SPE-176191-MS

Extracting Lessons Learned of 35 Water Shut-off Jobs in Mature Fields to

Improve Success Ratio of Water Shut-off Job
D. Permana, G. Ferdian, M. Aji, and E. Siswati, PT Chevron Pacific Indonesia

Copyright 2015, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE/IATMI Asia Pacific Oil & Gas Conference and Exhibition held in Nusa Dua, Bali, Indonesia, 20 –22 October 2015.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents
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Abstract
Increasing water production in a mature field means an increasing burden of operating cost because that
water production must be processed and treated before it is discharged or injected1. One common solution
to overcome this problem is conducting water shut-off jobs which consist of mechanical and chemical
water shut-off. Mechanical water shut-off is preferred to be carried out over chemical water shut-off
because of lower costs. An example of mechanical water shut-off is using cup packer, squeezing-cement,
and reperforating over a shorter interval.
In Central Sumatra, there were approximately 35 mechanical water shut-off jobs executed from 2010
– 2012. Those jobs were varied from mature fields having different geological conditions, recovery
mechanisms, drive mechanisms, and operating-rate conditions which may be influencing the rising of
current oil water contact (COWC).
Based on the completed water shut-off jobs, some lessons learned were to have perforation interval
above current oil water contact (COWC), have good historical water shut-off performance at surrounding
wells, have good production performance at surrounding wells, have good structural location, and have
by-pass oil identified from CO/RST log if it is below COWC. Those lessons learned will be applied to
upcoming water shut-off candidates.
This paper further explains of some lessons learned which will be useful for selecting candidate of
water shut-off jobs and compares the result of water shut-off jobs after implementing lessons learned
which significantly increased its success ratio and oil gain.

Introduction
There is no single definition of mature field, although different companies may apply their own definition
of mature field. Most of oil and gas production around the world is coming from mature field production
where the water continues to increase and oil continues to decline. Consequently, the operating expense
of a mature field is continuously increasing to treat water production that will eventually decrease field’s
profitability. Hence, decreasing water production will significantly help the operators reducing their
operating expense on water handling. One of common solutions to reduce water production is through
water shut-off jobs which can use either mechanical or chemical water shut-off.
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All work-overs including water shut-off jobs in mature fields are considered as base-business activities
which are regularly conducted by the operators to maintain mature field’s production. In the downturn of
oil and gas industry, lower oil price means that the oil price is getting closer to the lifting cost which leads
to lower revenue for the operators making them become more stringent to scrutinize their spending
activities including drilling, work-over and completion, and etc in order to effectively and efficiently
operate.
Adopting lessons learned and best practices from the industry or other operators is one of the ways to
accomplish effective and efficient operations. This paper describes the methodology used to analyze and
extract lessons learned from 35 water shut-off jobs that will improve success ratio and oil gain.

Methodology

There are two water shut-off types which are commonly used in the industry. Mechanical, which is
using either packer or squeezed-cement, and chemical water shut-off which is using chemical substances
e.g. polymer. In Sumatra Operations, most of the fields have reached their peak production and entered
mature phase of the lifecycle of oilfield. In here, mechanical water shut-off is still becoming preference
for water shut-off since it is considered cheaper than chemical water shut-off. During 2010 – 2012, there
were around 35 completed mechanical water shut-off jobs. Some parameters associated with well, G&G,
and economics are defined to be further analyzed one-by-one. These parameters are:
1. Oil Gain
Incremental oil caused by water shut-off job is calculated using average 90 days production after
job completed and is subtracted by average 30 days before job execution.
2. Delta Water Cut (WC)
Difference on water cut which is average 30 days before job execution and 90 days after job
completed.
3. Position of Perforation Interval to Current Oil Water Contact (COWC)2
Information of position of perforation interval to the current oil water contact (COWC) whether
well has perforation which is either above or below COWC. It can be taken from well log of last
drilling well information which is correlated to well candidate or last RST/ CO log from well
candidate or surrounding well’s candidate which is correlated to well candidate.
4. Well Structure Location2
Well’s position in the geological structure whether it is located in the flank or non-flank. It can be
seen from structural map of the well and sand which will be reduced its water production.
5. Water Cut (WC) at Offset Wells Producing at Proposed-Sand for Water Shut-off
Information of water cut (WC) at offset wells which are producing at proposed-sand for water
shut-off. It can be either higher or lower than proposed well and proposed sand.
In this study, two parameters which are Oil Gain and delta WC are used to define the success of water
shut-off job. The successful one is the job that resulted oil gain more than 12 BOPD indicating that the
job is economical and negative delta WC indicating that water rate has reduced. Meanwhile, the job which
got result either oil gain more than 12 or negative delta WC is considered as failure. The remaining three
parameters are used for further evaluation of WSO jobs why some jobs were successful and others were
failure and extracted to take the lessons that are learned from these jobs to be applied in the next WSO
job that will increase its oil gain and success ratio. There are additional parameters analyzed during
evaluation period which are followings:
1. Any Interval Producing with WC less than producing WC
If there is a well producing from more than 2 zones which have one or more interval having swab
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test with WC less than producing WC, the well can become WSO candidate after evaluating the
other parameters.
2. Unswept Interval based on CO/RST log
If there is well run CO/RST log and found that there were unswept intervals based on CO/RST log,
the well can become WSO candidate after evaluating other parameters
In order to extract the lessons learned, set of parameters data, Table 1, were prepared for each well and
analyzed well-by-well to find out why the jobs were successful and others were failure. Besides, baseline
of oil gain and success ratio was determined as comparison to measure the impact of applying lessons
learned in the WSO jobs, Table 2.

Table 1—Set of Analyzed Data by Well

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Table 2—Baseline Oil Gain (Mean and Median) and WSO Job Success Ratio

Analysis and Lessons Learned

As previously explained that each parameter of the last three parameters (position of perforation interval
to COWC, well structure location, and WC at Offset Wells Producing at Proposed-Sand) was evaluated
for each well to extract the lesson learned, then each well was grouped based on parameter’s difference
to see how significant the impact of each parameter. Pareto chart was used to compare the parameter
between successful and failure WSO and extract lessons learned that will be applied in the upcoming
WSO. The results are as follows:
1. Position of Perforation Interval to the COWC
Based on Pareto Chart Analysis, Figure 1, all wells whose perforation interval either partly or fully
above COWC have more successful and less failure than if it is located below COWC. Perforation
interval which is either partly or fully above COWC has the opportunity to conduct squeezed-
cement and reperforate with shorter interval at the top of perforation which is relatively far from
COWC or targeting the fining upward part of the sand body.

Figure 1—Comparison of Pareto Chart of Perforation to COWC

2. Structure Location
Based on Pareto Chart Analysis, Figure 2, all wells whose structure location located at the
non-flank area have more successful and less failure than if it is located in the flank area.
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Non-flank structure location is likely to have some parts of the sand which are either fully or partly
above COWC, especially if the OWC is evenly rising across the field.

Figure 2—Comparison of Pareto Chart of Structure Location

3. Offset Well WC to Proposed-Well WC

Based on Pareto Chart Analysis, Figure 3, all wells which have WC of offset wells producing from
the same sand with proposed-sand from well candidate less than WC of proposed-sand from well
candidate have more successful and less failure than if it is higher.
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Figure 3—Comparison of Pareto Chart of Offset Well WC to Proposed-Well WC

These data are required to be fulfilled to select WSO which will have higher success ratio. However,
if the wells can not fulfill those data, wells may still be able to be proposed for WSO if wells can have
either CO/RST showing unswept oil or any interval having swab test WC less than last well production
WC. In addition to those evaluated parameters, there are 2 other parameters which have been analyzed that
if wells have those data which are either CO/RST log showing unswept oil which may be located below
correlated COWC since the rising of OWC must vary depend heavily on how much fluid rate produced
from wells in the particular field or any interval having swab test WC less than last well production test
since the other intervals have significant difference of permeability and pressure that make higher
permeability and pressure are greatly dominating the fluid production. Another parameter that needs to be
fulfilled is historical of completed WSO on surrounding wells whether success or failure. If there is no
historical completed WSO in the field, wells may still be proposed for WSO with careful consideration
by assessing the cost of WSO compared with potential recoverable reserves that will be gained from
WSO. Summary of the flow process can be seen in the chart, Figure 4, as explained earlier. The result of
WSO using new flow process can be seen in the Table 3 and its comparison with analyzed data in the
Table 4. Based on the result in the Table 4, WSO using new flow process have higher success ratio which
has 76% success ratio compared with analyzed data which has 49% success ratio. Moreover, oil gain is
increasing both its mean and median value which are 35.2 BOPD incremental oil gain (from 29.17 BOPD
in the analyzed data) and 21.82 BOPD incremental oil gain (from 10.92 BOPD in the analyzed data),
respectively.
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Figure 4 —New Flow Process to Select Mechanical WSO Job

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Table 3—Completed WSO Jobs Using New Flow Process

Table 4 —Comparison between BEFORE and AFTER Using New Flow Process

Field Cases
There were several field cases discussed in this paper showing the application of new flow process to
select WSO job candidate either using combination of three parameters or using CO/RST or swab test data
showing any interval having WC swab test less than producing WC.

AFR_1
This well was drilled in 1974 and penetrated several productive sands which have been perforated and
interchangeably produced either commingle or single. It is located at the top of structure of AFR_1 field
as can be seen in the Figure 5.
SPE-176191-MS 9

Figure 5—Structural Map of AFR_1 Field

Besides, AFR_1 have perforation intervals still above COWC which was determined from AFR_1.18
as the latest drilled well in AFR_1 field, Table 5. Moreover, Offset wells WC at proposed-sand for WSO
at AFR_1 was lower than WC at proposed-well in the same field with the same sand production, Table
6.

Table 5—Perforation to COWC at AFR_1

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Table 6 —WC Offset Wells at Proposed-Sand Compared to Well Candidate at AFR_1

The difference of completion between before and after can be seen in the Figure 6. Those three
parameters have fulfilled the new flow process requirement. So, the result of WSO at AFR_1 was showing
good incremental oil production around 15 – 30 BOPD incremental oil gain and 3,000 BWPD water
reduction as depicted in the Figure 7.
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Figure 6 —Completion of AFR_1

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Figure 7—Normalization of Production of AFR_1 Comparing Before and After WSO Job

AFR_A38
This well was drilled in 1976 and penetrated several productive sands which have been perforated and
interchangeably produced either commingle or single. It is located mid-structure in the AFR_A field as
can be seen in the Figure 8. Besides, AFR_A38 have perforation intervals still above COWC which was
determined from AFR_A154 as the latest drilled well in AFR_A field, Table 7. Moreover, Offset wells
WC at proposed-sand for WSO at AFR_A38 was lower than WC at proposed-well in the same field with
the same sand production, Table 8.
SPE-176191-MS 13

Figure 8 —Structural Map of AFR_A Field

Table 7—Perforation to COWC at AFR_A38

Table 8 —WC Offset Wells at Proposed-Sand Compared to Well Candidate at AFR_A38

The difference of completion between before and after can be seen in the Figure 9. Those three
parameters have fulfilled the new flow process requirement. So, the result of WSO at AFR_A38 was
showing good incremental oil production around 250 – 300 BOPD incremental oil gain and 1,500 BWPD
water reduction as depicted in the Figure 10.
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Figure 9 —Completion of AFR_A38

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Figure 10 —Normalization of Production of AFR_A38 Comparing Before and After WSO Job

AFR_C209
This well was drilled in 2004 and penetrated several productive sands which have been perforated and
interchangeably produced either commingle or single. It is located at the top of structure of AFR_C field
as can be seen in the Figure 11. Besides, AFR_C209 have perforation intervals still above COWC.
Moreover, Offset wells WC at proposed-sand for WSO at AFR_C209 was lower than WC at proposed-
well in the same field with the same sand production, Table 9.
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Figure 11—Structural Map of AFR_C Field

Table 9 —WC Offset Wells at Proposed-Sand Compared to Well Candidate at AFR_C209

The difference of completion between before and after can be seen in the Figure 9. Those three
parameters have fulfilled the new flow process requirement. So, the result of WSO at AFR_C209 was
showing good incremental oil production around 25 – 40 BOPD incremental oil gain and 1,000 BWPD
water reduction.
SPE-176191-MS 17

Figure 12—Completion of AFR_C209

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Figure 13—Normalization of Production of AFR_C209 Comparing Before and After WSO Job

AFR_B1
This well was drilled in 2012 and penetrated several productive sands which have been perforated and
initially produced as commingle production. The well had good initial production which was 553 BFPD
and 291 BOPD (WC 47.4%). However, in the couple of days, WC was significantly increasing to WC
99.5%. This led to immediately prepare swab test program to identify which zone has water encroachment
and which zone has good oil production and became an example of WSO using swab test data as informed
in the new flow process, Figure 4, which is evaluating whether the well has any interval having WC swab
test less than producing WC. In this case, AFR_B1 swab test, Figure 14, showed that there was interval
producing 29 BPH with 10% WC which was less than producing WC which was already 99.5%. So,
packer was installed to isolate higher WC zone which was leaking from squeezed-zone. The result of
WSO at AFR_B1 was showing good incremental oil production around 250 – 300 BOPD incremental oil
gain and 700 – 800 BWPD water reduction as depicted in the Figure 15.
SPE-176191-MS 19

Figure 14 —Completion of AFR_B1

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Figure 15—Normalization of Production of AFR_B1 Comparing Before and After WSO Job

Conclusion
1. Methodology and its analysis can be easily adopted by others. However, some adjustment may be
needed to fit the operational, completion, and reservoir condition in each field.
2. New flow process was derived from extracting lessons learned from prior WSO and employing
process to achieve higher success ratio and oil gain compared to the old methodology.
3. Additional parameters which are any interval having swab test WC less than commingle-
production WC and identified unswept oil from CO/RST log can be used to alternatively select
WSO candidate if the first three parameters which are either perforation interval above COWC,
non-flank structural location, or WC at offset well higher than WC well-candidate could not be
fulfilled by well candidate.

Acknowledgement
The author would like to thank Chevron Pacific Indonesia for the permission and support given to the
author to publish and present the paper in the public forum.

References
1. Bailey, Bill, Crabtree, M., Tyrie, J., Elphick, J., Romano, C., Roodhart, L.: ⬙Water Control,⬙
Oilfield Review, Schlumberger, Vol. 12, No.1, Spring 2000
2. Permana, D., Fakhrizal, Nurwibowo, M.P.: ⬙Selection Criteria for Successful Water Shut-off
Treatment – Brown Field Success Story⬙ SPE-165753-MS, presented at SPE Asia Pacific Oil and
Gas Conference and Exhibition, Jakarta, Indonesia, 22 – 24 October 2013