The Critical Role of Grooving Before Cement Plug Sidetracking Operations

"Grooving" (or "slot cutting") is a preparatory operation performed on the top of a cement plug before initiating a sidetrack. It involves using a drill bit (preferably a short-gauge PDC bit​ or a mill-tooth cone bit) to grind a shallow slot or depression (30-80 mm deep) near the designed kick-off point. The operation is executed with extremely light weight on bit (0.5-1.5 tons), low rotary speed (30-50 rpm), and a low flow rate (0.6-0.8 m³/min). Its essence is to artificially create a miniature guiding "step" or ramp, providing an initial deflecting surface for the subsequent bit to prevent it from skidding on the flat cement surface or drilling straight down.

Eliminating the "Flat Surface Trap": A flat or irregular cement surface offers no natural deviation point, leading the symmetrically loaded bit to likely drill straight through the plug back into the old wellbore. Grooving creates the required asymmetry. The document cites field data: out of 4 wells in a block that sidetracked without grooving, 3 had initial azimuth deviations exceeding 15°, with 1 failing completely. In contrast, all 8 wells that performed grooving had deviations controlled within 5°.

Verifying Cement Plug Quality: The grooving process serves as a direct, real-time quality check. Parameters like rate of penetration, torque fluctuations, and the nature of cuttings returns​ are key indicators.

• Normal: Slow but uniform penetration, returns of fine cement particles, steady torque.
• Abnormal: Extremely fast penetration (cement strength too low), returns of large mud slugs (contaminated cement top), or severe torque spikes/bit bounce (hard streaks or debris).

Establishing a Controlled Kick-off Point: Without a whipstock, the only guidance for the bit is the asymmetry of its contact with the cement. The groove acts as this man-made guide, ensuring the bit naturally deviates along the groove's path, thus controlling the sidetrack's initial direction and preventing unpredictable "bit walking."

Preventing "Fallback" into the Old Wellbore: If the initial deviation angle is too small (<3°), the drill string can easily guide the bit back into the original wellbore after drilling through the plug, nullifying the sidetrack. A proper groove helps establish a sufficient initial angle to prevent this.

Bit Selection: Short-gauge, shallow cone PDC bits​ are preferred for easy penetration. Mill-tooth cone bits can be used with controlled WOB. Long-gauge PDC bits or full-hole bits are prohibited​ as they cannot create localized deflection.

Key Parameters (as per the table in the document):

Rotary Speed: 30-50 rpm

Weight on Bit: 0.5-1.5 tons

Flow Rate: 0.6-0.8 m³/min (just enough for cuttings transport)

Step-by-Step Process:

1. Run in hole to 0.5m above the cement top, circulate and clean the surface.
2. Slowly lower the bit, pausing every 0.1m to feel the contact.
3. Once weight indication shows contact, start the rotary table (30 rpm) and apply 0.5t WOB.
4. Maintain WOB and grind in place for 5-10 minutes to form a initial shallow pit.
5. Pick up the bit 0.2m, circulate and observe returns.
6. Lower the bit again, increase WOB to 1.0t, and grind for another 10-15 minutes to deepen and widen the groove.
7. Total grooving time should be 20-30 minutes, achieving a depth of ~50mm.

The "Three Observables" During Grooving:

Torque: Stable torque is normal. A sudden drop may indicate slipping or cement fracturing; a continuous increase may signal bit balling or sticking.

Cuttings Returns: Fine cement particles (1-3mm) are normal. Large chunks (>10mm) indicate excessive WOB or brittle cement. Mud returns signal contamination.

Rate of Penetration (ROP): During grooving, ROP should be 5-15 min/0.1m. An ROP faster than 2 min/0.1m indicates the cement is too soft, and operations must stop.

Post-Grooving Verification:

Visual Inspection (after pulling out): Cement grinding marks should be concentrated on one side (low side) of the bit, confirming an eccentric groove was formed.

Run-in-Hole Verification: When running back in, the bit should naturally "drop" into the groove, with a noticeable slight weight variation.

Deflection Test: Use sidetracking parameters (2-3t WOB, 40-60 rpm) to drill 0.5m and survey. An initial build rate ≥2°/30m confirms a successful groove.

Success Case: A directional well with a cement plug at 2700m. Using a φ152mm PDC bit, grooving was performed (40 rpm, 0.8t WOB, 15 min) to create a ~40mm deep groove. Post-grooving inspection showed clear eccentric wear. Subsequent sidetracking achieved a 2.5°/30m build rate and successfully left the old wellbore.

Common Critical Errors:

Excessive WOB during grooving​ (e.g., 3-4t): This drills a straight hole instead of grinding a groove. The core is "grinding," not "drilling."

Insufficient grooving time​ (e.g., 3-5 minutes): Results in only a surface scratch, which gets smoothed out. Minimum 20 minutes is recommended.

Neglecting to clean the cement top​ before grooving: Contaminants interfere with groove formation and can cause bit balling.

Grooving before the cement has fully cured: Cement must achieve sufficient compressive strength (≥3.5 MPa, verified by samples or CBL) to allow proper grooving.

The success of cement plug sidetracking depends on "stability," not speed. The grooving process, while adding about 30 minutes, acts as essential insurance for the entire operation-ensuring the cement plug is not wasted and the new wellbore does not fall back into the old one.

The key mnemonic provided is: "Use light pressure and slow rotation to grind a shallow groove; observe fine cuttings and eccentric wear; begin sidetracking only after achieving a groove depth of about 50mm." For more detailed information, please don't hesitate to contact Vigor team for more detailed product information.