# Issues Concerning the Use of Horizontal Wells in the Injection of Carbon Dioxide for Geologic SequestrationOffice of Water (4606M)
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### Issues C oncerning the Use of Horizontal Wells in the Injection of Carbon Dioxide for Geologic Sequestration

1.0 Introduction

Horizontal wells are wells drilled at an angle that deviates from the vertical by an angle approaching

or equaling 90 degrees . Typically, these horizontal wells are directionally drilled using advanced drilling technologies. The horizontal portion of the well bore is generally drilled parallel to the bedding of the geologic formation, although the angle of deviation may vary depending on the lo cal geology. Horizontal wells have been used in many applications including oil and gas production, enhanced oil recovery, coalbed methane production, hazardous waste remediation, and recently for geologic sequestration. (Hazardous waste remediation wells are typically shallow wells drilled in unconsolidated soil not exceeding a few hundred feet in depth. Therefore

, they are not applicable to the situations that will be encountered in geologic sequestration and are not discussed further in this paper.) This paper focuses on directionally drilled horizontal petroleum production wells, as they are most similar to wells that may be used in geologic sequestration projects. A brief overview of the technology will be presented followed by a discussion of how horizontal wells differ from vertical wells and how those differences may affect the permitting and operation of carbon dioxide
GS wells.

1.1 History

The first horizontal oil wells were drilled in the early to mid 20 th century. Use of these wells was limited, until technological advancements, particularly in directional drilling techniques , began
to reduce the costs and facilitated the use of horizontal well installation in the 1980s. Use of horizontal well technologies has increased since then. As of Decemb er 2002, there were over 17,000 horizontal wells drilled in the United States. In general, d ecreasing costs have been the main factor in the increased use of horizontal wells. Initially

, well drilling costs were substantially higher for horizontal wells, with costs up to 7 times the cost of a vertical well (EIA 1993). Advanced technology and experience have dropped the costs significantly. Today

, horizontal well costs are typically between 1.4 to 3 times the costs of a vertical well and can approach equ al costs as a crew gains experience in a formation (Joshi 1991, Knoll 2005). The cost differential per foot drilled also drops as the vertical well depth increases.

1.2 Types of Wells

A h orizontal well can either be drilled as a new well or from an existi ng vertical well ; such a well
is termed a re

- entry well. Wells drilled as horizontal wells typically are drill ed vertically to a point called the kickoff point, then angle d toward the target formation

, as shown in Figure 1. The deeper the kickoff point, the more difficult it is to control the direction of the well (Inglis 1988). A horizontal well is drilled horizontally through the target formation, and it is generally possible to move the well bore up or down within a formation to take advantage of geol ogic
features. A re

- entry well is drilled in the same manner, but an existing vertical well bore is usedPage 2 as the starting point. A hole is drilled through the casing and the well bore is extended from that point . Either type of well can have multiple laterals extending from a single starting point. Examples of this type of well bore are shown in the rightmost pane l of Figure 1 and in Figure 2.

Figure 1. Comp arison of Horizontal and Vertical Well Types.

Source: http://www.trident4

- 22.ca/index.php?page=pad

- based

- drilling

Figure 2. A Re

- entry Horizontal Well

http://www.aoc.co.jp/e/tec/tec

- main.html

Horizontal wells are often categorized based on the turn radius or the build angle. The turn radius of a horizontal well is the radius it takes the well bore to turn 90 degrees. The build angle is the angle the well bore can turn in one foot. The build angle is equal to 90 degrees divided by Source:Page 3 the turn radius. Table 1 shows the different types of wells and their typical turn radii and build angles.

Table 1. Categories of Horizontal Wells and Their Characteristics Well Type T u rn Radius (Ft.) Build Angle (degrees/ft .)

Ultra

- short Radius 1 – 2 45 – 60

Short Radius 20 – 40 1 – 2

Medium Radius 300 – 800 0.06 – 0.2

Long Radius 1,000 – 3,000 0.02 – 0.06

Source: Joshi (1991)

Ultra

- s hort and short radius wells are most common in re

- entry wells and are useful on small leases where large horizontal distances cannot be covered. They can be drilled relatively short lengths. Medium radius wells are most common and are widely versatile. Long radius wells are less common but can achieve great horizontal distances of 30,00 0 feet or more.

1.3 Applications

The main advantage of horizontal wells is that they enable a longer segment of the well completion to come into contact with the target formation (since these targets are commonly horizontal or nearly horizontal) and they have a smaller footprint for the above ground operation. For production wells, this can lead to higher production rates (since the oil or gas has a shorter distance to flow to the production well) and for injection wells, this can allow lower injection press ures and better injectivity. The method can also be useful to reach formations
that are under obstacles such as a river or a lake, or in heavily populated areas which make a vertical well impractical. Some common applications are:

**•** Fractured formations (instead of in porous media)

**•** Formations with high water or gas production (coning)

**•** Low permeability formations

**•** Formations that are thin in the vertical direction

**•** Formations used for gas production and enhanced oil recovery

In enhanced oil recovery operatio ns, several configurations are possible. An operation can use horizontal injection wells with a vertical recovery well or it can use horizontal injection and recovery wells.

Horizontal wells have also been used for carbon sequestration at Sleipner, Wey burn, and In Salah, as described below:Page 4 **•** The Sleipner project injects via a single horizontal injection well that is 12,310 feet long and 3,816 feet deep ( saline a quifer CO 2 s torage). The well has a surface casing extending to 1,919 feet measured depth and a production casing to 7,831 feet measured depth
, which marks the top of the injection formation (Baklid et al. 1996). The well is cemented for 8,530 feet along the well bore. The well is equipped with a
downhole shut

- off device. Because the well in jects wet carbon dioxide with some hydrogen sulfide content

, the exposed portions are constructed of 25 percent

c hromium ( Cr ) steel.

**•** The Weyburn project injects through 37 wells including several dual leg (i.e., two horizontal segments departing from a s ingle well bore) horizontal injection wells (Basin Electric Power Cooperative 2009 , White et al. 2004). The wells are approximately 4,600 feet deep. The Weyburn field also anticipates increasing the use of horizontal recovery wells ( Bennaceur et al

. 2004 ) . In total the Weyburn project has 105 horizontal wells, including both production and injection wells containing 175 lateral legs. The Weyburn project began using single leg horizontal wells and then moved to dual, triple, and quadruple leg wells. A t ypical single leg well has a kickoff point that is 4,500 feet deep with a 3,280

- foot medium radius horizontal section. The well is typically cased to the kickoff point and then is left open hole after that. The dual leg wells are kicked off from
the hori zontal portion of the first leg with the two legs typically running parallel. Quadruple leg wells add two more parallel well bores in the opposite horizontal
direction of the first two well bores.

**•** The In Salah project uses three horizontal injection wel ls that are 6,000 feet deep in a
60

- foot

- thick aquifer.

2.0 Overview of Technology

Advances in horizontal well technologies
, including drilling technology, logging and measurement, and completion technology
, have increased the use of horizontal wells.

2.1 Dri lling Technology

Traditional ly constructed vertical wells are drilled by attaching a drill bit onto straight sections of drill pipe. The pipe typically comes in 30 foot sections that are threaded or attached to each other, usually in two or three sections . The pipe is rotated at the surface on the drilling floor

, providing the torque for the drill bit. Drilling fluids are injected through the pipe and the assembly containing the drill bit and pumped up the annulus between the pipe and well bore to

remove the rock cuttings. The same technique can be used to drill long radius horizontal wells. The difference is that with directional drilling technology the drill pipe is deflected to the desired angle using a variety of tools. Jet drilling, whipstocks, an d “ rebel bars ” are among the equipment that can be used to bend the trajectory of the drill pipe (Inglis 1988).Page 5 The increased u se of downhole motors was one of the technologies that allowed horizontal wells to become commonplace. Downhole motors, sometimes called mud motors, are attached to the bottom of the drill pipe and rotate the drill bit from the bottom of the hole instead of having to rotate the entire drill pipe from the surface to the bottom of the hole. Downhole motors decrease the torque placed on the drilling assembly caused by rotating the drill pipe through the curved portion of the well bore. They also allow finer control of the drill bit and its direction. Downhole motors also allow the use of coiled tubing in place of the traditional drill pipe . Since downhole motors do not require rotation from the surface, rigid drill pipe is not required. Instead, coiled tubing
, flexible tubing that comes coiled on large spools can be used. The flexibility of the coiled tubing allows drilling at sharper angles than does rigid tubing. Also
, because the coiled tubing can be fed into the hole continuously
, the drilling does not have to be stopped to attach new segments of drill pipe. This greatly saves time and improves the efficiency of drill ing
, which decreases costs significantly. Ultra

- short radius wells can be drilled using various types of jetting technologies.

2.2 Logging and Measurement Technology

Drilling horizontal wells with precision requires sophisticated measurement techniques. The
d ri ll bit position is critical to ensur ing that the well’s course can be plotted and corrections can be made, if necessary. Accelerometers and magnetometers are used for drill bit positioning. These pieces of equipment supply deviation checks as required in the Class VI injection regulations (§146.87(a)(1)). Other equipment is used as well to measure variables such as temperature, pressure, weight on the bit, torque, drill speed, mud volume, and type and severity of vibrations. These instruments can provid e pressure and temperature readings required under §146.86(b)(1). Signals are transmitted to the surface via mud telemetry, electro

magnetic telemetry, or occasionally by hard wires run through the entire length of drill pipe.

In addition, advanced geo physical well logging equipment is now available to facilitate logging while drilling. Logging instruments are capable of measuring properties of the formation during drilling. Instruments can measure spontaneous potential, gamma ray, seismic, sonic, res istivity, and record photographic data to provide information including formation density, porosity, composition, and pressures. These instruments can provide information for both the resistivity and cement logs required at §§146.87(a)(2) and 146.87(a)(3)

.

Medium and long radius wells should also be able to accept the equipment and instrumentation required to perform mechanical integrity tests required at §146.87(a)(4). As long as the well bore is cased and cemented as required, pressure and tracer tes ts can be conducted. The equipment described in the previous paragraph can also be lowered into the well bore at times other than drilling. Equipment to measure temperature, pH, conductivity, and perform video inspections of the casing is also available. Therefore the full suite of mechanical integrity tests described in the GS regulation will be available for horizontal wells.Page 6 2.3 Completions

T here are three possible ways that the production zone of a horizontal petroleum recovery (production) well can be completed :

**•** Open hole

**•** Slotted liners

**•** Traditional cementing and perforations Open hole completions do not place any structural well components or equipment in the well bore. This type of completion requires that the formation be stable enough to avoid collapse of
the rock formation into the well bore. It is a fairly common practice in short and ultra

- short
radius wells.

Slotted liners are liners with pre

- formed holes or slots which are inserted into the well bore. They provide structural stability and c an limit the amount of fine material entering the well

. They can be fitted with a gravel pack, which can further reduce fine particle production . Liners
will allow fluid in the annulus between the liner and the formation and therefore do not seal off

one zone from another. Packers can be inserted along the liner to provide isolation between

zones. Completions in this manner are not, however, amenable to well stimulation techniques, such as hydraulic fracturing to increase formation permeability and inje ctivity.

Traditional cementing of casing in a horizontal well can be accomplished in medium and long radius wells. Any cement that can be used in a vertical well may be used for medium and long
radius horizontal wells. The production casing and cement are then perforated using shaped explosive charges. Cementing is necessary if the well is to be stimulated using fracture techniques. Under the Class VI rule, a ll stimulation, including hydraulic fracturing, must be approved by the Director. Owners or operators must submit a proposed stimulation program as part of the permit application under §146.82(a)(9); the plan must demonstrate that any stimulation activities will not compromise the integrity of the confining zone. The oil industry
has established 6 levels of integrity for horizontal well completions (Charlez and Breant, 1999)

. They are:

**•** Level 1 – Open hole for both the vertical section and the horizontal well bores.

**•** Level 2 – Main vertical bore is cased and cemented with open hole horizontal bore s.

**•** Level 3 – Main vertical bore is cased and cemented, horizontal bores are cased but not
cemented.

**•** Level 4 – Both vertical and horizontal well bores are cased and cemented.

**•** Level 5 – Both horizontal and vertical well bores are cased and cemented, the junc tions are hydraulically isolated using packers.

**•** Level 6 – The horizontal and vertical well bores are cased and cemented, the junctions are an integral part of the casing.Page 7 Levels 1 and 2 are considered to have neither mechanical nor hydraulic integrity. Le vels 3 and 4 are considered to have mechanical integrity but lack hydraulic integrity. Levels 5 and 6 have both mechanical and hydraulic integrity and are necessary in situations where there is complex geology with multiple pressures, fluids, and strat a. Class VI wells are required to be cemented to the injection zone (§146.82(b)(3)), therefore they must hav e a minimum of either level 3 or 4 integrity depending on whether the horizontal portion of the well is below the caprock or not.
Of course

, providi ng higher levels of integrity will provide increased protection against potential fluid migration.

3.0 Unique Considerations for the Use of Horizontal Wells

In many ways, the construction, monitoring, and maintenance of horizontal wells are very similar to
th ose of conventionally drilled vertical wells. In several areas, however, there are differences which can
affect activities such as well completion, well integrity testing, and well logging.

3.1 Turn R adius

The turn radius of a well (i.e.

, the deflection from the vertical of a well) influences the type and size of equipment that can be lowered into the well. Longer, less flexible equipment requires a larger turn radius to successfully insert the equipment into the well. Although advances are continuously bei ng made, generally it is not considered feasible to cement short and ultra

- short radius wells (Joshi 1991). The types of geophysical logging equipment that can be used in short
and ultra

- short radius wells are also limited. Medium radius wells may limit the use of some larger pieces of equipment. However, technology has advanced to the point where most traditional activities such as logging and cementing can be carried out. In general, long radius wells can use any equipment used in traditional vertical w ells. Use of short or ultra

- short radius wells may be an issue where there are very deep USDWs and it is undesirable to kickoff the well above the USDW. They may also be an issue if land availability is limited

, such as in heavily populated areas or offsh ore platforms.

3.2 Loads on Casing

Casing in curved and horizontal portions of a horizontal well is subject to physical loads not imposed on the casing of traditional vertical wells (Cernocky and Scholibo, 1995; Chen et al. 1990). Extra stress is placed on th e casing through the angled portion of the well because of the bend of the casing. Additional force can also be placed on the casing in the horizontal portion if the formation is unconsolidated or the formation collapses into the open well bore. The casi ng can also experience higher friction and torque when being installed through the bend. These factors necessitate careful design of the casing and may require use of stronger materials or use of casing with thicker walls to prevent damage.Page 8 3.3 Formation Dam ag e

During well drilling, drilling fluid can be forced into the surrounding formation along with cuttings from the drilling. This can reduce formation porosity and decrease production from the formation (for production wells) or the injectivity of the for mation (for injection wells). While formation damage is a consideration when drilling any well, it is a critical consideration in horizontal well drilling, construction, and operation because of the longer drilling times and the additional gravitational force on the fluids (Powell et al

. 1995 ; Sabins 1990).

One technique that can lessen the damage to the formation is called underbalanced drilling. In traditional drilling, the pressure of the drilling fluid is maintained at a pressure slightly higher
th an the reservoir pressure in order to prevent gas blowouts. In underbalanced drilling, the pressure is maintained slightly lower than the formation pressure so that less drilling fluid is squeezed into the formation. Some drilling fluids may also lessen formation damage associated with horizontal well drilling. In addition, development techniques can be used to clean the drilling fluids and cuttings out of the formation. Options for development include acid treatment, mechanical scraping, and fracturing (Joshi 1991).

3.4 Cementing

During cementing of a horizontal well, solids can settle along the bottom portion of the long horizontal segment of the well. This can cause cement channeling and may result in an
i nadequate cement job. Free water in cement can also settle out and result in compromised cementing. It can also be more difficult to keep the pipe centered in the well bore in horizontal wells, and pipe eccentricities can be more frequent and lead to flaws in the cement job. For this reason, it is i mportant that the wellbore be cleaned of any drilling fluids or cuttings before the cement job begins. The cement should also have no free water content

, as determined by conducting the American Petroleum Institute free water test at an angle of 45 degrees or greater (Joshi 1991). Keeping the drilling fluid turbulent, using special drilling fluids, and maintaining consistent fluid velocities around the pipe have been found to limit solids channeling and result in better cement jobs (Powell et al

. 1995; Lockyer et al. 1990; Sabins 1990). Centralizers can also be used to keep the pipe centered in the well bore during cementing. The Class VI regulation require s the use of centralizers to the injection zone, see §146.86(b)(3) ; supporting guidance will contain more details on the use and placement of
centralizers.

3.5 Mechanical Integrity Tests with Multip l e Laterals

Pressure tests done for mechanical integrity require blocking off the casing to monitor for pressure changes. This is not possible in laterals tha t are level 1 or 2 and are not cased. In wells with multiple, cased laterals, required mechanical integrity tests will be more complex
, as each of the bores will need to be plugged.Page 9 4.0 Potential Impacts of Horizontal Well Issues on Geologic Sequestration an d Pr otection of USDWs

Horizontal wells present some unique challenges and require specific considerations in comparison to vertical injection well construction and operation. When constructing, permitting, and operating
Class VI wells, owners or operators and permitting authorities need to keep these challenges in mind. All of these challenges can be addressed with proper well design and planning.

Because short and ultra

- short radius wells are not usually constructed with cement in place, owners and oper ators of the se wells will find it impossible to meet the requirements to case and cement the well from the injection zone to the surface (§146.86(b)(3)) unless the kickoff point for the well and the entire horizontal portion of the well lie entirely within the injection formation. These wells may
also not be able to meet the logging and integrity testing requirements. Therefore, i n general, short
and ultra

- short radius wells are not practical for use in Class VI wells. Medium and long radius wells,
howev er, are more than capable of being cemented and accommodating logging equipment. Such longer radius wells should be able to reach the same points as short radius wells if owners or operators start the kickoff point at a higher elevation.

Horizontal injection wells may require stronger casing than vertical injection wells. Class VI regulations require that the cement and casing materials be of sufficient structural strength and last the life of the project (§146.86 (b) (1)). For horizontal wells
, this means that the well will need to be designed to withstand additional forces from the rock column and from bending in addition to the traditional collapse, burst, compressive, and tensile forces used to design vertical wells. Well designers should calculate these forces and include them in their designs. Permitting authorities will need to consider these additional forces in determining that the strength requirement has been

met.

While formation damage can be a problem with horizontal drilling, it should not af fect the integrity of the well or the containment of the carbon dioxide. It will only affect the injectivity (and the viability of the well for use in injection). Stimulation of GS wells

, includ ing hydraulic fracturing, may be permitted if injectivity is low . All stimulation techniques must be designed and monitored in such a way that the confining zone is not fractured (§146.88(a)) ; §146.87(d)(1) requires that the fracture pressure of the confining layer be determined during the construction of the well before operation is allowed. Modeling programs exist which can calculate fracture lengths for hydraulic fracturing and can be used to design the fracture so the confining layer is not fractured.

Pipe eccentricities, free water, formation damage, and solids channeling can lead to poor cement jobs in horizontal wells. The u se of centralizers, underbalanced drilling, and proper use of mechanical scraping and acid washes can eliminate many of these problems. As a practical matter, if existing horizontal wells are used for GS, they are likely not cemented from the injection zone to the surface, but are likely cemented through USDWs. In such cases, the well may need to have additional cement placed to seal unsealed formations or to repair existing defects

. The Class VI rule at §146.87(a)(3)(ii) require s submission of cement logs

, which should show any defects thatPage 10 occurred during cementing. The logs will give confidence that the cement job was completed satisfactorily. Any defects found can be repaired us ing cement squeezes before the well is allowed
to operate.

If multiple laterals are used in horizontal wells, the laterals may complicate mechanical integrity tests. Each lateral will need to be cemented to perform the tests properly. To perform the tes ts , the

o wner or operator will need to isolate each lateral with packers and perform the test individually on
that lateral.

Table 2 presents a summary of requirement s for Class VI injection wells and potential considerations and challenges that owners or operators of Class VI horizontal injection wells will encounter and address during well construction and operation

.

Tab l e 2 C lass VI Rule Requirements and Potential Considerations with Horizontal Wells.
Class VI Requirement Potential Considerations f or Horizontal GS Wells

§146.86 Construction requirements Solids channeling, formation damage, free water, and pipe eccentricities can lead to poor cement jobs. Review of cement logs will be critical. Casing and cement materials may need increased str ength over vertical wells.

S hort and ultra

- short radius wells cannot meet the requirements to case and cement the well from the injection zone to the surface.

§146.87 Pre

operational logging, sampling, and testing Limited in short radius wells. No major issues in medium and long radius wells although cement logs will be more important items to review in permit approval.

D eviation checks, MITs, and other logs required during drilling (at § 146.87(a) and (b)) can be performed using measurement -while-d rilling and logging -whiledrilling techniques.

§146.89 Mechanical integrity Logging may be limited in short radius wells. Pressure tests not possible in unlined horizontal portions. No issues with medium and long radius wells. More complex with mult iple laterals.

Equipment to measure temperature, pH, conductivity, and perform video inspections of the casing is all available. The full suite of MITs required in the GS regulation will be available for horizontal wells.

5.0 Summary

While there are unique challenges associated with drilling and operating horizontal wells, use of horizontal wells for Class VI GS is technically feasible

, and such wells may be used by Class VI well owners or operators in their GS projects . Owners or operators of technically s ophisticated, Class VIPage 11 horizontal wells may not be able to use some techniques such as short radius drilling. However, with appropriate planning and well design, horizontal wells can and have been used for GS. Tools such as logging while drilling, underba lanced drilling, specially designed cements, centralizer placement, and cement logs along with proper design procedures will enable construction of wells that can meet the Class VI regulations.Page 12 6.0 References

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- 61.

Baklid, A., R. Korbol, and G. Owren. 1996. Sleipner Vest CO 2 Disposal, CO 2 Injection into a Shallow Underground A quifer. Presented at 1996 SPE Conference.

Basin Electric Power Cooperative. 2009. http://www.basinelectric.com/Energy_Resources/Gas/CO2_Sequestration/

Cernocky, E.P., and F.C. Scholibo. 1995. Approach to Casing Design for Service in Compact Reservoirs.
Presented at SPE Conference

Charlez, P.A., and P. Breant. 1999. The Multiple Role of Uncoventional Drilling Technologies from Well Design to Well Productivity. Presen ted at 1999 SPE Conference.

Chen, Y.C., Y.H. Lin, and J.B. Cheatham. 1990. Tubing and Casing Buckling in Horizontal Wells. Journal of Petroleum Technology v. 42

.

Energy Information Agency. 1993. Drilling Sideways – A Review of Horizontal Well Techno logy and its Domestic Application.

Inglis, T.A. 1988 . Petroleum Engineering and Development Studies: Volume 2 Directional Drilling.
Kluwer Publishers

Joshi, S.D. 1991. Horizontal Well Technology. Pennwell Books, Tulsa, OK

Knoll, R.G. 2005. Lessons from Canadian Natural Gas Horizontal Wells. Presented at Petroleum Technology Transfer Council 2005 meeting.

Lockyer, C.F., D.F. Ryan, and M.M. Cunningham. 1990. Cement Channeling: How to Predict and
Prevent. Presented at SPE Conference.

Powell, J. W., M.P. Stephens, J.M. Scheult, T. Sifferman, and J. Swazey. 1995. Minimization of Formation Damage, Filter Cake Deposition, and Stuck Pipe Potential in Horizontal Wells through the Use of Time

Independent Viscoelastic Stress Fluids and Filtrates. Presented at SPE Conference.

Sabins, F.L. 1990. Problems in Cementing Horizontal Wells. Journal of Petroleum Technology v. 42 #4.

SACS. Best Practices Manual.Page 13 White, D.J., G. Burrowes, T. Davis, Z. Hajnal, K. Hirsche, I. Hutcheon, E. Majer, B. Rostren, and S. Whittaker. 2004. Greenhouse Gas Sequestration in Abandoned Oil Reservoirs: The International Energy Agency Weyburn Pilot Project. GSA Today v. 14.