Reservoir Continuity For Eor Application
Authors: Wu R.S. , Patel R.R. , Batycky J.P. , Sykes M.D.H. 
Published: March 2010

Reservoir continuity is an important parameter in assessing the economic and technical viability of enhanced oil recovery (EOR) projects. Earlier uses for continuity focused mainly on infill potential and hence was calculated between all adjacent wells. This paper summarizes a technique designed specifically to address continuity between an injector and its producers. Various pattern types and injector locations can be evaluated. Continuities are pore volume-weighted by pattern and can also reflect porosity and permeability considerations.

Introduction

Enhanced oil recovery reserve estimates and solvent /chase gas requirements are directly affected by reservoir continuity. It is therefore imperative that the engineer be able to estimate reservoir continuity when evaluating the economic and technical viability of an EOR project.

Most studies in the literature(1–4) to date have used continuity to justify infill drilling. Hence, continuity calculations have been performed between all adjacent well pairs. Another constraint inherent in the standard approach for continuity calculations is the use of the same cut-off porosity as is applied to quantify original-oil-in-place (OOIP). The importance of low but finite vertical permeabilities in improving miscible solvent conformance, by reducing gravity override, has been illustrated by Stone(1).

The movements of fluids in a miscible flood are directly affected by the continuities between an injector and each of its surrounding producers. Hence, it is the intent of this paper to highlight the fact that for EOR projects, continuity calculations should reflect the unique needs of EOR including the requirement that injector-producer continuity be volume-weighted to arrive at a continuity value for each pattern.

We have developed a computer program that can access pool geology data bases to make the necessary continuity calculations. The high speed of computation (less than 15 cpu seconds for large pools) allows assessing how continuity is affected by porosity/permeability cut-offs, injector locations, and pattern types during the design stages of an EOR project.

Historical Background

Ghauri et al.(2) and Stiles(3) concluded that the single most important factor affecting the water flood performance of several West Texas carbonate reservoirs was the discontinuous nature of the pay zones which prevented the water flood from contacting a large amount of pore volume in the reservoir. An outgrowth of this observation was the concept of "continuous or net floodable " pore volume including attempts to quantify this factor.

Stiles(3) presented the first relationship for quantifying the continuity of pore volume. Figure 1 illustrates a simplified geological model of porous beds in a reservoir. Stiles defined the continuity between two wells as the ratio of the continuous porous rock volume to the total porous rock volume. Based on this definition, the continuity for the geological section shown in Figure 1 can be calculated as follows:

Equation (1) (Available In Full Paper)

Nomenclature is presented at the end of this paper.

Further studies by Stiles and George(4) concluded that all net pay, even though continuous, would not necessarily be floodable because of irregularities of the bed geometry. In the case of Bed I in Figure 1, based on computer model studies, only 75% of the pore volume that is not present at both wellbores was estimated to be floodable.