Analisa Pressure Draw-Down Dan Pressure Build Up Test: RABU, 27 JUNI 2007 12.30 - 16.30

ANALISA PRESSURE DRAW-DOWN
DAN PRESSURE BUILD UP TEST
RABU, 27 JUNI 2007

12.30 – 16.30
OBJECTIVES
To understand basic principle of commonly performed

pressure test; Build up and drawdown test with simple
example of PBU & PDD analysis; The assumptions
and limitations of the tests and demonstrate how to
make interpretation from those test using package
software in obtaining the Information from well testing.
INFORMATION GATHERED FROM WELL TEST
Reservoir characteristics that can be calculated from a well test include, but are not limited
to, the following:
1. Reservoir Pressure buildup curves can be extrapolated to obtain static reservoir

pressure.
2. Permeability is a measure of the ability of the reservoir rock to transmit fluid flow.
The permeability measured by a well test is the effective permeability of the
reservoir rock for the produced fluid.
3. Transmissivity is a measure of the ability of the reservoir to transmit the fluid
contained within it. It is a function of both reservoir rock and fluid properties.
4. Skin Factor is a quantitative measure of the degree to which the permeability in the
immediate vicinity of the well bore has been altered as a result of the drilling,
completion, and production process.
5. Damage Ratio is the ratio of the theoretical production rate to the actual production
rate measured during the test. It is an indication of the degree to which the well's
productivity can be improved by removing the skin damage induced during drilling
and completion of the well.
6. Productivity During a test, the productivity of the well is measured at a flowing
pressure that may or may not represent a reasonable producing pressure. The test
results can be used to predict the productivity of the well at any desired flowing
pressure.
INFORMATION GATHERED FROM WELL TEST. CONT..
7. Radius of Investigation is the approximate radial distance from the well

bore that is investigated by the test; the test analysis results represent the
average properties of the reservoir within this radius.
8. Reservoir Anomalies within the radius of investigation include barriers and
fluid contacts. Permeability changes or layered reservoirs are often
reflected in the pressure behavior observed during a test. This information,
when used in conjunction with other data, can often help in defining the
exact type of anomaly.
9. Reservoir Depletion If an observable pressure depletion occurs during a
test, then a small reservoir has been penetrated and most likely would not
contain commercial quantities of hydrocarbons.
PRINCIPLE OF DRAWDOWN AND BUILDUP TRANSIENTS
All well tests undergo a transient or infinite-acting radial flow period at some
point in the test. As a result, an analysis technique based on this flow regime
would be universally applicable as long as this flow regime could be
recognized on the data. The following sections demonstrate how to
recognize and analyze this flow regime.
When a well is opened to flow for a

drawdown test or shut in for a buildup, a
pressure gradient, or transient, is
established between the well bore and
the reservoir. This pressure transient
propagates into the reservoir at a speed
that is directly dependent on the
reservoir's rock and fluid properties.
PRESSURE TRANSIENT PROPAGATION IN A RESERVOIR
FLOW REGIME CATEGORIES
PRESSURE DRAWDOWN TEST
A pressure drawdown test is simply a series of bottom-hole pressure

measurements made during a period of flow at constant production
rate.
Usually the well is closed prior to the flow test for a period of time sufficient to
allow the pressure to stabilize throughout the formation, i.e., to reach static
pressure.
As discussed by Odeh and Nabor,1 transient flow condition prevails to a value of real
time approximately equal to
Semi-steady-state conditions are established at a time value of

FLOW HISTORY CLASSIFICATIONS
Three periods for analysis:
• Transient or early flow period

is usually used to analyze
flow characteristics;
• Late transient period is more
completed; and
• Semi-steady-state flow period
is used in reservoir limit
tests.
A. TRANSIENT ANALYSIS - INFINITE-ACTING RESERVOIRS
An ideal constant-rate drawdown test in an infinite-acting reservoir is modeled by the

logarithmic approximation to the Ei-function solution:
familiar form of the pressure drawdown equation

A plot of flowing bottom-hole pressure data versus the logarithm of flowing time
should be a straight line with slope m and intercept p1hr . Semilog straight line
does appear after wellbore damage and storage effects have diminished.
The slope of the semilog straight line may be given by
The intercept at log t = 0, which occurs at t = 1,

is determined from
Semilog pressure drawdown data plot.

STRAIGHT LINE BEGINNING
The beginning time of the semilog straight line

may be estimated from log-log plot of [log (pi-
pwf)] versus log t.
When the slope of the plot is one cycle in dp
per cycle in t, wellbore storage dominates and
test data give no information about the
formation.
The wellbore storage coefficient may be
estimated from the unit-slope straight line
using the following equation:
or
where dt and dp are the values read from a

point on the log-log unit slope straight line.
Vu is the wellbore volume per unit length in barrels per foot. Duration of wellbore
unloading can be estimated from
The apparent wellbore radius rwa may be estimated by
Radius of investigation at the beginning and end of the apparent middle time line may
be checked by
B. LATE TRANSIENT ANALYSIS – BOUNDED (DEVELOPED)
RESERVOIRS
Pressure behavior at constant rate in a bounded reservoir can be represented by
a plot of log (pwf — p) versus t should be linear with slope magnitude:
and intercept
plot of log (pwf - p) versus t will be linear provided the value of p is known. Usually it is
not. This means that a trial-and-error plot must be made using assumed pe values. That
value which yields the best straight line on the log (pwf - p) versus t plot is chosen as
the correct p value.
After determining the correct p value, kh can be estimated from the intercept value b
by
The pore volume (drainage volume) of the well Vp determined from the slope of plot, in
barrels, given by
The equivalent drainage radius re is given by
The skin factor can be found from
Where P is the average reservoir pressure. The pressure drop across skin zone is given
by
EXAMPLE: Analyzing Late Transient Drawdown Test
The pressure drawdown data were obtained from a 50-hours drawdown test in an oil well.
Before this test, the well has been shut-in and the pressure is allowed to build up to a
stabilized value of 1895psi. Other data pertinent to the test are as follows: qo =
750stb/day; h = 15 ft; Viso = 0.9 cP; Poro = 0.12; rw = 0.29 ft; ct = 17.5 x 10^psi-1; Bo =
1.245 rb/stb.
Find the average reservoir pressure, intercept, slope, permeability k, pore volume, skin
factor and pressure drop across skin.
Prepare late transient analysis plot
1. Choose various values of average pressure,pr = 1300, 1400, 1460, and 1490 psi.
2. Plot Log (pws-pr) versus time in hours on semilog paper.
3. If the curve is concave downward, estimated value of PR is too low, conversely, if the
curve is concave upward, the estimated value of PR is too large. Trial-and-error
procedure until a straight line is obtained.
4. Find the intercept and slope of the straight line.
Semilog Late-transient analysis plot, extended pressure drawdown test.
1. Find the intercept and slope values as
2. Estimate the formation permeability k
3. Estimate the reservoir (drainage) volume Vp.

4. Estimate the skin
C. SEMI STEADY-STATE ANALYSIS - RESERVOIR LIMIT TEST
If a pressure drawdown test is run for a long period of time, the pressure
follows semi-steady-state behavior, which starts when the curve for that
shape presented by Matthews et al.
TECHNIQUE FOR ANALYZE RESERVOIR LIMIT TEST
1. Plot both pwf versus log t and pwf versus t.

2. From semilog plot determine m and p1hr (extrapolate if necessary). If
3. desired, these quantities may be used to calculate kh/u and skin factor s
using standard techniques.
4. From the linear plot find slope, m*, pint and tpss.
Estimation of Reservoir Limit
1. Calculate the drainage volume, Vp

2. Calculate CA using Table B-2 or Figure B-8.
3. Estimate the drainage shape and well location.
or
EXAMPLE : Analyzing Single-Rate, Single-Phase Pressure Drawdown Test
A constant-rate drawdown test was run in an oil well with the following characteristics: qo =
250stb/day, Viso = 0.8 cP, Bo= 1.136rb/stb, co = 17.0 x 10-6 psi-1, poro = 0.039, h = 69 ft, pi =
4412 psi, and rw = 0.198 ft. Last flowing time 460 hr.
From the test data given, estimate the formation permeability, skin factor, pressure drop across
skin, flow efficiency and reservoir pore volume.
SOLUTION
Plot (pi — pwf) versus log time (semilog data plot)

Plot (pi — pwf) versus log time (log-log data plot)
Plot pwf versus time (Cartesian data plot)
1. From log-log data plot, determine
MTRl —»time at the beginning of transient period = 12 hours

MTR2 —> time at the end of transient period = 150 hours
Show the value of MTR on semilog plot
2. Estimate the formation permeability k
3. Check the radius of investigation at the beginning and end of the apparent
middle time line to ensure that we are sampling a representative portion of the
formation.
4. Estimate the skin & Flow efficiency
Single-rate drawdown test - log-log data plot.
Single-rate drawdown test – Semi-log data plot.
Single-rate drawdown test – Cartesian data plot.
5. Estimate the reservoir (drainage) volume Vp, find slope of the curve from linear plot
6. Estimate Reservoir Shape
From Table : Ca 10.9 ~
Well location in a square drainage area Well location in a 4x1 rectangular area.
COMPUTATIONAL MODEL
A computational model that describes the relationship between the pressure, flow rate
and reservoir rock and fluids properties In its simplest form is the diffusivity equation for
a well in the center of a circular, homogeneous, horizontal reservoir, uniform thickness
and a 1 phase fluid that obeys Darcy's law.
While the equation cannot be solved directly, indirect techniques provide a satisfactory
estimate using numerical computation.
The actual reservoir model is constructed from the basic

equation but take into account the geometry of the reservoir
(radial, linear, elliptical or spherical), number and types of
fluids and the flow regime.
The flow geometry for that area affected by the test can be the most common model
used to represent the pressure behavior of the reservoir is radial flow, where all flow
occurs radially toward the well between impermeable upper and lower boundaries at a
constant surface flow rate. The interpretation of test data will yield average reservoir
properties even when reservoir heterogeneities exist.
WELL TEST DATA INTERPRETATION
There are three major steps to a unified approach to well test data interpretation:
1. Identification of the proper model for the classification of the

reservoir (finite, infinite, homogeneous, dual porosity, dual
permeability, skin, fractures).
2. Specific analysis and calculations to estimate well and reservoir
characteristics.
3. Verification of results to ensure that the process resulted in the best
answer.
Well test interpretation is based on patterns of pressure change and the

derivative of pressure change to identify the type of reservoir behavior.
AN EFFECTIVE WAY TO INTERPRET PRESSURE DATA
1. Use a Diagnostic graph.

A diagnostic graph is a plot of the pressure change and the derivative of the
pressure change versus time on log-log paper. It can be used to identify each
flow period. Behavior specific plots can be useful to identify heterogeneous
behavior, effects of wellbore storage and high and low conductivity fractures
2. Calculate basic reservoir parameters, such as the kh, skin and well bore
storage capacity. This can be done conventionally, through the use of
specific plots, or with type curves.
3. Compare the result from well to well and from time to time with the
same well for matching or method is to prepare and analyze a
dimensionless semi-log plot .
4. Used calculated parameters as input simulator and run to see if it
matches the input data.

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ANALISA PRESSURE DRAW-DOWN

DAN PRESSURE BUILD UP TEST

RABU, 27 JUNI 2007

To understand basic principle of commonly performed

1. Reservoir Pressure buildup curves can be extrapolated to obtain static reservoir

7. Radius of Investigation is the approximate radial distance from the well

When a well is opened to flow for a

A pressure drawdown test is simply a series of bottom-hole pressure

Semi-steady-state conditions are established at a time value of

Three periods for analysis:

• Transient or early flow period

An ideal constant-rate drawdown test in an infinite-acting reservoir is modeled by the

familiar form of the pressure drawdown equation

The slope of the semilog straight line may be given by

The intercept at log t = 0, which occurs at t = 1,

Semilog pressure drawdown data plot.

The beginning time of the semilog straight line

where dt and dp are the values read from a

The apparent wellbore radius rwa may be estimated by

Pressure behavior at constant rate in a bounded reservoir can be represented by

a plot of log (pwf — p) versus t should be linear with slope magnitude:

The equivalent drainage radius re is given by

The skin factor can be found from

Prepare late transient analysis plot

2. Estimate the formation permeability k

3. Estimate the reservoir (drainage) volume Vp.

1. Plot both pwf versus log t and pwf versus t.

Estimation of Reservoir Limit

1. Calculate the drainage volume, Vp

Plot (pi — pwf) versus log time (semilog data plot)

1. From log-log data plot, determine

MTRl —»time at the beginning of transient period = 12 hours

2. Estimate the formation permeability k

From Table : Ca 10.9 ~

The actual reservoir model is constructed from the basic

1. Identification of the proper model for the classification of the

Well test interpretation is based on patterns of pressure change and the

1. Use a Diagnostic graph.

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