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Primary cementing involves displacing the drilling fluid from the annulus outside the casing and replacing it by cement slurry. Incomplete displacement can compromise zonal isolation, impacting both productivity and abandoning costs of the well. Washouts and similar irregularities are often generated during drilling, but their effect on displacement and cementing is poorly understood. We report results from full-scale cementing experiments and numerical simulations for an eccentric annulus geometry with a washout. Inclined annulus test assemblies were constructed by placing 7-in tubing inside 9 5/8-in casing and letting the inner tubing rest on the low side of the casing, supported by the tubing collars. The geometric irregularity is in the form of a 16-in casing segment welded onto the assemblies to represent a local washout. The assemblies were cemented by displacing a water-based spacer by denser, more viscous conventional cement slurry. Numerical simulations of cement placement have been performed using an open-source computational fluid dynamics platform. The quality of the hardened cement was investigated by leakage testing followed by visual inspection of the annulus after cutting the assemblies open at designated positions. Leakage paths are visible along the interface between cement and the outer wall along the upper side of the annulus. There is no obvious contamination of the cement after the washout due to mixing with the water-based spacer. Numerical simulations are in good qualitative agreement with experimental observations, including correctly predicting the presence of residual spacer fluid inside the washout. The experiments were performed at modest flow rates so the density difference between the fluids will largely compensate for downward casing eccentricity in the regular part of the assemblies. Buoyancy becomes even more pronounced inside the washout due to lower axial fluid velocity, resulting in strong downward slumping of the dense fluid and a fully three-dimensional displacement flow.
Primary cementing involves displacing the drilling fluid from the annulus outside the casing and replacing it by cement slurry. Incomplete displacement can compromise zonal isolation, impacting both productivity and abandoning costs of the well. Washouts and similar irregularities are often generated during drilling, but their effect on displacement and cementing is poorly understood. We report results from full-scale cementing experiments and numerical simulations for an eccentric annulus geometry with a washout. Inclined annulus test assemblies were constructed by placing 7-in tubing inside 9 5/8-in casing and letting the inner tubing rest on the low side of the casing, supported by the tubing collars. The geometric irregularity is in the form of a 16-in casing segment welded onto the assemblies to represent a local washout. The assemblies were cemented by displacing a water-based spacer by denser, more viscous conventional cement slurry. Numerical simulations of cement placement have been performed using an open-source computational fluid dynamics platform. The quality of the hardened cement was investigated by leakage testing followed by visual inspection of the annulus after cutting the assemblies open at designated positions. Leakage paths are visible along the interface between cement and the outer wall along the upper side of the annulus. There is no obvious contamination of the cement after the washout due to mixing with the water-based spacer. Numerical simulations are in good qualitative agreement with experimental observations, including correctly predicting the presence of residual spacer fluid inside the washout. The experiments were performed at modest flow rates so the density difference between the fluids will largely compensate for downward casing eccentricity in the regular part of the assemblies. Buoyancy becomes even more pronounced inside the washout due to lower axial fluid velocity, resulting in strong downward slumping of the dense fluid and a fully three-dimensional displacement flow.
Annular gas pressure, also known as sustained casing pressure (SCP), is a common problem and potential threat to the safety of personnel and equipment, as well as to the environment. Improved means of primary cementation and life-of-the-well simulations show promise for preventing future annular gas-flow problems. However, there has been little success in eliminating SCP, once developed, without jeopardizing the economic life of a well. Possible solutions to SCP in the form of remediation include bullheading cement, injection of zinc bromide brines into the well's annulus, or use of expensive resins to seal the annulus. A proposed solution that can compensate for the drawbacks of the above options and has shown promise on small-scale physical models is palletized alloy-metal. This solution involves placing palletized alloy-metal into the well's annulus, heating the alloymetal above its melting point, and then allowing the alloy-metal to cool. These steps form a continuous alloy-metal plug in the well's annulus. Data and conclusions documented in this paper are from full-scale pipe-in-pipe and pipe-in-sandstone geometry models having the following scope:Geometry of 5 1/2×8 1/2-in. pipe-in-sandstone and 10 3/4×13 3/8-in. pipe-in-pipe configurationsModel length limited to 15 ft and deviated at 30° from verticalAlloy-metal pellets placed and activated with water-based drilling fluid present in annuliTemperature limited to 200°F Results are also documented by dissecting the models and photographing the exposed cross-sections. Information gathered in this testing will help with the field introduction of this technology. Introduction SCP is a growing problem among offshore wells, leading to expensive shut-ins and remediations on many wells. Poor primary cementing, inadequate cement coverage, gas/water influx during or after cement placement, mud-cake shrinkage, and the development of stress-induced microfractures and microannuli are all cited as potential factors in SCP. The time between well completion and the onset of SCP can indicate the more likely causes. While there are several suspected culprits for SCP, pressure and temperature cycles are high on the list. Casing growth and contraction that result from production cycles and stimulation operations can de-couple the bond between the cement and casing. These forces can also stress-crack the cement. Both scenarios can create small pathways for high-pressure, lowvolume communication of annular gas to the surface, but the inaccessible nature of these pathways limits remediation options. Early onset mechanisms can include the following:Gas-cutting of the cementGas movement through a free-water channelCasing/tubing connection leaksInadequate cement coverage Late onset SCP can result from the following:Channels of bypassed mudStress cracks in the annular cement sheathShrinking or drying of the mud filter-cakeCasing/tubing connection leaks These factors are well documented, appearing frequently in past research. The following sections summarize a few main causes.
Experimental methods are still indispensable for fluid mechanics research, despite advancements in the modelling and computer simulation field. Experimental data are vital for validating simulations of complex flow systems. However, measuring the flow in industrially relevant systems can be difficult for several reasons. Here we address flow measurement challenges related to cementing of oil wells, where main experimental issues are related to opacity of the fluids and the sheer size of the system. The main objective is to track the propagation of a fluid-fluid interface during a two-fluid displacement process, and thereby to characterize the efficiency of the displacement process. We describe the implementation and use of an array of electrical conductivity probes, and demonstrate with examples how the signals can be used to recover relevant information about the displacement process. To our knowledge this is the most extensive use of this measurement method for studying displacement in a large-scale laboratory setup. Optical measurements and visual observations are challenging and/or costly in such large-scale systems, but can still provide qualitative information as shown in this article. Using electrical conductivity probes is a robust and fairly low-cost experimental method for characterizing fluid-fluid displacement in large-scale systems.
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