Effective mud removal is a prerequisite to attain the cement coverage necessary for good zonal isolation. Because of this, the oilfield industry has dedicated considerable attention to the topic of mud displacement over the past 60 years. The first 2D annular displacement simulator was introduced in the 1990s and it is now widely available. The results are satisfactory for simpler configurations. However, for deeper wells with complex trajectories such as highly deviated or horizontal wells, the models start to show their limits. This paper discusses the advancements in mud displacement simulation that overcome the limitations of the previous generation simulator and provide a more realistic simulation in highly deviated and horizontal wells. A new generation simulator now provides high-fidelity results via a combination of: 1) a pipe displacement model, accounting for fluid contamination inside the pipe; 2) a high-resolution annular displacement model, accounting for the complex 3D annulus shape with full determination of axial and azimuthal flows; and 3) a stiff-string centralization model based on the finite-element method, predicting casing position in a 3D wellbore. A primary cementing operation for a horizontal well was studied and an unprecedented congruence was witnessed between predicted fluids annular concentration maps and ultrasonic cement log. The simulator was also able to predict complex channeling patterns in the annulus. These results allow a better understanding of the cement placement technique and provide means to optimize the sequence of fluids to achieve effective mud displacement in the well. Enabled by advancements in today’s computing capabilities, the new simulator is able to simulate both simple and highly complex scenarios more realistically. Finally, the new model allows better planning and decision making to achieve zonal isolation and well objectives.
The cement sheath of an oil or gas well is designed to prevent fluid communication between the drilled formations during the productive life of the well through post-abandonment. However, even when cement initially provides good hydraulic seal, zonal isolation can deteriorate after initial placement due to microannuli formation. In many cases, a microannulus can be formed because of wellbore temperature changes, wellbore pressure changes, or set cement bulk shrinkage.Microannuli are considered serious concern in gas wells because, unlike liquid, gas flows easily to surface through very narrow microannuli. Cementing operations which fail to provide adequate zonal isolation may result in loss of isolation which poses a hazard to both personnel and the environment. A cement that has ability to expand post set, can help prevent the formation of microannuli. The concept is that the expanding cement will fill microannuli and ensure good bonding either between the formation and the cement or between casing and the cement. This paper discusses the application of expanding cement in two large gas fields; located in South Sumatra. In those fields, wells are produced using 7-in production tubing, with 13 3/8-in casing and 9 5/8 in liner as production strings. Production strings are constantly exposed to changes in pressure and temperature, both during drilling and during production. These changes can create stresses on the cement sheath and may create microannuli. There is a particular focus on the isolation around 13 3/8-in production string: unlike the 9 5/8-in production liner string, the 13 3/8-in string relies only on cement to isolate the gas zones, without any packer. This string needs to provide isolation is critical to ensure that the well can perform as designed and pose no hazard to personnel and to the environment. The paper also discusses the design steps and considerations, as well as evaluation results.
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