Decline Curve Analysis in Plain English
By PlainSight — Insightful Actions · Updated July 2026 · ~8 min read
Every well tells the same story in a different accent: it comes on strong, falls off fast, then settles into a long slow tail. Decline curve analysis is simply the discipline of describing that story with three or four numbers, so you can compare wells, forecast what is left, and spot the one that is behaving oddly.
What a decline curve is
Plot a well’s production rate against time and you get a curve that falls away from an early peak. Decline curve analysis fits a mathematical shape to that history so the remaining life can be projected. The classical framework, still the industry workhorse, comes from J. J. Arps in 1945 and is described by three parameters.
The three Arps parameters
qi — initial rate. Where the fitted curve starts. Often related to, but not identical with, the peak rate you actually measured.
Di — initial decline rate. How steeply production falls at the start, usually quoted as a percentage per year.
b — the decline exponent. The shape of the curve. b = 0 gives exponential decline (a straight line on a log plot), b = 1 gives harmonic, and values between produce hyperbolic decline. Conventional wells often sit near the low end; unconventional shale wells commonly fit higher b values, sometimes above 1 in early life.
Why the b-factor matters more than it looks
The b-factor decides how fat the tail is, and therefore how much oil or gas the model says you will ultimately recover. A small change in b produces a large change in the forecast — which is exactly why it deserves scepticism. A high b fitted to eighteen months of early shale data, extended blindly for thirty years, will predict a well that never stops producing.
The standard defence is a terminal decline: fit hyperbolic behaviour early, then switch to a fixed minimum exponential decline once the curve flattens beyond a chosen threshold. Without that switch, hyperbolic forecasts drift into physically implausible territory.
Estimated ultimate recovery
EUR is the area under the fitted curve — cumulative production to date plus everything the model projects from here to an economic limit. Two honest cautions belong with every EUR you produce:
- It is a model output, not a measurement. Change b, change the terminal decline, or change the economic limit, and the number moves substantially.
- Early-life fits are unstable. The less history you have, especially before the curve has clearly turned, the wider your real uncertainty is regardless of how tight the fit looks.
Reading your own production export
You need very little: a date, a well identifier, and a rate or volume per period. Monthly production is fine for most purposes and is less noisy than daily. Once loaded, the questions worth asking are:
Initial rate and time to peak
What to look for: how quickly the well reached its peak, and how sharp that peak was.
A well that peaks late may have been choked deliberately, or may have had flowback or facility constraints. Comparing peak timing across a pad often says more about operations than geology.
First-year decline
What to look for: the percentage drop from peak across the first twelve months.
This is the most comparable single number between wells, and far more robust than a fitted b in early life. It is also what most people mean when they casually say “a well declines 60% in year one.”
Fit quality and the outliers
What to look for: months that sit well off the fitted curve.
Deviations usually have an operational explanation — downtime, a workover, artificial lift installation, offset frac interference, or a shut-in. Flagging them matters, because feeding them into a fit quietly corrupts the forecast.
Well-to-well comparison
What to look for: how one well ranks against its peers on the same curve shape.
Normalizing by lateral length or by proppant loading makes comparison fairer. A well below its peers on the same completion design is a genuine question worth asking; one below peers with a very different design is simply a different well.
Connecting decline to completion design
The interesting analysis is rarely the curve alone — it is the curve set against how the well was built. Lateral length, stage spacing, proppant intensity, and landing zone all correlate with both initial rate and decline behaviour. A completion that buys a higher initial rate but a steeper decline may or may not win on EUR, and that is precisely the question worth putting numbers to before the next pad is designed.
Three honest limits
- Decline curve analysis is empirical. It describes what a well has done and extrapolates. It contains no physics about the reservoir, so it cannot anticipate a change in behaviour it has never seen.
- It assumes conditions stay broadly constant. A change in choke management, artificial lift, or offset development can invalidate a fit that looked excellent yesterday.
- It is not a reserves report. Booked reserves follow defined standards and professional judgement. Treat your own analysis as an operational read, not a formal estimate.
Let PlainSight fit your production data
Upload a production export and PlainSight fits decline curves per well, reports initial rate, decline, and b-factor, estimates recovery, compares wells against peers, and can fold in completion data such as lateral length and stage count. It runs entirely in your browser — your well data never leaves your device.
Try it free on your own numbers →
Frequently asked questions
- What data do I need for decline curve analysis?
- A date, a well identifier, and a production rate or volume per period. Monthly data works well and is less noisy than daily. Public state data works fine if you do not have your own.
- What is a typical b-factor?
- It depends heavily on the play and the drive mechanism. Conventional wells often fit low b values near exponential behaviour, while unconventional shale wells commonly fit higher values, sometimes above 1 early in life. Because a high b inflates long-term forecasts, most practitioners impose a terminal decline.
- How much history do I need before a forecast is meaningful?
- Enough for the curve to have clearly turned and settled. Very early fits are unstable, and confidence improves substantially as the well moves past its steepest decline.
- Is decline curve analysis the same as booking reserves?
- No. Reserves reporting follows defined standards and professional judgement. Decline curve analysis is an operational and screening tool that informs, but does not replace, that process.
This guide is general technical information, not engineering, investment, or reserves advice. Production forecasts are model outputs with real uncertainty and should be reviewed by a qualified petroleum engineer before being relied upon for economic or reporting decisions.