Application of Fluid Contact Match in Material Balance Study in a Water Injection Reservoir to Estimate STOOIP and Initial Reservoir Contacts

Abstract

Abstract Original hydrocarbon in-place and aquifer influx for a reservoir can be estimated using the classical Material Balance Equation (F = N*Et + We). Being a single equation with many unknowns, this makes the solution to the equation non-unique with a wide range of OHCIP estimates. Including fluid contact tracking helps to constrain the range of OHCIP and better define the initial conditions of the reservoir. The objective of the material balance study was to estimate the STOOIP and initial reservoir fluid contacts for two waterflooded reservoirs. The volumetric estimates of STOOIP for the proved area (defined by the HKO and ODT) gave very optimistic recovery factors to date. This paper will discuss the methodology used for estimating the initial reservoir condition by using fluid contacts tracking to constrain the material balance equation for production history match. Reservoir A2 was discovered with no initial fluid contacts seen in the reservoir prior to the onset of production. Water injection started in the reservoir after fourteen years of production. Oil RF to date from the reservoir is over 61% of the volumetric STOOIP estimate (265 MMSTBO) using the first fluid contacts seen in the reservoir post-production. Reservoir A3 was discovered with an OOWC and HKO with volumetric STOOIP estimate of 295 MMSTBO. Initial water injection started in the reservoir after fourteen years of production. Oil RF to date from the reservoir is 71%. Integrating fluid contacts tracking in the material balance study helped constrain the STOOIP estimates and give more realistic values (45 – 50% for reservoir A2 and 60 – 65% for A3). The STOOIP and initial contact estimates from this study were used as basis for comparison with the dynamic simulation study and the results were within 5%. These case studies show that applying fluid contact matching in material balance can help with better estimates of STOOIP and initial reservoir fluid contacts. This is of great importance to the optimum development of these reservoirs and has led to the identification of new drill opportunities in each reservoir.