Capturing Complex Dynamic Behaviour in a Material Balance Model
Authors: Mazloom Jalal , Foley William Leslie , Frizzell Dominique F. , Tosdevin Michael Andrew , Sibley Michael 
Published: February 2016

Abstract
Sometimes, a simple and quick material balance method is preferred to using a numerical simulation model. This preference can be justified when preparing the development plan and production optimization for a collection of hydrocarbon reservoirs (lean and rich gas condensate, oil rim and gas cap), some connected to an aquifer, and the reservoirs cannot be modelled separately. This situation can occur when multiple gas reservoirs are needed to be developed in order to provide enough gas for a particular project. A significant drawback of this modelling approach is the simplification introduced when a single tank model (Material balance method) is being used instead of a fine grid simulation model. The material balance method assumes every well contacts all hydrocarbons and that geological heterogeneity is not a factor in recovery. It is necessary to know how reliable are final gas and condensate recovery factors and gas, condensate and water production profiles predicted by a material balance model. In this study, we address all these uncertainties. A sensitivity analysis has been carried out on different aquifer strengths, gas condensate richness, and reservoir heterogeneity which are related to the real and field data set. Introduction of a generic method for selecting the important input data to the material balance model (relative permeabilities and well productivities) in order to have reliable results is the target of this study. The material balance results are compared to a fine grid simulation. It is observed that using the introduced method, the effect of reservoir heterogeneity and aquifer influx on final gas recovery factor can be captured in a material balance model.

Introduction
Predictions of oil and gas reservoirs behavior and hydrocarbon production profiles from them are crucial steps for planning fields development. Although it is believed numerical simulation (3 dimensional models) gives more reliable results than a material balance (zero dimensions) evaluation, a material balance method can be utilized in an acceptable range of uncertainties. Material balance has been used as a reliable tool for calculating hydrocarbon volume initial in place and reservoir drive mechanism and prediction production profile1, 2.
Sometimes material balance can be used for narrowing down uncertainties around in place volume and compartmentalization and presence of faults before simulation3. Recent years have witnessed efforts for improving the material balance method4, 5. Also, some studies have shown that material balance can be utilized for performance prediction of gas condensate reservoirs6. However, it is still important to understand whether field performance, as predicted by a tank model, is reliable enough for making a financial investment decision. In this paper, reservoir performance and production profiles predicted by material balance and 3D simulation model are compared with each other. It is explained how the tank model can capture the effect of aquifer and condensate drop-out on reservoir performance if the model is tuned properly.

Model construction and sensitivity analysis

1. Simulation model construction
A heterogeneous 3D geological model that had been constructed in GOCAD was selected as the reservoir model. General characteristics of the model can be found in table 1.