2 Well plugging and abandonment techniques
The purpose of permanent well abandonment is to isolate permeable and hydrocarbon bearing formation to the purpose of e.g. protecting underground resources, preventing potential contamination of potable water sources and preclusion of surface leakage. Abandonment aims at restoring the natural integrity of the formation that was penetrated by the wellbore. Abandonment procedures were developed in the oil and gas industry where several techniques were designed to prevent interzonal communication and fluid migration. If a well is not properly abandoned, it may provide pathways for brines, hydrocarbons or other fluids to migrate up the well and into shallow drinking wateror to surface.
The configuration of an abandoned well commonly comprises a surface casing often extending to depths below the lowermost drinking water aquifer, and a (set of) production string(s) running to the target formation. The annuli between casing and formation, and between different casing strings in general is cemented at least to an extent. Abandonment plugs consist of tailored cement types that may be supported by mechanical plugs.
In order to effectively seal a well, a stepwise procedure for plugging and abandoning wells was described by Fields and Martin (1997), addressing the removal of equipment and tools, cleaning the wellbore, plugging and testing.
2.1 Preliminary activities
2.1.1 Removing downhole equipment
The first step when starting a well abandonment operation is removing existing tools. This can be done using an existing drilling or conventional workover rig with the capacity to pull out of hole all downhole equipment previously used by the operator, such as production tubing, downhole pumps and packers. If tool removal is not possible due to stuck or lost equipment, well abandonment strategies have to be revised and approved by concerned authorities.
2.1.2 Wellbore cleanout
After the removal operation, the wellbore needs to be cleaned from fill, scale and other debris. To this purpose the wellbore is flushed by a circulation fluid with sufficient density to control pressure and with the physical properties that enable the removal of debris. Dependent on the specific conditions additional tools or additives may be required to successfully clean the hole.
The principal technique applied to prevent cross flow between permeable formations is plugging of the well, creating an impermeable barrier between two zones. Well plugs are being used for several different operations in oil and gas industry, such as lost circulation control, formation testing, directional/sidetrack drilling, zonal isolation and well abandonment (Smith, 1993). The scope of this study is restricted to the latter two applications. Well plugs can be either cement or mechanical plugs. Specifications of well plugs and abandonment are prescribed by regulatory authorities.
In the oil and gas industry the most common material used for plugging wells is Portland cement, which is placed in the well as slurry that hardens under influence of the in-situ temperature and pressure. A cement plug consists of a volume of cement that fills a certain length of casing or open hole to prevent vertical migration of fluids. Cement satisfies the essential criteria of an adequate plug; it is durable, low-permeable and relatively inexpensive. Furthermore, it is easy to pump in place, has a reasonable setting time and is capable of tight bonding to the formation and well casing surface preventing fluid flow along these interfaces (Calvert and Smith, 1994). In some regions, such as onshore North America, cast iron bridge plugs are as common as cement plugs.
In 1953 the American Petroleum Institute (API) defined standards on the grade and quality of different types of cement to be used in oil and gas industry (Table 2.1). API specifies that an adequate cement plug should have a compressive strength of at least 1,000 psi and a maximum liquid of 0.1 mD. The different classes of API cement are based on the downhole temperature at the depths where the cement is to be placed. Cement slurries are designed to meet API definitions and recommended practices, as well as to satisfy its specific performance criteria. To this purpose additives (e.g. sand, bentonite or dispersants) may be added to the Portland cement to enhance specific properties. Dispersants reduce the water/cement ratio, providing higher strengths and lower permeability. Accelerators may also be added to the cement to increase the early strength of the plug (Calvert and Smith, 1994). However, in many parts of the world, API well cement is difficult or impossible to obtain and construction cements are applied (Rogers et al., 2006).
|Class A||Intended for use from surface to 6,000 feet (1830 m) depth2 when special properties are not required. Available only in ordinary type (similar to ASTM C 150, Type I) 3|
|Class B||Intended for use from surface to 6,000 feet (1830 m) depth, when conditions require moderate to high sulfate-resistance. Available in both moderate (similar to ASTM C 150, Type II) and high sulfate-resistant types.|
|Class C||Intended for use from surface to 6,000 feet (1830 m) depth, when conditions require high early strength. Available in ordinary and moderate (similar to ASTM C 150, Type III) and high sulfate-resistant types.|
|Class D||Intended for use from 6,000 feet to 10,000 feet (1830 m to 3050 m) depth, under conditions of moderately high temperatures and pressures. Available in both moderate and high sulfate-resistant types.|
|Class E||Intended for use from 10,000 feet to 14,000 feet (3050 m to 4270 m) depth, under conditions of high temperatures and pressures. Available in both moderate and high sulfate-resistant types.|
|Class F||Intended for use from 10,000 feet to 16,000 feet (3050 m to 4880 m) depth, under conditions of extremely high temperatures and pressures. Available in both moderate and high sulfate-resistant types.|
|Class G&H||Intended for use as a basic well cement from surface to 8,000 feet (2440 m) depth as manufactured or can be used with accelerators and retarders to cover a wide range of well depths and temperatures. No additions other than calcium sulfate or water or both, shall be interground or blended with the clinker during manufacture of Class G or H well cement. Available in moderate and high sulfate-resistant types.|
1 Reproduced courtesy of the American Petroleum Institute from API Spec. 10 ”API Specification for Materials and Testing for Well Cements.”
2 Depth limits are based on the conditions imposed by the casing-cement specification tests (Schedules 1, 4, 5, 6, 8, 9) and should be considered as approximate values.
3 ASTM (American Society for Testing and Materials) C 150: Standard Specification for Portland .Cement.
In general a minimum of three cement plugs are placed during abandonment operations. These include a cement squeeze at the level of the perforations, a plug that is usually located near the middle of the wellbore, and a surface plug installed at shallow levels (Fields and Martin, 1997). Additional plugs may be placed depending on the specific conditions. However, proper plugging of a well is not always successful at first attempt. A long standing standard was that an average successful plug at the bottom of a wellbore would require 2.4 attempts (Crawshaw and Frigaard, 1999).
In the current operational practice, there are three major methods for well plug placements, i.e. the balanced plug method, the dump bailer method and the two-plug method. API recommends the abandonment to be chosen after an analysis of the well for probable risks and potential problems with respect to the different abandonment techniques (Englehardt et al., 2001). However, in reality ideal practices are not always successfully achieved.
2.2.1 Balanced plug method
The balanced plug or displacement method is the most common placement method. This technique involves the drillpipe or tubing to be placed at the planned plug base depth. Subsequently the cement slurry is placed on top of a mechanical device (such as a bridge plug) or viscous fluid or mud serving as the plug base. The slurry is pumped through the tubing until the level of the cement in the annulus is equal to that inside the casing (Figure 2.1). To prevent mud contamination, a spacer fluid is pumped ahead and behind the slurry. Once the plug is balanced, the tubing is pulled out of the slurry (Nelson and Guillot, 2006).
This method is one of the simplest techniques used in oil industry and is often used for placement of the middle plug of a well (Fields and Martin, 1997). The main problem is that of potential cement contamination. This can be minimized by using appropriate plug base material, such that downward migration of the cement plug is prevented (Nelson and Guillot, 2006).
2.2.2 Cement squeeze method
The presence and good quality of the primary cement sheath at the plugging level is essential for realizing zonal isolation. If cement behind the casing is lacking or inadequate, several methods can be applied to remediate the cement sheath and achieve isolation. API recommends using one of the three following cement squeeze methods (Englehardt et al., 2001): Squeeze cementing, block cementing or circulating cement.
Squeeze cementing involves the process of forcing by pressure a cement slurry into a specified location in a well through perforations in the casing or liner. Once the slurry encounters a permeable formation, the cement solids are filtered out of the slurry as the liquid phase is forced into the formation matrix in the form of cement filtrate. Squeeze cementing is a remedial cementing technique used to repair flaws in primary cement or damage incurred by corrosive fluids. A properly designed squeeze-cement operation will fill the relevant holes and voids with cement filter cake that will cure to form an impermeable barrier.
Block cementing is used to isolate a permeable zone. To this purpose the sections above and below the target formation are perforated and squeezed (Nelson and Guillot, 2006). This procedure is often applied before starting production of permeable zones.
A circulating squeeze involves circulating cement between two sets of perforations, isolated in the string by a packer or cement retainer. The operations consist of an initial circulation with water or acid as receding fluid, a subsequent circulation of the interval with a cleaning wash fluid, and pumping and displacing of the cement slurry. This method is a low pressure squeeze. Except for some increase in hydrostatic pressure resulting from the increasing cement column in the annulus, no pressure buildup is associated with this type of cement squeeze. The exact amount of required cement is unknown, leading to the use of excess cement. This holds the risk that cement slurry enters the casing above the packer or retainer. If this cement would cure, the tubing may become stuck in the hole (Nelson and Guillot, 2006).
Alternatively, at abandonment an operator could cut and pull the casing and isolate the remaining stub using the displacement method.
2.2.3 Dump Bailer Method
A dump bailer is a tool that contains a measured quantity of cement, lowered into the wellbore on a Figure 2.1). Applying this method, the cement is placed on top of permanent bridge plug placed below the desired plug interval. The bailer is opened by touching the bridge plug or by electronic activation and the cement is dumped on the plug by raising the bailer. As the slurry is stationary during its descent, considerations are required regarding special slurry-design (Nelson and Guillot, 2006).(Fields and Martin, 1997;
The dump bailer method is usually used for setting plugs at shallow depths, but can also be used at greater depths by using properly retarded cement systems. The advantages of this method are that the depth of the cement plug is easily controlled, and it is relatively inexpensive. A disadvantage is that the available quantity of cement is limited to the volume of the dump bailer. However, this can be solved by performing different runs. Furthermore, as the cement slurry is stationary in the bailer during its descent, special slurry design considerations are required to prevent slurry gelation or instability (Nelson, 1990).
2.2.4 Two-plug method
The name of the two-plug method is derived from the fact that the cement plug is placed with two (top and bottom) wiper or cementing plugs (Figure 2.2). This method is described by Nelson and Guillot (2006) and involves a special tool setting a cement plug in a well at a calculated depth, with a maximum of accuracy and a minimum of cement contamination. The tool comprises a bottom hole landing collar installed at the lower end of the drill pipe, an aluminium tail pipe, a bottom wiper plug (carrying a dart), and a top wiper plug. The application of cementing plugs enables the effective separation of the cement slurry from other fluids, reducing contamination and maintaining predictable slurry performance. The bottom plug is launched ahead of the cement slurry to clean the drill pipe and to minimize contamination by fluids inside the casing prior to cementing. A diaphragm in the plug body ruptures by increased pump pressure to allow the cement slurry to pass through after the plug reaches the landing collar. The top plug is pumped behind the cement slurry to isolate the cement from the displacement fluid. This plug has a solid body that provides positive indication of contact with the landing collar and bottom plug through an increase in pump pressure. The top plug then prevents cement from flowing up into the tubing string, meanwhile permitting reverse circulation. The drill pipe is pulled up until the lower end of the tail pipe reaches the calculated depth for the top of the cement plug.
2.3 Cement plug evaluation
After the well is plugged, testing is required to ensure that the plug is a placed at the proper level and provides zonal isolation. The plug can be verified by tagging its top, pump pressure testing or swab testing.
Tagging the top of cement (TOC) can be done through the employment of a drill pipe, wire line, work string or tubing. The procedure, recommended by API, is a very straightforward operation with the benefit that no additional pressure needs to be put on the wellbore. It enables exact determination of the top of the plug. Tagging the plug with open-ended pipe can also be applied for testing the cement plug’s integrity. However, disadvantages of this method comprise the concentration of the load on the area where the pipe hits the cement, the required incorporation of corrections for buoyancy and friction when using the pipe weight, and potential weight insufficiency for shallow plugs (Fields and Martin, 1997). Furthermore, a plug may be tested to be rigid at the top, while it shows less strength further down, leading to potential fluid migration over time (Smith, 1990).
Alternatively, pressure test can be executed using pump pressure. In this case the pressure is exerted uniformly on the plug and no corrections are required. The application of pump pressure provides more accurate data on the pressure, which could also be monitored over time (Fields and Martin, 1997). However, the associated changes in pressure itself could initiate casing integrity problems if the well cannot sustain the enforced pressure changes. Furthermore, it could lead to loss of wellbore control if conditions are not static (Englehardt et al., 2001).
Another pressure testing method is swab testing or swabbing. This technique involves running of a swabbing tool that reduces the pressure in the wellbore above the plug to levels below the pressure gradient from the isolatedbelow the plug. Subsequently fluid levels and pressure are monitored to ensure adequate isolation. Swabbing is more time-consuming relative to the other methods.
2.3.1 Plugging effectiveness
Nicot (2008) describes the risk of leakage through abandoned wells as a function of regulations toward drilling and abandonment enforced at the time of plugging, of the diligence expressed by the operator during the plugging, and of the materials used in the plugging operation. Inadequate well design, well construction or plugging/abandonment performance may lead to poor isolation. Primary causes of failure are connected to mud contamination as a result of poor mud removal (most common), unstable cement slurries, insufficient slurry volume, and poor job execution (Nicot, 2008).
These problems may be more abundant among older wells as plugging improvements were developed over time. First, the two-plug method, limiting potential mud contamination, was patented by Halliburton in 1922. Around 1930 the use of centralizers was introduced, enabling more uniform distribution of cement in wells through tubing (Smith, 1976; in: Nicot, 2008). Furthermore, before the invention and widespread use of a caliper survey instrument in the 1940s the exact quantity of cement needed for a thorough job was not available. In addition, before testing of the plug became a requirement, the plug location was not checked but assumed to be located as planned (Nicot, 2008).
Also advances of cementing materials over time enhanced plugging effectiveness, as reported in Nicot (2008): Before 1928, only a single type of cement was available for plugging. In 1940, cement manufacturers provided two types of Portland cement and three additives. After the late 1940s, the number of commercially available additives progressively increased. Since then, the American Petroleum Institute (API) has devised several classes of cement for different well conditions (pressure, temperature, and chemical conditions and, in particular, resistance to sulfate attack). National standards on cements for use in wells in the USA were published by API in 1953, enabling operators to tackle more difficult cementing jobs and ensure better cement placement. Due to the inadequate quality of cements used in advance of the implementation of the API standards, wells plugged with cement prior to 1952 are often not considered to have effective cement plugs (Ide et al., 2006). Although the main composition resembled that of today’s plugging cement, important additives like bentonite, dispersants and accelerators were not incorporated, leading to difficulty in constraining cement hardening, and consequently contamination of cement with drilling muds.
It should be noted that the implementation alone of regulations and guidelines does not necessarily imply that these were followed in full, since regulations have not always been strictly enforced over time in different areas in the world.
2.4 Abandonment practices
In order to develop a comprehensive overview of applied well abandonment techniques, common practice of well drilling, plugging and abandonment needs to be evaluated in different regions throughout the world. To this purpose, a questionnaire was developed and distributed amongst experts active in operating and service companies, consultancies, regulator bodies and research institutes involved in oil well drilling and abandonment activities worldwide. This section comprises a concise description of the results of the well abandonment survey. However, the collected amount of data proved to be insufficient to extrapolate and investigate regional differences.
2.4.1 Overview of well characteristics from representative fields or basins
Respondents provided information on well abandonment practices in Europe (onshore and North Sea), North America and Australia. Representative fields or basins mostly show well densities ranging between 1 to 100 wells/km2, while some records from North Sea gas fields indicate less than one well per square kilometre. Well densities in some high viscous oil fields exceed 100 wells/km2. Most of the wells in the representative fields are dated post-1960 and drilled to depths over 5,000 ft. A significant number of fields holds wells extending over 10,000 ft depth. This is obviously reflected by resulting pressures, showing a roughly even distribution over pressures from 1,000 psi to values higher than 5,000 psi.
2.4.2 Data availability
The majority of respondents indicated that for almost all wells (90-100%) data is available on well location (coordinates), present status of the well (i.e. operating, abandoned, etc.) and the well configuration. The latter comprises information on cased depths, top of cement, plug lengths, but also on the materials applied (i.e. type of cement, steel grade). A single respondent indicated that data is available only for 30-70% of the wells.
2.4.3 Drilling and completion
Various steel grades are used for casing, such as J55, K55, L80, N80, C95, P110 and Q125. In general the application follows guidelines on H2S content, temperature and pressure conditions. In corrosive environments, it is common practice to use Cr-13 type of steel (e.g. L80Cr-13), and/or corrosion inhibitor fluids. All respondents indicated that, prior to placement of annular cement, the borehole is prepared or cleaned. Subsequently, a primary cement sheath is placed over some length of the well, generally ranging from 30 to 90% of the wellbore length (one instance states 90-100% of the well being cemented). Approximately half of the respondents indicate that during drilling and completion no leakage occurs, while others indicate 10-30% of wells showing initial leakage (i.e. sustained casing pressure, gas migration outside the casing), as a result of casing corrosion or wear, poor cement coverage, improper slurry design or overpressurization of the wellbore.
All respondents reported to perform well abandonment according to regional or national regulatory frameworks or, in absence of these, according to international guidelines of OSPAR or the London Convention. Variations in regulations are reflected in the differences between plugging requirements. While some regulations demand records (e.g. pressure tests and/or cement bond log) on the status or integrity of the wells prior to abandonment, others have no specifications on this topic. The majority of regulatory frameworks stipulates plugs to be emplaced using the balanced plug method, whereas the dump bailer method and a choice between the balanced plug and the two-plug methods are noted on a single occasion. The prescribed minimum number of plugs ranges from 1 to 3, with minimum lengths between 8 to 100 m. Regulated plug testing methods generally involve either a weight test or pressure test, possibly in combination with a tag test. Rarely specific requirements are in place for corrosive environments, but if so these comprise the application of Cr-13 steel or inhibiting fluids in the wellbore.
Between operators there is disparity regarding their approach to well abandonment with respect to potential future applications of fields and wells. Although the majority of operators does not take into account second life applications, some started recently to evaluate the field’s value for future purposes prior to abandonment. In general, company practices closely reflects governing regulations, although operators may apply more stringent measures to secure(e.g. longer plug lengths) especially in corrosive environments (e.g. applying corrosion resistant measures and materials). This observation confirms the appropriateness of evaluating regulatory requirements as an adequate proxy for well abandonment practice.
While most respondents indicated that leakage is seldom observed in abandoned wells, some incidences (amounting between 0 to 30% of the abandoned wells) of sustained casing pressure or gas migration outside of the casing is reported, probably due to e.g. poor cement coverage, improper abandonment, overpressurization of the well, or micro-channelling in the cement.