Gas Injection for Upstructure Oil Drainage
Authors: Strickland R.F. , Morse R.A. 
Published: December 2007

Any attic oil project tries to displace with gas the oil located above the structurally highest well in the reservoir. Results from a numerical simulator show that the most important controllable variable in attic oil recovery is the volume of gas injected during each cycle. A method is presented for calculating the required gas injection volume for each cycle presented for calculating the required gas injection volume for each cycle of an attic oil project.

Introduction
Oil recovery from high-relief reservoirs can be increased by downstructure gas injection. In this process, often called "attic oil recovery," gas is injected in the structurally highest well in the reservoir. The injected gas will migrate upstructure, forming a secondary gas cap and displacing oil downward.In high-relief reservoirs, which typically occur around "piercement type" salt domes, field development is complicated by the lack of control on the upper limits of the structure. To ensure a commercial completion, operators usually locate development wells a safe distance from the estimated sand/salt interface. In reservoirs with an active bottomwater drive, much of the updip oil will not have been produced when the highest well in the structure waters out. Sidetracking or drilling new wells is expensive and risky. It would be almost impossible to locate new wells to drain all updip oil adequately. Reservoirs with steep dip and high permeability allow injected gas to migrate to the most permeability allow injected gas to migrate to the most inaccessible areas of the pinchout, providing good lateral coverage. Injected gas may be traded economically for oil as long as the price of a unit volume of reservoir oil is sufficiently greater than the price of a corresponding unit volume of reservoir gas.Many authors have reported on field applications of the attic oil recovery process. In 1971, Combs and Knezek published theoretical guidelines and field data concerning the maximum gas/oil segregation rate and the minimum and actual gas requirements to recover 1 bbl oil.This paper presents results of a numerical model study of an attic oil recovery process and defines the variables that control the reservoir performance of a successful project. A method is presented to calculate the required project. A method is presented to calculate the required gas injection volumes.

Numerical Model
The numerical model used in this study is a conventional beta model that was modified to simulate the attic oil recovery process properly. The model includes these features: three-phase flow of oil, water, and gas; gas/oil solubility; gas/oil and oil/water capillary pressure; three-phase relative permeability with hysteresis effects; variable bubble-point scheme; variable flow rates; gravity effects, both areally and vertically; conformance to any reservoir geometry; and variable rock and fluid properties. properties. Even with the advent of relatively efficient three-dimensional reservoir simulators, computing costs for a meaningful study can become expensive. A two-dimensional areal model can simulate the same problem, provided the distribution and flow of fluids in the vertical dimension is included implicitly in the areal model. Several authors' have reported on the theory and calculation techniques of "pseudofunctions." Jacks et al. presented a technique for calculating the "dynamic pseudofunctions" that could be applied over a wide range of How rates and initial fluid saturations.
JPT
P. 1323