Bentonite Plugs Research Articles

Mechanical Performance of Bentonite Plugs in Abandonment Operations of Petroleum Wells

This study aims to evaluate how the operational procedure adopted for pellet placement and the exposure to subsurface conditions influence the mechanical integrity of bentonite plugs used as barrier elements in the abandonment of petroleum wells. To this end, the plugs were formed by hydrating the pellets directly in water, simulating the onshore procedure, while the offshore plugs were obtained from pellets hydrated in deionized water after immersion in diesel or olefin, which are suggested as displacement fluids. The plugs obtained were tested by compression and adhesion tests. These mechanical tests were also carried out for specimens obtained from plugs exposed to four formulations of synthetic formation waters. The results obtained demonstrated that, in the offshore procedure, the previous contact with olefin may adversely affects the mechanical stability of bentonite plugs, while plugs formed from pellets immersed in diesel presented satisfactory mechanical properties. However, the contact with formation water evidenced that the onshore plug presents superior resistance than the offshore plug previously immersed in diesel. The highly successful performance of the onshore plug was attested by the maintenance of the compressive strength, which exhibited a maximum reduction of 13%, even after exposure to the most saline formation waters.

Modelling the piping-assisted erosion of clay barriers

Recent laboratory and field experiments have provided evidence of the erosion and piping of clay-based barriers. Predicting these phenomena is essential for the performance assessment of bentonite barriers and containments. This paper presents a non-local multi-physics model for bentonite erosion induced by piping flow that includes swelling, detachment of particles and co-transport of detached particles with piping flow. The erosion is controlled by the balance between the cohesive strength, which depends on the swelling, and the shear force, which depends on the water velocity and chemistry. The accuracy of the model is tested by comparison of simulation results with experimental data from pinhole tests and material erosion in boreholes. It is demonstrated that the model predicts accurately the total mass eroded by the piping flow. For example, the results show that the mass loss induced by piping-assisted erosion during the installation of a bentonite plug can reach 10·3% of the original mass, which may significantly reduce the sealing capacity of bentonite plugs in boreholes. The results of simulations show that the eroded mass depends on the borehole diameter and flow rate. The minimum flow rates required to erode 10% of the original mass are 0·8 l/s and 0·045 l/s for borehole diameter, db = 56 mm and 160 mm, respectively. These results demonstrate how the proposed formulations can be used to quantify the piping-assisted erosion of clay barriers, such as buffers, backfills and plugs considered for geological disposal of higher activity waste, and the sealing of investigation boreholes and abandoned geo-energy wells.

Evaluation of Bentonite Application for the Abandonment of Deep Geo-energy Wells

Hydrated bentonite is considered an alternative subsurface sealing/plugging material for deep geo-energy wells. However, the knowledge related to this application and the corresponding properties of bentonite is still lacking. This includes the mechanical properties at the interface of bentonite plugs with the adjacent materials (surrounding rock or casing steel) and the mechanical stability of plugs under downhole in-situ conditions. In this work, we performed experiments investigating the interface shear properties and shear strength of a bentonite plug under various settings for deep geo-energy applications, such as hydrocarbon and geothermal wells. The interface’s shear properties against various adjacent materials and fluid conditions were characterized. The influence of chemical exposure, the salinity of the curing fluid, fluid pH, pressure, and temperature on bentonite’s mechanical stability was evaluated in a small- and large-scale setting. The latter was performed using realistic casing sizes and placement methods, relevant for the field application. The experimental results show that the averaged shear strength of the bentonite plug interface is 13.3 kPa and 9.1 kPa when cured in freshwater and seawater, respectively. The increase in strength with increasing curing pressure, temperature, and fluid pH was characterized for the first time. The interfacial properties of cohesion and friction angle vary with different surrounding materials. They are also influenced by the saturating condition and salinity of the saturating fluid. Based on the experimental results, a bentonite plug with a minimum length of 15–43 m placed in casings of 7–5/8″ to 13–3/8″ would be sufficient to meet the necessary criteria of the Dutch regulators as an isolating material for well abandonment.

Influence of tailor-made TiO2/API bentonite nanocomposite on drilling mud performance: Towards enhanced drilling operations

A novel tailor-made TiO2 nanoparticles-bentonite (TNBT) nanocomposite-enhanced drilling mud was tested. The drilling mud showed excellent performance; (i) lubricity was enhanced, (ii) thin and well-intact mud cakes were created, (iii) coating was provided on shale and bentonite plugs, and (iv) clay and shale swelling were mitigated, hence these features ensured a platform for sustainable oil and gas well drilling. The synthesised TNBT nanocomposite followed the one-third bridging rule, i.e., these nanoparticles can bridge nanopore throats in shale pellets and American Petroleum Institute (API) standard bentonite pellets. The tailor-made TNBT nanoparticles were characterized by X-ray diffraction, and scanning electron microscopy. The coefficient of friction and filtrate loss of TNBT-enhanced muds were determined with a lubricity tester, and an API filter press, respectively. Additionally, shale and clay swellings were determined with a linear swell meter. The coefficient of friction was reduced after 0.8 g of TNBT was added to 350 ml drilling mud, resulting in a 35% enhancement in mud lubricity. The new mud also drastically mitigated the swelling in shale and clay when compared to conventional mud. The swelling inhibition was attributed to an increase in the mud viscosity and improved nanopore plugging characteristics, which resulted in less water infiltration. Thus, drilling mud supplemented with the tailor-made TiO2 nanoparticles-bentonite nanocomposite showed superior characteristics and a good potential for field operations.

Failure modes for hydrated bentonite plugs used in well decommissioning operations

This paper aims to theoretically investigate and characterize the failure mechanism of hydrated bentonite plugs, when used as a sealing material in wellbores. Using the proposed theoretical models, it was determined that when hydrated bentonite plugs failed due to friction at the walls, created by internal swelling pressure or through internal shear failure, the resulting failure pressure was linear with the plug height (H), coefficient of friction or internal shear strength, and inversely proportional to plug diameter (D). On the other hand, if the friction at the walls was created by plug weight then the failure pressure was a function of H2/D. However, in this case, the frictional strength increased parabolically with height, and, at reasonable plug heights, the frictional strength became greater than shear strength, and resulted in failure due to internal shear failure, which also made the failure pressure a function of H/D. This suggests that plug strength is usually a linear function of H/D. Moreover, the coefficient of friction and internal shear strength are dependent on final moisture content of the hydrated plug, the salinity of the hydration water, pressure, temperature and hydration time. The strength of a bentonite plug is inversely proportional the total volume by which it expands during hydration and takes in excess of 185 days to achieve this state. Plug strength decreased in saline water, although even at elevated concentrations sufficient strength remained to produce and adequate seal.From very limited data, shown here, Queensland bentonite, from the 5D zone of Amcol's Gurulmundi mine, appeared to have similar plugging strength as Wyoming bentonite.

Gas transport modelling at different spatial scales of a geological repository in clay host rock

The work presented in this article is a part of the international European Commission FORGE project which studied the behaviour of repository gases in the context of radioactive waste disposal. Experimental work is essential to understand the main transport mechanisms for gas and to determine the main geophysical parameters. However, while laboratory and rock experiments can help to investigate the short- and partly the middle-term behaviour of gas in a rock formation, long-term predictions have to be based on numerical simulations. Numerical simulation of long-term gas behaviour in a deep geological repository was one of the aims in the FORGE project. The objectives of the FORGE modelling were to describe the state-of-the-art consideration of gas in performance assessment, and to propose an updated treatment of gas issues in long-term safety assessments for high-level radioactive waste and spent nuclear fuel. Three benchmark exercises for a hypothetical geological repository in clay host rock ranging in scale from a single disposal cell to the whole repository were defined. To provide added value to this benchmark, a feature not yet well represented in typical gas simulations was introduced: the explicit representation of the interfaces between the excavation disturbed zone and the engineered elements within the excavation, such as waste canisters, bentonite plugs, and access drifts. In order to assess gas transport behaviour at the whole repository scale, models were developed with ‘mathematical’ or ‘numerical’ upscaling techniques for small-scale features. The assessment across different modelling scales revealed the main long-term gas migration pathways and led to the conclusion that the explicit representation of interfaces is not important.

Small Angle Neutron Scattering Investigation of Compressed Bentonite Plugs

Bentonite plugs manufactured with and without a polymer binder were studied using Small Angle Neutron Scattering (SANS) and Ultra Small Angle Neutron Scattering (USANS) techniques. We have subjected the samples to hydration by evaporating water and then applied CD4 methane at a Zero Average Con-trast (ZAC) pressure (about 48.3 MPa/7000psi) to measure the accessible and inaccessible pores and channels. The data show that introduction of moisture has an immediate effect on the smallest pores, with swelling preventing access by the methane; however the larger network appears to be little affected by the addition of the moisture. The addition of a binder to the plug creates a greater number of open pores in the micron and above size regime, indicating that the polymer is vital in creating larger channels, which aids the movement of mois-ture throughout the plug, at the expense of sealing the smallest ones. This sug-gests that the polymer binder helps in the self-healing process in the clays and makes the structure more stable. The process of plug manufacturing and drying produces a chaotic system of aggregates in the sample, which forms packed clusters 130 nm in size. Addition of moisture destroys this structure. Addition of water to the dried plug material showed that at 18 weight% a dual layer of water has formed in the montmorillonite layer of the clay, separating the montmorillonite layers by 17 A. Increase to 33 weight% of water has added an extra layer with basal spacing of 19 A. The clay hydration with vapor and directly deposited liquid appear to delaminate some of the clay affecting the fractality index at the micron sizes. Mechanical compression had a significant effect on the polymerised sample, encouraging larger water-conducting channels to form.

Characterisation of Geomechanical Properties of Bentonite Clay Used for Plug and Abandonment Operations of Coal Seam Gas Wells

Compressed bentonite in the form of pellets or plugs is used for the abandonment of production wells for the oil and gas industry. The design of the abandonment systems is based on the hydro-mechanical behaviour of the compressed bentonite defined by mechanical parameters that are used from published data rather than quantified for the used material by laboratory investigations. This paper presents an experimental study on characterising the swelling and shear strength behaviour of raw and polymer (polyvinylpyrrolidone, PVP) treated bentonite. Dislodgement tests consist of three hydrated bentonite plugs inserted in steel casings with the failure mechanism characterised. The bentonite used comes from a local mine (in Queensland, Australia) and is comparable to other bentonites usually used for the abandonment of wells or for other problems where mineral sealing is required (e.g. basal clay barriers of landfills). The experiments have shown that polymer treated bentonite shows significantly larger shear strengths than raw bentonite with simultaneously less swelling. More compressed samples also showed higher shear strengths and less swelling. The dislodgement tests have characterised for the first time the cascaded failure mechanism of a series of plugs forming an abandonment system. This investigation is the first step towards the development of an improved design for abandonment systems for wells using bentonite plugs.

Gas migration properties through a bentonite/argillite interface

Among diversified industrial uses, see for instance Koch [Koch, Appl. Clay Sci., (21) 2002], and following positive in situ experiments, compacted bentonite blocks are potential candidates for sealing nuclear waste repositories, thanks to their swelling ability in a wet environment. As requested by Andra (French Agency for Nuclear Waste Management) and complementarily to in situ experiments, an original experimental laboratory set-up was designed in order to reproduce the introduction and swelling of bentonite plugs inside an argillite host rock. Once the argillite/bentonite interface is established, an increase in storage tunnel gas pressure is simulated and the interface gas migration pressure (or gas critical pressure) is evaluated. More precisely, a first experimental set-up provides bentonite swelling pressure and kinetics (i.e. mainly hydraulic cut-off, time to reach asymptotic swelling pressure and value of asymptotic swelling pressure) at given initial compaction and saturation rate. This phase is preparatory to reproducing the introduction and subsequent swelling of a bentonite plug inside the argillite host rock, which uses a similar test rig. Experimental results of water permeability and gas critical pressure are provided for MX80 compacted bentonite associated to Bure Callovo-Oxfordian argillite.

Saturation development in the bentonite barrier of the Mock-Up-CZ geotechnical experiment

This paper reports on both the methodology and the results of the measurement of saturation development during the operation of the Mock-Up-CZ experiment. This experiment simulates the engineered barrier system hydration process of the Swedish KBS-3V vertical disposal system. The Mock-Up-CZ model consists of a barrier of bentonite blocks and a heater which simulates the radioactive waste. The potential inflow of granitic water from the natural barrier is simulated by employing a flooding system installed on the inner side of the experimental tank. The whole experiment is enclosed in a cylindrical pressure tank the construction of which allowed for the high pressure caused by the considerable swelling of the bentonite. Six measurement profiles along the vertical profile of the tank allow the continuous measurement of the various on-going processes. A number of sensors were placed inside the bentonite barrier (40 thermometers, 50 hydraulic pressure cells and 37 humidity sensors). The experiment commenced on 7th May 2002. In the first phase, the bentonite barrier was loaded only with the heat created by the heater for a period of six months. During the second phase, which commenced on 4th November 2002, in addition to the continuing thermal activity, the barrier was saturated with synthetic granitic water. This phase will most likely be concluded at the end of 2005. It has been found that of all the geotechnical measurements currently being performed, the measurement of changes in water content is the most challenging. The results obtained from employing any of the usually accepted methods for the measurement of indirect water content are often somewhat unreliable. Despite the problematic nature of these methods, 37 humidity sensors were installed in the barrier. Moreover, the construction of the experimental tank provided for sampling locations with access points allowing core sampling by means of drilling. These samples allow the determination of the water content profile of the barrier using a simple gravimetric method. The advantage of this method is that it provides an accurate and detailed description of changes in the barrier’s water content in a cost-efficient way. After core drilling, the openings thus created are immediately re-sealed with bentonite plugs thereby allowing for the repeated exploitation of sampling points. By combining the results of indirect water content measurement using sensors and the results obtained from the laboratory water content analysis of core samples, accurate information on barrier saturation has been provided on a continuous basis since the launch of the experiment. Approximately 90% saturation (in the vicinity of the measurement points) had been reached by October 2003, the saturation process measurement results corresponding to developments in swelling pressure. Surprisingly, considerable differences were detected in swelling pressure values. It is expected that an explanation for this problem will be found during the eventual dismantling of the experiment. The fact that identical water content values have been recorded (October 2004) independent of distance from the heater would seem to prove that full saturation is approaching. Interestingly, temperature has not been found to affect the degree of saturation.

LOT—in situ diffusion experiments using radioactive tracers

An experiment series at Äspö Hard Rock Laboratory, called “Long Term Test of Buffer Material”, LOT, are carried out at Äspö Hard Rock Laboratory to validate models of clay buffer performance at standard KBS-3 repository conditions and to quantify clay buffer alteration processes at adverse conditions. In conjunction with the tests of the bentonite, cation diffusion tests using caesium and cobalt are performed. Each test contains 38 cylindrical blocks of bentonite clay with a hole in the middle which are placed around a copper rod in a vertically drilled hole at a depth of 450 m. In each test, four identical cylindrical bentonite plugs, doped with 1 MBq 57Co and 134Cs, respectively, are inserted in the fifth block from the bottom. The system was left to be saturated with ground water before heating of the central copper rod started to simulate the thermal activity of radioactive waste. The experiments continued for 14 months before the bentonite blocks were drilled out using over-core drilling technique. The lowest blocks were cut up, sparsely in the outer layer, and in cubic centimeters, big samples closer to where the activity was inserted. All samples were analyzed using a gamma spectrometer to get a three-dimensional picture of the activity distribution. The system was then fitted to a diffusion model to obtain apparent diffusivities for the two cations. The apparent diffusivity for cobalt agrees well with those obtained in other in situ experiments and in laboratory studies, while the value for caesium is lower than expected. This can be due to that the clay was not fully water saturated during the experiment.

Mass transport through defected bentonite plugs

Compacted bentonite-based materials are important barriers in many waste containment strategies. To function as effective barriers, however, these materials must maintain their low water permeability and molecular diffusivity for long periods of time under a variety of environmental conditions. Here we examine the permeability and diffusivity of compacted bentonite plugs that were either slotted, to mimic fractures, parallel to the direction of mass flow or heated at 150 and 250°C for several weeks at various moisture contents before testing. The dry density of the plugs ranged from about 0.9 to 1.3 Mg/m 3. The results show that the saturated hydraulic conductivity and diffusivity (for I − and Cs +) of the treated or ‘defected’ bentonite plugs are essentially the same as those of untreated plugs at similar densities. This provides confidence that compacted bentonitic materials can funntion effectively as barriers for long periods of time under a range of environmental conditions.

Soil-water sampling using pan and deep pressure-vacuum lysimeters

A description of two soil-water sampling devices and success achieved in obtaining soil water on a routine basis at depths of 1 to 36 feet below land surface are described. In one facility, galvanized, 16 gage metal pans, 12 × 15 inches, with a copper spout, are driven into the wall of an open trench to intercept gravitational water at 1-foot intervals to a depth of 17 feet. Water samples are diverted to sample bottles attached to the inside walls of a protective housing by way of plastic tubing. Flooring is required to prevent quicking of stratified residual silt loam to sandy loam soils at the base of the trench. From 4 to 6 inches of water are irrigated adjacent to the trench on a weekly basis and a floor drain or sump pump is required to prevent flooding of the sampling trench. The deep trench lysimeter provides gravitational water samples on a routine basis when waters are artificially applied to the site but failed to yield samples during the growing season in the absence of irrigation under Pennsylvania's climate.A modified version of a commercially avaiable lysimeter constitutes the second device which can extract soil water under tension from depths in excess of 50 feet. The commercial lysimeter contains a porous ceramic cup attached to a 2-foot section of plastic pipe with a one-hole rubber stopper. In the modified version a two-hole rubber stopper is used and two copper tubes are inserted through the stopper, one for evacuating the tube, the other to force out water. This improved soil-water sampler has been named a pressure-vacuum lysimeter or “suction lysimeter.” One or more lysimeters may be installed in a 6-inch diameter drill hole. The ceramic tip is embedded in a pulverized silica deposit to insure hydraulic continuity with the soil-water reservoir. Native soil, bentonite plugs, or grout plugs are used to backfill holes to prevent channeling. A hand pump may be used to evacuate the tube and to blow out samples. When sufficient soil-moisture is available 500 to 970 mls of water may be obtained on a weekly basis. During prolonged dry periods, no samples can be collected when soil water has been depleted around the ceramic point. This may be four to eight weeks after pan lysimeters fail to yield water samples.

Self-Diffusion of Exchangeable Cations in Bentonite

AbstractA special integral form of the solution of Fick's law was applied to study the self-diffusion of Na, Cs, and Ca ions in Wyoming bentonite plugs by means of a radioactive tracer technique. A simple procedure for measuring diffusion coefficients was developed by depositing a thin radioactive tracer film on the surface of clay mineral plugs at controlled temperatures. Thin sections of the clay plugs were removed and radioactivity measurements made on the portion of the plugs remaining. The self-diffusion coefficients of the ions were evaluated by a graphical method from the solution of Fick's law at special boundary conditions. Under oven-dry conditions at 105°C diffusion of Na ion in a bentonite film was observed, while in the same experiment, SO42− ion did not diffuse. In a clay-water system, however, the anion diffused much more rapidly than the cation. Based on the experimental results of observing Na22- and S35-tagged Na2SO4 diffusion, different diffusion paths were proposed using a picture of the clay matrix as a structure with “channels.” Diffusion coefficients of exchangeable cations decreased with increasing clay contents. From the results of activation energy and self-diffusion coefficients of Na and Ca ions at various clay-water concentrations, the relative “hopping distances” of diffusing ions were calculated and discussed. It is suggested that in the self-diffusion of exchangeable Na ion in bentonite the ion could be hopping from one individual clay plate to another, and the Ca ion from one “package” of plates to another “package” of plates.