Oil and Gas Decline Curves Explained for Investors

What Is a Decline Curve?

A decline curve is a mathematical model that predicts how a well's production rate decreases over time. Every well starts with a peak rate (initial production, or IP) and then declines as reservoir pressure drops. The rate and shape of that decline tell you how much oil you'll ultimately recover and how fast revenue will shrink.

For investors, the decline curve is the most important tool for evaluating whether revenue projections are realistic. If someone shows you a well that produces 50 barrels per day forever, walk away. Every well declines.

The Three Arps Decline Models

J.J. Arps defined three decline curve types in 1945. Exponential decline: production drops at a constant percentage rate each month. Common in conventional vertical wells after initial flush production.

Hyperbolic decline: production drops quickly at first, then the decline rate itself slows over time. The b-factor controls deceleration — typical b-factors for shale wells range from 0.5 to 1.5. Higher b-factors mean slower long-term decline and more ultimate recovery.

Harmonic decline: a special case where b-factor equals 1. Some conventional reservoirs with strong water drive exhibit this pattern.

IP Rate vs. Terminal Decline

Two numbers matter most: initial production rate (IP) and terminal decline rate. IP tells you what the well produces in its first 30-90 days — when revenue is highest. But IP alone tells you almost nothing about long-term value.

Terminal decline is what the well settles into after 12-24 months. A conventional Oklahoma well might have terminal decline of 8-12% per year. A tight shale well might decline 15-25% annually before flattening. The terminal rate determines how long the well remains economic.

What Investors Should Look For

When reviewing an offering document, ask: What IP rate is the operator using? Is it based on offset wells or theoretical models? A good operator provides type curves from at least 5-10 offset wells in the same formation.

What b-factor did they use? Higher b-factors produce more optimistic projections. What's the assumed economic limit — the rate below which operating costs exceed revenue? Compare projected EUR against offset wells. If the operator claims 200,000 barrels when nearby wells average 120,000, they need to explain why.

How Decline Curves Affect Returns

A well with high IP and steep decline gives strong revenue in years 1-2 but drops off quickly. A well with moderate IP and gentle decline provides steadier income over 15-30 years. For tax purposes, the steep-decline scenario can be advantageous: your IDC deduction happens in year one when revenue is highest.

At BassEXP, we model decline curves conservatively using offset well data from wells we've drilled in the same formations. We'd rather show realistic projections and have the well outperform.