Pressure and Fluid Contacts · Module P3
Gradients and implied densities
Each fluid in a connected column plots as a straight line of pressure against true vertical depth. The module covers the least-squares fit, why it has to run on true vertical depth, typing tests by fluid by hand rather than trusting an automatic guess, converting each slope into an implied density, and stating the confidence interval that shows how far a few tests can be trusted.
- Level
- Intermediate
- Position
- Module 3 of 8
- Video
- About 20 minutes, narrated
- Format
- Narrated video lesson with captions and slide reading view; with the certified option, a quiz whose pass unlocks the next module.
What the module covers
- One least-squares line per fluid
- Why every fit runs on true vertical depth
- Typing tests by fluid, and the limits of automatic typing
- Implied specific gravity from a gradient
- Confidence intervals and the number of tests
- Water gradient and salinity
Learning outcomes
After the module you should be able to:
- Fit gradients that read as fluid densities, with their uncertainty stated.
- Read a gradient as a specific gravity and judge whether it is physical
- Explain why a high R² on three tests is weak evidence
A short preview
Divide a fitted gradient by 0.433 and the answer is the specific gravity of the fluid behind it. A slope of 0.320 psi/ft reads as 0.74, a plausible oil; 0.455 psi/ft reads as 1.05, a brine. The check matters because a straight line through three points nearly always looks good: R² can pass 0.99 while the 95 percent interval on the slope still runs from gas to water. More tests over a longer column, rather than a better R², is what narrows that interval.
Who it is for
Petrophysicists, reservoir engineers and geoscientists who screen formation pressure tests, pick fluid contacts and carry them into volumes for exploration, appraisal and development decisions.
Prerequisites: None required; working familiarity with well logs and volumetrics helps. The hands-on steps use the Pressure Analysis panel in Formation Evaluation.
Take the module in the Learning Hub
Pressure and Fluid Contacts: Specialty course, sold separately: eight modules that turn a formation tester report into fluid contacts you can defend, from screened tests and gradients to capillary entry height and a contact range in the volume. The lesson opens in the Learning Hub once the track is purchased; graded quizzes and the certificate come with the certified option.