Studies of Offshore Reservoir With an Interfaced Reservoir/Piping Network Simulator

Abstract

Summary A reservoir simulator and a piping network simulator have been interfaced to study reservoir deliverability. The interfaced programs are used for ongoing studies of several large reservoirs. An example of a recent study is presented to illustrate program usage. The interfaced system proves useful primarily for reservoirs producing through complex gathering systems. The system provides better forecasts of reservoir deliverability and is a useful tool in designing the gathering lines. Introduction A simulation model that combines both reservoir and surface facility calculations offers increased accuracy in predicting reservoir deliverability. This concept was presented by Dempsey et al. for a gas/water system. Simolte and Hearn described a recent application for study of a single-phase gas reservoir.This paper describes an interfaced reservoir/surface-facility simulation system used to study large offshore oil reservoirs that produced through complex gathering systems. A previous papers reported test cases run on an early version of the simulation program. The program subsequently has been refined to make practical and routine calculations for reservoir models with several thousand grid blocks and several hundred wells. Current usage is for ongoing design and deliverability studies of several major reservoirs.Fig. 1 shows an example of a hypothetical simulation model typical of recent application. The grid is a portion of the reservoir simulator grid. The platform locations and the surface piping network are superimposed. Several wells are connected to each platform.The formulations of reservoir and network simulators have been well documented in the literature and will not be described in detail in this paper. We will explain the following.1. The calculation procedure and approximations adopted to minimize central processing unit (CPU) time.2. Our technique for converging the reservoir and network simulator calculations.3. Applications of the system to example problems. Iterative Procedure A rigorous iterative procedure for the reservoir/network calculations was presented in Ref. 1 for gas deliverability calculations. Rigorous Iterative Solution For an accurate solution, the reservoir variables provided to the network simulator should represent the average or midtime-step values for the time step. To ensure that the correct values are used, it is necessary to iterate between the reservoir and network models within each time step.1. For each well, estimate average of midtime-step Pe, cut, and J based on initial conditions or extrapolation from previous time step.2. Solve network model given values from Step 1 to obtain individual-well flow rates. The step includes a calculation traverse for pressure drop from well bottom to network outlet several times until well rates converge. Step 2 represents a subiteration to develop well rates that satisfy the boundary conditions of Pe and fixed network-outlet pressure.3. Solve reservoir model for current time step using flow rates computed in Step 2 to obtain end of time-step pressures and saturations. Compute end of time step, cut, J, and P, for each well. JPT P. 399^