Abstract

Decline analysis is the most used technique for forecasting reserves. Although decline analysis for gas fields has been shown to have a strong theoretical background, decline analysis for multiphase situations is less clear. This paper suggests when it is appropriate to use decline analysis for waterfloods and what is happening physically when decline trends develop. It also shows field situations which follow clear trends and allow us to use decline techniques to diagnose field behaviour. This paper presents field cases for waterfloods and suggests a theoretical analysis that ties in well with the observations from these field examples.

Field cases as well as analytical analysis/simulation(1) generally support harmonic or hyperbolic decline for late stage waterflood behavior. In other words, reservoir factors generally lead to hyperbolic or harmonic decline late in the waterflood life. However, that is not to say that exponential (b=0) or "super" exponential decline (b<0) never occur. When they do occur, usually non-reservoir factors are involved.

This paper also shows how incremental oil recovery can be calculated using decline methods accounting for changes in fluid and injection rates.

The waterflood decline correlation period should have the following criteria:

  • the watercut should be greater than 50%

  • the voidage replacement ratio should be close to one

  • well count should be relatively constant

  • injection and fluid production rates should be relatively constant

  • the reservoir pressure should be relatively constant

  • producing well pressures should be constant

  • the GOR should be relatively constant

  • the volume of water injected should be greater than 25% of the hydrocarbon pore volume.

In order to properly access future waterflood performance, we need to estimate what is controlling the oil decline rate. After substantial water breakthrough has occurred, the oil rate profile is usually controlled by:

  • relative permeability

  • changing volumetric sweep

  • water handling constraints

  • fluid rates handling constraints

  • permeability/injectivity in the near wellbore regions

  • well positions.

Most successful waterfloods are hopefully in reasonably good continuity reservoirs with moderate to high permeabilities, therefore well interference is very likely. However, because of well interference in such moderate to high permeability reservoirs, individual well decline analysis is to be used with caution. We would therefore recommend using an aggregate analysis of a group of wells as a more realistic scenario rather than a sum of individual wells.

Introduction

Diagnosis of the character of producing wells is the key skill that petroleum engineers in an operating environment require. This characterization and corresponding reservoir forecast/diagnosis task is most often performed using the technique of decline analysis. Because of its ease of use, decline analysis has a huge appeal as a forecast method and is accordingly the most widely used technique.

The pioneering work on production decline analysis was performed by Arps(2) in 1944. Equations for pressure decline were formulated empirically based on statistical analysis of production data obtained from non-fractured reservoirs. Production decline was considered to be proportional to pressure decline through assumptions of constant wellbore pressure and constant productivity index.

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