This study presents a method of determining the initial oilin- place -N, ratio of initial gas to oil - m, reservoir permeability K, and Skin Factor S from the combination of solution to the material balance equation and pressure transient analysis theory using the cumulative production history, individual well rate history and PVT data of the reservoir with limited well test pressure data.
Equations derived from solutions to analytical pressuretransient analysis equations can be used to estimate from production history of the reservoir and well production rate history with as few as a single well test pressure data, the initial oil-in-place-N, ratio of initial free gas to oil -m, average reservoir permeability-K, and Skin effect- S within the drainage area of the well. The solution method considers the variable rates production history of each of the wells and combines this with a global pressure data analysis approach as opposed to the usually employed approach of applying each method in isolation, which often produce erroneous results.
The analysis procedure allows every detail of the reservoir development, rate changes at every well from inception, global (reservoir wide) and local (individual well) fluctuations in pressure to be included in the overall results. Local variations in reservoir rock properties at the well locations are also accommodated in the method.
By modifying the analysis technique used for solving the linear material balance, a method is derived that allows the determination at the same time; the original oil-in-place, N and initial ratio of free gas to oil in the reservoir, m without the previous knowledge of one or the other from an independent source, a requirement that's often needed for such solution.
Few methods are available for estimating the reserves for oil and gas reservoirs, one of which is the material balance method (MBE). These estimates help in making the critical decisions concerning depletion plans and development strategies regarding the reservoir. The MBE makes these estimates by considering different time intervals in the production history of the reservoir and maintaining that there exist a volumetric balance in the reservoir at these different time intervals.
Several methods have been developed and published on applying the MBE to various types of reservoirs and solving the equation to obtain the initial-oil-in-place (N) and the ratio of the initial free gas to oil (m) in the reservoir. One of such methods is the straight-line method popularized by Havlena and Odeh4,5 which instead of considering each time interval and corresponding production data as being separate from other time interval, combines all time intervals under consideration and obtain a solution that satisfies all the intervals together. In applying the straight-line method however, it is usually required that an independent source of determining the value of m exist. This study presents a method of simultaneously determining the value of N and m from production and well test data.
No matter the method used to solve the MBE, a common and important requirement is the need to accurately estimate the average reservoir pressure at the various time intervals.