457503602-5-Gas-Well-Testing-and-Analysis-pptx.pptx

EE053-3.5-3
Gas Engineering
Gas Well Testing and Analysis

Module Code and Module Title Title of Slides
Outline
• Introduction
• Flow-after-flow test
• Isochronal test
• Modified isochronal test
• Time to stabilization
• Radius of investigation
• Classifications, limitations and use of Deliverability test
Slide <2> of 13

Introduction
• Deliverability testing refers to the testing of a gas well to measure
its production capabilities under specific conditions of reservoir
and bottomhole-flowing pressure (BHP).
• A common productivity indicator obtained from these tests is the
absolute open flow (AOF) potential. The AOF potential of a well
is defined as the rate at which the well will produce against a
zero backpressure. It cannot be measured directly but may be
obtained from deliverability tests.
• Most common types of deliverability tests:
– Flow-after-Flow test
– Isochronal test
– Modified isochronal test
Slide <2> of 13
VIDEO

Deliverability equations based on theoretical methods are :
1. Pressure solution technique
2. Pressure-squared technique
3. Pseudo pressure technique
4
Theoretical Method

Empirical Method
• Where C is the flow coefficient and n is the deliverability exponent.
The equation reveals that a plot of Δ(P 2) = P 2
R — P2
wf versus qsc on log-
log scales should result in a straight line having a slope of 1/n.
• Once a value of n has been determined from the plot, C can be
calculated by using data from one of the tests that falls on the line.
• Then AOF is calculated. AOF is used for these tests. AOF is maximum
rate at which the well could flow against the theoretical back pressure at
the sand face.
Slide <2> of 13

Module Code and Module Title Title of Slides Slide <2> of 13

Flow-after-Flow Test
• Sometimes called gas backpressure or four-point tests, are
conducted by producing the well at a series of different
stabilized flow rates and measuring the stabilized BHFP at the
sandface.
• Each different flow rate is established in succession either with or
without a very short intermediate shut-in period. This type of
test was developed to overcome the limitation of long testing
times required to reach stabilization at each rate.
• The stabilized rate and pressure are recorded; the rate is then
changed and the well flows until the pressure stabilizes again at the
new rate. The process is repeated for a total of three, four, or five
rates.
Slide <2> of 13

A plot of typical flow-after-flow data
9

Stabilized Flow Test Analysis:
A flow-after-flow test was performed on a gas well located in a
low-pressure reservoir. Using the following test data, determine
the values of n and C for the deliverability equation, AOF, and
flow rate for = 175 psia.
10
wf
P
Example 1
Test qsc (mscfd) Pwf (psia)
- 0 201
1 2730 196
2 3970 195
3 4440 193
4 5550 190

Solution
201 psia is PR (Reservoir pressure or Initial pressure)
Solve for ΔP2:
Test 1, Pwf =196 , then PR
2 - Pwf
2 = (201)2- (196)2 = 1985 = 1.985*103 psia
2 - Pwf
2 = (201)2- (195)2 = 2376 = 2.376*103 psia
2 - Pwf
2 = (201)2- (193)2 = 3152 = 3.152*103 psia
2 - Pwf
2 = (201)2- (190)2 = 4301 = 4.301*103 psia
Solve for n;
Slide <2> of 13

From test 4, calculate C:
Therefore, the deliverability equation is:
12

13

Exercise 1
flow rate for = 465 psia.
14
wf
P
Test qsc (MSCFD) Pwf (psia)
0 0 650
1 5230 590
2 6750 575
3 7380 560
4 8650 510

Exercise 2
flow rate for pwf = 335 psia.
Slide <2> of 13
0 0 520
1 3640 405
2 4270 420
3 5150 395
4 6350 375

Exercise 3
flow rate for Pwf = 850 psia.
Flow-after-flow Test data are shown in the following Table.
0 1300
1 12570 1220
2 13700 1178
3 14660 1120
4 15470 1050
16

Isochronal Test
• Isochronal test is a series of a single point tests developed to
estimate stabilized deliverability characteristics without actually
flowing the well for the time required to achieve stabilized conditions.
• This test consists of producing the well at a fixed flow rates with
flowing periods of equal duration.
• It is conducted by alternately producing the well, then shutting in the
well and allowing it to build to the average reservoir pressure. The
well is then flowed at a second rate for the same length of time.
• The isochronal test is based on the principle that the radius of
investigation established during each flow period is not a function of
the flow rate but depends only on the length of time for which the
well is flowed.
Slide <2> of 13

Example 2
• An isochronal test was conducted on a well located in a reservoir that had an
average reservoir pressure 𝑃R of 1952 psia. The well was flowed on four choke
sizes, and the flow rate and flowing bottom-hole pressure were measured at 3 hr
and 6 hr for each choke size.
• An extended test was conducted for a period of 72 hr at a rate of 6.0 MMscfd, at
which time Pwf was measured at 1151 psia. The slopes of both the 3-hr and 6-hr
lines are apparently equal. Determine n, C, deliverability equation and AOF.
Slide <2> of 13
t = 3 hr t = 6 hr
qsc (mscfd) Pwf (psia) Pwf (psia)
2600 1793 1761
3300 1757 1657
5000 1623 1510
6300 1505 1320
6000 Ext. flow, t = 72, Pwf = 1151

Solution
• Use the first and last points on the 6-hr test to calculate n:
• Using the extended flow test to calculate C:
• Given the data in the following table, the deliverability equation
for in mscfd is:
• To calculate AOF, set = 0:
sc
q
wf
P

Deliverability data plot 21

Inflow Performance Relationship curve
22

• For some low-permeability wells, the time required to obtain stabilized
shut-in pressures may be impractical.
• Objective of modified isochronal tests is to obtain the same data as in
an isochronal test without using the lengthy shut-in periods required
for pressure to stabilize before each flow test is run.
• The modified isochronal test is essentially the same as the isochronal
test, except the shut-in periods separating the flow periods are equal to
or longer than the flow periods.
• Isochronal tests are modelled exactly; modified isochronal tests are not.
However, modified isochronal tests are used widely because they save
time and money and because they have proved to be excellent
approximations to true isochronal tests.
23
Modified Isochronal Test

Modified Isochronal test
24

25
Isochronal
Modified
Isochronal

26

Exercise 4
A modified isochronal test was conducted on a well located in a reservoir that
had an average reservoir pressure of 2050 psia. The well was flowed on four
choke sizes, and the flow rate and flowing bottom-hole pressure were
measured at 6 hours for each choke size.
An extended test was conducted for a period of 72 hour at a rate of 9.0
MMscfd, at which time Pwf was measured at 1270 psia. Using the data in the
following Table, calculate the values of n and C for the deliverability equation,
AOF, and flow rate for Pwf = 850 psia. Generate an IPR curve for this well.
Test qsc (Mscfd) Pwf (psia)
1 6500 1890
2 7600 1805
3 9200 1700
4 10400 1600
Extended flow 9000 1270
t=72 hour
27

28
Gas Production

29
• The inflow performance/deliverability is a measure of the
reservoir’s ability to produce gas to well bore.
• IPR is used to describe the relationship between gas production
and Bottom Hole Flowing Pressure (BHFP).
• Performance curves characterizing a gas production system are
very useful tools, and is used to visualize and graphically predict:
– The effects of declining reservoir pressure
– Changes in tubular size, increasing water production
– To install gas compressors
– To forecast future production
Inflow Performance Relationship
(IPR)

30
The equation introduced by Rawlins and Schellhardt where:
Q = gas flow-rate, MMscfd
Pr = average reservoir pressure, psia
Pwf = bottom-hole flowing pressure, psia
C = stabilized performance coefficient, constant
n = numerical exponent, constant
IPR

31
• Eqn. reveals that a plot of (P 2) = P 2
R — P2
wf versus qsc on log-
log scales should result in a straight line having a slope of 1/n.
• Once a value of n has been determined from the plot, C can be
calculated by using data from one of the tests that falls on the
line.

32

33
By taking various Pwf values, the corresponding q (gas flow rates) can
be calculated and then plotted.

Pressure Transient Analysis
• Also known as Well Test Interpretation or Well Test Analysis.
• Defined as the analysis of pressure changes over time, especially
those associated with small variations in the volume of fluid. In most
well tests, a limited amount of fluid is allowed to flow from
the formation being tested and the pressure at the formation
monitored over time. Then, the well is closed and the pressure
monitored while the fluid within the formation equilibrates.
• Pressure transient analysis is mainly used for reservoir
characterization in Exploration and Appraisal stage. During an
exploration well test, hydrocarbon is produced for the first time and
flared using temporary facilities (platform rig) to evaluate the potential
and reserves of a field. The analysis of these pressure changes can
provide information on the size and shape of the formation as well as
its ability to produce fluids.
Slide <2> of 13

Application of Pressure Transient
Analysis
• Productivity tests are conducted to:
– Determine well deliverability
– Characterize formation damage and other sources of skin effect
– Identify produced fluids and determine their respective volume ratios
– Measure reservoir pressure and temperature
– Obtain representative fluid samples suitable for PVT analysis
– Evaluate completion efficiency
– Evaluate workover or stimulation treatments.
• Descriptive reservoir tests are conducted to:
– Assess reservoir extent and geometry
– Determine hydraulic communication between wells
– Characterize reservoir heterogeneities
– Evaluate reservoir parameters.
Slide <2> of 13

Types of Pressure Transient Tests
Single-well Tests
• Involve only one well in which the
pressure response is measured
following a rate change.
• A common single-well test:
Pressure buildup test.
– Placing a bottomhole pressure
(BHP) measuring device in the well,
and shutting in the well.
– Following shut-in, the BHP builds-up
as a function of time, and the rate of
pressure build-up is used to
estimate well and formation
properties.
– From a pressure build-up test,
average reservoir pressure and
permeability in the well’s drainage
area can be estimated.
Multiwell tests
• When the flow rate is changed in
one well and the pressure
response is measured in one or
more other wells, the test is
called a multiwall test.
• Concept: to produce from or to
inject into one well, the active
well and observe the pressure
response in one or more offset
wells.
• Multiwell tests are designed to
estimate both permeability and
porosity in the drainage area of
the wells and quantify reservoir
anisotrophy.
Slide <2> of 13

• Stabilization time is defined as the time when the flowing pressure is
no longer changing or is no longer changing significantly.
• Physically, stabilized flow can be interpreted as the time when the
pressure transient is affected by a no-flow boundary, either a natural
reservoir boundary or an artificial boundary created by active wells
surrounding the tested well.
• Stabilization is more properly defined in terms of radius of
investigation.
• Radius of investigation, rinv, means the distance that a pressure
transient has moved into a formation following a rate change in a
well.
38
Time to Stabilization

Module Code and Module Title Title of Slides Slide <2> of 13
The pressure in the wellbore continues to decrease as flow
time increases. Simultaneously, the area from which fluid is
drained increases, and the pressure transient moves further
out into the reservoir.

• From then that is, rinv = re, and stabilization is said to have been
attained. This condition is also called the pseudo-steady-state. Pressure
does not become constant but the rate of pressure decline does.
• Time to stabilization can be determined by:
Slide <2> of 13
= average reservoir pressure
R
P

• If the time to stabilization is of the order of a few hours, a
conventional backpressure (flow-after-flow) test may be
conducted.
• Otherwise one of the isochronal tests is preferable. The
isochronal test is more accurate than the modified
isochronal test and should be used if the greater accuracy
is required.
41

Radius of Investigation
• Defined as the point in the formation beyond which the
pressure drawdown is negligible, is a measure of how far a
transient has moved into a formation following any rate
change in a well.
• Stabilized flowing conditions occur when the radius of
investigation equals or exceeds the distance to the no-flow
boundary of the well.
• As long as the radius of investigation is less than re,
stabilization has not been reached and the flow is said to be
transient.
Slide <2> of 13

• Gas well tests often involve interpretation of data obtained in
the transient flow regime when for rinv < re. If rinv = re , the flow
is pseudo-steady-state.
• When the radius of investigation reaches the exterior
boundary, re, of a closed reservoir, the effective drainage
radius is given by
rd = 0.472 re
43
Radius of investigation after t
hours of flow is
Radius of Investigation

Example 3
Solution:
Time to stabilization is:
The radius of investigation is:
44
= 0.032
𝑘𝑃𝑅𝑡
ϕμ𝑔
= 0.032
6.282(3700)(147.2)
0.1004(0.02350)
= 1219 ft

Exercise 5
• Given the following data, calculate time to stabilization and
radius of investigation.
k = 5.014 mD
Pr = 3500 psia
Φ = 0.1113
μg = 0.02350 cp
re = 1800 ft
t = 152.4 hr
Slide <2> of 13

• In designing a deliverability test, oil operators must collect and
utilize all information which includes:
 logs
 drill-stem tests
 previous deliverability tests conducted on that well
 production history
 gas and liquid compositions
 temperature
 core samples
 geological studies
46
Classifications, Limitations, and use of
Deliverability Test

• Knowledge of the time required for stabilization is a very
important factor in deciding the type of test to be used for
determining the deliverability of a gas well.
• This may be known directly from previous tests, such as
drill-stem or deliverability tests, conducted on the well or
from the production characteristics of the well.
• If such information is not available, it may be assumed that
the well will behave in a manner similar to neighbouring
wells in the same pool, for which data are available.
47

Figure.4.9 Types, limitations, and uses of deliverability tests.
48

Modified Isochronal Test
Slide <2> of 13

END OF LESSON
Slide <2> of 13

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