hydrate-related drilling hazards and their...

2nd National Iranian Conference on Gas Hydrate (NICGH)

Semnan University

Hydrate-Related Drilling Hazards and Their Remedies

Milad Poorfaraj Ghajari

Sahand University of Technology *[email protected]

Alireza Sabkdost Sahand University of Technology

[email protected]

Hesam Taghipoor soghondikolaee Sahand University of Technology

[email protected]

Abstract Considerable fuel resource for the future, Transportation ease of gas hydrate (as natural gas phase state), likely

role in global climate change and potential drilling hazards are the main reasons for researcher’s attraction to

gas hydrate issues. The gas hydrates have been recognized as significant potential resources for the 21st century

fuel. However, from the drilling perspective, the gas hydrates seem as dangerous drilling hazards. Because of

the importance of drilling operation as the first attempt to access energy sources, it is necessary to pay more

attention to these hazards. The main objective of this article is to present a comprehensive review about the

drilling problems related to hydrate formation in drilling operations and remedies of problems for understanding

the problem in petroleum industry. Some of the notable problems, explained in this article, include wellbore

stability, plugging chokes, kill lines, BOP, gas cut mud and sea floor stability. Different methods for the gas

hydrate suppression during drilling operations and removing blockage practices are perused in this article.

Keywords: Gas Hydrate, Drilling Hazards, Well Problems, Remedies

Research Highlights

This articles is an up to date literature review about hydrate-related drilling hazards.

Useful solutions for drilling hazards remedies were presented.

This study is operational for Iranian gas hydrate bearing field.

mailto:[email protected]

Hydrate-Related Drilling Hazards and their Remedies

1. Introduction

Gas hydrate are ice-like compound in which hydrocarbon gas molecules become trapped within

a lattice of water molecules under high-pressure and low-temperature condition (Solan, 1990) [1].

Hydrates were first observed by Davy in 1810.They were introduced to the petroleum industry

in 1934 by Hammerschmidt as substances which were responsible for the freezing of gas

transmission lines [2]. Methane, ethane, propane, n-butane, i-butane, hydrogen sulfide, nitrogen,

and carbon dioxide are well-known hydrate-forming components [3]. Gas hydrate is thought to

be the largest reservoir of organic carbon on Earth and a source of dissolved organic matter to the

oceans (Kvenvolden, 1993; Whelan et al., 1999) [1] . Current estimates predict that the amount of

gas sequestered in hydrates varies between 100,000-200,000 trillion cubic feet (TCF) (Collet,

1997) [4]. This amount of energy trapped in gas hydrates all over the world is about twice the

amount found in all recoverable fossil fuels today [5].

The conditions necessary for the stability of gas hydrates are moderately low temperatures and

moderately high pressures. These conditions could exist offshore in shallow depths below the ocean

floor and onshore beneath the permafrost [5].Due to the low temperature and high pressure

environment of the seabed, most of the deepwater gas wells will encounter gas hydrate problems

if no hydrate prevention is implemented [6].the required water for hydrate formation can come

from two main sources: drilling fluid or formation water produced with gas influx [2]. Hydrate

formation in shallow-water and onshore wells usually results from the presence of produced water

[3]. The other water sources are such as Condensed water from natural gas, Water from invaded

mud filtrate and Water from water or gas-water transition zones [6].

As hydrocarbon exploration and development moves into deeper water and onshore

environments, it becomes increasingly important to quantify the drilling hazards posed by gas

hydrates [7]. As shown in figure 1 most of the hydrates recovered in nature are offshore although

there are a few hydrates deposits found on land (permafrost) [5].


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Figure 1: worldwide distribution of Hydrate Deposits [8]

As a result of increased deepwater drilling, the potential for natural-gas-hydrate problems

during drilling has increased in recent years [9]. It is very likely that the continuous understanding

of gas hydrate from a drilling perspective could actually improve the success in producing the

enormous resource trapped in these formations.

2. Drilling problems due to gas hydrate

There have been documented cases of hydrate-related well trouble such as gas kicks, blowouts,

subsidence, stuck pipe, gas leaks outside casing, and inadequate cement jobs (Yakushev and

Collett, 1992) [10]. These problems are categorized in two main groups: 1- Wellbore instability

problems 2- Well control problems. Each group is described in details at the following.

2-1. Wellbore instability problems

Gas hydrate dissociation in the wellbore may result in gasification of the drilling fluid. Lowering

mud density, changing mud rheology, lowering hydrostatic pressure, hydrate dissociation and

wellbore instabilities (like hole enlargement and wellbore collapse) are the results of mud

gasification [7]. The amount of gas hydrate that can dissociate will depend significantly on both

initial formation characteristics and bottomhole conditions (like mud temperature and pressure)

[7].


Open-hole instability caused by hydrate dissociation may produce zones of decreased shear

strength in sediment, where sediment can become unconsolidated or over-pressured due to gas

build up and fluid expulsion (Durham et al., 2003; Winters et al., 2001; Winters et al., 2002) [10].

Hole enlargement is the result of hydrate dissociation (gas release) in the openhole section of the

well. Figure 2 shows the schematic of this problem.

Figure 2: Schematic of Gas Release problems in Gas hydrate drilling [11]

Casing collapse is another dangerous problem in the drilling of gas hydrate bearing formations.

Hydrate dissociation may occur behind the surface casing. The casing may collapse if the pressure

in the hydrate exceeds the differential collapse pressure. However, the volume created by the

dissociation of hydrates may be filled with cement, and this may reduce the risk of casing collapse

[12]. If the casing has enough collapse strength, the released gas moves upward behind the casing

and gas leakage will be observable at the sea floor or sea level in offshore drilling and wellsite in

onshore drilling. Instability at seafloor and near-surface interval is the result of hydrate

dissociation. Hydrate dissociation may produce failure planes along gas migration pathways and

weakened zones that destabilize under natural triggers such as gravitational loading and seismic

activity (Kayen and Lee, 1991) [10]. Figure 3 shows the schematic of gas leakage problem.


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Figure 3: Schematic of Gas Leakage problems in Gas hydrate drilling [11]

2-2. Well Control Problems

In deep-water drilling rigs, the risers are partially insulated with the floatation material attached

to them, while the BOPs and choke and kill lines are exposed to sea water. As a result it is more

likely for hydrate to form inside the BOPs and the choke and kill lines [13]. In a well control

situation, the kick fluid leaves the formation with a high temperature, with an extended shut-in

period it can cool to seabed temperature, with high enough hydrostatic pressure at the mudline,

hydrates could form in BOP stack, choke and kill line, as have been observed in field operations

[14].

the formation of natural gas hydrates during deepwater-well-control operations can have

several such adverse effects as [2]:

1. choke and kill-line plugging, which prevents their use in well circulation;

2. plug formation at or below the BOP's, which prevents well-pressure monitoring below

the BOP's;

3. plug formation around the drill string in the riser, BOP's, or casing, which prevents

drill-string movement;

4. plug formation between the drillstring and the BOP's, which prevents full BOP closure;

and

5. plug formation in the ram cavity of a closed BOP, which prevents the BOP from fully

opening.


These problems are represented in figure 4.

Figure 4: Pictorial representation of some notable problems encountered while drilling through gas

hydrate formation [5]

In the rare cases, the water needed for hydrate formation comes from the water-based drilling

mud itself. The loss of water from the mud causes flow properties to deteriorate severely. In the

most extreme scenario, all solids will settle out, leaving little or no fluid in the wellbore [9].

3. Remedies for the drilling problems

According to the mentioned problems, avoiding hydrate formation is the best remedy. Hydrate

formation in the well equipment can be avoided by modification of drilling fluid formulation and

optimization of drilling operations .Sometimes hydrate formation is unavoidable and it blocks the

kill-lines, Bop and chokes. At these situations, hydrate melting is the main method of removing

blockage. There are four basic schemes for hydrate melting.

3-1. Avoid hydrate formation

Techniques adopted so far to avoid the risks of drilling in hydrate zones include the following

(Freij-Ayoub et al. 2007; Birchwood et al. 2005, 2007):

1- keeping the temperature above, or the pressure bellow hydrate formation conditions

2- Cooling the drilling fluid

3- Adding chemical inhibitors and kinetic additives to the drilling fluid to prevent hydrate

formation and to reduce hydrate destabilization in the formation


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4- Increasing the mud weight to stabilize the hydrates, but avoiding fracturing

5- Accelerating drilling by running casing immediately after hydrates are encountered and

using a cement of high strength and low heat of hydration

6- Managing the wellbore temperature by controlling the circulation rate [7]

In drilling operations, good primary control of the well will prevent kicks and keep the wellbore

free of gas. The most practical way to stop hydrates forming during deepwater production

operations is to prevent reaction of gas with water by use of chemical inhibitors [12].The

inhibitors may cause one or more of the following effects:

1. Delay the appearance of the critical nuclei (kinetic inhibitor)

2. Slow the rate of hydrate formation (crystal modifier)

3. Prevent the agglomeration process (crystal modifier) [14]

The salt and glycerol contents of water in mud dominated hydrate formation. Other mud

additives, such as bentonite, barite, and polymers, collectively promoted hydrate formation to a

lesser degree [9]. Salts are effective hydrate inhibitors and their inhibitive effect, on weight bases,

are a function of molecular weight, valency and degree of ionization. Their effectiveness can be

ranked, on weight basis, as follows: NaCI > KCI > CaCl2 > NaBr > Na-Formate > Calcium Nitrate

[14]. NaCl is the best thermodynamic inhibitor compared to NaBr, Na-Formate, KCl and CaCl2.

Among the glycols, ethylene glycol shows the best performance compared to AQUA-COLTMS,

GEO-MEGTMD207 nad HF-100NTM [14]. Ethylene glycol is a better inhibitor due to more

hydroxyl groups being available to make hydrogen bonds with the water molecules and hence make

it more difficult for the water molecules to participate in the hydrate structure [14]. The literature

review indicated that 20-23 wt% Nacl/polymer drilling fluid systems are the most commonly used

drilling fluid formulations for deep water drilling [14].

Surfactants or alcohols are known to decrease the surface tension of water. Lowering the surface

tension of water enhances the rate of gas diffusion in the bulk water during hydrate formation.

Hydrate-crystal growth is controlled by the rate of gas diffusion from the bulk of water to the crystal

surface. Consequently the presence of these components in the water results in rapid hydrate

growth [14].

the net effect of the drilling-mud components was to promote or to increase the temperature

at which hydrates were stable [9]. It was suggested that compounds such as PHPA and Bentonite

are thermodynamic promoters since they keep the hydrate stable at higher temperatures relative to

pure water [14].

Increasing the mud density increase increases the pressure at the hydrate layer and controls the

dissociation of hydrates during drilling. By using cooler mud, the mud column does not become


the heat source for the dissociation. Franklin suggested drilling with lower mud weight allowing

the hydrate to decompose and controlling dissociation rate [15].

The rate of penetration is directly proportional to the amount of gas released when drilling

through gas hydrate [5]. So ROP, WOB and mud circulation flowrate are some parameters which

should be optimized to ovoid hydrate formation.

Some new technologies to consider in deepwater or offshore drilling for avoiding hydrate

hazards may include: [16]

Managed Pressure Drilling (MPD)

Slim and Insulated Marine Riser

Drilling the Top Hole in Deep Water

Underbalanced Drilling

Drilling With Casing (DWC)

Using gas hydrate pills which are concentrated, highly-inhibitive formulations is useful solution.

These pills can be placed in the BOP stack and choke and kill lines and are utilized when a gas

kick is encountered during a drilling operation or when the drilling location is abandoned during

several hours due to adverse weather or technical faults. These fluid are usually formulated to be

much more hydrate suppressive than drilling fluids [17].

3-2. Removing Blockage

According to the hydrate phase diagram in figure 5, hydrate melting can be achieved by

changing the hydrate state (changing P & T) to the instability state of hydrate.


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Figure 5: Phase diagram illustrating three basic hydrate melting Schemes [13]

Figure 6: methods of hydrate blockage removing

As figure 6 shows, hydrate melting, as a main removing blockage method, can be achieved by

using four basic schemes such as [13]:

1. Mechanical, by applying direct mechanical force such as drilling or differential pressure.

Mechanical removal by drilling or jetting seems to be the safest way to remove hydrates

plug. The preferable and most available means to mechanically clear a plug inside kill and

choke line will be a coiled tubing fitted either with a nozzle or a mud motor [18].

Removing Blockage

Mechanical

Drilling Blockage

Jetting fluid

DepressurizationProduction from free-gas zone

(below hydrate zone)

Chemical Using Inhibitors

Thermal

Radial heat tracing

pipe warm-up

Hot water circulation


2. Depressurization, means reducing the pressure over the hydrate plug to a pressure below

the hydrate equilibrium pressure at the prevailing temperature. So the hydrate blockage

starts to dissociate at the boundary subjected to the pressure reduction. The most common

depressurization technique envisions drilling through the hydrate layer and completing the

well in the free-gas zone. Gas production from this layer leads to pressure reduction

and decomposition of the overlying hydrate [12].

3. Chemical, by using inhibitors like methanol, salts or glycol into direct contact with the

hydrate blockage to destabilize the hydrate. Alcohols and glycols are well known hydrate

thermodynamic inhibitors [13]. It is reported that methanol and a calcium chloride solution

were successfully injected for remediation to reopen flow paths in Messoyakha Field [19].

4. Thermal, increasing hydrate temperature above the hydrate equilibrium temperature.

Radial heat tracing, pipe warm-up, hot water circulation thorough coiled tubing, using

acetylene frame, downhole electric heater and heat generating fluids are some available

options to remove a hydrate blockage from the choke and kill lines [13]. A patented

thermochemical method –Self Generated Nitrogen (SGN) is new technic which applies the

heat to dissociate the crystallized hydrate. Heat application around the body of the

equipment enabled them to dissociate and release the tree cap by means of its regular

retrieving tool [12].

4- Conclusion

Overcoming drilling hazards guaranties the successful well completion and production

operations with the lowest cost. Recognition the type of well problem helps us finding the most

suitable solution. Hydrate-related drilling hazards are categorized in two main groups: 1- Wellbore

instability problems 2- Well control problems. Hole enlargement, wellbore collapse, casing

collapse, seafloor instability are some problems referred to wellbore instability in hydrate issues.

Hydrate formation inside equipment causes choke, kill-line, BOP and formation plugging and so

well control hazards. The main solution of overcoming drilling problems is avoiding the occurrence

of problems. Adding salts, glycols and inhibitors to drilling fluid and regarding some points in

drilling operations can avoid hydrate-related drilling problems to some extent. Despite of

precautions, hydrates are formed in the wellbore and block the equipment. Hydrate melting by


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different Mechanical, Depressurization, Chemical and Thermal methods will remove hydrate

blockage.

References

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Gulf of Mexico, OTC paper 12111, presented at the 2000 Offshore Technology Conference held in

Houston, Texas, 1–4 May 2000.

[2] Barker J.W , Gomes R.K, Formation of Hydrates During Deepwater Drilling Operations, SPE

paper 16130, Journal of Petroleum Technology, March 1989.

[3] Hale Arthur.H, Dewan Ashok K.R, Inhibition of Gas Hydrates in Deepwater Drilling, SPE paper

18638, SPE Drilling Engineering , June 1990.

[4] Diaconescu C.C, Knapp J.H , Gas Hydrates of the South Caspian Sea, Azerbaijan: Drilling Hazards

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[8] Kvenvolden, K.A., Gas Hydrates-Geologic Perspective and Global Change, Review of

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[9] Kotkoshle T.S, et al., Inhibition of Gas Hydrates in Water-Based Drilling Muds, SPE paper 20437,

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[12] Catak E., Hydrate Dissociation during drilling through in-situ hydrate formations, Master of

Science Thesis, Technical University of Istanbul, May 2006.

[13] Yousif M.H, et al., Hydrate Plug Remediation: Operations and applications for Deep Water

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Conferences and exhibition in San Antonio, Texas, 5-8 October 1997.

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Occurrence and Recovery, Butterworth, Woburn, MA., p.115, 1983.


[16] Hahhegan, P.E., et al.: MPD-Uniquely Applicable to Methane Hydrate Drilling, SPE paper 91560

presented at the 2004 SPE/IADC Underbalanced Technology Conference and Exhibition, Houston,

Texas, U.S.A., 11- 12 October 2004.

[17] Halliday, W., Clapper, D. and Smalling, M., New Gas Hydrate Inhibitors for Deep Water Drilling

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[19] Makogon, Y.F., Hydrates of Hydrocarbons, Pennwell Publishing Co., Tulsa, Oklahoma, 1997.

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