Casing & Cementing Operations

Module 5: Casing & Cementing Operations 

5.1 Casing Design Principles

Casing strings are critical for well integrity, providing structural support and zonal isolation.

Casing Strings & Functions:

1. Conductor Casing:

• First string set shallow (50–300 ft) to stabilize surface formations.

• Prevents washouts and supports subsequent casing.

2. Surface Casing:

• Isolates freshwater zones (typically 1,000–5,000 ft).

• Anchors BOP stack for well control.

3. Intermediate Casing:

• Isolates unstable formations or pressure transitions.

• Mitigates lost circulation or kicks in deeper sections.

4. Production Casing:

• Final string reaching reservoir depth.

• Provides conduit for hydrocarbons and well completion.

Casing Design Calculations:

• Burst Pressure:

• Maximum internal pressure (e.g., gas influx) casing must withstand.

• Formula: Burst Resistance = Yield Strength × (OD/t) (OD = outer diameter, t = wall thickness).

• Collapse Pressure:

• External pressure from fluid/formation loads.

• Governed by API Collapse Rating Tables (varies with diameter/grade).

• Tension Load:

• Weight of casing string + bending forces in deviated wells.

• Safety factor: 1.6–2.0 for static loads; higher for dynamic (tripping).

Design Workflow:

1. Select casing size based on wellbore diagram.

2. Calculate burst/collapse/tension for worst-case scenarios.

3. Choose material grade (e.g., L-80, P-110) and connection type (e.g., API, premium).

5.2 Cementing Process

Cementing isolates zones, prevents leaks, and supports casing.

Slurry Design:

1. Additives:

• Retarders: Extend thickening time for deep wells (e.g., lignosulfonates).

• Accelerators: Speed up setting in shallow zones (e.g., calcium chloride).

• Lightweight Agents: Reduce density for weak formations (e.g., bentonite, foam).

• Dispersants: Improve flow (e.g., sulfonated polymers).

2. Thickening Time:

• Measured via Consistometer (simulates downhole conditions).

• Must exceed operational time + safety margin (typically 2–4 hours).

Primary Cementing Techniques:

1. Single-Stage Cementing:

• Cement pumped through casing shoe in one circulation.

• Used for shallow or simple wells.

2. Multi-Stage Cementing:

• Uses differential valves to cement isolated sections separately.

• Ideal for long intervals or pressure-sensitive zones.

3. Liner Cementing:

• Cementing a liner (not full casing) tied back to surface.

• Common in deep wells to save cost.

Critical Steps:

• Mud Removal: Pre-job spacer fluids clean the wellbore.

• Centralization: Casing centralizers ensure even cement distribution.

• Pressure Testing: Verify zonal isolation post-cementing (e.g., CBL/VDL logs).

Key Takeaways:

• Casing design balances burst/collapse/tension loads with cost and safety factors.

• Cement slurry is tailored to well depth, temperature, and formation needs.

• Cementing techniques vary based on well complexity; poor cement leads to sustained casing pressure (SCP) or blowouts.

• API Standards (e.g., 5CT, 10A) govern material specs and testing.