Decline Curve Analysis

 Introduction

Globally, the oil and gas production profiles differ considerably. When a field starts

production, it builds up to a plateau state, and every operator will want to remain in

this stage for a very long period of time if possible. But in reality, it is practically not

possible, because, at a point in the life of the field, the production rate will eventually

decline to a point at which it no longer produces profitable amounts of hydrocarbon

as shown in Fig. 7.1. In some fields, the production build-up rate starts in the first few

years, most fields’ profiles have flat top and the length of the flat top depends on

reservoir productivity.

Some fields have long producing lives depending upon the development plan of

the field and reservoir characteristics such as the reservoir, drive mechanism. Wells

in water-drive and gas-cap drive reservoirs often produce at a near constant rate until

the encroaching water or expanding gas cap reaches the well, thereby causing a

sudden decline in oil production. Wells in gas solution drive and oil expansion drive

reservoirs have exponential or hyperbolic declines: rapid declines at first, then

leveling off.

Therefore, decline curve analysis can be defined as a graphical procedure used for

analyzing the rates of declining production and also a means of predicting future oil

well or gas well production based on past production history. Production decline

curve analysis is a traditional means of identifying well production problems and

predicting well performance and life based on measured oil or gas well production.

Today, several computer software have been built to perform this task and prior to

the availability of computers, decline curve analysis was performed by hand on

semi-log plot paper. Several authors (Rodriguez & Cinco-Ley (1993), Mikael

(2009), Duong (1989) have developed new models or approach for production

decline analysis. Agarwal et al. (1998) combined type curve and decline curve

analysis concepts to analyse production data. Doublet et al. (1994), applied the

material balance time for a field using decline curve analysis.

Furthermore, as stated by Thompson and Wright (1985), decline curve is one of

the oldest methods of predicting oil reserves with the following advantages:

• They use data which is easy to obtain

• They are easy to plot

• They yield results on a time basis, and

• They are easy to analyze.

7.2 Application of Decline Curves

• Production decline curve illustrates the amount of oil and gas produced per unit

of time.

• If the factors affecting the rate of production remaining constant, the curve will be

fairly regular, and, if projected, can give the future production of the well with an

assumption that the factors that controlled production in the past will continue to

do so in future.

• The above knowledge is used to ascertain the value of a property and proper

depletion and depreciation charges may be made on the books of the operating

company.

• The analysis of the production decline curve is employed to determine the value

in oil and gas wells economics.

• Identify well production problems

• Decline curves are used to forecast oil and gas production for the reservoir and on

per well basis and field life span.

• Decline curves are also used to predict oil and gas reserves; this can be used as a

control on the volumetric reserves calculated from log analysis results and

geological contouring of field boundaries.

• It is often used to estimate the recovery factor by comparing ultimate recovery

with original oil in place or gas in place calculations

Causes of Production Decline

• Changes in bottom hole pressure (BHP), gas-oil ratio (GOR), water-oil ratio

(WOR), Condition in drilling area

• Changes in Productivity Index (PI)

• Changes in efficiency of vertical & horizontal flow mechanism or changes in

equipment for lifting fluid.

• Loss of wells

7.4 Reservoir Factors that Affect the Decline Rate

• Pressure depletion

• Number of producing wells

• Reservoir drive mechanism

• Reservoir characteristics

• Saturation changes and

• Relative permeability.

7.5 Operating Conditions that Influence the Decline Rate

• Separator pressure

• Tubing size

• Choke setting

• Workovers

• Compression

• Operating hours, and

• Artificial lift.

As long as the above conditions do not change, the trend in decline can be analyzed

and extrapolated to forecast future well performance. If these conditions are altered,

for example; through a well workover, the decline rate determined during

pre-workover will not be applicable to the post-workover period.

7.6 Types of Decline Curves

Arps (1945) proposed that the “curvature” in the production-rate-versus-time curve

can be expressed mathematically by a member of the hyperbolic family of equations.

Arps recognized the following three types of rate-decline behavior:

Exponential decline

• Harmonic decline

• Hyperbolic decline

Arps introduces equations for each type and used the concept of loss-ratio and its

derivative to derive the equations. The three declines have b values ranging from 0 to

1. Where b ¼ 0 represents the exponential decline, 0 < b < 1 represents the

hyperbolic decline, and b ¼ 1 represents the harmonic decline (Fig. 7.2).

The plots of production data such as log(q) versus t; q versus Np; log(q) versus

log(t); Np versus log(q) are used to identify a representative decline model.

7.6.1 Identification of Exponential Decline

If the plot of log(q) versus t OR q versus Np shows a straight line (see figures below)

and in accordance with the respective equations, the decline data follow an expo￾nential decline model.

Mathematical Expressions for the Various Types of Decline

Curves

The three models are related through the following relative decline rate equation

(Arps 1945):

Relationship Between Nominal and Effective Decline Rate

The nominal decline rate (Di) is defined as the negative slope of the curvature

representing the natural logarithm of the production rate versus time

Cumulative Production for Exponential Decline

The Integration of the production rate over time gives an expression for the cumu￾lative oil production as:

Steps for Exponential Decline Curve Analysis

The following steps are taken for exponential decline analysis, for predicting future

flow rates and recoverable reserves (Tarek, 2010):

• Plot flow rate vs. time on a semi-log plot (y-axis is logarithmic) and flow

rate vs. cumulative production on a cartesian (arithmetic coordinate) scale.

• Allowing for the fact that the early time data may not be linear, fit a straight line

through the linear portion of the data, and determine the decline rate “D” from the

slope (b/2.303) of the semi-log plot, or directly from the slope (D) of the rate￾cumulative production plot.

• Extrapolate to q ¼ qt to obtain the recoverable hydrocarbons.

• Extrapolate to any specified time or abandonment rate to obtain a rate forecast and

the cumulative recoverable hydrocarbons to that point in time

7.7.2 Harmonic Decline Rate

Cumulative Production for Harmonic Decline

The expression for the cumulative production for a harmonic decline is obtained by

integration of the production rate. This is given by:

Hyperbolic Decline

The hyperbolic decline model is inferred when 0 < b < 1

Hence the integration of