Conventional displacement methods such as waterflooding do not work effectively in densely fractured reservoirs. The high fracture permeability prevents significant pressure differentials across oil bearing matrix blocks leading to negligible oil drive. In such reservoirs one has to rely on natural mechanisms like capillary imbibition or gravity to recover oil from the matrix rock. In Middle East fractured carbonates, the matrix rock is commonly oil-wet or mixed wet and only gravity drainage remains a feasible process. However, permeabilities are usually low, <10 mD, resulting in low gravity drainage production rates with high remaining oil saturation and/or capillary holdup.
Thermal EOR methods have the potential to improve the gas oil gravity drainage (GOGD) rate and ultimate recovery. For shallow fractured reservoirs, it is feasible to inject steam into the fracture system, in the process known as Thermally Assisted GOGD (TAGOGD). Steam will condense as it contacts cooler matrix rock, resulting in a steam front that develops in a stable way through the fractures. Conductive heating of the matrix will result in oil expansion, viscosity reduction, solution gas drive and stripping effects. No viscous pressures are building up, and steam drive does not occur. For reservoirs containing viscous oil, the viscosity reduction effects are most important. When steam is injected in light oil reservoirs, solution gas drive and stripping effects potentially become dominant.
In this paper we analyse the effect of the different recovery mechanisms. We discuss the results of stack simulations for light oils and for a range of fracture spacings with reference to our previous results on viscous oils. We compare single-porosity simulations of a fracture-matrix stack system with dual-permeability simulations. The dual-permeability modeling requires special techniques to capture transient effects.
The connected fracture network in densely fractured reservoirs has a strong impact on reservoir displacement mechanisms. Once a gas cap is established in the fracture system, the oil will drain down the matrix rock driven by gravity and into the fracture system below the fracture GOC or at matrix flow barriers. In the fracture system the oil forms a (thin) rim that can be produced. Production rates achieved with a GOGD process are often low due to low matrix rock permeabilities, capillary hold-up and re-imbibition effects. Capillary hold-up also negatively impacts ultimate recovery.