The classical Material Balance equation (F = N*Et + We) is a zero-dimensional reservoir modeling methodology that is used to estimate original oil-in-place volume (N), gas cap size, and aquifer influx (We). The material balance equation is a single equation with many unknowns (e.g. N, m, We); thus the solution to the equation is inherently non-unique. In other words a range of original oil in place (STOOIP) gives good match of the material balance equation. The range of STOOIP solutions of the material balance equation is often too high. In this paper a methodology is suggested for reducing the uncertainty in the material balance derived STOOIP values. In the proposed methodology, the material balance equation is further constrained by history matching the average fluid contacts in the reservoir. Three case studies were used to illustrate the application of material balance and fluid contact match in STOOIP estimation. The first case study is a synthetic reservoir with a STOOIP of 100 MMBO, initial gas cap and aquifer influx. The result shows that solving only the material balance equation gives a very wide range of STOOIP of 80 - 300 MMBO, while including the fluid contact match reduced the STOOIP range to 80 – 125 MMBO. The second case study is a real reservoir with initial gas cap and aquifer influx. Using material balance alone the STOOIP range was 80 – 800 MMBO whereas including the fluid contact match gave a lower STOOIP range of 100 – 120 MMBO. The last case study also shows a reduction in the range of STOOIP estimation from 50 - 500 MMBO for solving the material balance equation alone to 90 - 120 MMBO when the fluid contacts are history matched. These case studies show that uncertainty in the material balance STOOIP estimates are greatly reduced by matching fluid contacts.
Recent reservoir studies have shown great strides in capturing complex multi-tank reservoir systems using Material balance (MBAL®) model especially when these tanks are hydraulically connected. The preference of a simple and quick material balance method over numerical simulation is justified when time and budget constraints may hamper full field simulation modelling. The subject reservoir for this study is a mature hydraulically connected tank under waterflood. This paper compares reserves estimation using predictive material balance and dynamic simulation after history matching both models.Results from this paper shows that for multi-tank modeling using of hydraulically connected reservoirs using MBAL, transmissibility modeling & aquifer modelling are key factors in achieving reasonably reliable MBAL models. A good understanding of the reservoir fault frame work and connectivity, adequate & reliable BHP, injection and production data are very important for attaining a good history match & model calibration. Results of this study show a good comparison between the Reserves estimate from both MBAL and that from dynamic simulation. Under the appropriate conditions, Predictive Material Balance is a quick alternative tool to reservoir simulation in reservoir performance prediction especially when time & resources are limited. Also, predictive material balance analysis should be applied in reservoir where the existing wells have adequate data to build reliable inflow & out flow models.
As Oil fields containing stacked reservoirs are developed, it is not uncommon for these reservoirs to be in pressure communication. The extent of pressure depletion may determine the viability of potential new drills. Possible communication of the X1 reservoir with the Y1 reservoir was initially suspected when a Material Balance study earlier conducted on the X1 reservoir did not yield a reliable historical pressure match using mapped volumes of the X1 thereby suggesting potentially larger Oil in Place number in the X1 reservoir. Also, the structural overlay of the X1 over the Y1 reservoir having compatible hydrocarbon contacts seemed to also indicate possible communication between the X1 and Y1 reservoirs. The conclusion of the Material Balance study recommended a more detailed probe into the X1 reservoir to ascertain the impact of communication on the reservoir development strategy. A simulation study was conducted primarily to evaluate the impact of communication uncertainty on the new drill recovery. Three scenarios were considered namely: Communication of the Y1 with the X1 through the aquifer; Communication of the Y1 with the X1 through the hydrocarbon and the Y1 completely isolated from the X1. The history match indicates reservoir communication through the aquifer as the most plausible scenario. However, due to active reservoir drive mechanism of gas Cap expansion and strong aquifer support in the X1 reservoir and possibly same in the Y1, all three scenarios did not adversely affect the potential recovery in this instance with recoveries between 50-59%. Hence incorporating surveillance information from offset producers is vital in evaluating production potential of zones which may be unproduced but not virgin.
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