Reading a pressure build-up test
Go from raw gauge data to permeability, skin and boundaries by reading the derivative plot step by step.
The picture
Radial flow
What you see: The derivative is flat.
What it tells you: The pressure disturbance is moving outward in all directions. The flat level gives permeability-thickness: kh = 70.6 q B μ ÷ level, in oilfield units. This is the most valuable part of the test.
Synthetic example data, made for teaching.
The workflow
Collect and check the data
In: Gauge pressure, rate history, fluid and rock propertiesOut: Clean pressure data and a correct rate history
You need pressure versus time, every flow rate before the shut-in, and the properties B, μ, ct, φ, h and wellbore radius. Plot the raw pressure first and look for gauge drift, gaps and spikes.
Pick the build-up period
In: Pressure and rate historyOut: Shut-in start time and the build-up data
Mark the exact time the well was shut in. Everything after it is the build-up. The flow time before it, tp, is needed for the Horner plot.
Draw the diagnostic plot
In: Build-up pressure dataOut: Log-log plot of Δp and its derivative against Δt
Plot the pressure change since shut-in and its Bourdet derivative on log-log axes. The shape of the derivative is the fingerprint of the reservoir, and you read it before you calculate anything.
Identify the flow regimes
In: Diagnostic plotOut: Labeled wellbore storage, radial flow and boundary periods
Early on, pressure and derivative fall on a unit-slope line: wellbore storage. Then the derivative humps and settles to a flat line: infinite-acting radial flow. Later changes in the derivative show boundaries: it doubles for a sealing fault and rises steeply for a closed reservoir.
Calculate kh and skin
In: Radial flow line, rate, fluid propertiesOut: Permeability, skin factor, extrapolated pressure p*
Use the flat derivative level (or the Horner slope m) in oilfield units: k = 162.6 q B μ ÷ (m h). Skin comes from the pressure at one hour, S = 1.1513 [ (p1hr − pwf) ÷ m − log(k ÷ (φ μ ct rw²)) + 3.23 ]. Extrapolating the Horner line to infinite shut-in time gives p*.
Match a model and report
In: Interpretation values, a modelOut: Match on the diagnostic plot and a results table
Choose a model that reproduces the whole derivative, not only the radial section. Adjust until pressure and derivative match together, then report the results with the model type and the assumptions.
What you hand over
Reservoir pressure, permeability-thickness, skin, and any boundaries seen, with the model match shown on the log-log plot.
Background
What a pressure test tells you
When you shut in a well, the pressure rises as the reservoir recovers. How that pressure rises over time carries information about the rock and fluid several tens or hundreds of meters from the well. A pressure transient analysis turns that curve into numbers: permeability-thickness (kh), skin (damage or stimulation near the well) and boundaries.
The plot that does the work
The log-log diagnostic plot shows the pressure change and its derivative together. Read it from left to right, like a story about how far the pressure disturbance has traveled:
- The wellbore has to fill up first (wellbore storage).
- The disturbance enters the reservoir and settles into radial flow.
- The disturbance reaches something: a fault, a boundary, a change in rock.
Use the interactive plot on this page to see where each period appears.
Decide the model before the numbers
The most common beginner error is to start calculating before reading the plot. The derivative shape tells you which model to use, and a number calculated from the wrong period is wrong however carefully you calculate it.
Common mistakes
- Using the wrong rate history, which distorts the whole derivative.
- Picking a straight line on the Horner plot without confirming that it is radial flow on the derivative.
- Ignoring wellbore storage and calling an early straight line radial flow.
- Using the wrong net thickness h, which gives permeability that is off by the same factor.
- Calling a boundary from a test that was too short to reach it.
Check yourself
Try this workflow in PulseTech
Analyze drill stem tests and pressure transients.
Prefer a walkthrough? Request a guided demo. Studying? See the student discount.