7.2 Cementing Casing and Liner: BHP Changes, Verification & Flow During Cementing

Cementing turns the casing into part of the secondary barrier envelope, but during the job itself the bottom-hole pressure keeps changing as fluids of different densities are pumped. The IWCF Level 2 syllabus asks candidates to describe those changes, how a successful job is confirmed, and what to do if the well flows during cementing.

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How Bottom-Hole Pressure Changes During a Cement Job

A typical primary cement job pumps a sequence of fluids down the casing and up the annulus:

While pumping, circulating friction also adds to BHP (an equivalent circulating density effect), so the pump rate is part of the design. Heavy cement inside the casing can also U-tube (free-fall) against the lighter fluids in the annulus, making returns briefly faster than the pump rate and then pausing; the crew should expect this and report anything different.

Worked Example: Spacer and Slurry Effects

A well at 10,000 ft TVD has 12.0 ppg mud (hydrostatic $0.052 \times 12.0 \times 10{,}000 = 6{,}240\text{ psi}$) and a formation pressure of 5,950 psi, giving 290 psi of overbalance.

• If 800 vertical feet of annulus fill with 8.4 ppg water wash, BHP falls by $0.052 \times (12.0 - 8.4) \times 800 = 150\text{ psi}$ to about 6,090 psi, still overbalanced.
• If 2,000 vertical feet fill with the wash, BHP falls by $0.052 \times 3.6 \times 2{,}000 = 374\text{ psi}$ to about 5,866 psi, below formation pressure: the well can kick. Hence the limit on wash volume, or a weighted spacer.
• When 2,000 vertical feet of annulus later fill with 15.8 ppg cement, BHP rises by $0.052 \times (15.8 - 12.0) \times 2{,}000 = 395\text{ psi}$, which must stay below the fracture pressure at the shoe.

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Confirming the Cement Has Been Placed Successfully

A poor cement job leaves a primary barrier failure behind the casing: channels, a low cement top or an unsealed shoe can let formation fluid flow up the annulus, appear as sustained casing pressure, or lead to a blowout. The Montara blowout (2009) followed a casing-shoe cement barrier that was accepted without verification.

The IWCF syllabus lists three checks that the cement went where intended:

1. Correct returns: full returns throughout the job. Losses mean cement may have gone into the formation and the top of cement may be lower than planned.
2. No back flow: after the top plug is bumped and pressure is released, little or no fluid should flow back from the casing. Back flow means the float equipment is not holding and the heavier cement in the annulus is trying to U-tube back into the casing; pressure must then be held on the casing (the cement head valves kept closed) until the cement sets.
3. Correct quantity and weight: the slurry density is checked while mixing, the volume mixed and pumped matches the plan, and the plug is bumped at the calculated displacement.

These show placement, not strength. To verify that the set cement forms a confirmed barrier, it is pressure tested: for example the casing pressure test against the cemented shoe track, and later the shoe test (FIT or LOT) after drilling out. Where a barrier must hold pressure from the formation side, such as a liner top, an inflow test may also be required.

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If the Well Starts to Flow During Cementing

A well can flow during cementing because a light wash reduced hydrostatic, because losses lowered the fluid level, or because the cement lost hydrostatic while setting. The syllabus gives the same three-step response as for running casing:

1. Stop operations: stop pumping.
2. Secure the string: close the valves on the cement head or the safety valve on the string so nothing can flow up the casing.
3. Shut in the well: close the annular preventer (or rams that fit the pipe across the stack), open the choke-line (HCR) valve against the closed choke, and record pressures.

The supervisor then decides how to continue, for example finishing the displacement through the choke while holding back-pressure, because cement left in the string or annulus will soon set. After the job, the well is monitored (flow checks on the annulus) while the cement develops strength.

For the Level 2 crew, the duties are to watch returns and the trip tank throughout the job, compare them with the cementing programme, and report any gain, loss or unexpected flow to the Driller immediately.