Characterization of Annular Cement Permeability of a Logged Well Section Using Pressure-Pulse Decay Measurements

The cement behind casings is an important barrier element in wells that should provide zonal isolation along the well. The hardened cement does not always isolate permeable formations, either due to placement issues or loads that over time compromise the integrity of the barrier. The modern method used to characterize the annular material is ultrasonic logging which provides essential information concerning the type of material behind casing, but no measurement of the annular permeability. This study provides permeability characterization of a casing-cement sandwich joint retrieved from a 33 years old production well that has been logged at surface using a state-of-the-art ultrasonic tool. The joint contains an interval of low-permeable cement that previously has prevented permeability measurement by gas injection. A pressure–pulse decay test method has now been performed that is based on monitoring the evolution of a pressure pulse through the joint. Long-term pressure measurements show communication through the entire joint and are in qualitative agreement with the log. A pressure diffusion model is used to estimate local permeability along the joint, enabling comparison of log response and permeability. The low-permeable region is relatively short, situated directly on top of a casing collar, and has permeability that is orders of magnitude lower than the cement above and below. In the longer term, results from this and related studies can be used as input for future sustained casing pressure evaluations or for quantifying seepage risk behind casings for abandonment designs.publishedVersio

Fluid Migration Characterization of Full-scale Annulus Cement Sections Using Pressure-Pulse-Decay Measurements

Fluid migration behind casings is a well integrity problem that can result in sustained casing pressure, undetected leaks to the environment, and potentially very challenging remediation attempts. Understanding the geometric dimensions and extent of annular migration paths is important for diagnosing and effectively treating fluid migration and sustained casing pressure problems in wells. In this study, permeability and micro-annuli sizes in two full-scale cemented annulus test sections are measured using transient pressure-pulse-decay and steady-state seepage measurements. One of the studied sections is a cemented 9 5/8-in. and 13 3/8-in. casing section from a 30 years old Norwegian North Sea production well. A model for predicting the transient pressure decay in annular sections with non-uniform permeability is presented and the permeabilities of the two sections are determined by fitting the transient model to pressure measurements at either side of the test sections and at selected axial positions. For both sections, measured micro-annulus sizes are within the range of effective wellbore permeabilities based on sustained casing pressure records and previous vertical interference tests from other wells. The test sections display measurable axial permeability variations with the bottom part of these vertical sections having the lower permeability. For the retrieved casing section, the axial permeability variation occurs close to the middle of the test section and is attributed to the top-of-cement. Increasing internal casing pressure is found to slightly reduce the equivalent micro-annulus size, indicative of fracture-like response of the migration paths. Using two independent test protocols, we have measured effective permeabilities as well as local permeability variations in full-scale test sections and found consistent results. The study suggests that the transient test procedure can be used to more effectively characterize low-permeable annular cement where it is otherwise time-consuming or difficult to establish steady-state flow conditions.publishedVersio

Giant current-driven domain wall mobility in (Ga,Mn)As

We study theoretically hole current-driven domain wall dynamics in (Ga,Mn)As. We show that the spin-orbit coupling causes significant hole reflection at the domain wall, even in the adiabatic limit when the wall is much thicker than the Fermi wavelength, resulting in spin accumulation and mistracking between current-carrying spins and the domain wall magnetization. This increases the out-of-plane non-adiabatic spin transfer torque and consequently the current-driven domain wall mobility by three to four orders of magnitude. Trends and magnitude of the calculated domain wall current mobilities agree with experimental findings.Comment: Final version accepted by Physical Review Letter

A numerical study of density-unstable reverse circulation displacement for primary cementing

Primary cementing of the casing string is the operation where the annular space behind the casing is displaced to a cement slurry. Once hardened, the cement should form a solid annular barrier and provide zonal isolation behind the casing. Reverse circulation cementing involves injecting the cement slurry directly into the annulus that is to be cemented, displacing drilling fluid down the well. This will normally represent a density-unstable situation with an increased risk of inter-mixing of fluids and slurry contamination compared to conventional circulation cementing. This study addresses the reverse circulation displacement mechanics, and is based on a reverse circulation field case where the quality of the hardened cement has previously been established by characterization of two retrieved joints. We use 3D numerical simulations to study possible displacement conditions and compare findings qualitatively to the actual cement. Additional simulations indicate the importance of imposed flow rate and viscous stresses in suppressing the destabilizing effect of buoyancy. A simplified one-dimensional displacement model provides reasonable predictions of the front propagation speed in vertical, concentric annuli and correct identification of conditions that result in backflow of lighter fluid. To the best of our knowledge, this study is the first numerical study undertaken to better understand density-unstable displacements in annular geometries.publishedVersio

An Integrated Modeling Approach for Vertical Gas Migration Along Leaking Wells Using a Compressible Two-Fluid Flow Model

Gas migration behind casings can occur in wells where the annular cement barrier fails to provide adequate zonal isolation. A direct consequence of gas migration is annular pressure build-up at wellhead, referred to as sustained casing pressure (SCP). Current mathematical models for analyzing SCP normally assume gas migration along the cemented interval to be single-phase steady-state Darcy flow in the absence of gravity and use a drift-flux model for two-phase transport through the mud column above the cement. By design, such models do not account for the possible simultaneous flow of gas and liquid along the annulus cement or the impact of liquid saturation within the cemented intervals on the surface pressure build-up. We introduce a general compressible two-fluid model which is solved over the entire well using a newly developed numerical scheme. The model is first validated against field observations and used for a parametric study. Next, detailed studies are performed, and the results demonstrate that the surface pressure build-up depends on the location of cement intervals with low permeability, and the significance of two-phase co-current or counter-current flow of liquid and gas occurs along cement barriers that have an initial liquid saturation. As the magnitude of the frictional pressure gradient associated with counter-current of liquid and gas can be comparable to the relevant hydrostatic pressure gradient, two-phase flow effects can significantly impact the interpretation of the wellhead pressure build-up

Experimental investigation of laminar and turbulent displacement of residual oil film

Residual oil films on pipe walls are a common occurrence in industrial processes, and their presence can significantly impact system efficiency and performance. However, the mechanisms that govern oil film removal by an immiscible displacing fluid from the internal walls of pipes under different flow regimes, including laminar and turbulent flows, are not yet fully understood. In this study, we investigated the impact of displacing fluid flow regime, injected volume, displacement time, and wall shear stress on the efficiency of residual oil film removal in a pipe. We first verified the applicability of our developed oil film measurement method for the use in vertical pipes, and found that gravity did not significantly affect the long-term oil film removal process. We verified that our results from the laminar cases agree with the theoretical thin-film limit scaling under reasonable assumptions of constant shear stress and negligible surface tension. We then examined the displacement efficiency of residual oil film under laminar and turbulent flow regimes. Our experimental results revealed that the onset of turbulence of displacing fluid played an important role in the efficient removal of residual oil film, with an optimal range of Reynolds numbers (7000–8000) when the injected volume of displacing fluid is limited. Furthermore, we explored the combined effect of wall shear stress and displacement time on the displacement process under different turbulent flow regimes. We found that the intermediate turbulent regime was the most efficient for achieving cleaning in a limited time, while the highly turbulent regime proved to be the most effective for achieving complete cleaning over a longer time period. These findings have important implications for oil recovery and pipeline maintenance and provide valuable insights into optimizing the removal of residual oil film in pipes.publishedVersio

Identification of nonlinear conservation laws for multiphase flow based on Bayesian inversion

Conservation laws of the generic form ct+f(c)x=0 play a central role in the mathematical description of various engineering related processes. Identification of an unknown flux function f(c) from observation data in space and time is challenging due to the fact that the solution c(x, t) develops discontinuities in finite time. We explore a Bayesian type of method based on representing the unknown flux f(c) as a Gaussian random process (parameter vector) combined with an iterative ensemble Kalman filter (EnKF) approach to learn the unknown, nonlinear flux function. As a testing ground, we consider displacement of two fluids in a vertical domain where the nonlinear dynamics is a result of a competition between gravity and viscous forces. This process is described by a multidimensional Navier–Stokes model. Subject to appropriate scaling and simplification constraints, a 1D nonlinear scalar conservation law ct+f(c)x=0 can be derived with an explicit expression for f(c) for the volume fraction c(x, t). We consider small (noisy) observation data sets in terms of time series extracted at a few fixed positions in space. The proposed identification method is explored for a range of different displacement conditions ranging from pure concave to highly non-convex f(c). No a priori information about the sought flux function is given except a sound choice of smoothness for the a priori flux ensemble. It is demonstrated that the method possesses a strong ability to identify the unknown flux function. The role played by the choice of initial data c0(x) as well various types of observation data is highlighted.publishedVersio

Influence of fluid viscosity hierarchy on the reverse-circulation displacement efficiency

Reverse-circulation cementing is an alternative strategy for well cementing where the cementing fluids are injected directly into the annulus from the surface. This cementing strategy can reduce downhole circulation pressures compared to conventional circulation cementing and potentially eliminate the need for retarders in the cement slurry. In reverse-circulation operations, the fluid hierarchy will normally involve density-unstable combinations along the annulus. Since the annular geometry prevents the mechanical separation of fluids, reverse-circulation cementing is associated with a risk of slurry contamination and mixing during placement. Although reverse-circulation cementing has been known for several decades and is used for cementing of both onshore and offshore wells, it remains unclear whether conventional circulation job design guidelines apply to reverse-cementing or indeed how fluid properties should be optimized for such operations. The purpose of the current study is to contribute to the understanding of buoyant annular displacements, with a particular focus on the role of viscosity hierarchy on the annular displacement in vertical and near-vertical annuli. We present a combined experimental and numerical study of density-unstable downward displacements in a downscaled, narrow concentric annulus. A transparent annulus flow loop was used to conduct downward displacements. A high-speed camera and a mirror arrangement were used to track the displacement. Numerical simulations of the experiments and selected other cases were performed using the open-source OpenFOAM computation framework. We study Newtonian and mildly shear-thinning fluids, and our study aims to determine whether it is more efficient to use a displacing fluid with higher viscosity or lower viscosity than the displaced fluid while maintaining a constant average viscosity for the fluid pair. The experimental and numerical results, which are in good qualitative agreement, demonstrate that the viscosity hierarchy of the fluids significantly affects the displacement flow features. Our results show that a more viscous displaced fluid leads to faster growth of the instabilities and, as a result, less efficient displacement. Oppositely, we observe less tendency for finger growth and a more diffusive mixing region for more viscous displacing fluids. The effect of the viscosity hierarchy can get stronger by increasing the inclination of the annulus and the viscosity difference between the fluids from 0.006 to about 0.02 Pa s. The findings can assist in the selection of fluid properties for future reverse-circulation displacement operations.publishedVersio

Development of a novel experimental technique for the measurement of residual wall layer thickness in water-oil displacement flows

The effective removal and displacement of fluids is important in many industrial and environmental applications, such as for operation and cleaning of process equipment, fluid injection in porous media for oil recovery or aquifer remediation, or for achieving subsurface zonal isolation in new or abandoned wells. The accurate measurement of the residual fluid wall film left behind after displacement by a cleaning fluid is a long-standing challenge, particularly so for very thin fluid films where the thickness can be of the order of micrometer. We focus on the characterization of oil films left on the wall of a horizontal pipe after the pipe has been displaced by water, and develop a novel, non-intrusive analytical technique that allows the use of relevant pipe materials. The oil that originally occupies the pipe is stained by a hydrophobic dye Nile red, and an intermediate organic solvent is used to collect the residual oil volume that remains after displacing the pipe with a known volume of water. Finally, ultraviolet-visible spectroscopy is used to measure the Nile red concentration in the collected fluid, which is proportional to the residual volume of oil in the pipe. We demonstrate the methodology by conducting experiments where the displacing fluid is injected at two different imposed velocities, and where the injected fluid volume is varied. As expected, we find a gradual thinning of the oil film with increasing injected fluid volume. We compare the measured film thicknesses to a displacement model based on the steady velocity profile in a pipe, and find that experiments consistently produce smaller film thicknesses. This developed technique allows quantification of displacement and cleaning mechanisms involved in immiscible displacements at laminar, transitional and turbulent regimes, for different non-Newtonian fluid pairs, and for different realistic pipe materials and surface roughnesses.publishedVersio

Pipe Viscometer for Continuous Viscosity and Density Measurement of Oil Well Barrier Materials

The barrier material is a crucial component for wells, as it provides mechanical support to the casing and prevents the uncontrolled flow of formation fluids, ensuring zonal isolation. One of the essential prerequisites for the success of cementing an oil and gas well is the efficient removal of in-situ fluids and their adequate replacement by the barrier material. The quality of the mud displacement is affected by both the density and the viscosity hierarchy among subsequent fluids. Consequently, accurate and reliable measurement of fluid properties can help ensure consistent large-scale mixing of cementing fluids and verification that the properties of the mixed fluid are according to plan. In this paper, we investigate the implementation of a pipe viscometer for future automated measurements of density and viscosity of materials for zonal isolation and perform a sequential validation of the viscometer that starts with small-scale batch mixing and characterization of particle-free calibration liquids, followed by conventional Class G cement and selected new barrier materials. Finally, a larger-scale validation of the pipe viscometer was performed by integrating it into a yard-scale batch mixer for in-line characterization of expanding Class G oilwell cement mixing. In all cases, flow curves derived from pipe viscosity measurements were compared with offline measurements using a rheometer and a conventional oilfield viscometer. After a series of measurements and comparisons, the investigated in-line measurement system proved adequate for viscosity estimation. The flow curve of the barrier materials showed results similar to measurements using a conventional viscometer, validating the proposed test configuration to continuously measure the rheological behavior of the barrier material. The pipe viscometer flow curves are generally found to be in good quantitative agreement with independent viscometer characterization of the fluids, although some of the pipe viscometer measurements likely exhibited entrance length effects. Future improvements to the pipe viscometer design involve the assessment of even longer pipe sections to allow full flow development at the highest shear rate range and possibly different pipe diameters to improve the measurement resolution of low-shear rate viscosity.publishedVersio