A Simple Approach to Dynamic Material Balance in Gas-Condensate Reservoirs

• Published in: Oil & gas science and technology 2014, Vol.69 (2), p.307-317
• Main Authors: Heidari Sureshjani, M., Gerami, S., Emadi, M.A.
• Format: Article
• Language: English
• Subjects: Applied sciences; Crude oil, natural gas and petroleum products; Energy; Exact sciences and technology; Fuels; Physics
• ISSN: 1294-4475; 1953-8189
• DOI: 10.2516/ogst/2012022

In traditional material balance calculations, shut-in well pressure data are used to determine average reservoir pressure while recent techniques do not require the well to be shut-in and use instead flowing well pressure-rate data. These methods, which are known as “dynamic” material balance, are developed for single-phase flow (oil or gas) in reservoirs. However, utilization of such methods for gas-condensate reservoirs may create significant errors in prediction of average reservoir pressure due to violation of the single-phase assumption in such reservoirs. In a previous work, a method for production data analysis in gas-condensate reservoirs was developed. The method required standard gas production rate, producing gas-oil ratio, flowing well pressure, CVD data and relative permeability curves. This paper presents a new technique which does not need relative permeability curves and flowing well pressure. In this method, the producing oil-gas ratio is interpolated in the vaporized oil in gas phase (Rv) versus pressure (p) data in the CVD table and the corresponding pressure is located. The parameter pressure/two-phase deviation factor (p/ztp) is then evaluated at the determined pressure points and is plotted versus produced moles (np) which forms a straight line. The nature of this plot is such that its extrapolation to point where p/ztp = 0 will give initial moles in place. Putting initial pressure/initial two-phase deviation factor (pi/ztp,i) (known parameter) and estimated initial moles (ni) into the material balance equation, average reservoir pressure can be determined. A main assumption behind the method is that the region where both gas and condensate phases are mobile is of negligible size compared to the reservoir. The approach is quite simple and calculations are much easier than the previous work. It provides a practical engineering tool for industry studies as it requires data which are generally available in normal production operations. However, it is only applicable when average reservoir pressure approaches dew point pressure and falls below it. The methodology is validated using synthetic production data for several examples. In addition, the method is evaluated through estimation of average reservoir pressure and original gas in place from actual field data. The results show a fairly good agreement in gas in place obtained from the new method and that of volumetrically calculated value for this field. Lors de calculs traditionnels d’é