Decline curve
- Also known as
- decline curve analysis, DCA, Arps decline
A decline curve is a plot of a well's production rate over time, showing how output falls as the reservoir is drained. Decline curve analysis fits a mathematical model to that history to forecast future production and estimate ultimate recovery.
What is a decline curve?
Almost every oil and gas well produces most strongly at first and then less over time, as reservoir pressure falls and the rock around the well drains. Plotting production rate against time gives the decline curve. Its shape tells engineers how the well is performing and, projected forward, how much it is likely to produce.
Decline curve analysis
Decline curve analysis fits a mathematical model to a well’s production history. The most widely used models come from J. J. Arps, who in 1945 described three forms:
- Exponential decline: the rate falls by a constant percentage each period.
- Hyperbolic decline: the percentage decline itself slows over time, controlled by a parameter usually called the b-factor.
- Harmonic decline: a special case of hyperbolic decline.
The fitted curve is extended to an economic limit, the rate below which the well no longer pays for itself, to estimate remaining reserves and estimated ultimate recovery (EUR).
Shale well declines
Wells in unconventional reservoirs decline very steeply in their first year or two, as rock close to the fractures drains, then flatten. Their early behavior often fits hyperbolic models with high b-factors, which, extended indefinitely, overstate long-term production. Engineers therefore commonly switch to an exponential terminal decline later in the forecast. Because early data can fit several curves, EUR estimates for young shale wells carry considerable uncertainty.
Why decline curves matter
Decline curves underpin reserve estimates, well economics, and decisions about when to intervene. A treatment, workover or artificial lift change that raises production or flattens the decline is judged by how it shifts the curve, and by the added volume it produces over the rest of the well’s life.
Frequently asked questions
In Arps’ hyperbolic model, the b-factor controls how quickly the decline rate itself slows. A b-factor of zero gives exponential decline; higher values give a curve that flattens more over time.
Most early production comes from rock close to the hydraulic fractures, which drains fast. Flow from the very tight rock farther away is slow, so rates fall steeply before leveling off.
Sources
- decline curve analysis, SLB Energy Glossary
- Analysis of decline curves, J. J. Arps, Transactions of the AIME, 1945
- Decline curve analysis, SPE PetroWiki