part 8- type curve analysis

8/13/2019 Part 8- Type Curve Analysis

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Type Curve Analysis-WTA 1

MOnur September 2013


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U on com letion of this section, the student should be able to:

1. Identify wellbore storage and middle time regions on type

curve.

2. Identify pressure response for a well with high, zero, or

negative skin.

.

buildup tests using drawdown type curves.

4. Calculate wellbore storage coefficient, permeability, and skin

factor from type curve match.



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If we rearran e terms in the Ei-function solution, we can rewrite

the equation in terms of dimensionless variables.

For the line-source solution, we define the dimensionless

pressure, pD, the dimensionless time, tD, and the dimensionless

radius, r .



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Althou h we did not realize the im ortance at the time, we defined

skin factor to be dimensionless.

To account for wellbore storage, we define an additional

dimensionless variable - the dimensionless wellbore storage

coefficient, CD.

It is important to notice the similarity between Pd and S..



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The Grin arten t e curve describes the ressure res onse under

the following assumptions:

1. Constant rate production from a

2. Vertical wellbore in an

3. Infinite-acting homogeneous reservoir that is

. e w s ng e p ase qu o sma an cons an

compressibility

5. Infinitesimal skin that may be modeled with an apparent

wellbore radius

6. Constant wellbore storage coefficient

The Gringarten type curve was specifically developed for

drawdown tests in oil wells. We will see that we may use it (withsome limitations) to analyze pressure buildup tests in addition to

drawdown tests, and to analyze gas well tests as well as oil well

tests.



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In the Grin arten t e curve, the time is lotted as t /C , and the

dimensionless wellbore storage coefficient and the skin factor are

combined into a parameter CDe2s.

Each value of the parameter CDe2s describes a pressure response

having a different shape.

We call the set of curves with different values of CDe2s a type

curve, and the individual curves for different values of CDe2s are

called stems.

Note the similarity between any adjacent pair of curvescorresponding to different values of CDe2s. This similarity causes

a lot of uncertainty when doing type curve analysis using pressure

curves only.



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If we calculate the lo arithmic derivative of the semilo

approximation to the line source solution, we find that the result is

a constant that depends on flow rate, fluid properties, and rock

properties.

The logarithmic derivative of the dimensionless form of the same

equation is a constant with the value 0.5.

Note that the logarithmic derivative of pressure has the same

units as pressure.



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Just as we constructed a dimensionless t e curve with different

stems corresponding to different values of CDe2s, we can

construct a derivative type curve from the logarithmic derivative of

the pressure type curve.

The shapes of these stems are much more distinctive than those

for the pressure type curve.



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Each of the stems on the Grin arten t e curve exhibits

characteristic behavior.

At early times, the pressure and pressure derivative fall on a unit-

slope line. During this period, the pressure response is

completely determined by the wellbore properties. Permeabilitycannot be estimated if the only data available lies within this

WBS-dominated period.

After WBS effects have ceased, the derivative follows a horizontal

line. This is referred to as the middle time region. Permeability

may be estimated whenever there is 1/2 log cycle or more of data

in the middle time region.

ere s a rans on per o e ween e un s ope ne an e

middle time region. During this transition, both WBS andreservoir properties influence the pressure response. It is

sometimes possible to estimate permeability using data during

WBS and the transition, but the results are not as reliable as

when there is data l in in the middle time re ion.



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Skin factor may be estimated qualitatively by the shape of the

.

High skin factor

The pressure derivative rises to a maximum and then falls

sharply before flattening out for the MTR. The pressure curve rises along a unit slope then flattens out

.

The pressure and pressure derivative are separated by ~2 log

cycles after the end of WBS.

Little or no skin factor

The pressure derivative rises to a maximum, then falls only

sli htl before flattenin out for the MTR.

The pressure and pressure derivative are separated by ~1 log

cycle after the end of WBS.

Negative skin factor

The pressure derivative approaches a horizontal line from

below.


The pressure and pressure derivative both leave the unit slope

line early, but take a long time to reach the MTR.


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part 8- type curve analysis

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