is used to displace the oil from the reservoir. The drive mechanisms are pressure depletion of the reservoir pressure, in the form of solution-gas drive, gas cap drive, natural water drive, or fluid and rock expansion, or gravity drainage. In this stage, only 10-30 % of the OOIP (oil originally in place) is produced (Castor et al., 1981). Secondary recovery is usually initiated when primary production is declining, adding additional energy to the reservoir in order to maintain pressure or provide a more efficient oil displacement. Gas injection and waterflooding are two secondary methods, the latter being the most common. The recovery factor after the secondary stage is usually 30-50 % of OOIP (Castor et al., 1981). The tertiary recovery stage, comprising miscible gas injection, chemical injection and thermal energy methods, typically takes over when secondary recovery becomes uneconomical. However, oil production does not always follow this chronological order. Therefore, the term ‘Enhanced Oil Recovery’ (EOR) is nowadays more widely used than ‘tertiary recovery’. Another term, IOR (Improved oil recovery) is also often seen, and in addition to EOR this term includes reservoir characterization, reservoir management and infill drilling (Green and Willhite, 1998). The EOR methods most commonly target the oil left in the waterflooded reservoir, which especially in carbonate reservoirs can be a substantial amount. There are five categories of EOR processes (Green and Willhite, 1998): mobility-control (polymers, foams), chemical (surfactants, alkaline agents), miscible (hydrocarbon solvents, CO2), thermal (steam, in-situ combustion) and other processes, such as microbial EOR, immiscible CO2 etc. Seawater injection should perhaps be categorized under other processes, as it is a superb EOR fluid to chalk. This will be discussed later in chapter 3...