Practical Decline Analysis Part 1-Uses And Misuses
Authors: Laustsen D. 
Published: March 2010

Introduction

Decline analysis is one of the most widely used methods of reserve interpretation in the industry. Properly used, it is also one of the most reliable. However, in my experience reviewing reserve reports, it is commonly misused. This discussion paper has been prepared in two parts. The first part presented herein provides an overview of methods and theory, then identifies common misinterpretations and rules to avoid these misinterpretations. The second part goes on to discuss various interpretation techniques to identify production problems and development opportunities.

Methods

There are two main types of decline interpretation methods, namely "type curve matching" and "curve fitting." Both methods can be used in depletion drive pools which are characterized by transient and pseudo steady-state flow regimes. Figure 1 illustrates the transient and pseudo steady-state flow regimes on dimensionless scales. The transient period occurs prior to the drainage radius reaching boundary conditions with pseudo steadystate flow occurring thereafter.

Only curve fitting is used in pressure supported pools such as waterfloods, miscible floods, and water drives. Pressure support decline behaviour is more complex and is generally characterized by multiple flow regimes. An example of this is a waterflooded pool which initially produced under transient and pseudo steadystate flow, then steady state flow after commencement of water injection and finally post water breakthrough flow behaviour.

Type Curve Matching

The type curve matching method was developed by M.J. Fetkovich (reference 1984 SPE paper 13169) and consists of converting and plotting production data in dimensionless variables then overlaying curves to obtain a type curve match. The match of the transient portion of the curve is used to characterize permeability and skin factor. The inflection point in the type curve identifies the start of PSS flow regime which is used to quantify drainage area. Finally, the matching of the Arps depletion stem in the pseudo steady-state flow regime is used to quantify recoverable reserves.

The advent of commercial PC based decline packages has made this method much more popular in recent years.

A key observation in this technique is that the transient decline behaviour does not relate to the PSS or depletion decline behaviour. This is an important consideration when dealing with low permeability reservoirs which have long transient periods (i.e., 2 years + for 0.1md reservoir).

Curve Fitting

The curve fitting method refers to numerically fitting a curve through historical production data with the premise that future production decline will be represented by this numerical relationship. The equation most commonly used was developed by Arps in 1944 to represent a constant flowing pressure solution to a well of fixed drainage radius, (equation available in full paper)

The best fit can be either exponential when b approaches 0, hyperbolic when b&gt; and b &lt;, or harmonic when b=1.

The best fit can be computer calculated or visual. Visual best fit exponential decline is based on a straight line arithmetic rate vs. cumulative production plot. Visual best fit harmonic decline is based on -a straight line log rate vs. cumulative production plot.