A Versatile, Fully Implicit, Black Oil Simulator With Variable Bubble-Point Option

Abstract

ABSTRACT A stable and efficient, general-purpose, variable bubble point black oil simulator is presented. The program utilizes the method of substitution of variables for modeling gas phase appearance and disappearance. Paralleling its application in conventional reservoir simulation, recent work has shown that the present model forms a solid basis for the development of more specialized simulators containing unique options. The objective of this paper is to present a general description of the simulator and to describe in detail the numerical methods and specific programming techniques utilized. At the heart of the model is the variable bubble point formulation. This employs relatively complex Jacobian building routines and switching schemes. These are specially designed for efficiency and acceleration of convergence. The automatic timestep selector for thermal simulation presented by Sammon and Rubin has been modified to accommodate the substitution of variables in black oil simulation. Techniques for implementation of this timestep selector are presented. To solve the system of nonlinear flow equations, a modified Newton-Raphson procedure is presented. In comparison with the classical Newton-Raphson procedure, the present method requires a larger number of iterations per time step, but overall computation time is significantly reduced. A series of tests are presented to compare the classical and the modified Newton-Raphson procedures and to illustrate the efficiency and stability of the model.