Example Data Table
Sample inputs and selected outputs to validate your setup.
| Model | qi | Di | b | Duration | Step | Rate at end | Cumulative at end |
|---|---|---|---|---|---|---|---|
| Hyperbolic | 1200 bbl/d | 0.30 per month | 0.75 | 120 months | 6 months | ~84.7 bbl/d | ~129,000 bbl |
| Exponential | 800 m³/d | 0.08 per month | 0 | 60 months | 3 months | ~7.2 m³/d | ~9,910 m³ |
Formula Used
How to Use This Calculator
- Select a decline model that matches your analysis approach.
- Enter qi and Di using the same time unit basis.
- If using hyperbolic, set a realistic b-factor for the well.
- Choose duration and step size to control the forecast table resolution.
- Optionally set an economic limit rate to estimate EUR to that cutoff.
- Press Submit to view results above, then export CSV or PDF.
Why decline forecasting matters for engineering decisions
Decline forecasting converts observed production behavior into forward rates, volumes, and reserves for budgeting, facility sizing, and surveillance. A repeatable workflow reduces bias when comparing wells, pads, or lift changes. This calculator returns rate and cumulative at each step, enabling screening before detailed simulation. Early points capture steep drawdown; later points quantify tail contribution and planning risk. Use consistent history matching windows and exclude shutdown periods to prevent distorted parameter estimates during fitting.
Choosing a model: exponential, hyperbolic, or harmonic
Exponential decline assumes constant nominal decline and often fits boundary dominated periods that plot as straight lines on semilog scales. Hyperbolic decline adds a b-factor that softens early decline and extends the tail; values between 0 and 1 frequently provide practical fits. Harmonic decline is the b equals one case, producing a strong tail and logarithmic cumulative growth.
Parameter discipline and unit consistency
Inputs must share the same time basis: if Di is per month, duration and step must be months. A quick check is that forecast rate should never increase with time. If it does, review Di units, sign, and b. Smaller steps improve resolution but add rows; larger steps summarize trends faster. Precision settings control rounding for cleaner reporting.
Economic limit, time to cutoff, and EUR interpretation
An economic limit rate is a threshold where costs, constraints, or commercial terms make continued production unattractive. Solving q(t) equals qe yields time to limit and an implied EUR to that cutoff. Treat this as a planning metric, because interventions, downtime, and operating strategy can shift the decline. Run sensitivities around qe, Di, and b to frame uncertainty. If qe exceeds qi, the cutoff is immediate, and EUR equals zero incremental production today.
Using the forecast table for validation and communication
The table provides incremental volumes between steps, supporting monthly or quarterly production and cash flow estimates. Compare predicted cumulative to measured cumulative at known dates to validate fit quality. Document model choice, parameters, start date, and scenario notes so exports remain traceable. When communicating outcomes, state the cutoff basis alongside end-of-forecast rate and cumulative to avoid confusion across teams.
FAQs
What does the b-factor represent in hyperbolic decline?
The b-factor controls curvature: higher b slows early decline and lengthens the tail. It often reflects flow regime and operational influences, so use fit-to-data and engineering judgment rather than a single default value.
Why do my results look unrealistic at long times?
Long tails can dominate cumulative if b is high or if the economic limit is too low. Re-check Di units, reduce b, shorten horizon, or set a realistic cutoff rate that matches operating and facility constraints.
How should I pick the time step?
Use smaller steps for fast-changing early decline and larger steps for long-term planning. A common approach is monthly for the first year and quarterly after, then compare totals with measured production for validation.
Is EUR the same as proved reserves?
No. EUR here is a model-based estimate to a chosen cutoff. Proved reserves require regulatory definitions, commercial certainty, and validated performance. Treat this output as a screening and planning indicator.
Can I use this for gas, water, or any unit system?
Yes. The math is unit-agnostic if you keep time basis consistent. Set rate and volume labels to your unit system, and interpret cumulative in the corresponding volume units.
Why is exponential sometimes preferred over hyperbolic?
Exponential uses fewer assumptions and can be stable when data show constant nominal decline. It may also avoid overly optimistic tails. If hyperbolic fits only by pushing b very high, reassess the dataset and constraints.