OILWAT - Microcomputer Program for Oil Material Balance With Gas Cap and Water Influx

Abstract

Summary This paper describes an interactive computer program, OILWAT, for material-balance calculations in oil reservoirs with gas cap and water influx. A material-balance calculation uses production and pressure data to calculate original oil in place (OOIP) and aquifer characteristics. This formulation can be very useful in a history match to define reservoir and aquifer parameters before a simulation study. It also can be used to predict future pressure and primary production. The use of different material-balance methods usually results in different OOIP estimates. OILWAT helps solve this problem by combining several material-balance methods with various aquifer options (small pot, steady state, and unsteady state) and comparing them statistically. Panel input and graphical output are available in the program. Several important features are (1) variable bubblepoint, which enables very good history match during repressuring of the depleted reservoirs; (2) variable rock compressibility for abnormally pressured reservoirs; (3) an option to calculate original gas-cap or aquifer size provided that OOIP is known; and (4) applicability to saturated and/or undersaturated reservoirs. In contrast to most material-balance programs, OILWAT permits the use of a single data set for a reservoir initially at undersaturated condition that subsequently drops below bubblepoint.