Introduction to Permanent Plug and Abandonment of Wells

This open access book offers a timely guide to challenges and current practices to permanently plug and abandon hydrocarbon wells. With a focus on offshore North Sea, it analyzes the process of plug and abandonment of hydrocarbon wells through the establishment of permanent well barriers. It provides the reader with extensive knowledge on the type of barriers, their functioning and verification. It then discusses plug and abandonment methodologies, analyzing different types of permanent plugging materials. Last, it describes some tests for verifying the integrity and functionality of installed permanent barriers. The book offers a comprehensive reference guide to well plugging and abandonment (P&A) and well integrity testing. The book also presents new technologies that have been proposed to be used in plugging and abandoning of wells, which might be game-changing technologies, but they are still in laboratory or testing level. Given its scope, it addresses students and researchers in both academia and industry. It also provides information for engineers who work in petroleum industry and should be familiarized with P&A of hydrocarbon wells to reduce the time of P&A by considering it during well planning and construction

A method for assessing drilling fluid induced formation damage in permeable formations using ceramic discs

When drilling an oil and gas or geothermal well, the formation's ability to produce or flow may be reduced because of exposure to the drilling fluid during the drilling operation. To evaluate such formation damage, core flooding tests are typically conducted using representative samples of rock to measure the change in the formation permeability in the zone near the wellbore. Disadvantages of core flooding tests include time and cost of a test and potential limited access to representative cores. Therefore, core flooding tests are generally not practical to use for screening and adjustment of drilling fluid compositions when a high number of tests are planned. A method has been suggested to allow for time and cost-effective testing of mass change of ceramic discs, such that a high number of tests may be completed within a limited timeframe and budget. However, so far only limited testing had been conducted to understand the potential for measuring permeability change. In the present study, the method was applied to test for change in permeability of ceramic discs following HTHP tests. A reverse flow of fluid was applied to lift off the filter-cakes and then a breaker fluid was applied. Thereafter the permeabilities to air and water and the dry disc mass was measured and compared with the original value to detect any changes. The repeated tests showed very high correlations between changes in permeability to air, permeability to water and changes in disc mass, ranging from 0.906 to 0.984. The tests were repeated by different researchers and the results of the two test-series showed high correlations between the original and repeated test series. The overall results provide a high degree of consistency and confirmed findings in past research conducted on core flooding tests. Present study inferred that the simplified method for assessing formation damage produces consistent results and may be used as a cost-effective method for comparing different drilling fluids and methods for removing the filter cakes, ahead of potential core flooding tests.publishedVersio

Invasion of CaCO3 particles and polymers into porous formations in presence of fibres

Formation damage can occur through migration of drilling fluid particles and polymers into porous formations. A methodology for assessing formation damage was applied to measure invasion of CaCO3 and polymers into porous formations, where the CaCO3 particles size had been selected using established particle size selection methods. Tests were conducted with and without the presence of a cellulose-based additive, to study if the fibres could reduce the fluid loss and limit the formation damage. Input factors such as applied differential pressures, ranging from 6.9 to 34.9 MPa (1000-5000psi), and median pore-throat openings of discs were also varied to investigate which parameters affected the significance of the formation damage. The results showed invasion of CaCO3/ground marble into the formation and that particle size selection methods used to reduce fluid loss also led to formation damage. Further it was discovered that the presence of fibres limited the invasion of both CaCO3 and polymers into the porous formations when the D90 of the fibres were ≥3/2 times the pore-throat size, and that higher applied pressures led to larger formation damage. The fluid loss tests also showed both lower total fluid losses and lower fluid loss rates over time with the fibres added to the fluids, indicating that the filter-cake permeability was reduced with the addition of the fibre particles.publishedVersio

Comparison of Lost Circulation Material Sealing Effectiveness in Water-Based and Oil-Based Drilling Fluids and Under Conditions of Mechanical Shear and High Differential Pressures

A study was conducted to assess the performance of granular and fibrous lost circulation materials as preventative treatments and in remedial treatment of lost circulation in water-based and oil-based drilling fluids. For the preventative treatments, a factor that introduced increased mechanical wear on the particles was added to the hot-rolling process, to identify signs of deterioration of performance of certain materials. The study of remedial treatments of lost circulation was conducted on slotted discs with apertures of 750 µm and up to 5 mm and with a differential pressure of up to 34.5 MPa (5000 psi). To compare the sealing pressures of the different tests, a simple statistical analysis was introduced to differentiate between the peak holding pressures and the sustainable holding pressures of the various material and fluids combinations. The material degradation studies showed that CaCO3-based lost circulation materials rapidly experienced significant particle degradation after exposure to fluid shear and mechanical degradation and that this considerably reduced the sealing performance of the materials. Also, synthetic graphite-based products showed clear signs in particle size degradation and a significant reduction in sealing performance. Cellulose-based products showed superior resistance toward mechanical wear and only small changes in sealing performance. When comparing water-based and oil-based fluids, it was clear that granular lost circulation materials showed considerably lower sealing efficiency in oil-based drilling fluids compared to water-based drilling fluids. In contrast, cellulose-based materials showed similar sealing performance in oil-based fluids and water-based fluids.publishedVersio

Viscosity Models for Drilling Fluids: Herschel-Bulkley Parameters and Their Use

An evaluation is presented of the practical usage of the Herschel-Bulkley viscosity model for drilling fluids. If data from automatic viscosity measurements exist, the parameters should be selected from relevant shear rate ranges to be applicable. To be able to be used properly, viscosity measurements must be measured with a sufficient accuracy. It is shown that a manual reading of standard viscometers may yield insufficient accuracy. It is also shown that the use of yield point/plastic viscosity (YP/PV) as measured using API or ISO standards normally provide inaccurate viscosity parameters. The use of the Herschel-Bulkley model using dimensionless shear rates is more suitable than the traditional way of writing this model when the scope is to compare different drilling fluids. This approach makes it also easier to make correlations with thermodynamic quantities like pressure and temperature or chemical or mineralogical compositions of the drilling fluid.publishedVersio

Effect of Zn2+ and K+ as Retarding Agents on Rock-Based Geopolymers for Downhole Cementing Operations

Geopolymer material has a potential to function alongside Portland Cement as an efficient cementitious material for well cementing and plug and abandonment applications. Geopolymer material requires retarding agents to be displaced into the well while considering the properties required to maintain efficient zonal isolation through superior mechanical properties. Chemical admixtures affect the material structure and can, in some cases, jeopardize material integrity if not engineered properly to suite downhole conditions. The present article shows the effect of Zn2+ and K+ species have as retarding agents on slurry, mechanical, and microstructural properties. The approach has been carried out to obtain a preliminary overview of how retarding agents can behave in mix design slurries where eventually sealing performance was examined. Samples were cured and examined for periods of 1, 3, 7, 14, and 28 days at downhole conditions. The results obtained confirm a retardation effect by the addition of Zn2+ and K+ species and some shortcomings in early strength development due to a poisoning mechanism by Zn2+ species. This phenomenon indicates the formation of Ca-Zn phase that can hinder the nucleation of the geopolymeric gel structure. No significant effects were observed on the microstructural development throughout the curing period. The effect of Zn2+ species was also observed in increasing threshold for hydraulic sealability. It may be concluded that the tested retarding agents require furthermore development to minimize shortcomings in mechanical properties specifically early strength development.publishedVersio

Bonding Mechanism of Zonal Isolation Materials to Clean and Rusted Casing

In oil and gas and geothermal well construction, a cementitious material is pumped in the wellbore to provide zonal isolation and support the casing during the life cycle of the well. Thus, the cementitious barrier materials must be durable in terms of chemical and mechanical properties and have chemical compatibility with casing pipe. The complex region of casing-cement interface is considered a key parameter to fulfill long-term zonal isolation. This interface must be chemically stable and impermeable to block unwanted formation fluid communication. Shortcomings of conventional Portland cement under operational conditions and increasing sensitivity to its carbon footprint are motivations for a green alternative. Bond strength and sealability of cement with steel surface have been measured previously. But few research works cover surface characterization and morphological analysis of barrier materials and the connected steel surface. This study provides a full picture of selected alternative materials in terms of shear bond strength, hydraulic sealability, and interface morphology analysis of the materials. Materials include API Class G cement, an industrial expansive cement, noncement-based pozzolanic material, geopolymer, and thermosetting resin. Also, clean and rusted steels were considered as a representative for the casing pipe in the field. The samples were prepared under elevated pressure and temperature. The results proved that higher shear bond strength is not an indication of good sealability, and the ingredients used to mix slurries have a critical role in the structure of the interfacial zone between casing and barrier material.publishedVersio

Fundamentals and physical principles for drilled cuttings transport – Cuttings bed sedimentation and erosion

The increasing necessity of challenging wellbore structures and drilling optimization for improved hole cuttings cleaning has been growing along time. As a result, operator companies have been researching and applying different hole cleaning techniques. Some of these are applied as traditional rules of thumb but are not always suitable for the new and up-coming challenges. This may result in inefficient hole cleaning, non-productive times, pipe stocking and low rate of penetration (ROP), among other problems. Here are presented some results and improvements for hole cleaning optimization obtained by the different research groups. The different authors mainly focus on specific cuttings transport parameters and sometimes combination of some of them. For this reason, there has not been a study that takes into account all of the different factors at the same time to accurately predict the cuttings bed height, formation and erosion, critical fluid velocity and properties and other key parameters. Consequently, there is a lack of understanding about the relation between different factors, such as the cohesiveness of the drilled cuttings with the different interstitial drilling fluids within the cuttings-bed. This relation can be analyzed establishing a wet-granular approach to obtain more efficient cuttings transport mechanism in challenging conditions.publishedVersio

Application of the quemada viscosity model for drilling fluids

A number of different models are used to describe the shear rate dependent viscosity of drilling fluids. Most, such as the Herschel-Bulkley model, have a purely empirical basis. The Quemada model, while still empirical, is based on physical principles. It is based on the notion that structural units develop in the fluid at low shear rates which are then partially broken down as the applied shear rate increases. In the current work, drilling fluid rheological data are fitted to the Herschel-Bulkley and the Quemada model. The development of the Quemada model and the calculation of each model parameter are presented. We show that the Quemada model better fits measurements over a wider range of shear rates than the Herschel-Bulkley model. We describe how to select the parameters of the Quemada model. Knowing the difficulty of obtaining a known shear rate for fluids with yield stresses, we discuss how this can affect the quality of the Quemada model fit. Furthermore, in principle, the Quemada model is not applicable in presence a non-zero yield stress. Therefore, we show how to handle the yield stress using a (very high) zero shear rate viscosity.acceptedVersio

Rheological Compatibility of a Hardening Spacer Fluid and Oil-based Drilling Fluid

In the placement process of the cement slurry, treatment fluids such as the spacer are pumped ahead of the cementitious slurry to minimize the contamination of the slurry by drilling fluid and ensure superior bonding to the casing and formation. The spacer discussed in this work can harden with time and act as a settable spacer. This characteristic can be an advantage for well integrity if some spacer pockets are left in the annulus. Rheological compatibility of different mixtures of the spacer with oil-based drilling fluid (OBDF) has been studied using a rheometer, and the resulting R-factor, which indicates the degree of compatibility between fluids, has been calculated. An increase in the flow curve was observed for the mixture of the fluids. However, based on the R-index, these fluids are compatible with displacement in the wellbore. A nonionic surfactant, typically used in conventional spacers acting as an emulsifier and a water-wetting agent, was used in the hardening spacer design. The results show that the addition of OBDF to hardening spacer containing surfactant can increase viscoelasticity. Hardening spacer containing surfactant can successfully reverse the OBDF emulsion. By performing a small-scale mud displacement experiment, we observed that surfactant can improve the wall cleaning efficiency of the spacer while having minimal impact on the bulk displacement.publishedVersio