Hydrate-Related Drilling Hazards and Their Remedies
Milad Poorfaraj Ghajari,Alireza Sabkdoost ,Hesam Taghipoor soghondikolaee
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.
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].
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. 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.
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
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.
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].
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 different Mechanical, Depressurization, Chemical and Thermal methods will remove hydrate blockage.
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