Overview
A kill sheet is the essential well control document that pre-calculates the pressures and volumes needed to safely circulate out a kick and kill a well. Two primary methods are used: the Driller's Method (two-circulation) and the Wait & Weight (Engineer's) Method (one-circulation). Both use the same fundamental well control principles but differ in execution sequence and time to kill.
Theory
When a kick occurs, formation fluid enters the wellbore because formation pressure exceeds hydrostatic pressure. To regain control:
- Shut in the well (record SIDPP and SICP)
- Calculate kill mud weight
- Circulate out the kick while maintaining constant bottomhole pressure (BHP)
- Kick density < 3 ppg → gas kick
- Kick density 3–7 ppg → oil/condensate kick
- Kick density > 7 ppg → saltwater kick
- Grace, R.D. (2003). Advanced Blowout and Well Control. Gulf Publishing.
- Watson, D., Brittenham, T. & Moore, P.L. (2003). Advanced Well Control. SPE Textbook.
- IWCF Well Control Training Manual.
- PetroWiki — Well control: https://petrowiki.spe.org/Well_control
The kill mud weight must generate enough hydrostatic pressure to balance formation pressure without exceeding fracture gradient at the shoe.
Formulas
Kill Mud Weight
KMW = OMW + SIDPP / (0.052 * TVD)
| Symbol | Description | Units |
|---|---|---|
| KMW | Kill mud weight | ppg |
| OMW | Original mud weight | ppg |
| SIDPP | Shut-in drill pipe pressure | psi |
| TVD | True vertical depth of well | ft |
Initial Circulating Pressure (ICP)
ICP = SIDPP + SPR_slow
where SPR_slow = slow circulating rate pressure (recorded before kick).
Final Circulating Pressure (FCP)
FCP = SPR_slow * (KMW / OMW)
Drill Pipe Pressure Schedule (Wait & Weight)
Linear decrease from ICP to FCP over the number of strokes to fill the drill string:
DP_pressure(n) = ICP - (ICP - FCP) * (n / strokes_to_fill_DS)
Maximum Allowable Surface Pressure (MAASP)
MAASP = 0.052 * (Fracture_MW - MW) * Shoe_TVD
Kick Identification
Kick_density = MW - (SICP - SIDPP) / (0.052 * kick_height)
Volume Calculations
Drill string capacity = (ID^2 / 1029.4) * length (bbl)
Annular capacity = ((Dh^2 - OD^2) / 1029.4) * length (bbl)
Worked Example
Given: OMW = 10.0 ppg, TVD = 10,000 ft, SIDPP = 300 psi, SICP = 450 psi, SPR at 30 spm = 600 psi, pit gain = 20 bbl.
Step 1: Kill mud weight:
KMW = 10.0 + 300 / (0.052 * 10,000) = 10.0 + 0.577 = 10.58 ppg → round up to 10.6 ppg
Step 2: ICP:
ICP = 300 + 600 = 900 psi
Step 3: FCP:
FCP = 600 * (10.6 / 10.0) = 636 psi
Step 4: MAASP (fracture MW at shoe = 14.0 ppg, shoe TVD = 5,000 ft):
MAASP = 0.052 * (14.0 - 10.0) * 5,000 = 1,040 psi
Step 5: Kick type (kick height from annular volume = 200 ft):
Kick_density = 10.0 - (450 - 300) / (0.052 * 200) = 10.0 - 14.4 = -4.4 ppg → GAS KICK
Driller's Method vs Wait & Weight
| Aspect | Driller's Method | Wait & Weight |
|---|---|---|
| Circulations | 2 (circulate kick out, then kill) | 1 (kill mud displaces kick) |
| Time to start | Immediate | Must wait for kill mud to be mixed |
| Casing shoe pressure | Higher (gas reaches shoe in old mud) | Lower (kill mud behind gas) |
| Complexity | Simpler (constant DP pressure first circ) | More complex (DP pressure schedule) |
| When to use | Small kicks, simple geometry | Large kicks, narrow-margin wells |
Valid Ranges
| Parameter | Typical Range |
|---|---|
| SIDPP | 0 – 3,000 psi |
| KMW increment | 0.2 – 2.0 ppg above OMW |
| SPR (30 spm) | 200 – 1,200 psi |
| MAASP | Must be > SICP to safely shut in |