A First-Look Method For Analyzing Mature Waterfloods
Authors: Krasey Russell 
Published: March 2010

A first-look analysis procedure/or determining (he theoretical waterflood area sweep has been devised for the Pembina Easyford Cardium Unit No. 1. This method may also apply to other mature waterfloods in the Pembina field. The new method uses as its base a simple permutation of Darcy's Law and uses easy-to-determine reservoir factors such as production, reservoir pressure, and the time to breakthrough. The procedure uses a qualitative method for determining vertical sweep efficiencies based on changes in gamma ray response. The new analysis procedure can be used as a guide in developing a better understanding of the present reservoir behaviour based on its past performance. The information that is provided may be used to determine if more elaborate analysis methods are warranted.

Introduction

The simple first-look analysis procedure for determining areal sweep of mature waterfloods was developed during ongoing analysis of Pembina Easyford Cardium Unit No. 1. The method utilizes field variables and requires mature water floods with a long production history, to smooth out minor changes in production practice over short periods of time. The method does not use or require any difficult-to-determine reservoir factors such as permeability.

The use of the model is limited to Pembina Cardium mature waterfloods. The well density and pattern do not modify the method of analysis. The first portion of the report outlines and gives explanation for the factors which have to be determined, either qualitatively or quantitatively. The second portion of the report outlines the calculated waterflood behaviour for the Pembina Easyford Cardium Unit No.1.

Analysis Procedure

Breakthrough Analysis

The following method for analyzing existing mature Pembina Cardium water floods is proposed in an attempt to describe waterflood behaviour qualitatively. The information gained is relatively inexpensive to produce and can be used as a "first-look" screening method.

Waterflood Recovery Equation

The general equation for calculating expected recovery from a water flood is as follows.

(1) Recoverable Oil  =  OOIP × E V × E A × E D

where:

EV = Vertical Sweep Efficiency

EA = Areal Sweep Efficiency

ED = Displacement Sweep Efficiency

OOIP = Original Oil in Place

Since EA is the factor that this analysis is trying to determine, the other factors, ED. EV, must be determined first.

Displacement Sweep Efficiency

Displacement of the moveable oil in a reservoir by injected water can be simulated by the use of fractional now curve. The ED is the ratio of the moveable oil displaced at breakthrough to the total volume of moveable oil in the reservoir.

Equation (2) Available In Full Paper.

where Sor is the residual oil saturation.

The value of ED ranges from 0 to 1. A value of 1 would indicate true piston-like displacement of the moveable oil.

Vertical Sweep Efficiency

The calculation of EV is qualitative based on changes in the shale content. Generally, in the Cardium formation, shale concentration increases with reservoir depth. Because shale content in the reservoir and permeability are related, changes in shale content will be used to determine vertical sweep efficiency for a non-homogeneous pay zone.