The purpose of this paper is to investigate the effect of injection rate, gravity force and connate water saturation on the mechanism of gravity drainage.
Gravity drainage is the most important mechanism in oil recovery from the fractured reservoirs. During gas injection gravity drainage may either become more effective or retard the process. Finding the optimum injection rate is our main goal in this study.
In our experiments nitrogen and kerosene were used as gas and oil respectively. The core samples were prepared from outcrop of Asmari formation of Iran, which was a fractured carbonate rock.
The injection gas rate is an important factor affecting on the oil recovery. We found a critical rate in which the gas rate is lower or higher than critical, the oil recovery decreases. However the recovery factor at critical rate is the maximum. This phenomena was discussed based on the competition of different forces of viscous, capillary and gravity. These forces cause the displacement change from the stable piston like displacement to an unstable trapping displacement. Observation showed that different amount of the gravity forces have no effect on oil recovery before gas breakthrough; however, with more gravity force, more oil production was observed after breakthrough. Also, we observed irreducible water saturation change during gas injection in our carbonate rocks.
Gravity drainage is one of the major issues in naturally fractured reservoirs. Knowledge of this mechanism has been progressively improved since sixties1. In fractured oil reservoirs submitted to gas injection; gravity drainage is the main mechanism of oil production from the matrix blocks. This mechanism is important in the development of many oil reservoirs with large dip angle2. For example Dumore and Schols3 reported an extremely low residual oil saturation of 5% in high permeability sandstone cores after gravity drainage2.
Slobod and Howlett4,5 have studied the effects of gravity segregation in vertical unconsolidated porous media under miscible conditions in the laboratory. The main variables in their study were the viscosity ratio, the density differences and the rate of flow. The aim of this work is to improve the understanding of the dynamics of gravity drainage and injection rate on oil recovery from Iranian carbonate rock during immiscible gas injection.
The rock samples were prepared from outcrop of Asmary limestone. Asmari limestone of Lower Miocene to Oligocene age represents most of the oil reservoirs of Iran which is a very tight and highly fractured formation. Two different cores were used in this study. The permeability of models was measured by core holder system and a Darcy's set up. Because of the very low permeability matrix, the measurements were repeated with another Hassler sleeve type core holder. Oil and gas relative permeabilities were measured experimentally by the unsteady state method (figure 2). The porosity of models was measured by the saturation method. Dimension and petrophysical properties of cores are given in Table 1.
Kerosene was used as the oil phase, nitrogen was used as displacing gas and distilled water was used as connate water in all tests.
