The effect of miscibility conditions on Gas Oil Gravity Drainage; an experimental study in a simulated fractured medium using CO2 as a displacement agent.

Abstract

Production from fractured reservoirs is low because many displacing fluids sweep the fractures, but do not produce oil from the matrix blocks. In case of water-wet reservoirs it is possible to inject water that displaces the oil from the matrix blocks into the fractures. For oil-wet reservoirs water displacement is inefficient. Therefore this thesis investigates the possibility to inject high pressure gas, such as nitrogen, flue gas and CO2. The expected mechanism is gas-oil gravity drainage, where the oil is displaced in the matrix blocks by gravity and produced in the fractures. Secondary mechanisms occurs due to dissolution of the gaseous components in the oil, which leads to swelling, IFT reduction and viscosity reduction.

The method of investigation is through experiments, where a core is placed inside a core holder with free space all around it representing the fracture. The injection point was on the top centre, whereas the production point was at the bottom centre. All injection rates were 5 ml/minute at the experimental conditions. A total of eight experiments successful experiments were performed. There were two kinds of cores, viz., with sizes of (diameter x height) of (0.03 m x 0.1 m) for core-A, and (0.04 m x 0.2 m) for core -B.

Experiments (1-3) in core-A, with carbon dioxide, nitrogen and flue gas at low (45 bar) pressures did not lead to any discernible oil production. Experiment (4) that used CO2 at 60 bar in core A, resulted in 62% recovery. Experiment (5) used CO2 at 80 bars also in core A resulted in 85% recovery.

Experiment 6 that used CO2 in core B at 45 bar resulted in 38% recovery. CT scan observations showed that CO2 entered from top in piston like displacement until capillary-gravity equilibrium was attained. Experiment 7 that used CO2 in core B at 80 bars resulted in almost complete recovery as determined by a post-mortem observation. However only 50% of the liquids were collected, and it was inferred that the other part was produced along with the gas phase. Experiment 8 that used CO2 in an mixture of heptane, decane and iodo-decane, resulted in a recovery of 85%. CT scan data showed that the CO2 entered from the bottom, but there was a transition zone between the gas that entered and the initial oil.

A finite element model was developed that uses the Brinkman equation both in the fracture and the matrix for the case that we have miscible conditions. Comparison between experiment and modelling shows a qualitative agreement.