Life of Well Isolation Implemented in Major North Sea Development
Abstract: fax 01-972-952-9435. AbstractThe Buzzard field operated by Nexen is the largest oil discovery in the North Sea since the early 1990s. About 27 production wells, of approximately 9,000 ft total vertical depth (TVD), will be drilled during the field life. Maximized life of well isolation is imperative. The production liners are being cemented with a life-of-well engineered solution.The reservoir section consists of two discrete oil-bearing sandstones. Oil recovery will be maximized by water injection from subsea… Show more
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Carbonate reservoirs are often characterized by high pressure and high content of H2S and CO2. For these reasons, drilling the reservoir is the most challenging activity of such fields and long-term zonal isolation across the reservoir section is one of the primary requirements. In the example well considered for this study, the production liner is set at a depth of approximately 4,500 meters and the mud density is 16.2 lb/gal (1.95 g/cc). After a production liner is cemented, the well undergoes several operations such as fluid displacement, casing/liner pressure tests, stimulations, production, and injection; these operations create load on the cement sheath. Carbonation of neat Portland cement systems in CO2 environments is well known in the industry. The carbonation is of significant concern if the CO2 can enter the cemented annulus. The surface area of the cement sheath that contacts CO2 should be minimized to help prevent carbonation. This can be achieved by reducing the permeability, preventing the formation of cracks and micro-annulus, and reducing the components in the cement sheath prone to attack from CO2. To assure long-term sealing properties of the cement sheath during the life of the well, a cement formulation has been developed to be mechanically durable and chemically resistant to aggressive environments. The cement system discussed in this paper was designed to withstand the stresses imposed by changes of pressure regime during the well life by improving elasticity and thus helping prevent damage to the cement sheath. In addition, the potential for carbonation was limited by reducing the components in the slurry formulation that could react with CO2. Mechanical properties and resistance to CO2 environments of the cement system were tested in the laboratory. The cement system was successfully evaluated in the yard test. Cement sheath analysis, slurry design and testing are discussed. The results presented in this work should help in the design and implementation of solutions to contain reservoir fluid and injected fluids, including in the presence of H2S and CO2. Introduction The main objectives for cementing are to:support the casing.protect the casing from shock loads (tubular collapse).provide a pressure-tight seal between zones containing either different pressure regimes or fluid content for the entire well life.protect the casing from corrosion.seal off zones of lost-circulation or thief zones. To meet these objectives, the properties required in the cement slurry and the set cement sheath includes the following:Stable at the given density—no free water and no settling.Easily mixed and pumped.Provide adequate thickening time, fluid loss, and gel strength.Meet the optimum rheological properties required for mud removal.Impermeable to annular fluids while curing.Develop strength quickly after placement in the annulus.Develop mechanical properties to help ensure well integrity for well life.Bond to casing and formation.Have low permeability to resist reservoir fluid migration and attack.Stable under downhole conditions of temperature, pressure, and chemical exposure.
Carbonate reservoirs are often characterized by high pressure and high content of H2S and CO2. For these reasons, drilling the reservoir is the most challenging activity of such fields and long-term zonal isolation across the reservoir section is one of the primary requirements. In the example well considered for this study, the production liner is set at a depth of approximately 4,500 meters and the mud density is 16.2 lb/gal (1.95 g/cc). After a production liner is cemented, the well undergoes several operations such as fluid displacement, casing/liner pressure tests, stimulations, production, and injection; these operations create load on the cement sheath. Carbonation of neat Portland cement systems in CO2 environments is well known in the industry. The carbonation is of significant concern if the CO2 can enter the cemented annulus. The surface area of the cement sheath that contacts CO2 should be minimized to help prevent carbonation. This can be achieved by reducing the permeability, preventing the formation of cracks and micro-annulus, and reducing the components in the cement sheath prone to attack from CO2. To assure long-term sealing properties of the cement sheath during the life of the well, a cement formulation has been developed to be mechanically durable and chemically resistant to aggressive environments. The cement system discussed in this paper was designed to withstand the stresses imposed by changes of pressure regime during the well life by improving elasticity and thus helping prevent damage to the cement sheath. In addition, the potential for carbonation was limited by reducing the components in the slurry formulation that could react with CO2. Mechanical properties and resistance to CO2 environments of the cement system were tested in the laboratory. The cement system was successfully evaluated in the yard test. Cement sheath analysis, slurry design and testing are discussed. The results presented in this work should help in the design and implementation of solutions to contain reservoir fluid and injected fluids, including in the presence of H2S and CO2. Introduction The main objectives for cementing are to:support the casing.protect the casing from shock loads (tubular collapse).provide a pressure-tight seal between zones containing either different pressure regimes or fluid content for the entire well life.protect the casing from corrosion.seal off zones of lost-circulation or thief zones. To meet these objectives, the properties required in the cement slurry and the set cement sheath includes the following:Stable at the given density—no free water and no settling.Easily mixed and pumped.Provide adequate thickening time, fluid loss, and gel strength.Meet the optimum rheological properties required for mud removal.Impermeable to annular fluids while curing.Develop strength quickly after placement in the annulus.Develop mechanical properties to help ensure well integrity for well life.Bond to casing and formation.Have low permeability to resist reservoir fluid migration and attack.Stable under downhole conditions of temperature, pressure, and chemical exposure.
Zonal Isolation Achieved in Kashagan Field Through Integrated Approach
The Kashagan Field is located in the northeast end of the Caspian Sea (Kazakhstan), and its carbonate reservoir is characterized by high pressure and high content of H2S and CO2. The primary purpose of the production liner is zonal isolation to contain reservoir and injected fluids. The challenges are magnified in these wells because of the presence of corrosive fluids, varying properties of rock, and high pore-pressures. Carbonation of neat Portland cement systems in CO2 environments is well-known. Carbonation is a significant concern if CO2 enters the cemented annulus. The surface area of the cement sheath that contacts CO2 should be minimized to help prevent carbonation. This can be achieved by reducing permeability, preventing formation of cracks and microannuli, and reducing the components in the cement sheath prone to attack from CO2. Zonal isolation was achieved by designing and deploying a cement sheath that had 1) high structural integrity, 2) low permeability, and 3) was stable when exposed to downhole conditions of chemicals, temperature, and pressure. The structural integrity helped prevent the formation of microannuli and cracks during well operations. This feature, combined with low permeability, prevented the formation fluids from entering the annulus. The chemical stability helped prevent any deterioration of the cement sheath if it were to come in contact with downhole fluids such as CO2. The production liner was set at ±4,500 meters and the mud density was ±1.95 g/cc. When mud was swapped with seawater during the completion operation, it resulted in significant pressure decrease inside the casing. The cement sheath was designed to withstand completion and subsequent operations during the well-life to maintain structural integrity and help secure zonal isolation. The cement system was formulated to withstand well operations by improving the elasticity, reducing hydration volume shrinkage, and engineering optimum expansion of the cement sheath. The potential for carbonation was decreased by lowering the permeability and reducing the components in the cement sheath that could react with CO2. The cement system was tested in the laboratory for elasticity, resistance to CO2, and expansion under downhole conditions. If the well were to be operated beyond its design limits, then there would be a risk of cement sheath failure leading to creation of microannuli and small cracks, and potential influx of formation fluid into the annulus. The cement system is designed to react with formation fluids and respond in such a way to seal small cracks and microannuli automatically, without well intervention. The cement system has been successfully deployed in six wells. The cement bond logs were excellent and verified the successful cement slurry placement on all jobs, and subsequent successful well operations confirmed zonal isolation. The same cement system is being deployed in additional wells. Cement slurry design, field deployment, and reservoir performance is discussed. The results presented in this work should help in the design and implementation of solutions to achieve zonal isolation, even in a corrosive environment. Introduction The challenges to zonal isolation in the Kashagan field are both chemical and mechanical, resulting from exposure of cement sheath to reservoir fluid containing CO2 and H2S and stresses from extreme well operations. After detailed study of the challenge, it was evident that an integrated approach was needed to achieve zonal isolation. The geology, reservoir fluid characteristics, drilling parameters, and details of well operations, such as production and injection, are all important and need to be considered to design and deploy a reliable cement system. A multidisciplinary team was assembled to solve this challenge.
Sustained casing pressure (SCP) is a major risk encountered in most wells in India, particularly in developmental oil and gas fields situated in the northwest area of the country. The inability of conventional cement systems to withstand cumulative stresses from completion and production phases might be one of the primary reasons for the failure of expected barriers during zonal isolation, which can lead to SCP. This case study reviews a flexible cement system used in an oil and gas field located in northwestern India, including extensive laboratory evaluation, operational design, execution, and well evaluation. The field requirement was to design a gas-tight, lightweight slurry for a production zone to treat losses and address issues of SCP. A finite element analysis (FEA) structural simulation was performed, which modeled the lifetime operation of the well. Including elastomeric and tensile strength enhancement materials in the cement system was recommended based on results from structural simulation to reduce risks of SCP. Incorporating these materials can enhance a cement system's mechanical properties (i.e., increase Poisson's ratio and tensile strength and decrease Young's modulus). The slurry was designed with cement, hollow spheres, lost circulation material (LCM), a tensile strength enhancer, and elastomeric material; the cement slurry was batch mixed to ensure homogeneity. The case study examines the impact of the fluid rheological hierarchy, pump rates, and multiple bottom plugs on cement slurry displacement efficiency using computational fluid dynamics (CFD). A finite-difference, three-dimensional (3D) displacement simulator was used to predict the flow behavior of fluids, both inside and outside the casing, throughout the operation using actual operational parameters. Based on two-dimensional (2D) hydraulics analysis, various factors, including rheology, pumping rates, and fluid density, were adjusted to maintain equivalent circulating densities (ECDs) below the fracture pressure to help avoid losses. A tailored cement design, which created a stable gas-tight resilient cement system, helped minimize risks to production by achieving effective zonal isolation. This was confirmed through a post-operation review. The cement bond log, including a microseismogram and circumferential visualization, indicated that the cement was adequately placed around the casing. The increase in the cement system tensile strength helped ensure wellbore integrity, even in high-stress/strain load cycle production environments. Cementing operation success was confirmed by excellent cement bond logs and zero SCP observed between the production and surface well casing annulus. The life-of-the-well tool is an engineered, proactive, and interventionless zonal isolation solution to help extend well economic life, thus preserving production while reducing or even eliminating costly remediation (Ravi et al. 2002).
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