This paper describes the results of a series of tertiary, immiscibie, CO 2 corefloods of Wilmington field Pliocene reservoir rock containing heavy oil (± 14 ° API [±O. 97 g/cm 3] and ±480 cp [±480 mPa' s]). An initial set of corefloods defined the recovery potential of the CO 2 injection, while a series of later tests served to define the process more accurately as applIed III the field. In an attempt to understand the displacement mechanism, simulator matching of one of the later, more refined groups of corefloods was performed. The corefloods and simulator work indicate that the incremental recovery is more than can be accounted for by oil-viscosity reduction and crude-oil swelling. The improved performance is attributed to more favorable displacement characteristics and the presence of a free gas saturation in the cores.

This paper describes the solution of the equations for two-dimensional, two-phase, immiscible flow by variational methods. The formulation of the equations and the Galerkin procedure for solving the equations are given. procedure for solving the equations are given. The results of numerical experiments for one-dimensional, two-dimensional areal, and two-dimensional cross-sectional examples are presented. In each case, the results are compared with finite-difference solutions for the same problem. The ability to track sharp fronts is demonstrated by the variational approach. The time approximation used is shown to be stable for difficult problems such as converging flow and gas percolation. Also, the variational solution is shown to be percolation. Also, the variational solution is shown to be insensitive to grid orientation. Introduction In practical applications in the petroleum industry, the nonlinear, partial differential equations for fluid flow through a porous medium are currently solved almost exclusively by finite-difference methods. Variational or Galerkin (the terms are used interchangeably here) methods for solving these equations offer the potential advantage of higher-order accuracy at lower computational cost.This paper describes research on the solution of the equations for two-phase immiscible fluid flow using variational methods. The literature on the application of these methods to immiscible fluid flow is sparse. Douglas et al. describe solution of the one-dimensional immiscible displacement problem using cubic-spline basis functions and solving simultaneously for pressure and saturation as the dependent variables. They concluded pressure and saturation as the dependent variables. They concluded that the method was practical and that better answers are obtained with the same computational effort than by finite-difference methods. They also concluded that their choice of basis functions was probably not optimal. Verner et al. discuss the solution to the one-dimensional problem using "parabolic basis elements" (C degrees quadratic-basis problem using "parabolic basis elements" (C degrees quadratic-basis elements). Using the same data as was used by Douglas et al., they concluded that the parabolic, finite-element, spatial approximation gives results similar to the cubic splines for the same number of degrees of freedom. McMichael and Thomas solved the equations for three-phase, multidimensional immiscible flow. They solved simultaneously for the three-phase potentials as dependent variables. Although they stated that a general three-dimensional program with variable-basis function capability was developed, program with variable-basis function capability was developed, the examples they presented were two-dimensional areal. Also, piecewise linear basis (Chapeau) functions were used in their piecewise linear basis (Chapeau) functions were used in their example problems. The numerical experiments presented by McMichael and Thomas were limited to two relatively simple problems. They concluded that the Galerkin method requires significantly more work per time step than a finite-difference model, but that larger time steps could be taken. Vermuelen discussed the solution of the two-phase immiscible flow equations by simultaneously solving for the wetting- and nonwetting-phase pressures using a semi-implicit, first-order time approximation. Vermuelen's example problems used piecewise linear-basis functions. Based on one of these examples, piecewise linear-basis functions. Based on one of these examples, he concluded that the Galerkin technique appears to be less accurate than the finite-difference method for problems of water tongue displacement. In addition to the above work on two-phase immiscible flow through porous media, several authors have discussed the application of variational methods to miscible displacement problems and single-phase flow problems. SPEJ P. 27

Spivak, Allan,* Member SPE-AIME, Chevron Oil Field Research Co., La Habra, Calif. Abstract This paper describes a study of gravity segregation (underrun or override of injected fluids) in two-phase, secondary recovery displacement processes. Reservoir simulation was used to investigate the factors that influence gravity segregation and the magnitude of gravity effects for both water floods and gas floods. The degree of segregation for a given set of conditions was determined bycomparing the results of two-dimensional cross-sectional with one-dimensional horizontal calculations, andcomparing the results of three-dimensional vs two-dimensional a real calculations. The degree of segregation is quantitatively described by the dimensionless number E defined as (Rbt)no gravity -(Rbt)gravity E =, (Rbt)no gravity where Rbt is recovery at breakthrough. Gravity segregation effects in two-phase displacement processes were found to increase withincreasing processes were found to increase withincreasing permeability (either horizontal or vertical),permeability (either horizontal or vertical),increasing density difference,increasing mobility ratio,decreasing production rates. anddecreasing level of viscosity for a fixed viscosity ratio. A series of calculations was made in which the parameters that affect gravity segregation were varied. The effect of each parameter on the degree of gravity segregation was observed. The effect of both stratified and random heterogeneity was also studied. A correlation between the degree of gravity segregation and the dimensionless groups G = 0.00633 and M = mobility ratio was established. This correlation is based on the results of the simulator calculations and a detailed analysis of the equations for three-dimensional, two-phase. immiscible, incompressible flow. The correlation can be used to determine qualitatively whether gravity segregation will be a significant factor in a given flooding process. It can also be used to determine whether the assumption of vertical equilibrium is valid in the simulation by a two-dimensional a real simulation model of reservoirs where fluid saturations vary significantly in the vertical direction. Introduction For many years, gravity segregation in reservoir processes was not accounted for because it could processes was not accounted for because it could not be adequately handled in reservoir engineering calculations. The advent of reservoir simulation provided the capability to handle gravity, and it provided the capability to handle gravity, and it became apparent that gravity effects could significantly affect reservoir performance. This paper describes a study in which a three-dimensional paper describes a study in which a three-dimensional (3-D), two-phase, incompressible simulator was used to look at gravity effects in displacement processes. The objectives of this study wereto processes. The objectives of this study wereto determine what factors influence gravity segregation and in what way;to look at the magnitude of gravity segregation effects;to compare reservoir performance calculations with and without gravity performance calculations with and without gravity effects. PREVIOUS WORK ON GRAVITY PREVIOUS WORK ON GRAVITY SEGREGATION IN DISPLACEMENT PROCESSES Craig et al. did experimental work to study the effects of gravity segregation during water, gas, and solvent flooding. As a result of their studies, they concluded that segregation of fluids due to gravity effects could result in oil recoveries at breakthrough as low as 20 percent of those otherwise expected. They also concluded that performance may in some cases be influenced to a greater degree by heterogeneity than by gravity effects. They were not able to directly compare performance under a given set of conditions with and without gravity. SPEJ p. 619

The Long Beach Oil Development Co. (LBOD) tertiary immiscible C02 project began in March 1982 with the injection of a mixture of about 85 % CO 2 /15% N2 into the Tar zone of Fault Block V in the Wilmington field, Los Angeles County, CA. The 330-acre [134-ha] project had been waterflooded since 1961, and the water cut was more than 95%. Gas was injected alternately with water for a period of about 5 years. This paper reviews project performance from inception to the end of 1987.