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A new coiled-tubing-conveyed tool uses special beads to clean in situ completions.
In three areas of the world, insoluble scale deposits were removed from wellbores in which previous removal attempts using conventional mechanical and chemical means had proven unsuccessful. Results were accomplished with a scale removal system that combines two recently developed technologies: an advanced jetting tool assembly that can be configured for specific scale situations and a unique abrasive material.
The Jet Blaster system combines a rotating-head jetting tool with abrasive particles called sterling beads. The system is supported by a software package that helps the engineer select the optimum head, nozzle size, flow rate and pressure for each job conducted, maximizing scale removal and wellbore clean-up efficiency.
The scale problem
Scale can become a problem when wells produce water along with oil and gas. When incompatible waters mix downhole, dissolved minerals supersaturate produced water during critical changes in pressure and temperature.1 The scale precipitates from the water, adheres to metallic surfaces and forms hard, layered deposits.
Water production also can be a problem during supplemental recovery operations. Surface water injected to boost reservoir pressure can produce scale when the water breaks through in a producing well. Injected seawater is especially problematic as it promotes the growth of barium or strontium sulfates, both of which are virtually insoluable.2
Early scale growth will roughen the surface of production tubing, increasing friction pressure and reducing flow rate. Production rates are further reduced as scale grows and decreases the cross-sectional flow area (Figure 1).
Conventional removal methods
Scale-removal methods must be quick, nondamaging to the wellbore and formation and effective at preventing scale recurrence. An inappropriate removal method can actually promote scale reprecipitation.
The best technique for scale removal depends on the quantity and type of scale present. Scale types have different strengths and textures, varying from soft, delicate layers, whiskers or crystals having high microporosity to rock-like layered deposits that have low permeabilities and porosities. Well completion design also can influence the scale removal procedure selected.
While some scales, such as calcium carbonate, can be dissolved using acid treatments or washes, their use can cause well and tubing damage. Additionally, acids require inhibitors and thorough well cleanouts. Moreover, scale often is not homogeneous, but rather a mixture of different scale types. In this case, an acid solution will dissolve only the soluble scale, leaving the inert scales intact.
For acid-intolerant wells and scales that are resistant to chemical removal, mechanical scale removal methods can be used. Conventional solutions for mechanical removal are positive displacement motors and mills, impact hammers with mills, or pure-liquid jetting techniques. However, mills and similar cutting tools can jeopardize tubing integrity during treatment. Moreover, their sizes are limited by a well's smallest restriction. Also, neither mills nor impact hammers allow for internal profile or diameter variations, which often are present in modern completions. Therefore, they are best suited for cleaning straight, unobstructed pipes rather than irregular completion hardware.3
Downhole liquid jetting tools use multiple jet orifices or an indexed jetting head to achieve full wellbore coverage. Although certain jetting systems use acid, their application is limited to acid-soluble scales and wells that are more tolerant to acid. Although pure-water jetting is effective for removing softer deposits, its effectiveness decreases in hard-scale applications. Moreover, when water jets alone are used to clean scales, they do so by removing them in large chunks, which can jeopardize hole-cleaning objectives.
The same problem is encountered when scale is milled. Cutting size cannot be controlled, resulting in lumps too large to be easily circulated from the wellbore.
Jetting with abrasive-laden slurries is an alternative to pure liquids. Adding a small concentration of solids, 1% to 5% by weight, to a water jet can improve its ability to cut through scale. Sand has been used as an abrasive with some jetting applications. However, because steel is prone to ductile failure, the rough particles can damage tubulars; their integrity can be completely destroyed if a tool fails to continue its penetration during jetting operations.
The search for a better abrasive
A study at Schlumberger Cambridge Research investigated the physical interactions between abrasive particles and standard tubing materials.2 Studying the interaction between an individual particle and a target surface showed differences between the failure mechanisms of brittle scale and ductile steel. A sharp, particle-like sand will erode a ductile material with a cutting and plowing action. However, a hard, round particle will bounce off the surface, leaving just an impact crater. Scale exhibits brittle failure in that a particle's impact will fracture the material and ultimately cause substrate failure. Unlike ductile steel, breaking brittle scale is independent of particle shape.
These findings prove that using rounded rather than sharp particles for the abrasive material can effectively remove scale without eroding the steel tubing as well. However, the findings also confirm that while rounded particles reduce steel damage compared to sharp ones, they do not eliminate it. Erosional effects of abrasives are dependent not only on particle shape, but also material hardness. Glass beads can damage tubulars because they are significantly harder than steel. On the other hand, reducing particle hardness too much lessens its effectiveness. The abrasive material must be hard enough to effectively cut scale deposits while leaving steel tubulars intact with minimal damage.
Other parameters such as material friability also factor into the search for the most suitable abrasive. While many spherical particles have sufficient hardness, their low durability renders them ineffective for removing scale. Instead, these highly friable particles shatter upon impact with the substrate, lacking the necessary energy to effectively remove scale.
Extensive theoretical and experimental study enabled the research team to specify the physical properties of an ideal abrasive for removing scale with minimal steel damage. While sterling beads match abrasive sand's exceptional scale-removal performance, they are 20 times less erosive to steel. Also, unlike sand, the material inflicts minimal tubular damage even when prolonged jetting occurs in one spot. The spherical particles have a high-fracture toughness and low friability. Acid-soluble and nontoxic, they simplify cleanup operations compared to other materials.
Making a better jetting tool
The abrasive research was performed in conjunction with jetting tool development. Engineers developed the viscous fluid-controlled jetting tool, which has a rotating head with a speed-control system for regulating rate of penetration (ROP). Its jet-nozzle characteristics are optimized for use with sterling beads.
The main components of the bottomhole assembly are the nozzle head, swivel, high-pressure filter and drift ring (Figure 2). Designed for removing scale from the tubing, the drift ring helps prevent progress downhole until the deposit has been removed, thus providing positive indication at surface of fullbore cleaning. By controlling ROP, the drift ring further ensures full tubing-diameter cleaning with minimal damage to the steel surface.4
The jetting tool and beads comprise a system of coiled tubing-conveyed intervention techniques designed to remove most scale types. The blaster system provides three solutions for removing deposits for a wide range of conditions:
the jet-blasting technique uses the new jetting tool with nonabrasive fluids for soft-deposit removal applications;
the scale-blasting technique uses the jetting tool with the special abrasive beads for hard-scale removal; and
the bridge-blasting technique uses a powered milling head and abrasive jetting for completely scale-plugged tubulars.
The blaster system also includes software that designs a scale-removal strategy. The program identifies correct tool configuration and geometry (drift ring, nozzle and nozzle head size) based on well conditions to predict and optimize jetting power and head penetration rate. It also can determine the optimum abrasive concentration for the specific scale deposit situation.
Applying the system
The scale-blasting technique was used in the North Sea to remove hard barium sulfate deposits identified by multifinger caliper logs.4 Decreasing flowing pressure threatened a multiple-mandrel, gas-lift completion well, because insufficient gas pressure remained to reach the only active valve in a side-pocket mandrel. Increasing water production would reduce the well's life unless a damaged gas-lift mandrel could be changed out. However, thick scale deposits had accumulated on two gas-lift valves in the well, preventing the damaged valve from being removed and replaced.
Previous attempts with solvents had been ineffective in removing enough scale to allow kickover tools to engage and latch the valve. The coiled-tubing-conveyed, special-abrasive jetting technology cleaned the damaged side-pocket mandrel in addition to the other valve at a rate of 100 ft/hr. The damaged valve was successfully retrieved and replaced, thus sustaining production and extending well life.
In a tapered production-tubing well completion in Gabon, thick barium sulfate deposits on five gas-lift mandrels did not allow them to be easily changed out. Gauge-cutter runs revealed scale buildup that bridged the tubing and blocked access to the lower section of the well, which had been shut-in since 1994. Numerous mechanical and chemical scale removal attempts had proven unsuccessful.
The scale-blasting technique with beads successfully cleaned the gas-lift mandrels, allowing their replacement and permitting production for the first time in 3 years. Expected production targets were exceeded, with rates reaching 2,000 b/d shortly after treatment. Records were made as well - 6,500 ft (1,983 m) of tubing were cleaned in 25.5 hours total jetting time, with penetration rates averaging 600 ft/hr to 900 ft/hr.
Recently, the system was used on a high-producing well in the Gulf area. Scale problems restricted and then prohibited flow, resulting in 5,000 b/d of lost production from the short string. A slickline confirmed the scale buildup from depth of 4,380 ft (1,336 m). Combined with the beads, the jet-blaster technique completely removed the scale, cleaning the wellbore in one run and restoring accessibility.
The author thanks Schlumberger for permission to publish this article and Aaron McDonough and Leo Burdylo for their contributions to this article.
1. Crabtree, M., Eslinger, D., Fletcher, P., Miller, M., Johnson, A. and King, G.: "Fighting Scale - Removal and Prevention," Schlumberger Oilfield Review, Autumn 1999.
2. Johnson, A., Eslinger, D. and Larsen, H.: "An Abrasive Jetting Scale-Removal System," SPE paper No. 46026.
3. Bittner, S., Zemlak, K. and Korotash, B.: "Coiled Tubing Scale Removal of Iron Sulfide - A Case Study of the Kaybob field in Central Alberta," SPE paper No. 60695.
4. Tailby, R., Ben Amor, C. and McDonough, A.: "Scale Removal From the Recesses of Side-Pocket Mandrels," SPE paper No. 54477.