Practical Decline Analysis Part 2-Identification And Quantification of Development Opportunities
Authors: Laustsen D. 
Published: March 2010

Introduction

This paper is the second of a two-part discussion on decline analysis. The first part in last month's issue presented a discussion of decline methods and theory as well as a review of limitations of the methods and rules to avoid misinterpretations. This second part presents various interpretation techniques to identify production problems and development opportunities.

Identification and Quantification of Development Opportunities

The most reliable way of quantifying development opportunities is to compare recovery factors derived from decline extrapolations with theoretical recovery factors. When reserves derived by decline analysis fall short of theoretical values, a development opportunity exists. Other means of identifying development opportunities is from a review of the character of the production history. Observations to consider in various types of reservoirs are described below:

Solution Gas Drive Oil Pools

Increase in oil decline rate without significantly increasing GORs

Declining oil rates and increasing GORs are expected in a solution gas drive reservoir as pressure declines. If oil rates fall without increasing GORs the decreased productivity is likely caused by inefficient bottom hole pumping conditions or formation damage rather than pressure loss. A bhp change or wellbore stimulation may therefore be warranted. An approximate quantification of the incremental reserves associated with the remedial action would be the difference between the current decline extrapolation and a previous extrapolation prior to the wellbore problem.

A comparison of a well's decline behaviour with offset wells in the same pool is often useful in identifying wellbore problems.

Shallow or harmonic decline trends

Shallow or harmonic decline trends indicate either low permeability, wellbore damage or large reservoir size. In either case, workovers or additional drilling should be examined to at least shorten the reserve life of the pool.

Water Drive Reservoirs

Declining fluid rates

Declining fluid rates in water drive reservoirs is usually caused by inefficient bottom hole pumping conditions or formation damage. A bhp change or wellbore stimulation may therefore be warranted. An approximate quantification of the incremental reserves associated with the remedial action would be the difference between the current oil rate decline extrapolation and the oil cut decline extrapolation.

Shallow or hannonic decline trends where recovery is governed by water coning

Shallow decline trends after water breakthrough indicate either low permeability or large reservoir size. In either case, additional drilling should be examined to at least shorten the reserve life. Harmonic decline trends typically indicate that interference between producing wells is not occurring. Infill drilling should therefore be examined. Incremental recovery associated with such a program should be rationalized volumetrically considering the shape of the cones.

Waterfloods

Declining fluid rates

As in water drive reservoirs, declining fluid rates may be caused by inefficient bottom hole pumping conditions or formation damage which can be resolved by either bhp changes or wellbore stimulation. An additional cause could be bottom hole pressure declines resulting from insufficient voidage replacement. In such instances, injection rates and voidage calculations should be rechecked and an examination made for potential 'thief zones.