To achieve a successful oil well cementing and ensure zonal isolation in oil well cementing, efficient mud displacement is crucial. Inadequate mud removal can lead to problems like cement channeling, permitting the intrusion of hydrocarbons and communication between permeable zones. The following factors are essential for ensuring a good cement bond:
- Conditioning the Drilling Mud: Before removing the drill pipe from the well, it’s essential to reduce the mud’s viscosity to the lowest practical level. Focus should be on decreasing the low shear-rate rheology of the mud, such as gel strengths and yield points. However, caution should be exercised to maintain the mud rheology above the minimum level required to suspend weighting agents like barite.After the casing is run, additional mud conditioning is necessary to remove gelled mud that may have formed beneath the casing due to poor centralization. Typically, two to three hole volumes suffice for this purpose, but the actual requirement depends on the mud’s viscosity and casing centralization. Gelled mud becomes increasingly challenging to remove over time.It’s important to note that the removal of gelled mud requires shear stress to overcome its strength. This shear stress can be generated through pipe movement or the mobile mud. Increasing mud flow rate or modifying drilling fluid properties can enhance shear stresses. Ideally, mud should be conditioned during circulation before casing installation to minimize gelled mud issues.
- Casing Movement: Whenever possible, casing should be reciprocated or rotated. Various studies have shown that pipe movement enhances displacement efficiency by breaking up gelled mud pockets. The debate continues regarding whether casing reciprocation or rotation is more effective. For liners, rotation is recommended, considering liner setting and gas swabbing concerns. Common guidelines suggest reciprocating 20-40 ft strokes over 2-5 minutes or rotating at rates of 10-40 rpm.
- Centralization: When running casing in deviated wells, eccentricity can lead to trapped pockets of mud on the low side of the wellbore, potentially causing cement channeling or incomplete zonal isolation. To ensure unhindered circulation beneath the casing, it is advisable to maintain a minimum standoff of 70%, especially after optimizing mud rheology and displacement rates. Centralizers should be positioned to allow casing reciprocation or rotation in cases where casing movement is necessary. A centralizer program should consider the entire casing string’s mechanics, along with buoyancy and density differential effects during displacement. If washouts are expected, centralizer quantities should be adjusted to account for the increased hole size.
- Displacement Rate: Turbulent flow in the displacing fluid leads to highly effective displacement. However, achieving turbulence across an eccentric annulus can be challenging, and it often results in gelled mud remaining in the narrow section of the annulus. In cases where turbulence is feasible, a higher displacement rate should be employed for the best results. In situations where turbulence cannot be maintained, a well-designed laminar displacement can still achieve effective results. Careful consideration of density, viscosity, and annular flow rate is crucial for laminar displacements.
- Washes and Spacers: In a successful primary cementing operation, the cement slurry must displace the fluid surrounding the casing. The incompatibility of mud and cement can lead to channeling or viscous masses. To address this issue, an intermediate fluid is used as a pre-flush to remove drilling mud from the annulus. Optimal mud removal is achieved with a simple wash, which creates turbulence at low annular velocities. However, weighted spacers may be necessary for well control, and a combination of a thin wash with a weighted spacer can effectively remove mud. When turbulent wash or spacer is used, a minimum contact time of 10 minutes should be ensured.
In conclusion, a successful oil well cementing operation relies on addressing these critical factors to ensure efficient mud displacement, proper centralization, casing movement, effective washes and spacers, and appropriate displacement rates. Tailoring these considerations to the specific well conditions is essential for optimal results.
References
Baker, R. (2001) A primer of oilwell drilling: A basic text of oil and gas drilling. 6th edn. Austin, TX: Petroleum Extension Service, Continuing & Extended Education, University of Texas at Austin.
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Bourgoyne, A.T. (1986) Applied drilling engineering. Richardson, TX: Society of Petroleum Engineers.
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