An Analytical Bottomwaterdrive Aquifer Model for Material-Balance Analysis

Summary

In this study, we present a new analytical model to predict water influx from finite bottomwaterdrive aquifers (BWDAs). The new model can be used for aquifer representation in reservoir simulators and material-balance analysis. The new model is computationally fast. The model has been verified against the results from the Coats (1962) and Allard and Chen (1988) models. Using the new model, we compare the cumulative water influx from bottomwaterdrive (BWD) and edgewaterdrive (EWD) aquifers.

In the second part, we couple the new aquifer model with the material-balance equation (MBE) and demonstrate its use in reserves estimate. Production data from a BWD oil reservoir simulated by Allard and Chen (1988) is analyzed. We conducted a McEwen analysis (1962) to predict the reserves.

Introduction

Hydrocarbon reservoirs may be classified on the basis of their drive mechanisms. In the reservoirs adjoined by water aquifers, waterdrive may be the primary production mechanism. In these reservoirs, production of hydrocarbons causes a pressure drop in the hydrocarbon/water interface. Because of this pressure drop, the aquifer reacts by encroaching into the reservoir and filling its pore spaces. The invasion of reservoir rock by aquifer water may have a significant impact on reservoir performance. Therefore, water influx into hydrocarbon reservoirs must be predicted accurately as a function of time, pressure history at the reservoir/aquifer interface, reservoir/aquifer size ratio, and aquifer characteristics.

To forecast the performance in a waterdrive reservoir, an aquifer model simulating the fluid flow in the aquifer and flow from the aquifer into the reservoir is needed. Aquifer models may be classified on the basis of flow regimes and flow geometries. In terms of flow regimes, aquifers may be grouped as pot model, steady-state models, pseudosteady-state models, and unsteady-state models. On the basis of flow geometry, the aquifer/reservoir systems may be categorized as zero-dimensional, linear, and radial. The aquifer models considering radial-flow geometry may additionally be sorted as EWD or BWD.

Aquifer models are commonly used in two different reservoir-engineering applications, reserves estimation and reservoir simulation. The main functionality of aquifer models is to predict cumulative water influx in material-balance computations. Along these lines, the Allard and Chen (1988) model, among others, has been incorporated into commercial software. In the large-scale simulation studies, the number of gridblocks may be reduced to have reasonable computation time and expense. Typically, in such large-scale simulation cases, the aquifer is represented implicitly by use of an analytical model. In commercial reservoir simulators, several analytical models are available for implicit representation of aquifers.