# decline curve.pdf

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7/29/2019 Decline Curve.pdf

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Introduction

Graphic method graphs papers to sophisticated software.

Means of predicting Oil well production behavior at different points in

time based on the oil well production history.

Is a traditional mean of identifying well production problems and

predicting well performance and life based on measured oil well

production.

Curve fitting of performance with certain standard curves leads to

extrapolation to predict potential future performance of the reservoir

assuming the same trend.

Basic tool for recoverable reserves estimation.

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Decline in Production

Loss of reservoir pressure.

Change in produced fluid ratio.

Increase in secondary fluid volume entering the wellstream.

ange n r ve mec an sms.

Saturation and permeability changes.

Decline might be caused by politics, malfunctions, sabotage, depletion and

other factors.

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Decline in Production

Politics-driven decline usually disappears once the political tensions have

been resolved, and this was clearly seen after the oil crises of the 1970s

when Middle East resumed their oil export to the western countries.

e r v ng orce e n ec ne can e po t ca or soc oeconom c,

representing manmade restrictions on the utilization of a reservoir

In a similar way, economics-driven decline might be seen in fields where

lack of payments, service, modernization and investments has reduced the

production flow.

Also in this case, decline usually disappears once more investments have

been made or the economic situation returned to normal.

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Advantages

Data is easy to obtain.

Easy to plot. Doesnt require any kind of knowledge of reservoir

characteristics.

Yields results on a time basis.

Easy to analyze.

Low cost.

Time efficient.

Easily programmable.

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Limitations

Often dismissed as qualitative exercise.

Poor quality production history may lead to bad interpretation.

A simplistic view of decline curve can lead to wide misuse often caused by

misunderstanding.

Decline curves are relevant for boundary layer flow only.

Change in operating condition usually change the shape of the curve

Interpretation of future production for low permeability , multilayered or

fractured reservoir is very difficult because of highly variable and

uncertain effects.

Changes in operating conditions and any potential changes have to be

taken into account while developing equation.

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Limitations

Some production history is required. Usually 10% reserves should have

been produced before applying decline curve for good results.

Doesnt encompass all reservoir properties in prediction.

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Types of Decline

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Exponential decline curve (Arps Model)

Decline rate, D, is constant, the production is said to follow an exponential

decline.

Differential form

b-exponent term - the time-rate-change of the reciprocal of the decline

rate i.e.

or

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Exponential decline curve (Arps Model)

On integration from 0 to t and Di. t o D

On substituting D

For exponential curve exponent term b=0 which implies that decline rate

remains constant i.e. D= Di. Putting b=0 in above equation, we get

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Exponential decline curve (Arps Model)

Integrating we get

Taking log on both sides, we get

This gives an equation of a straight line with logarithm of production rateon y axis and time on x axis. Intercept on y axis: lnqi and Slope: -D

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Exponential decline curve (Arps Model)

Cumulative production

This too, gives a straight line when plotted as Flow Rate vs. Cumulative

Production. Slope: -D. Intercept on y axis :qi

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Curve Characteristics

Flow rate/Production rate vs Time

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Curve Characteristics

Flow rate/Production rate vs Cumulative Production

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Simple straight-line relationship, makes the easiest to recognize, and the

simplest to use, when compared to the other decline curves (hyperbolic or

harmonic).

Also the most commonly used of all the decline curves.

Curve Characteristics

- . ,

initially, the well is in transient flow or flush production.

Decline curve analysis is used to model stabilized flow, not transient flow;

hence, the early time data will often not follow the trend of the stabilized flow

period.

Theoretically, well producing at constant backpressure, exponential decline

is equivalent to boundary dominated flow, which occurs only after the end

of transient flow.

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Ideally plots of log-rate vs. time, and rate vs. cumulative production should

both result in straight lines from which the decline rate can be determined.

However, if the flow rate is intermittent (may be due to market

restrictions, a well only produces two weeks in any given month and is shut-

Curve Characteristics

, - .

declines (because it does not account for shut-in durations), whereas the rate

vs. cumulative production will give the correct straight line and decline rate.

Emphasis should be given accordingly to the plot of rate vs. cumulative

production graph, in preference to the log-rate vs. time graph.

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Nominal decline rate method

Lets define nominal decline rate as

or

Also, we know

For equal time interval t=1

Therefore

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Nominal decline rate method

Parameters can be redefined as

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Presence of transient condition

Water influx and influence of gas cap expansion mechanism

Change in compressibility and mobility ratios of reservoir fluids

Deviation from Exponential Curve

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