Engineering lesson
SIDPP versus SICP what each pressure represents
SIDPP and SICP are pressures at different flow paths. Under the usual static assumptions, their difference reflects different hydrostatic columns; substituting casing pressure for drillpipe pressure can distort a kill-density calculation.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Write two balances to the same bottom point
In a simplified shut-in example, the drillpipe contains a known 12 ppg column over 10,000 ft TVD. A 500 psi SIDPP implies 6,740 psig at the bottom: 6,240 psi of fluid head plus 500 psi at surface.
If the annular head is only 5,800 psi because its fluid distribution differs, the same bottom pressure requires 940 psi SICP. Both readings can be consistent; they need not be numerically equal.
What the drillpipe calculation assumes
The inferred density increment is SIDPP/(0.052 × TVD). The calculation presumes a credible static drillpipe fluid column and an interpretable stabilized pressure. Float effects, trapped pressure or an uncertain column can break that inference.
SICP is valuable diagnostic information, but the simplified example cannot infer influx height, composition or volume from that reading alone. Multiple distributions can produce the same hydrostatic deficit.
Separate diagnosis from arithmetic
A higher casing pressure does not by itself prove a higher formation pressure than the drillpipe balance gives. First reconcile both paths to the same depth and reference.
This lesson explains a static balance, not a shut-in or circulation procedure. Observed pressure changes over time require additional operational information beyond these fixed teaching columns.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Annular head (psi) = 5800. SICP (psig): 940; SIDPP (psig): 500.
- SICP (psig)
- SIDPP (psig)
Worked example
Two paths to one static bottom pressure
Bottom pressure fixed at 6,740 psig; drillpipe head 6,240 psi, so SIDPP = 500 psig. Annular head varies.
Relation and unit basis
SICP = 6,740 − annular head
Both surface pressures are gauge readings on the same atmosphere; the columns connect to the same bottom datum.
1. Define the baseline
Annular head (psi) = 5800
Bottom pressure fixed at 6,740 psig; drillpipe head 6,240 psi, so SIDPP = 500 psig. Annular head varies.
2. Select the relation
SICP = 6,740 − annular head
Both surface pressures are gauge readings on the same atmosphere; the columns connect to the same bottom datum.
3. Substitute the values
6,740 − 5,800 = 940 psig; 6,740 − 6,240 = 500 psig
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
SICP (psig) = 940; SIDPP (psig) = 500
Different surface readings can balance the same bottom pressure when fluid heads differ.
Result and interpretation
SICP (psig) = 940; SIDPP (psig) = 500
Different surface readings can balance the same bottom pressure when fluid heads differ.
The comparison changes annular head while holding the inferred bottom pressure constant. SIDPP remains tied to the known drillpipe column.
Exact scenario data
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Annular head (psi) | SICP (psig) | SIDPP (psig) |
|---|---|---|
| 5,400 | 1,340 | 500 |
| 5,600 | 1,140 | 500 |
| 5,800 | 940 | 500 |
| 6,000 | 740 | 500 |
| 6,240 | 500 | 500 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. With bottom pressure 6,740 psig and annular head 6,000 psi, what SICP balances it?
Show the worked answer
740 psig. The bottom pressure is unchanged; the annular hydrostatic contribution increased.
2. Reproduce the baseline SICP (psig) without rounding intermediate values.
Show the worked answer
6,740 − 5,800 = 940 psig; 6,740 − 6,240 = 500 psig. SICP (psig) = 940. Different surface readings can balance the same bottom pressure when fluid heads differ.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Annular head (psi) changes from 5400 to 6240. Other controls remain as stated in the premise. SICP (psig) changes from 1340 to 500.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. Both surface pressures are gauge readings on the same atmosphere; the columns connect to the same bottom datum. Different surface readings can balance the same bottom pressure when fluid heads differ.
Before using the result
- Reconcile both paths at one bottom datum.
- Verify what fills the drillpipe.
- Do not infer influx properties from one pressure difference.
Different surface readings can balance the same bottom pressure when fluid heads differ.
Continue learning
Start with the broader workflow guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.