Gas Reservoir Simulation Using Semi-Implicit Rates With Multiple Compressor Stations

Abstract

ABSTRACT Reservoir simulation of oil or natural gas reservoirs enables the petroleum engineer to evaluate both the physical and economic consequences of various production scenarios (1). This paper describes a mathematical formulation devised to couple the line pressure drop and centrifugal compressor equations to a three-dimensional reservoir simulator. The assumptions applied are: The fluid flowing is a single phase gas.The compressor has a fixed horsepower and efficiency as a function of time.The delivery point pressure from the compressor station is known as a function of time. This method has been applied to a gas-water reservoir where the production consisted of essentially a single gas phase. There was a multiple well header at the inlet of each compressor station. Four separate compressor stations were used in the case history study although the method is general in the number of stations desired. The field problem was a three-dimensional system with two non-connected layers. A successful history match was obtained over a three-year production period, and multiple prediction runs were carried out in order to maximize ultimate gas recovery. This simulation technique will allow for multiple reservoir depletion by defining no-flow regions around each reservoir that is modeled. Simulation of gas storage reservoirs is also possible with this method to accurately predict requirements for compressor horsepower and well spacing as a function of desired deliv-erability. The gravity and frictional losses are accounted for in each tubing string to the surface. Individual layer rates are computed implicitly in the solution process with a gradient term to maintain a pressure differential between layers. Using this scheme, crossflow will occur between layers in a well-bore for areas where pressure differentials arise due to excessive production from one layer.