8. Tools and Techniques for Plug and Abandonment

In permanent P&A operations, there are situations where there is only a poor annular barrier or no annular barrier at all. When there is a long length of uncemented casing, a cut-and-pull operation can be the necessary option. In this operation, a circumferential cut is made of the casing, above a casing coupling, and then a spear is engaged inside the casing to pull the casing out of hole. The spear can be engaged hydraulically. For the traditional method, the pulling force is provided by the working unit through the workstring to the bottom hole assembly. However, the advancement of cut-and-pull techniques provides a new generation of tools, downhole hydraulic pulling tool anchors, to create large amounts of pulling force without fully engaging the working unit pulling capacity. As an example, by use of 1 psi hydraulic power, 300 psi is generated by the downhole hydraulic pulling tool anchors [1].

Ideally, the cut-and-pull operation is a single trip. However, there are challenges associated with it, which may require multiple trips. Such challenges are settled barite behind casing, scale depositions, collapsed formation, or unknown bond strength of the poor casing cement. Therefore, pipe retrieval requires high pulling capacity. The pulling capacity can be beyond the working unit or workstring capacity. It can also compromise the stability of the facility (i.e. working unit or platform). Therefore, multiple trip are performed by cutting the casing into short lengths. During casing pulling operations when the casing is moving, debris may fall down around the casing causing it to get stuck and even be irretrievable. Multiple trips expose personnel to several cut-and-pull operations and increases the risk of HSE issues. In addition, the retrieved pipe needs to be handled safely and disposed off properly.

The casing cut can be done using explosives, chemicals, mechanical cutters or using abrasive cutters. Regardless of which type of cutting technique is used, usually the cut is performed when the casing is under tension. Some of the challenges associated with explosive cutters are transportation, handling and storage, uncertainties related to eccentricity or stand-off of the device and damage of the outer casing, dispersion of force from the device, and shape of the resulting cut. Radial cutting torches, which use thermite derivatives to melt casing radially, can cut the casing partially or cut the pipe behind casing. Chemical cutters utilize chemicals which react with steel. Bromine triflouride is an example of such a chemical which is extremely hazardous for surrounding and personnel with irreversible health effects. The efficiency of chemical cutters can be affected by the presence of scale, poor spray pattern, or eccentricity of casing. Mechanical cutters are either electrical pipe cutters (see Fig. 8.1) or hydraulic pipe cutters (see Fig. 8.2). One of the advantages of mechanical cutters is the centralizer which holds the cutter in the pipe center and the risk of damaging the outer casing due to eccentricity is reduced.

For abrasive cutting techniques, abrasive cutting particles are injected into a water jet and wear away the production tubing, casing, drill pipe or drill collar. As this technique has advantages for cut and removal, especially wellhead cut and removal, the subject is covered in more detail, later in this chapter.

In this operation, casing is milled when a length of casing needs to be removed. Such circumstances may include slot recovery or sidetracking. The process of opening a window is typically done by a mill, however, milling with the use of an abrasive fluid jet has been studied [2]. In a P&A operation the required length is usually longer than required for sidetracking. Therefore, typically a section casing milling operation is carried out.

One of the main reasons limiting the use of rigless P&A operations is poor casing cement or uncemented casing. For conventional practice, a window is section milled and the operation is called section milling. The aim of section milling is to grind away a portion of casing and cement. While section milling the casing, the hole needs to be kept clean by removal of produced swarf and other debris. The term swarf is used for metal fillings or shavings created by the milling tool during the casing removal process. The opened window needs to be under-reamed to expose new formation. Then, a cement plug is placed. Section milling is a time consuming operation and difficult to execute safely and efficiently. The current rate of milling operations for 7-in. casing is typically around 7–9 (ft/h), with additional time taken for tripping, hole cleaning and cleaning of BOP cleaning. The operation increases risk and introduces different challenges. The fluids designed for section milling must have sufficient weight and viscosity to suspend and transport swarf to surface while keeping the opened hole stable. Sometimes, the required viscous profile of the designed fluids increases the ECD to exceed the fracture gradient, resulting in breaking the formation. This phenomenon may lead to fluid loss and subsequently swabbing and loss of well control. Presence of fluid loss also causes poor hole cleaning and risk of packing off the Bottom Hole Assembly (BHA) which can lead to sticking of the milling or under-reaming BHAs. Section milling is also affected by the location of casing couplings and casing accessories such as centralizers and scratchers. With current milling tools, there is a risk of splitting and buckling the casing, which effects performance and the ability to successfully mill the required interval. Swarf and skimmed casing, and debris can also damage the BOP and effect its functionality. At surface, the transported swarf must be separated and captured by use of handling equipment. Swarf needs to be handled and disposed off properly and as it has sharp angular surfaces, it introduces HSE challenges. Swarf therefore requires a special handling system with trained personnel who need to be equipped with protective equipment. The milling operation requires the use of a drilling rig which is costly. As the cutting tool is worn out after only some feet of milling, frequent trip out and in is often required, which is time consuming. An additional limitation of section milling is generated vibrations. Table 8.1 shows the milling data gathered from a well on the NCS. Due to challenges associated with conventional milling operations, some techniques or methods have been suggested as alternative solutions. These techniques include upward milling, PWC, melting the downhole completion and plasma-based milling.

Section milling is a proven technique which gives full access to the original formation for creating a rock-to-rock barrier. However, swarf transportation to surface and swarf handling on surface are time consuming and costly operations which are associated with HSE risks. If swarf could be left behind in the wellbore, section milling without swarf to surface, the section milling technique could be more efficient. Upward milling is a new form of section milling technique where the milling operation is performed while moving upward, cutting the swarf into small bits, where the swarf falls down into the wellbore. This system consists of a taper mill, auger section, section mill, emergency release disconnect, jet sub, left-hand rotating mud motor, drill collars, torque isolation assembly, spring loaded pads, spiral stabilizer, and intensifier [3, 4]. Figure 8.3 shows the main tools of upward milling bottom-hole assembly, from top (component 1) to bottom (component 11).

The key components of the upward section mill assembly are shown in Fig. 8.3. At the bottom of the planned milled window, the assembly opens its knives and creates the cut through the casing. Then, it mills upward to the desired depth, and finally retracts the knives at the top of the window. In conventional milling, the knives are retracted when pulling the workstring upward. However, the retraction mechanism for knives in the upward milling method is challenging as the knives cannot retract through upward movement.

Left-hand mud motor—Right-hand rotation upward milling increases the risk and chance of unscrewing casing collars, especially at intervals where uncemented casing exists. Therefore a left-hand motor provides the downhole left-hand rotation and the required torque for the section mill and auger. A design feature for such a motor is high-torque and low-speed. This type of motor may be used in combination with coiled tubing to carry out rigless P&A operations.

Auger—In order to improve the process of swarf movement into the rat hole and prevent bridging, auger sections are used (see Fig. 8.3). Casing inside diameter, auger outside diameter, fluid flowrate, density of fluid and system operational procedures are some of the parameters considered during the design process of the auger and which affect the efficiency of it.

When milling upwards, to prevent backing-off of casing collars, a left-hand rotation is necessary. It can be facilitated either by a left hand mud motor or by left-hand workstring to surface. An alternative flow path is also a requirement as swarf or cuttings need to be deposited below the milled section. Table 8.2 presents advantages and possible limitations of the upward milling technique.

Generally speaking, this technique was first taken into use in the 70’s to establish annular barrier whereby casing is perforated, washed and cemented [5]. Briefly, a perforation gun is run to the barrier depth where there no cement or poor cement behind casing. The casing is perforated, Fig. 8.4, and the gun is either left in hole or retrieved. In the next step, a washing tool is Run in Hole (RIH) and washes the annular space behind the perforated casing to remove the debris, settled mud and mud film [6, 7]. The washing process is carried out downward and a several times to obtain fresh formation. At surface, removed metal and debris can be seen and monitored on shakers which gives better control of the washing process. When washing is completed, an integrity test is performed to check the quality of washed and removed zone. If the integrity test is successful, the washing tool may either be left below the bottom perforations to function as a mechanical foundation for the cement plug, which is placed in the next step, or it is used as BHA for the cementing stage. For the washing tool to serve as a foundation, a packer is incorporated in the tool and once activated, stays in place. For the next step, spacer is pumped through the perforations. To do so, a new BHA may be used if the wash tool has already been released after the washing process. While pumping spacer, the work string is pulled out of hole. The process is known as pump-and-pull. The spacer is placed below the bottom perforations and extended to above the top perforations.

Nowadays, the perforation gun and washing tool are run in a single trip and when perforating is completed, an activation mechanism is engaged and drops the gun into the well rat hole. The single trip method saves time (Fig. 8.5).

For the next step of the operation, the BHA is placed below the bottom perforations and cement slurry is pumped, Fig. 8.6. After pumping some volumes to remove the spacer around the BHA, work string is pulled out of hole while pumping cement. The pumping of cement is continued with the calculated rate while pulling the work string with an optimal speed until cement and BHA reach above the top perforations. The BHA needs to be pulled out of the cement plug, to at least two stands above the top of cement. Then the well must be circulated to get clean.

One of the challenging parts of the PWC technique is the washing operation. The goal of washing is to remove any materials present behind the perforated casing. Wash fluid, which is a modified water-based fluid, should be pushed through the created perforations, and transport any materials presents out from the annular space. Currently, there are two different methods of washing: swab cup tool and jet tool. In the swab cup method, rubber plastic cups are installed below and above the injection nozzle present on the BHA, Fig. 8.7. Cups create a seal between casing and the work string and prevent the washing fluid traveling in the annular space between the casing and tool, Fig. 8.8a. In this way, wash fluid penetrates through the perforations into the annulus behind the casing and moves upward.

The jet tool method uses a jetting tool to wash and clean out debris by spraying wash fluid, Fig. 8.8b. The angle of jet nozzles and the exit velocity of wash fluid play an important role in the success rate of the washing technique. Centralization of the jet tool while washing could be a concern while for the swab cup tool, the cups partly act as a centralizer. Table 8.3 presents field data from a P&A operation where the PWC technique was used.

When pumping cement slurry through the perforations, the cement should fill the annular space behind the perforations. Displacement efficiency of spacer and placement of cement is a strong function of exit velocity and inclination of the casing, Fig. 8.9. The displacement efficiency, during washing and cementing, is a concern and matter of research. More theoretical and experimental work should be performed to understand the mechanisms involved. To improve the cement placement and force cement through the perforations, different tools have been designed. Creating a cyclone effect is one of the suggested methods, Fig. 8.10.

There are advantages and possible limitations for the PWC technique, Table 8.4. Lack of qualification methods are the most challenging limitations. With current technologies, to qualify a PWC job, the cement inside the casing is drilled out and casing cement placed during the PWC job is logged by employing sonic logs. However, holes created during perforating challenge the reliability of logging data, in addition to uncertainties associated with sonic logs and the interpretation of logging data in general. If the annular barrier is qualified, cement is placed inside the casing and when the cement has set, it is pressure tested and tagged.

During permanent plug and abandonment of Oil and Gas wells, the presence of the production tubing introduces challenges associated with logging cement behind the production casing, and cutting and pulling or section milling part of the production casing. Therefore, in conventional P&A methods, the production tubing needs to be retrieved, which is time consuming, costly and associated with risk. The limitations with cutting and pulling casing revolve around two main issues, the ability to effectively cut and retrieve casing and the manual handling of pipe at the surface. Current technology generally requires at least two BHA runs, one with a cutting assembly to cut the pipe at the required depth, then an additional run to retrieve (fish) the pipe above the cut. There are tools available that allow cutting and pulling in one run but a significant time reduction is not yet achieved. Many situations exist which make the pipe unrecoverable, even if the cut is fully successful. In such cases, section milling may be necessary. Challenges introduced by section milling have already been discussed in this chapter. The challenges related to section milling are proliferated by the type of production facility and working unit used for P&A. For offshore P&A activities, rigless P&A utilizing LWIV is a goal. The reason is a significant reduction of daily rental cost. Plasma-based technology may address some of these challenges. The development of plasma-based milling technology for through tubing well abandonment might be a potential solution. Generally speaking, plasma-based milling technology aims to disintegrate steel into small particles and transport the particles to surface [10].

In 1920’s, Irving Langmuir described a fundamental state of matter which, unlike the other three fundamental states of matter, does not freely exist, where an ionized gaseous substance becomes highly electrically conductive. In this state, the behavior of matter is dominated by long-range electric and magnetic fields. In 1928, Irving coined the term “plasma” for the new matter state. Lightning and fire are examples of plasma. Plasma can be produced artificially by subjecting some gases to a strong magnetic field or by heating them [11, 12]. The most common gases used for generation of plasma include: air, argon, nitrogen, hydrogen and carbon dioxide. A Plasma jet can be used for different processes such as plasma cutting, plasma arc welding, plasma spraying, etc. Plasma cutting is a process of cutting an electrically conductive material utilizing an accelerated jet of superheated electrically ionized gas, plasma, having a large kinetic energy [12]. Figure 8.11 shows a schematic of a thermal plasma DC torch based on a cathode ionizing a gas stream.

Downhole conditions and materials imply that, the plasma-based milling technology cannot utilize state-of-the-art conventional plasma torch technology. The most important difference compared to conventional plasma torch technology is that the electrical arc with temperatures of tens of thousands of degrees Kelvin heats the surface of target material directly. In addition, its radiation component is also more efficient, with minimalized heating of intermediate gas. The intermediate gas flow in conventional plasma torches reduces the efficiency of heat transfer into the rock. Moreover, the arc creates area-wide, relatively homogeneous heat flow from a spiral arc on the whole surface for a high-intensity disintegration process.

Figure 8.12 shows a process of tubing and casing section milling using plasma-based tools. The tool is deployed through the tubing to the target zone where the plug is to be set (Fig. 8.12a). The electric arc is ignited, plasma is created and the tool moves upwards while milling the tubing (Fig. 8.12b). After tubing milling, the tool is moved back to its starting position and then removes casing and cement layers (Fig. 8.12c). After the removal of both tubing and casing, the tool is pulled out of hole (Fig. 8.12d). The section is then ready for cement plug placement (Fig. 8.12e).

The combination of a high temperature large cross-section plasma torch and rotating electric arc is another generation of plasma generators, which might be an effective tool for casing milling. The process using plasma technology is based on a mixture of hybridized plasma, chemical and thermochemical processes resulting in fast metal degradation and removal. The main process responsible for the rate and effectivity of steel degradation and removal is high temperature oxidation supported by melting and evaporation. Nowadays, several studies and techniques deal with the effect of water steam and temperature on the steel removal rate for a wide range of input parameters. One can conclude that temperature and heat transfer were found to be the key factors in increasing the constant rate needed for the required thermo-chemical and thermo-physical processes. The proportional contribution of the processes results in a steel removal effect, which varies with changing temperature and brings the following basic features [10]:

Because of steel oxidative processes, a large amount of energy is released during oxidation reactions and recycled to the steel removal processes. In closed vessel conditions, the total energy consumed for steel removal is at least by 30–40% lower than the theoretical value needed for steel melting. Penetration rate is a strong function of total power put into the steel degradation processes and the environment [10, 13]. Theoretically, by increasing input power the steel removal rate should be increased slightly linearly up to its saturation point, which can be obtained only by experimentation.

Figure 8.13 shows a plasma-based tool, which is acting on a mono-structure multistring casing sample whereas casing cements support the casings. As shown in Fig. 8.13, the inner casing and cement layer have been completely removed on the chosen section. Experiments have proved that a 3.5-in. tool is capable of milling a wide range of casing sizes including 4½-in., 5½-in. and 7-in. [14].

Scaled testing in pressures up to 1450 psi has been reported. Based on the challenges associated with section milling challenges, several parameters like ROP, steel types and cuttings types from plasma milling processes have been analyzed. Experiments carried out at different boundary conditions show that the efficiency of cutting steel can be characterized, empirically, by one special parameter. This parameter, ε, describes the energy needed for total removal of a mass of steel under the physical conditions. The parameter, ε, has a statistical character as it summarizes the liberated energy coming from exothermic iron oxidation processes and the real electric energy supplied to the plasma generator. Therefore, it is evident that ε is always lower than the consumed electric energy. It was also found that ε is dependent on the degree or type of steel oxidation and hydrodynamic circumstances [14]. In order to determine the ROP, testing with a plasma generator has been carried out on two types of steels: carbon steel S355 and alloy steel with 20% Cr and 12% Ni. The value of ε is calculated from [14]:where U × I is the electrical power to plasma generator, m is the mass of the removed steel from the sample plate, and t is the time of the process.

A functional correlation has been reported between the steel removal rate (SRR [kg/h]) and plasma voltage U [V], current intensity I [A], plasma torch efficiency h [0–1] and net energy requirement per unit mass of removed steel ε [MJ/kg]:

In real casing conditions, in a water environment at low pressures, the value of ε is found to be in the range of 3–4 MJ/kg. When considering power output 250 kW, plasma torch efficiency 70% and net energy requirement per unit mass of removed steel 3 MJ/kg, the value of SRR is 210 kg/h [14]. This value means ROP 2.0–4.5 m/h for 9 5/8-in. casing section milling (depending on the wall thickness). This ROP is comparable to present-day section milling techniques, however the real difference is the fact that the plasma-based tool is able to mill various casing dimensions (as well as multiple strings) using one tool. This means a reduction in tripping and a significant increase in overall productivity.

For S355 steel, scanning electron microscopy (SEM) analysis clearly indicates the dominant presence of iron (II) oxide in the cuttings, Fig. 8.14. Structural analysis proved a heterogeneity between the formed oxidized and diffusive metallic layers in the cuttings. This resulted in differences in the thermal expansion coefficients of metal-oxide systems at the border of metallic and oxide layers. Therefore, hydrodynamic removal of such weakened multilayers could be realized relatively easily. In the case of alloy steel, the aforementioned differences in thermal expansion properties of metal-oxide multilayers are significantly higher due to a higher grade of chemical heterogeneity in the microstructure. Figure 8.14 shows samples of SEM image and Energy-dispersive X-ray spectroscopy (EDX) analysis of cuttings´ material formed during plasma-based steel removal process of S355 steel.

Apparently, the plasma-based technology is capable of removing carbon steel as well as steel alloys without significant obstacles. Recently, plasma milling in a high-pressure environment has been presented. Subsequently, the following topics associated with the plasma-based milling process of production tubing and/or casing were researched [15]:

Cement removal at pressures of up to 42 MPa using electrical plasma has been tested at laboratory scale. In the case of implementation for either water-based or oil-based fluids, no interference effects on the milling process are reported but due to the presence of drilling fluid contaminating the cement, the removal process seems to be enhanced. The degradation is increased due to different thermal conductivity of present materials. Likely, chemical reactions with a plasma-forming medium are more significant and drilling fluid degradation is stronger or drilling fluid is flushed by the dynamics of implementation of the plasma forming medium into the process. In addition, it is possible to retrieve data of increased electrolysis when the process takes place in a “muddy” environment. The electrolysis level increase is different for WBM and OBM. This gives an important input to the knowledge regarding the structure of the milled casing.

Experimentally it has been shown that plasma-based milling technology can remove production tubing with control line and clamps. Since control line removal is a challenge using conventional technologies, this ability is another advantage.

A well documented advantage is related to the production of small particles instead of swarf. Figure 8.15a shows a typical example of cuttings collected from the casings after the milling processes. Using a sieve analysis, the size distribution of cuttings after drying was evaluated, Fig. 8.15b.

The smaller particles are formed from small fragments of oxidized particles with an irregular shape. A fraction of bigger particles contains a larger number of globular particles having smooth surfaces. The ratio of cement particles is approximately the same for each size group. SEM-EDX analysis has been carried out for each size groups and it has been concluded that oxidation processes penetrate the steel volume. Figure 8.16a shows spherical particles identified as a ferrite material with small amounts of oxygen in the structure. Higher content of oxide is shown in the dark parts on the particles. Figure 8.16b, c show a visible inner structure of the oxide fragment. Advantages and possible limitations of plasma-based milling technology are listed in Table 8.5.

Explosive technology has been used for control of blowing wells, removal of conductors for well abandonment, removal of platform piling for salvage, and the removal of debris which may present a hazard to navigation and the fishing industry [17]. In this technique, shaped charge cutters are used to produce slot type cuts rather than producing holes in a classical manner. In the classical manner, conical lined shaped charges are used to produce perforations when completing oil and gas wells. The principal of charge cutters and conical shaped charges are the same but the charge cutters provide a linear cutting action (see Fig. 8.17). To create a cut in circular geometries (such as pipes and wellheads) circular cutters, which consist of two 180° hermetically sealed charges, are used (see Fig. 8.18). The circular charges can be used inside or outside circular geometries.

Generally, an explosive cutter system consists of three main parts: command unit, detonator, and charge. The command unit sends a signal via a shielded electrical cable to a detonator, and the detonator initiates the charge directly or via a cortex link. There are some advantages and possible limitations associated with use of explosive cutting for wellhead cut and removal, Table 8.6 (Fig. 8.19).

The petroleum industry is familiar with different hot cutting methods including oxygen-gas cutting, oxygen-arc cutting, thermic lance, plasma arc cutting, pyrotechnic cutting, and flame jet cutting. The hot cutting technique for land-based and underwater (wet) cutting is almost the same. However, due to presence of water, a gas pocket needs to be created between the torch and target. One main reason to create the gas pocket is that water dissipates the heat more than air and the cut efficiency is dramatically reduced. General advantages and possible limitations of hot cutting are listed in Table 8.7.

In the flame cut process, an oxygen-fuel flame burns in the gas pocket and heats a spot on metal. A jet of pure oxygen, which is located in the center of the heating flame, blows against the spot on the metal to oxidize it with pure oxygen. As the torch is moved, the cut is formed [18]. Hydrogen is the prime fuel gas used for underwater cutting. The oxygen-acetylene flame is another type of flame which generates more heat compared to the oxygen-hydrogen flame. The flame equipment is bulky and requires added skills. In addition, it will only cut through steel and cannot cut through stainless steel nor nonferrous metals such as aluminum, and bronze. This lack of cutting ability is due to the low degree of oxidation of such materials. The flames cut efficiency is a function of water depth. Therefore, the technique is not used as it was used in the old days. The advancement of arc cutting technology has resulted in reduced use of flame cutting.

The arc cutting technique is almost similar to the flame cut but instead of a flame, a plasma arc is the source of heat. The arc heats the metal and oxygen is blown through the electrode to oxidize the metal. Compared to the flame cutting technique, arc cutting is faster and easier to handle and use. However, it can only cut through carbon or alloy steel. A variation of arc cutting is plasma-arc cutting.

The plasma-arc cutter generates a large amount of heat which acts on a spot on the steel surface. A gas flow blows away the molten metal, Fig. 8.20. The plasma-arc is able to cut through thick metal devices with high speed. It can cut through steel, aluminum, copper, and stainless steel alloys, cement and multiple casings.

Generally speaking, mechanical cutting methods have limitations specially when there is no cement in the annular space between conductor and casing string. The lateral movement of one string creates a challenge for cutting the next string. Mechanical cutting is divided into different categories including diamond wire cutting system, milling cutter, sawing (guillotine saw), and grinding.

Abrasive methods have long been used in industrial and manufacturing processes to create cuts through rock, steel, and reinforced concrete [21]. Abrasive methods used in the petroleum industry to create cuts are categorized as sand cutting and abrasive water jet cutting. This categorization is based on the pressure used to create the cut. In a sand cutting technique, a high volume of particles are pumped at low pressure; however in abrasive water jet cutting, a low volume of solid particles are pumped at high pressure [22].

Light Amplification by Stimulated Emission of Radiation, broadly known as Laser, was coined by Gordon Gould in 1957. Generally speaking, lasers are devices which convert different kinds of energy to electromagnetic beams of monochromatic and coherent waves. Monochromatic means the output electromagnetic waves have a single output wavelength or in other words it means one color output. Coherent means that all the waves are in phase with one another. The generated waves span through the different regions including gamma, X-ray, ultraviolet, visible light, infra-red, microwave, and radio waves.

If the generated stream of electromagnetic beams have high enough energy, then they can create a cut on steel and rock samples. However, high-powered laser technology is required for such operations. The intensity of a laser beam depends on the wavelength of the beam. Common components of a laser are active medium, energy input (known as pump source), and feedback (laser cavity). An electron is pumped into a highly excited state and transit to a metastable region. As the electron loses its energy to return to its initial conditions, it generates photons in different directions. This process is known as spontaneous emission.

The efficiency of a laser cutter depends on several laser properties including discharge type, peak power, wavelength, average power, intensity, repetition rate, and pulse with the discharge type [25‐27]. The laser discharge can be pulsed or continuous. In pulsed discharge type, the optical power appears in pulses for a certain period of time at some repetition rate. However, in continuous type of discharge, the optical power appears continuously.

The main challenge associated with the utilization of laser cutters at downhole conditions is the presence of wellbore fluids. Downhole fluids are opaque, near-opaque, or even dark which are not conducive to laser cutting.