Predicting hydrocarbon recovery

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Predicting reservoir system quality and performance
Author	Dan J. Hartmann, Edward A. Beaumont
Link	Web page
Store	AAPG Store

The volume of hydrocarbon contained in a reservoir is a function of pore volume and water saturation (S_w). Reservoir size and porosity determine pore volume. Pore throat size (see Pore and pore throat sizes) distribution, pore geometry, and hydrocarbon column height determine S_w. Estimating hydrocarbon volume in place before drilling a well is a matter of predicting pore volume and S_w. Recovery of hydrocarbons depends on the efficiency of the reservoir drive mechanism. Predicting recovery depends on predicting reservoir quality and reservoir drive.

Calculating oil volume in place

To calculate volume of original oil in place (OOIP)in barrels from volume measured in acre-feet, use the following formula:

${\displaystyle {\text{$OOIP}}={\frac {7758\times {\text{A}}\times {\text{h}}\times \Phi \times (1-{\text{S}}_{\text{w}})}{{\text{B}}_{\text{oi}}}}}

where:

• 7758 = conversion factor from acre-ft to bbl
• A = area of reservoir, acres from map data
• h = thickness of reservoir pay, ft
• Φ = porosity (decimal, not percent)
• S_w = water saturation (decimal, not percent)
• B_oi = formation volume factor = 1.05 + (N × 0.05), where N = number of ft³ of gas produced per bbl of oil (gas-oil ratio or GOR). For example, if a well has a GOR of 1,000, then B_oi = 1.05 + (10 × 0.05) = 1.1.

Calculating gas volume in place

To calculate volume of original gas in place (OGIP) in cubic feet from volume measured in acre-feet, use the following formula:

${\displaystyle {\text{$OGIP}}=43,560\times {\text{A}}\times {\text{h}}\times \Phi \times (1-{\text{S}}_{\rm {w}})\times \left({\text{depth}}\times {\frac {0.43}{15}}\right)}

where:

• 43,560 = conversion factor from acre-ft to ft³

Estimating recoverable volume of oil or gas

Estimating recoverable oil or gas depends on predicting reservoir quality and reservoir drive. Reservoir analogs help narrow the range of values for variables that determine recovery factor (R.F.). Use the equation below to estimate the recoverable oil or gas in a reservoir:

${\text{Recoverable oil or gas}}={\text{OHIP}}\times {\text{R.F.}}$

where:

• OHIP = original hydrocarbons in place

Estimating recovery factor

Drive mechanism has the greatest geological impact on recovery factor. (See Drive mechanisms and recovery.) Narrowing the range in recovery factor is a matter of estimating how much difference pore type and reservoir heterogeneity impact the efficiency of the drive mechanism. To estimate the recovery factor, use the procedure below:

1. Decide which drive mechanism is most likely from the geology of the prospective reservoir system and by comparing it with reservoir systems of nearby analog fields or analog fields in other basins.
2. Multiply OOIP or OGIP by the recovery factor for the expected drive.
3. Narrow the recovery factor range by predicting the thickness of the reservoir by port type. Port type affects recovery rate. For example, in a reservoir with strong water drive and macroporosity, recovery will be up to 60%, mesoporosity recovery will be up to 20%, and microporosity recovery will be 0%.

Recovery factors for different drive types

The table below shows recovery factor percentages for different drive mechanisms for oil vs. gas reservoirs.

Reservoir drive mechanism	Percent ultimate recovery
	Gas	Oil
Strong water	30–40	45–60
Partial water	40–50	30–45
Gas expansion	50–70	20–30
Solution gas	N/A	15–25
Rock	60–80	10–60
Gravity drainage	N/A	50–70

See also

• Interpreting water saturation
• Interpreting Sw distribution in a reservoir
• Interpreting hydrocarbon shows

External links

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original oil in place

barrels

gas-oil ratio

original gas in place