This study was undertaken to develop a method to reliably predict the performance of high relief carbonate reservoirs in the Rainbow Field (Alberta) when produced by gravity controlled internal and external gas drives. These processes were investigated in the laboratory and a practical way of handling the results for field predictions has been established for the external gas drive.
For a medium GOR oil (800 scf/STB), primary depletion resulted in a complex diphasic flow with thermodynamical exchanges, which immiscible numerical simulations were unable to match.
Recent progress in laboratory technology has resulted in development of laboratory experiments which tend to reproduce the actual field conditions:
Actual core samples assembled in such a fashion as to make up a physical model representative of the reservoir understudy;
Reservoir fluids at reservoir pressure and temperature;
Fluid velocity in model similar to velocity in reservoir.
Due to the low velocities and the complexity of the technology, the duration of such experiments ranges from several weeks up to about one year.
The data from each experiment are analyzed with a numerical model in the same manner as for an actual field history. Since the porous media and fluid properties and boundary conditions are well known, the adjusting parameters concern only the physics of the diphasic flow. They are the basic parameters which also control the production mechanisms in the actual reservoir.
Two experiments were conducted to simulate the gas pressure maintenance process and one the primary depletion process. Since gravity segregation controls the production in the high relief Rainbow reservoirs, all the experiments were performed vertically with the production fluid withdrawn from the bottom of the model.
Figure 1 presents a diagram of the apparatus used in this study.
Two physical models were used: a short model of about 70cm in length and a long model of about Each model was made up of several 3-inch diameter cores from the Rainbow Reg River member. All the core samples were cleaned with chloroform and the main petrophysical properties were measured. Table 1 gives the detailed composition of each model.
In order to prevent fluids diffusing out of the model through the rubber sleeve during the experiment which lasted up to eight months, each core sample has coated with a lead -bismuth alloy and its two races were shaped on a lathe to obtain a good contiruity between two adjacent cores.
The core samples forming the petrophysical model a were stacked in the Hassler cell B, avacuated by vacaum, and saturated with heptane at reservoir pressure.(The total pore volume measured in these conditions is generally smaller than the sum of pore volumes measured at laboratory conditions due to the effects of rock compressibility and metal penetration into vugs.) A 5700 psi overburden pressure and 190 °F reservoir temperature were maintaired throughout the experiment.