Introduction

At the beginning of this text, the various hydrocarbon reservoirs were subdivided into four types. This contains a discussion on reservoirs that have only liquid phases initially present. The will consider oil reservoirs that have an initial gas cap. These two reservoir types differ significantly from the gas reservoirs. The differences stem from the composition of the reservoir fluids and result in a distinct primary production method—that of depletion drive. Compared to volumetric gas drive, depletion drive is a weaker primary production method and more factors, such as rock and water compressibility, must be considered in order to accurately predict the behavior of the reservoir. The same method, the material balance, will be used in this prediction; it will, however, require additional terms.

Oil in place for oil reservoirs can be calculated in two ways. If available, well and seismic data can be used to calculate the oil in place using techniques like those explained. Alternately, oil and gas production data combined with pressure and saturation data can be used in a material balance employing.