Estimation of Volumetric Sweep Efficiency of a Miscible Flood
Authors: Baker R.O. , McClernon L.L. 
Published: March 2010

Forecasting volumetric sweep efficiency and oil recovery of a miscible flood is difficult, yet reliable production forecasts are required for economic analysis. Conventional decline curve analysis cannot be applied until well after the miscible flood is underway, and may be inapplicable if oil production rates are constant or increasing. Numerical simulation can usually give reliable estimates of oil recovery but only after a time consuming history match. In addition, computer resources and manpower constraints can often limit the areal extent of a simulation model, and therefore limit the model's applicability for full field forecasts.

In this study, simple methods based on waterflood surveillance techniques were developed for estimating miscible flood volumetric sweep efficiency. These methods can be used to predict ultimate recovery and volumetric sweep for horizontal miscible floods in either strongly gravity-dominated systems, heterogeneous systems or viscous fingering dominated systems.

Miscible flood simulation studies and results from actual field projects show that a plot of gas-oil ratio (GOR) correlates well with cumulative incremental oil production. Such a plot can be used to estimate miscible flood incremental recovery. If the log GOR vs. cumulative oil is a straight line, this implies that the late oil rate decline will be either harmonic or hyperbolic depending upon total fluid production rate. These surveillance techniques can identify areas in which volumetric sweep and oil recovery is performing below expectations. These techniques allow an operator to continuously monitor performance and to make timely operational changes to maximize oil recovery.

Introduction

It is critical that a development team has an excellent knowledge of reserves remaining in a reservoir to plan optimum depletion strategies. Unfortunately the reservoir engineer has a difficult decision of predicting reserves from decline analysis or simulation. Determining incremental oil in a tertiary miscible flood by decline analysis is especially difficult because:Often daily fluctuations in oil rate may be greater than the incremental oil rates attributed to miscible floodingInfill drilling and well workovers are occurring simultaneously as miscible floods start up so it is difficult to attribute oil production to tertiary miscible responseInitial overall oil rates are constant or increasing These factors mean that conventional decline analysis cannot be applied until well after the miscible flood is underway.

For a more "accurate" answer, the reservoir engineer can use numerical simulation but this approach requires detailed reservoir knowledge, usually requires fine detailed grids and is expensive and time consuming. Therefore simulation is limited to only portions of the reservoir.

Nevertheless, large capital expenditure decisions need to be made on an ongoing basis. It is therefore critical to determine what the incremental recovery and volumetric sweep are in a timely fashion. Many surveillance techniques that have been developed for waterfloods and gas floods are applicable to miscible floods, with slight modifications. In this paper techniques are derived for gas flood and modified for miscible flood. The techniques are validated using numerical simulation where multiple simulation models are made increasingly more complex and realistic. Finally, field cases are also used to validate the theories.