Abstract
Abstract
Waterflooding often recovers little oil from carbonate reservoirs because carbonate reservoirs are usually oil-wet and fractured. Dilute surfactant solution injection into the fractures can improve oil production from the matrix by lowering the oil/water interfacial tension (IFT) and by altering the wettability of the rock to intermediate/water-wetting. A 3-D, two-phase, multicomponent, finite-difference, fully-implicit numerical simulator is developed that incorporates adsorption, phase behavior, wettability alteration, and related capillary pressure / relative permeability variations. The reliability of the simulator is established by comparing the core-scale simulation results with experimental results obtained in our laboratory. IFT reduction, wettability alteration and permeability are varied to study the sensitivity of the surfactant-aided gravity drainage to these process parameters. Wettability alteration drives the oil production at the early stage, but gravity is the major driving force afterwards. Surfactants which alter the wettability to a water-wet regime give higher recovery rates for higher IFT systems. Surfactants which cannot alter wettability give higher recovery for lower IFT systems. Extent of wettability alteration increases the rate of oil recovery. Recovery rate decreases with permeability significantly for a low tension system, but only mildly for high tension systems.
Introduction
More than half of the world's remaining oil reserves are said to be contained in carbonate reservoirs.1 Recovery from carbonate rocks is not efficient because of many factors such as reservoir heterogeneity, produced fluid type, drive mechanisms and reservoir management. Many carbonate reservoirs are naturally-fractured and oil-wet / mixed-wet.2,3 These factors lead to low recovery factor in waterflooding.4 Surfactant flooding (or "huff-n-puff") techniques are being developed5–11 to improve oil recovery from oil-wet/mixed-wet, fractured carbonate formations. Our previous studies identified surfactants that can recover more than half the original oil from an initially oil-wet core by IFT reduction, wettability alteration and negligible adsorption.8,10 The process was modeled and the simulator matched the experimental results at the laboratory-scale.9
There are three major forces for oil recovery: gravity, capillarity and viscous. Since the current process is an imbibition process, the viscous forces as compared to gravity and capillarity are very low. This is different from a flooding process where viscous forces are important. Mattax and Kyte12 have proposed a scaling group for capillarity-controlled imbibition. Ma et al.13 have proposed a modified scaling group which is given as:
Equation (1)
where, is dimensionless time, is the rock permeability, is the porosity, s is the interfacial tension, mw and mo are the viscosities of water and oil phases, respectively. Lc is the characteristic length for imbibition, and t is the actual time of imbibition. Using this dimensionless time, the experimental results obtained by Ma et al.13 for water-wet cases fit into a unique curve, which is referred to as the very strongly water wet (VSWW) curve.
The ratio of the capillary to gravitational forces termed as macroscopic inverse Bond number, is given by
Equation (2)
Schechter et al. 14 found that for < 1, gravity segregation dominates the flow and that for high values of >1, capillary forces are dominant. Hagoort15 has analyzed the 1-D gravity driven oil drainage, and the expressions for dimensionless time and recovery are:
Equation (3)
and
Equation (4)