Estimate original gas volumes from reservoir properties quickly. Switch units, compute Bg, and export reports. Compare scenarios and document decisions with consistent calculations today.
| Scenario | Area | Thickness | Porosity | Sw | NTG | Bg | Estimated OGIP |
|---|---|---|---|---|---|---|---|
| Example A | 120 acres | 65 ft | 12% | 28% | 85% | ~0.0050 | ~4.97 Bscf |
| Example B | 2.0 km² | 18 m | 15% | 35% | 90% | 0.0042 | ~14.60 Bscf |
This calculator uses the common volumetric method for original gas in place (OGIP):
When you choose Bg calculation, an industry approximation is used: Bg ≈ 0.02827 × Z × T(R) ÷ P(psia).
Area and net thickness are the largest volume drivers. For early studies, map-based areas often range from 20 to 2,000 acres, while net pay commonly falls between 10 and 150 ft across intervals. Porosity from core or log models typically spans 6–22%. Water saturation often sits in the 20–55% band; small changes materially affect gas saturation. Net-to-gross is commonly 60–95% and should reflect cutoffs applied consistently.
Bg converts reservoir volume to standard volume, so its accuracy is critical. When computed, Bg uses pressure, temperature, and Z. At 2,500–5,000 psia and 160–260°F, Z frequently ranges 0.75–1.05 for gas, but can deviate with rich gas or high CO₂. A 5% increase in Z increases Bg about 5%, reducing OGIP about 5%. Use a PVT report or an EOS-based correlation for final cases.
This tool normalizes area to acres and thickness to feet before applying the 43,560 ft²/acre constant. If you input 2.0 km², it becomes about 494 acres; 18 m becomes about 59.1 ft. Confirm that porosity, Sw, and NTG are entered as percentages, not fractions. If you enter Bg directly, keep it in reservoir ft³ per standard ft³. Mixed Bg units are the most common source of order-of-magnitude errors.
OGIP is best treated as a distribution, not a single point. Build low, base, and high cases by varying area, net thickness, porosity, and Sw within interpreted bounds. For example, ±15% on area, ±10 ft on net pay, ±2 porosity points, and ±5 saturation points can produce a wide spread. Document assumptions, then export results for peer review. If you have probabilistic inputs, run scenarios and summarize P10/P50/P90 externally.
The calculator reports OGIP in scf, Bscf, and BCM, plus recoverable gas using your recovery factor. Screening recovery factors often range 60–90% for depletion drive in simple reservoirs, but can be lower with tight rock, strong water drive, or operational constraints. Use recoverable volumes to sanity-check plateau targets and facility sizing. For reserves work, integrate material balance, decline analysis, and dynamic simulation to reconcile volumetrics with performance data.
OGIP is the estimated standard gas volume originally present in the mapped reservoir rock. It is calculated from area, net thickness, rock properties, saturations, and Bg.
Compute Bg when you have pressure, temperature, and a defensible Z-factor. Enter Bg directly when you already have a PVT-derived Bg at reservoir conditions and want consistent reporting.
The volumetric equation uses gas saturation as (1 − Sw). Increasing Sw reduces gas-filled pore volume linearly, so even a few saturation points can noticeably shift OGIP, especially in high-porosity reservoirs.
Use your best gross thickness and apply net-to-gross to represent pay-quality rock. If NTG is uncertain, run low/base/high NTG cases to understand its impact before committing to a single value.
Yes. Area supports hectares and km², and thickness supports meters. The calculator converts to acres and feet internally before computing OGIP, then reports results in scf, Bscf, and BCM.
It is suitable for screening and early evaluation. For booking, reconcile volumetrics with PVT, material balance, decline analysis, well tests, and dynamic simulation, and document uncertainty with auditable inputs.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.