Multi-Stage Hydraulic Fracturing Completion Design - Cédric Bella
Multi-Stage Hydraulic Fracturing Completion Design - Cédric Bella
Multi-Stage Hydraulic Fracturing Completion Design - Cédric Bella
This is to certify that Multi-stage hydraulic fracturing completion design, is a study for thesis
(Professional Master’s Degree (Specialty: Petroleum Engineering)) carried out and written by
BELLA AMBATINDA Cédric under my supervision.
Name of Supervisor:………………..……………………………………………………………..
Qualification:………………………………………………………………………………………..
Signature:…………………………...……………………………………………………………….
Date: ………………………………………………………………………………………………...
i
Dedication
To my parents
ii
Acknowledgements
I would first like to thank Mr. Brice ASOPJIO for giving me the opportunity to do my internship
in MA-ES. He was more than just a boss. His door was always open whenever I ran into a trouble
spot or had a question about my research. He consistently allowed this paper to be my own work
but steered me in the right direction whenever he thought I needed it.
I take this opportunity to express gratitude to all the staff of MA-ES. They were kind and
supportive.
I would like to thank Prof. François MVONDO OWONO for being my academic supervisor.
He made sure that this work had been done in conformity with the university’s rules.
I wish to express my sincere thanks to Mr. Gabriel KUIATSE, the promoter of ISA-EMT. He
founded this university and transmitted his passion for petroleum engineering.
I would also like to thank all the staff of ISA-EMT, especially Mr. Stéphane KOUM, for all the
efforts to ensure that we are receiving the best training.
I give special thanks to these families: BELLA, HYONG, MAMA, NAVARRO and KOHLRUS
for their love and support.
Finally, I must express my profound gratitude to my family and my friends for providing me with
unfailing support and continuous encouragement throughout my years of study and through the
process of researching and writing this thesis. This accomplishment would not have been possible
without them. Thank you.
iii
Table of contents
Certification.......................................................................................................................................i
Dedication........................................................................................................................................ii
Acknowledgements.........................................................................................................................iii
Table of contents.............................................................................................................................iv
List of tables....................................................................................................................................vi
List of figures.................................................................................................................................vii
Abstract...........................................................................................................................................ix
Résumé.............................................................................................................................................x
Introduction......................................................................................................................................1
Chapter 1: Generalities.................................................................................................................3
2.1. Methodology........................................................................................................................17
2.2. Tools....................................................................................................................................21
2.3. Data......................................................................................................................................22
3.1.1. Metallurgy................................................................................................................25
3.1.2. Seals..........................................................................................................................26
3.3.5. Bullnose....................................................................................................................33
3.6. Recommendations...............................................................................................................37
Conclusion......................................................................................................................................38
References......................................................................................................................................39
v
List of tables
vi
List of figures
Figure 1-1: Structure of MA-ES.......................................................................................................4
Figure 1-2: Typical casing and liner clean-out string.......................................................................5
Figure 1-3: Running gun string and perforating...............................................................................6
Figure 1-4: Gravel pack principle.....................................................................................................7
Figure 1-5: Multi-zone completion..................................................................................................8
Figure 1-6: Basic completion design................................................................................................9
Figure 1-7: Conventional Christmas tree.......................................................................................10
Figure 1-8: Schematic of the hydraulic fracturing procedure).......................................................11
Figure 1-9: Production and reserves enhancement from hydraulic fracturing in low permeability
reservoirs.........................................................................................................................................13
Figure 1-10: Schematic of the plug-and-perf method....................................................................14
Figure 1-11: Schematic of a ball activated fracking sleeve technique in a horizontal well...........15
Figure 2-1: First-pass material selection........................................................................................19
Figure 2-2: PowerDraw interface...................................................................................................22
Figure 2-3: Initial well schematic...................................................................................................24
Figure 3-1: Corrosion rates as a function of chromium content.....................................................25
Figure 3-2: A selection guide for the completion method in MSHF..............................................30
Figure 3-3: Proposed completion design........................................................................................31
Figure 3-4: Activation process of a ball-activated fracking sleeve................................................32
Figure 3-5: Non-prep toe valve......................................................................................................33
Figure 3-6: Isolation valve..............................................................................................................33
Figure 3-7: Bullnose.......................................................................................................................34
Figure 3-8: 4-1/2" liner initially.....................................................................................................34
Figure 3-9: Lower completion RIH................................................................................................34
Figure 3-10: Packers are set, and the first stage is ready to be pumped.........................................35
Figure 3-11: Fluid flow when treating zone 1................................................................................35
Figure 3-12: First ball is dropped in the well and guided...............................................................35
Figure 3-13: The first ball fracking sleeve is opened, and zone 2 is ready for treatment..............36
Figure 3-14: Treatment of zone 3...................................................................................................36
Figure 3-15: Treatment of zone 4...................................................................................................37
vii
Abbreviations and glossary
CT = Coiled tubing
MD = Measured depth
PNP = Plug-and-perf
RU = Rig up
RD = Rig down
TD = Total depth
vii
Abstract
Hydraulic fracturing is a reservoir stimulation technique that increases productivity and oil and
gas reserves in low permeability reservoirs. One of the main challenges facing the fracking
industry is fracturing multiple targets along a single wellbore. Because one zone has to be
isolated while treating another one, this is a delicate operation with huge costs. To lower the
completion cost, the simple idea is to perform the job in a single trip.
This research proposes a completion that can allow fracturing four zones in a single trip in the
field “X.” The steps to design a well completion for multiple fracturing are first to select the best
completion method then the required equipment and the materials that it is made of. After that,
the completion schematic must be drawn using PowerDraw 2019 in this case, and the summary
installation procedures explained. The data used to design the completion are the well trajectory,
the reservoir data including temperature, pressure and fluid properties, and the production and
injection strategy.
The results suggest that multi-stage hydraulic fracturing can be done in a single trip using the
ball- and-sleeve method. The most appropriate materials were found to be an alloy with 13% of
chromium as metallurgy and Hydrogenated nitrile for sealing elements.
ix
Résumé
Cette étude porte sur une proposition de complétion de puits capable de fracturer 4 zones en une
seule descente dans le champ « X ». Pour proposer la complétion du puits, il faut d’abord choisir
la méthode de complétion la plus adaptée, puis sélectionner les équipements et les matériaux
appropriés, capables de tenir sur la durée de vie du puits. Il faut enfin dessiner le schéma de
complétion et écrire la procédure de son installation. Les données utilisées pour le
dimensionnement des équipements sont la trajectoire du puits, les propriétés du réservoir et des
fluides produits et injectés et enfin la stratégie de production. Les schémas de complétion ont été
réalisés avec PowerDraw 2019.
Les résultats montrent que la méthode « ball-and-sleeve » permet de fracturer 4 zones en une
seule descente. Dans ce cas précis, la métallurgie recommandée est un acier contenant 13% de
chrome et les éléments d’étanchéité seront en nitrile hydrogéné.
x
Introduction
In the beginning, the purpose of drilling a well is to confirm the presence of hydrocarbons. It is
well known that at this stage, 7 wells out of 10 are dry. So there is no need to spend much money
on them. For the other wells, further research is conducted, and if the hydrocarbons reserve is
found to be commercially viable, then some special equipment must be installed in the well to
start production. This is known as completing the well. Completion is the interface between the
reservoir and the surface facilities (Bellarby, 2009). Without it, it is impossible to produce a well
safely and efficiently. The completion string must be designed to reach the production objectives.
In some cases, permeability is low, leading to low production rates. This requires hydraulic
fracturing to create a conductive path for the fluid from the reservoir to the well. This process can
significantly increase well productivity and oil and gas reserves. This topic is well addressed in
the literature (Bellarby, 2009; Economides et al., 2013). However, it deals with only one
reservoir. Sometimes several reservoir zones are to be produced with a single well as it is more
economical. Fracturing multiple zones is challenging because one zone has to be isolated while
treating another zone.
Hydraulic fracturing is a costly operation. In fact, an oil price of 50$ (fifty Dollars) per barrel is
needed to break even this type of projects (Kleinberg, et al., 2018). These costs include
completion costs, fluid and proppants costs, and pumping charges (Jabbari & Benson, 2013).
Multi-stage hydraulic fracturing (MSHF) costs are even higher due to longer operation times. In
recent years, there have been several studies to determine which completion method is the most
economical (Mathur & Kumar, 2016). Actually, the best completion method doesn’t exist. It all
depends on the application. But to reduce the overall cost of MSHF, the simple idea is to conduct
the operation in a single trip. The aim of this work is, therefore, to design a completion that can
perform hydraulic fracturing of multiple zones in a single trip. Is it technically feasible?
This research was conducted while doing our internship at MA-ES from 30 th September to 30th
November 2019. To design effective completion, we need to perform the following tasks:
1
- Draw the completion schematic and write installation procedures
This work is structured in three chapters. In chapter one, the fundamentals of well completion are
explained, equipment used in typical well completion and its role is presented. This is followed
by the principle of hydraulic fracturing and the demonstration that hydraulic fracturing can
improve well productivity and increase hydrocarbons reserve. At the end of this chapter, an
overview of the research on MSHF completions is presented. In chapter two, the steps needed to
design a well completion are explained, followed by the tools and data used in this work. In
chapter three, the proposed well completion is discussed, covering the completion method that
will be used, the selected materials, the drawing of the completion and the running procedures.
2
Generalities
Chapter 1: Generalities
This chapter begins with the presentation of MA-ES, the company where this research was
conducted, followed by a literature review on the topic of multi-stage hydraulic fracturing
completion design. This will introduce key concepts such as the definition of well completion, the
purpose of a hydraulic fracturing stimulation, and an overview of research in completion designs
for hydraulic fracturing.
Managing Director
Accountant
Administrative
assistant
IT Specialist
a. Wellbore cleanout
Well completion starts as soon as the production casing is landed and cemented in place for a
cased well and when the last section has been drilled for an open hole. In a cased well, the
very first
operation is to clean the casing. Drillers used a special fluid, referred to as “drilling mud” to
conduct their operations. This fluid must be replaced with a completion fluid that is best suited
for completion operations. Also, the casing itself must be cleaned to remove the remaining debris
in the well. This is critical where the casing will be perforated and where some special
equipment, such as packers, will be set. The process of casing cleaning involves the use of
mechanical scrapers and the circulation of drilling mud (figure 1.2). After removing debris, the
mud must be displaced by completion fluid until the fluid is clean: the Nephelometric Turbidity
Unit or NTU must be lower than 20 (Bellarby, 2009).
Figure 1-2: Typical casing and liner clean-out string (Bellarby, 2009)
b. Casing perforations
In a cased hole completion, a casing is set below the producing zone. Therefore, the casing must
be perforated to establish a connection between the reservoir and the well. To establish a path
with the hydrocarbons stored in the pay zone, a perforating gun is lowered to the target depth.
Wireline
is the preferred conveyance method for short and light gun strings. To deploy long and heavy
gun strings, a technique referred to as tubing conveyed perforating or TCP is required (ENI,
1999). Using sensors in part of the gun string, the gun can be accurately positioned in front of the
targeted reservoir. The charges in the gun are now ready to be ignited using the surface
perforating control equipment. Once the correct depth has been confirmed, the gun is fired. The
charges in the gun explode, providing a path through the formation, the cement and the casing to
the production tubing. This is illustrated in figure 1.3.
This gravel filters the produced sand particles and the screens keep the gravel in place. Screens
are made of perforated pipes covered with wires. The holes of these screens are large enough to
let the fluid flow through it, yet small enough to stop the gravel. The first equipment to be landed
in a gravel pack completion is its base. This can be a sump packer or a bridge plug. The role of
a sump packer or a bridge plug is to isolate the bottom of the well so that an unwanted fluid
cannot flow from below it upward. Above the sump packer or the bridge plug, there is generally a
bull plug to tag on it. On top of it, comes the screen assembly, which is comprised of screens
joints screwed together until the perforated zone is all covered. Then there is a need to add some
blank pipes to constitute a gravel reserve. Above that, the gravel pack extension allows the
gravel pack fluid circulation. This type of lower completion ends with a gravel pack packer. To
select the zones that are produced or injected, it is most frequent to use a sliding sleeve, a
landing nipple and at least two packers in a single string completion. Here, the packers isolate
the annulus and guide the produced fluids towards the tubing. The sliding sleeve will allow a
selective or commingled production. To produce the lower reservoir, the sliding sleeve must be
closed and to
produce the upper reservoir, a plug must be installed in the landing nipple, and then the sliding
sleeve must be opened with wireline or slick line (figure 1.5).
Top connector
Swab valve
Flow fitting
Figure 1-8: Schematic of the hydraulic fracturing procedure (Guo, et al., 2007)
Hydraulic fracturing is not a new technology. It was first successfully used in the late 1940s
(Speight, 2016). With the advances in drilling technology, hydraulic fracturing can be used in
conjunction with horizontal wells, and this made possible production of unconventional
reservoirs.
It was recorded that 95% of newly drilled wells in the US were hydraulically fractured
(Aminzadeh, 2019). If hydraulic fracturing has some much success in spite of its downsides
(environmentalists argue that hydraulic fracturing pollutes the groundwater and induce
earthquakes), it is because it can significantly increase the productivity of a well (Economides &
Nolte, 2000). For example, let’s assume a fractured well that has kfw = 2000 mD-ft, k = 1 mD, xf
= 1000 ft, rw = 0.328 ft and re = 1490 ft, where kf is the permeability along the fracture, k is the
reservoir permeability, w is the width of the fracture, xf is the fracture half-length, rw is the
wellbore radius and re is the drainage
𝑓
radius. The dimensionless fracture conductivity is 𝐶𝑓𝐷 = 𝐾 𝑤 (equation 1.1). In this case, CfD = 2.
𝑘𝑥𝑓
𝑟 ′𝑤
𝑠
( = −𝑙𝑛 𝑟
)increase
(equation 1.3). In this case, the skin factor is s = -6.75. Finally, the fold of
𝑤
(FOI) under steady-state radial flow, denoted as J/J0, where J and J0 are the productivity indexes
𝑟
𝑙𝑛( 𝑒 )
before and after fracturing, is 𝐽 = 𝑟
𝑟𝑤
(equation 1.4). In this case, J/J0 = 5.0. That means
𝐽0 𝑙𝑛( 𝑒 )+𝑠
𝑟𝑤
that the production rate will be five times bigger with the same pressure drawdown applied on the
well.
The equations 1.1, 1.2, 1.3 and 1.4 suggest that hydraulic fracturing will exhibit better results for
low permeability reservoirs. In fact, equation 1.1 shows that if reservoir permeability k is big
with all other parameters constant, then the fracture conductivity CfD will be small. Then using
equation 1.2, the effective wellbore radius r’w will be small. Equation 1.3 shows the absolute
value of skin factor |s| will be small. And finally, equation 1.4 shows that the fold of increase J/J0
will be small. The benefits of hydraulic are better illustrated in figure 1.8. It is shown that the
flow rate is increased after fracturing and also that the reservoir is produced longer, meaning
hydraulic fracturing permitted to tap into some additional reserves.
Figure 1-9: Production and reserves enhancement from hydraulic fracturing in low
permeability reservoirs (Economides, et al., 2013)
This method is simple and reliable, but it requires multiple runs of wireline and coiled tubing
intervention, thus leading to huge costs. The benefits and limitations of this method are shown in
table 1.1
Benefits Limitations
Unlimited number of stages Requires wireline or coiled tubing
Flexible placement stage Requires plug mill-out for full diameter
production
Limited refract options
Figure 1-11: Schematic of a ball activated fracking sleeve technique in a horizontal well
(Mathur & Kumar, 2016)
Table 1-2: Benefits and limitations of ball-and-sleeve completion
Benefits Limitations
Nonstop fracturing operations A limited number of stages
No mill-out required Reduced diameter
Stage placement is fixed once completed
To sum up, this chapter covered the operations done in well completions (casing cleaning, casing
perforation, running lower completion, running upper completion, installing a Christmas tree and
cleaning the well). The basic equipment to complete a well is presented, and the role of each tool
is given. Then the basic principle of hydraulic fracturing was explained, which is pumping a
special fluid made of water and additives in general until the pressure is high enough to fracture
the rock. The fractures will be kept open by injecting proppants in them. It was demonstrated that
hydraulic fracturing could significantly increase well productivity by increasing permeability
close to the wellbore. Finally, the most used completions techniques for multi-stage fracturing are
the plug- and-perf method and the ball-and-sleeve method. PNP requires running down a plug
after treating the lower zone. This technique can be costly since running the plug requires well
intervention and more rig time. The ball-and-sleeve method can allow fracturing multiple zones
in a single trip since the fracking sleeves can be opened by dropping the ball from the surface and
pumping fluid. The next chapter will present the methodology used to design completion for
MSHF and the tools and data used in this research.
Methodology, tools, and data
2.1. Methodology
The purpose of the methodology is to obtain a design of hydraulic fracturing for a multi-stage
single trip completion. This will require five steps that are: data gathering, material selection,
equipment selection, completion design schematic and writing of completion procedures.
17
Table 2-1: Minimum required data
a. Metals
Almost all the metal used in the industry is some form of steel. Steel is an alloy of iron and
carbon. The amount of carbon in steel is less than 2.5%, typically 0.3%. Other elements can be
added to improve corrosion resistance or strength. These alloying elements can be present up to
5% by weight in low-alloy steel. When these elements are in concentrations above 5%, the metal
is referred to as alloy steel. They are sometimes called corrosion-resistant alloys or CRAs. The
additional elements and their purposes are:
As a first pass, figure 2.1 can be used to define appropriate tubing metallurgy. This drawing is
from Sumitomo Metals and incorporates some proprietary grades.
To select metallurgy, first, calculate the partial pressures of H 2S and CO2. Then read on the graph
the recommended metallurgy. The selected metallurgy must be able to withstand the
environmental constraints and be available at a good price. So even if H2S and CO2 are the
parameters that have
the most influence on corrosion mechanisms, the impact of temperature and economics must also
be investigated.
b. Seals
Various seals are used in completions. The most common elastomers and their applications are
listed in the table below.
The seals must be chosen based on the fluids that will be present in the well. These fluids can be
the produced fluids, the injected fluids and the completion fluids. H2S is present in some
produced fluids, while methanol is usually injected to eliminate hydrates, ZnBr (Zinc Bromide) is
used as a completion brine, and HCl is required for acid stimulation.
Write a detailed procedure that includes all the information required for the completion
operation.
Write a summary procedure with a list of references for more detailed procedures.
The purpose of the procedures is to tell the completion installation team how to install the
completion in a safe, unambiguous way and to capture lessons learnt.
2.2. Tools
The software used to draw the completion schematic is PowerDraw 2019. It is a well schematic
drawing program for the oil and gas industry. It is a plug-in for Microsoft Visio. Therefore, Visio
is required before installing PowerDraw. The interface is presented in figure 2.2. There are
shapes for well and completions. We can drag and drop them to build a well schematic,
customize them, assemble them, calculate their depths, and write their descriptions in a table.
Figure 2-2: PowerDraw interface
2.3. Data
Note: To preserve the client’s data confidentiality, the field will be named “X.” Only the data
used to design the completion will be presented here (Table 2.1).
In that field, a horizontal well was drilled and perforated. The objective of this work is to design
the lower completion so that 4 reservoir zones can be hydraulically fractured in a single trip. High
injection pressures (above reservoir breakdown pressure) and flow rates are expected. After
treatment, the productions of the 4 zones will be commingled. The reservoir pressure is 4200 psi,
and the temperature is 115°C. The produced fluid is an oil with API gravity of 32. It is a
naphthenic type of fluid, and its aromatic fraction is very low. H 2S content is low (5 ppm), and
CO2 content is high (2.5 mole %). The measured depth at the toe of the well is 3500 ft. The used
data are summarized in table 2.3.
Table 2-3: Data for well completion design
The initial well schematic is illustrated in figure 2.3. The well is made of 2 casings (13-3/8”
surface casing at 750mMD, 9-5/8” intermediate casing at 1900mMD, both grades are L80) and 2
liners (7” liner at 2600mMD and 4-1/2” liner at 3500mMD, both grades are L80). The upper
completion is 4-1/2” with a tubing retrievable safety valve to control the well at 100mMD, a
chemical injection mandrel to allow chemical injection at 1600mMD, a gauge mandrel to
measure temperature and pressure at 2350mMD, a landing nipple above the production packer
and a landing nipple below the production packer to set plugs. The tubing size is 4-1/2” with a
nominal weight of 12 pounds per feet. Its grade is L80.The tubing string ends with a polished
bore receptacle or PBR and a seal assembly to seal in it. The PBR is used as a tieback for the 4-
1/2” liner so that the production string has a consistent diameter of 4-1/2”.
Methodology, tools, and data
Tubing hanger
Control line
Cp 30’’@150mMD
TRSV 4-1/2’’ @100mMD
Csg 9-5/8’’@1900mMD
Packer 7x4-1/2’’ @2400mMD
3200-3240mMD
3420-3450mMD
3360-
Liner 4-1/2’’@3500mMD
Liner 7’’@2600mMD 3380mMD
3300-3315mMD
24
Results and discussion
𝐻2𝑆 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (equation 3.2). At the reservoir pressure, it is therefore 4200 psi * 5^10-6, which is
0.021 psi. Using the first-pass selection graph (figure 2.1), the recommended metallurgy is an
alloy with 13% of Chromium (13Cr). Moreover, the reservoir temperature is 115°C, which is
239°F. At this temperature, Sumitomo Metals Industry (2008) found that 13Cr has a lower
corrosion rate than 9Cr (figure 3.1).
25
CO2 hydrates and dissociates to HCO3- and CO32-
𝐶𝑂2 + 𝐻2𝑂 ↔ 𝐻2𝐶𝑂3
𝐻2𝐶𝑂3 ↔ 𝐻+ + 𝐻𝐶𝑂3−
𝐻𝐶𝑂− ↔ 𝐻+ + 𝐶𝑂2−
3 3
CO2 diffuses to the metal surface and reacts cathodically at the surface by using electrons
and producing HCO3- and H2
1
𝐶𝑂 + 𝐻 𝑂 + 𝑒 − ↔ 𝐻 + 𝐻𝐶𝑂−
2 2 2 3
2
3.1.2. Seals
The selection of the appropriate elastomer in sealing elements depends on the following
parameters from table 2.2: temperature, H2S content, presence of methanol, hydrogen chloride,
zinc bromide, and aromatic hydrocarbons. The reservoir temperature is 239°F in the application
range of nitrile, the H2S content is 5 ppm, which is less than 10 ppm, and the fluid is not
aromatic. Furthermore, acid stimulation is not needed here, so there will be no presence of HCl.
And since there will be no injection of methanol (it is injected when there are hydrates in the well
and hydrates form only in low temperatures, which is not the case here), nitrile should be the
most appropriate elastomer for the sealing elements. But the operating temperature is too close to
the upper limit of nitrile temperatures. In this case, the best solution is to use hydrogenated nitrile
or HNBR. It has good physical properties and can resist higher temperatures.
3.2. Completion method
It was discussed before that the two most commonly used techniques for a multi-stage hydraulic
fracturing are the plug-and-perf method and the ball-and-sleeve method. In this case, the most
appropriate method is ball-and-sleeve. The plug-and-perf method requires multiple wireline and
coiled tubing runs to place the fracturing fluid. This increases the time and costs to perform the
fracture treatment. And the operation constraint was clearly to perform the fracture treatment in a
single trip so that less rig time is needed and high costs are saved.
a. Economic analysis
To estimate the cost of a method, we will add the cost of the equipment to the rig cost. The
pumping charges, fluid and proppants costs are the same for both plug-and-perf and ball-and-
sleeve. The results are shown in the following tables.
When comparing the cost of the required equipment for plug-and-perf and ball-and-sleeve, it
appears that plug-and-perf completion is most economical. But the plug-and-perf method requires
coiled-tubing interventions for perforating and setting plug. It was estimated that the time to rig
up (RU) and rig down (RD) between stages is 3 hours. It takes 1 hour to pump the treatment for
each stage. And the rig cost in shallow water offshore was estimated at 400,000$ per day. So one
hour will cost 16,667$ approximately. The operation costs for both projects are shown in the
following tables.
Now for ball-and-sleeve completion, the costs are 54,500$ for equipment and 66,667$ for
operation. The total cost is, therefore, 121,167$. For the plug-and-perf completion, the costs are
24,000$ for equipment and 266,667$ for operation. The total cost, in this case, is 290,667$.
In this case, ball-and-sleeve completion is more economical. So this is the completion that will be
designed, and its technical feasibility will be discussed here.
b. Equipment selection
In this case, the reservoir pressure is 4,200 psi. Using a safety factor of 10%, the maximum
working pressure is 4,620 psi. Therefore, equipment must be rated at 5,000 psi. Also, the working
pressure of the equipment should be above 125°C.
c. Completion method selection guide for similar designs
Our study made us implement the following selection guide for multi-stage hydraulic fracturing
completions. This can be used for similar completion designs. It will choose between the two
most used methods that are PNP and ball-and-sleeve.
The first parameter to evaluate is the well type. If it is an open hole, the completion
method must be the ball-and-sleeve.
If it is a cased hole, and the number of stages is low (less than 20), then the ball-and-
sleeve completion must be used
If the number of stages is between 20 and 40 and the production rates are low, then the
ball- and-sleeve completion must be used. Else, the plug-and-perf method must be used.
Finally, if the number of stages is higher than 40, then the plug and perf method must be
used
This selection guide is illustrated in figure 3.2. It is based on a comparison between economic
analysis of PNP and ball-and-sleeve. Since the flow rate is reduced in the ball-and-sleeve
completion, the net present value or NPV of PNP becomes greater than the NPV of ball-and-
sleeve for a high number of stages.
Figure 3-2: A selection guide for the completion method in MSHF
3.3. Well schematic
Tubing hanger
TRSV @100mMD
Csg 13-3/8’’
@750mMD
Csg 9-5/8’’
@1900mMD
Gauge mandrel @ 2350mMD
Lower completion
Production packer @ 2400mMD
Isolation packer Ball frac sleeve Isolation packer
Figure 3.3 illustrates the proposed completion design. In the lower completion, a ball and sleeve
method was proposed. This consists of 5 isolation packers, 3 ball-activated fracking sleeves, 1
pressure-activated fracking sleeve or Non-prep toe valve, 1 isolation valve and a bullnose.
3.3.1. Isolation packer
The proposed isolation packer is a hydraulically-set packer for the 4-1/2” liner. This packer sets
when sufficient internal pressure is applied to the packer. The packer expands and sets on the
liner. The role of this packer is to isolate the annulus between the completion string and the
casing in front of a producing zone. The only way to produce this zone is, therefore, to establish a
flow path between this annulus and the production string. 5 packers are used between the 4
producing zones to isolate between them.
3.3.5. Bullnose
The bullnose (figure 3.7) is a device used at the end of the string. Its primary function is to
prevent flow from entering the bottom end of the string, whilst its rounded nose design provides a
positives guide while running in hole.
Figure 3-7: Bullnose
The next step is to test all pumping lines to make sure they can deliver the required flow rates.
Then a ball is dropped to land in the ball seat of the isolation valve, and when the pressure
reaches the required pressure, the isolation valve closes. Now it is time to set the packers by
applying
internal pressure (figure 3.10). These packers will be set at a specified pressure. Of course, the
uppermost packer will set first, followed by the lower packers.
Figure 3-10: Packers are set, and the first stage is ready to be pumped
The next step is to pump the first stage to fracture the first reservoir. The fracturing fluid is
pumped from the surface, then through the lower completion. At this point, all the ball-activated
fracking sleeves are closed. The non-prep toe valve will open at the required pressure and allow
the fracturing fluid to enter the annulus between the casing and zone 1 (figure 3.11).
The next step is to treat zone 2 and isolate zone 1 at the same time. The smallest ball is dropped
in the well and guided by the fluid to the lowest ball seat (figure 3.12).
The ball reaches the first ball seat, and when enough pressure is applied to it, the sleeve shifts and
opens the ports (figure 3.13).
Figure 3-13: The first ball fracking sleeve is opened, and zone 2 is ready for treatment
The second stage is pumped from the surface through the lower completion. The ball-activated
fracking sleeves in front of zone 3 and zone 4 are still closed. The fluid will enter the annular
between the casing and zone 2. The ball prevents the fluid from going to zone 1. After treating
zone 2, the next size ball is dropped from the surface and reaches the seat of the ball-activated
fracking sleeve in front of zone 3. When enough pressure is applied, the ball-activated fracking
sleeve is opened, and zone 3 is treated (figure 3.14).
After treating zone 3, the largest ball is dropped from the surface and guided until it reaches the
seat of the uppermost ball fracking sleeve. When enough pressure is applied above the ball, the
sleeve shifts and the ports are open. Zone 4 is treated while the other zones are isolated (Figure
3.15).
Figure 3-15: Treatment of zone 4
Now that all the 4 zones are treated, the service tool can be pulled out of the hole. The balls will
dissolve after a few hours, and the treated zone can be produced at the same time.
3.6. Recommendations
In this case, the reservoir pressure is 4,200 psi. Using a safety factor of 10%, the
maximum working pressure is 4,620 psi. Therefore, equipment rated at 5,000 psi could
work, but since it is too close to the limit, we would recommend 10,000 psi rated
equipment. Detailed economic analysis must be conducted to find out if using 10,000 psi
equipment is profitable.
A fracture treatment design is tied to multiple disciplines, so the completion team must
work closely with production engineers to optimize pumping rates and pressures. Also,
corrosion engineers must be part of the team and carry further work to validate the
proposed metallurgy.
Conclusion
Hydraulic fracturing is a technology used since 1950 to produce tight oil. It consists of pumping a
special fluid at a pressure high enough to initiate cracks in the rock. This process can significantly
improve well productivity in low permeability reservoirs. For economic reasons, multiple
reservoir zones are often produced by a single well, and when there is a need to fracture them, it
is very challenging because one zone must be isolated while treating another zone. To reduce the
costs of operations, the question was asked if multi-stage hydraulic fracturing could be done in a
single trip. To make MSHF a successful operation, all aspects must be planned ahead. This
implies selecting the most appropriate materials, completion method and equipment. In the field
“X,” a completion design to fracture 4 zones in a single trip was required. We found out the best
materials were an alloy of steel with 13% of chromium for metallurgy and Hydrogenated nitrile
for sealing elements. Chromium adds resistance to steel in a corrosive environment, and at 239°F,
13% of Chromium gives better results than 9%. Both nitrile and hydrogenated nitrile could be
used in the presence of the produced fluid, completion fluid and fracturing fluid, but the
downhole temperature was very close to the upper limit of nitrile applications, hydrogenated
nitrile was finally validated. The best completion method, in this case, was the ball and sleeve
method because it is the only one that can be performed in a single trip, and it is economical. To
fracture the 4 zones in a single trip, 5 isolations packers, 3 ball-activated fracking sleeves, 1 non-
prep toe valve, 1 isolation valve and 1 bullnose will be needed. The packers will isolate the
annulus between the completion string and the casing, leaving this annulus as the only way to
fracture and produce a target zone. The ball- activated fracking sleeves will isolate the lower zone
while treating the upper zone, and later dissolution of the ball will permit commingled
production. The non-prep toe valve has the same role as the fracking sleeve, but it is more
economical to use it for the first stage. Finally, the isolation valve will allow setting packers.
This case was a typical case where the ball and sleeve completion was the best due to the lower
number of stages. To be able to use the ball-and-sleeve method for a high number of stages,
further research is required so that the limited use of balls can be replaced by another way of
opening the sleeves. RFID (radio frequency identification) could be an interesting option. Also, to
select the most appropriate equipment, further work must be done with the help of the production
engineers designing the fracture treatment and corrosion engineers for material selection.
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