Abstract

The use of carbon dioxide as an oil recovery agent in petroleum reservoirs has been investigated for many years. Both laboratory and field studies have established that carbon dioxide can be an efficient oil displacing agent.

The purposes of this research were to (1) study experimentally the miscibility of carbon dioxide and medium crude oil in d consolidated sandstone core and an unconsolidated sand pack, and (2) evaluate the effectiveness of carbon dioxide as a tertiary recovery agent in a waterflood added core.

A 15-foot-long consolidated core was made by Joining three individual 5-foot-long Berea sandstone cores with small diameter tubing, and a 14- foot-long unconsolidated sand pack was formed by filling a 2-inch-1.0. plastic tube with a known weight of sand. Electrodes were installed at one foot intervals along the core and sand pack so that fluid saturations could be determined by the resistivity method. The pore volume and the recoveries were obtained by material balance calculations.

In this study. Millican Field oil of 45 centiposes viscosity and 25 °API gravity was used to saturate the cares. Oil displacement experiments were conducted at a pressure of 2000 psig and a temperature of 120D F. The cares were first water flooded down to residual oil saturation, and then were flooded using a carbon dioxide slug of 30 percent of the pore volume pushed by water.

Recoveries by conventional water flooding were platted and compared to those obtained by carbon dioxide floods in both consolidated and unconsolidated systems. The distance from the inlet of the core for complete miscibility between carbon dioxide and oil was determined by the saturation pattern inside the core. The over-riding effect of carbon dioxide and relative permeability on displacement efficiency were also investigated.

Introduction

The problem of improving recovery from petroleum reservoirs has become increasingly urgent because of the sharp change in the world energy situation. There are still tremendous amounts of oil that are left unrecovered in the ground after application of present recovery methods. Therefore, improvements in tertiary recovery techniques are required. The technique utilizing carbon dioxide has received considerable attention in the past decade. Complete or total oil recovery from oil bearing reservoirs is not possible where displacement is made by immiscible fluids, such as water or low pressure gas, because capillary forces and 1nterfacial tension will result in the leaving behind of a fixed residual oil saturation. The ObV10US approach to total oil recovery on a pore-to-pore basis would be to reduce capillary forces to zero. This can only be accomplished if the displacing fluid is miscible with the displaced oil.

The utilization of carbon dioxide as a miscible displacing agent has been investigated in both the laboratory and field 1- 12 in the past decade. The analysis of these tests indicated that additional oil beyond that obtained by normal waterflooding could be recovered with carbon dioxide.

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