Flowing material balance (FMB) analysis is a practical method for determining original hydrocarbon volumes in-place. It is attractive because it enables performing material balance calculations without having to shut-in wells to obtain estimates of reservoir pressure. However, with some exceptions, its application is limited to single-phase oil and/or gas reservoirs over limited pressure ranges during depletion. In unconventional reservoirs, reservoir and/or production complexities may further restrict FMB usage. Among these complexities are significant production/injection of water, production resulting in higher Gas-Oil-Ratios and pressure drawdowns, geomechanical effects, and multi-well production effects. As a result, application of the conventional FMB to unconventional reservoirs may lead to significant errors in hydrocarbons-in-place estimation.

This paper first discusses the application of conventional FMB to the analysis of single or multi-phase flow in single or multi-well scenarios, and then provides a new, comprehensive version of the FMB to address the above-mentioned complications. For the new FMB, pseudo-pressure is used to account for two-phase oil and gas flow. In addition, by using a general material balance equation, water production/injection and multi-well effects are included in the analysis. The new FMB analysis approach is validated by comparing results against numerical simulation of multi-fractured horizontal wells (MFHWs). These comparisons demonstrate that, not only gas production, but also water production/injection, can have a significant effect on the calculated original in-place hydrocarbon volumes. The new FMB analysis approach provided herein successfully accounts for all flowing phases in the reservoir, and is demonstrated to be applicable for multi-well scenarios.

The methodology presented in this paper maintains the simplicity of FMB, yet accounts for multi-phase flow and multi-well complications. The developed FMB and the presented approach can be used by reservoir engineers to reasonably determine the original volumes of hydrocarbons in-place in both conventional and unconventional reservoirs.

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