Click to see subtopics:Volumetric analysis is a technique that uses geological observations and information to estimate original fluids-in-place. It is often referred to as a "static method” as it primarily sources its data from core samples, wireline logs, and geological maps. Volumetric calculations are typically used prior to production to estimate reserves, and after considerable production to determine the efficiency of recovery, the areal extent of the reservoir, and as a basis for advanced studies, such as reservoir simulations.
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Rock Volume |
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Pore Volume |
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Hydrocarbon Pore Volume |
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A comprehensive geologic study of the prospect is necessary to increase the confidence and reliability of determined reservoir properties, such as volume, porosity, and fluid saturations. In calculating the volume of the reservoir, accurate determinations of the areal extent and thickness must be made with respect to the geological structure and depositional environment. The use of isopach maps in combination with planimetering is a commonly used method in the determination of reservoir volume. Conclusions drawn concerning lithofacies and depositional settings are used to provide an assessment of porosity, while wireline log and core data provide the analyst with measurements of fluid saturations.
Oil reservoir calculations
| Original Oil-in-Place (OOIP) Calculations | ||
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Field Units [stb] |
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Where: A = acres h = feet φ = fraction Soi = fraction Boi = bbl/stb |
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Metric Units [m3] |
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Where: A = square meters h = meters φ = fraction Soi = fraction Boi = m3/m3 |
Gas reservoir calculations
Historically, in a gas reservoir, only free gas-in-place was considered. Because of this, only one name was required: OGIP. However, with the increasing use of adsorbed gas reservoirs in the industry, Fekete / IHS has adopted the name “OGIPF” to define the gas-in-place for a free-gas reservoir. Likewise, the name “OGIPA” is used to define the gas-in-place in a reservoir. The name OGIP has been retained to describe the total original gas-in-place.
Free gas equations
| Original Free Gas-in-Place (OGIPF) Calculations | ||
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Field Units [scf] |
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Where: A = acres h = feet φ = fraction Sgi =fraction Bgi = ft3/scf |
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Metric Units [m3] |
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Where: A = square meters h = meters φ = fraction Sgi = fraction Bgi = m3/m3 |
Adsorbed gas equations – shale reservoirs
Shale gas reservoirs usually contain much more adsorbed gas than free gas. Therefore, OGIP calculations for shale reservoirs should also account for adsorption. For shale reservoirs, total gas in place is calculated as:
OGIP = OGIPF + OGIPA
The following equations are used to calculate the original adsorbed gas-in-place (OGIPA).
| Original Adsorbed Gas-in-Place (OGIPA) Calculations | ||
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Field Units [scf] |
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Where: A = acres h = feet ρb = ton/ft3 p = psi(a) VL$ = scf/ton pL$ = psi(a) |
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Metric Units [m3] |
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Where: A = square meters h = meters ρb = g/cm3 p = kPa(a) VL$ = cm3/g pL$ = kPa(a) |
Adsorption saturation option
Adsorbed gas may occupy a portion of the measured porosity. To account for this, you can use the Adsorption Saturation option. If this option is selected, the calculation for OGIPA remains unchanged, but the calculation for OGIPF uses a decreased value for gas saturation. For information on the calculation for this adjusted Sg, see the shale adsorption correction.
For adsorption theory, see shale properties.
Calculations for liquid-rich gas reservoirs
With liquid-rich gas, it is important to calculate the original dry gas-in-place (OGIPd), as well as the original condensate-in-place (OCIP).
Original dry gas in-place comes from free gas and dissolved gas:
OGIPd = OGIPdF + OGIPS
Original condensate in-place comes from free condensate and vaporized condensate:
OCIP = OCIPF + OCIPV
The following equations are used to calculate OGIPdF, OGIPS, OCIPF, and OCIPV:
| Calculations for Liquid-Rich Gas | ||
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Field Units [scf, stb] |
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Where: A = acres h = feet ɸ = fraction Sgi, Sci = fraction Bgi = ft3/scf Bci = bbl/stb Rsi = stb/scf Rvi = scf/stb |
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Metric Units [m3, m3] |
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Where: A = square meters h = meters ɸ = fraction Sgi, Sci = fraction Bgi = m3/m3 Bci = m3/m3 Rsi = m3/m3 Rvi = m3/m3 |
Note that the formation volume factor used to calculate the original dry gas-in-place is Bdgi — the formation volume factor for dry gas. It is different from Bgi — the formation volume factor for recombined gas displayed in the Properties editor. The dry gas formation volume factor (Bgdi) is defined as the ratio of the gas phase volume at reservoir conditions (Vg) to the equivalent volume of its gas component at standard conditions. The formation volume factor for recombined gas is the value obtained using given gas correlations based on the recombined gas gravity. For more information, see recombination.