Determination of Relative Permeability and Trapped Gas Saturation for Predictions of WAG Performance in the South Cowden CO2 Flood
Authors: Wegener D.C. , Harpole K.J. 
Published: December 2007

Abstract
CO2 relative permeability, trapped gas saturation, and hysteresis effects are key parameters in determining injectivity and displacement efficiency in a miscible CO2 WAG injection project. Coreflood experiments were conducted to determine these parameters for samples from two major lithofacies in the South Cowden reservoir interval. This work was co-funded by DOE under the Class II Oil Program. The data provided key input parameters for simulation modeling and evaluation of the DOE sponsored advanced technology field demonstration project at the South Cowden Unit, Ector County, Texas.
Measurements of residual oil saturations following miscible CO2 displacement, trapped CO2 saturations, and endpoint relative permeabilities of CO2 and water (both before and after CO2 displacements) are presented for the primary facies. The secondary facies was determined not to be amenable to a WAG injection process. The corefloods were conducted at reservoir conditions (98 F and 1800 psig) using live oil and brine. Magnetic resonance imaging (MRI) was used to screen core plugs for internal, "hidden" heterogeneities prior to flow testing. Trapped gas saturations in the main reservoir facies at South Cowden varied from approximately 20-25 3 % PV. Measured CO2 relative permeabilities were significantly lower than oil relative permeabilities at comparable water saturations. Water relative permeabilities after CO2 displacement were appreciably reduced compared with values observed prior to CO2 injection. The impact of these key parameters on stimulation model predictions of CO2 flood performance is illustrated using a typical pattern simulation model. Experimental procedures and apparatus are presented along with a method for estimating uncertainties in the trapped gas saturation measurements.

Introduction
Effective management of a reservoir being considered for CO2 miscible WAG injection benefits from extensive evaluation of its projected performance via reservoir simulation. The degree to which a sophisticated reservoir simulator is successful in predicting reservoir performance is directly related to the quality of the input data, which includes geological descriptions along with the reservoir rock and fluids properties. Such input may include water and CO relative permeability data, trapped gas saturation, and relative permeability hysteresis effects.
The influences of these parameters in miscible and immiscible WAG processes have been discussed in Refs. 1-8. Observed trends have not always been consistent, even among west Texas reservoirs. As a result, some degree of laboratory work should probably be included in the planning of each miscible or immiscible WAG project. The objectives of this paper are to present the results of South Cowden miscible WAG laboratory experiments and to demonstrate how the generated data impact predictions of reservoir performance via reservoir simulation.

Experimental
Core Samples. The core plugs used in this experimental work were taken from native state cores which had been drilled with a bland mud of neutral pH. The core plugs were cut and stored in deaerated lease crude. A large number of core plugs were available from which to select the plugs used in the coreflood experiments. The core plugs were divided into two broad geologic lithofacies groups:a burrow-mottled dolopackstone "chaotic" facies anda fusulinid dolowackestone "moldic" facies
The burrow-mottled chaotic facies has been characterized as having well connected intergranular and intercrystalline porosity, while the fusulinid moldic facies has been characterized as having poorly interconnected moldic porosity.
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