Oil in place, abbreviated as \(N\) and \(OIP\), is the total quantity of crude oil that is originally found in a subsurface reservoir. It encompasses all the oil contained in the rock formations within a specific reservoir, regardless of whether it can be technically or economically recovered. Determining the reservoir oil in place is crucial for understanding the potential productivity of an oil field and involves a combination of geological, petrophysical, and engineering data.
Oil in Place Formula
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\( OIP \;=\; \dfrac{ 7758 \cdot A \cdot h \cdot n \cdot (1 - W_s ) }{ B_o } \) (Oil in Place) \( A \;=\; \dfrac{ OIP \cdot B_o }{ 7758 \cdot h \cdot n \cdot (1 - W_s ) } \) \( h \;=\; \dfrac{ OIP \cdot B_o }{ 7758 \cdot A \cdot n \cdot (1 - W_s ) } \) \( n \;=\; \dfrac{ OIP \cdot B_o }{ 7758 \cdot A \cdot h \cdot (1 - W_s ) } \) \( W_s \;=\; 1 - \dfrac{ OIP \cdot B_o }{ 7758 \cdot A \cdot h \cdot n } \) \( B_o \;=\; \dfrac{ 7758 \cdot A \cdot h \cdot n \cdot (1 - W_s ) }{ OIP } \) |
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| Symbol | English | Metric |
| \( OIP \) = Oil in Place | \(bbl\) | - |
| \( A \) = Reservoir Area, acres from map data | \(acre\) | - |
| \( h \) = Reservoir Thickness | \(ft\) | - |
| \( n \) = Porosity | \(dimensionless\) | - |
| \( W_s \) = Water Saturation | \(dimensionless\) | - |
| \( B_o \) = Oil Formation Volume Factor (Barrels of Oil per Barrel of Reservoir Oil) | \(bbl \;/\; STB\) | - |
Oil in place is determined using well-established reservoir evaluation methods, most commonly the volumetric method. This approach integrates geological and petrophysical properties of the reservoir, including bulk rock volume, porosity (the fraction of rock volume that is void space), water saturation (the portion of pore space occupied by water rather than hydrocarbons), and the formation volume factor (which accounts for the change in oil volume between reservoir and surface conditions). The resulting estimate quantifies the total hydrocarbons stored in the reservoir pore system, typically expressed in stock tank barrels (STB) or cubic meters.
It is critical to distinguish oil in place from recoverable reserves. Only a fraction of the oil in place can be produced, depending on reservoir drive mechanisms, fluid properties, rock characteristics, and the recovery methods applied (primary, secondary, and enhanced recovery techniques). The ratio of recoverable oil to oil in place is defined as the recovery factor, which varies widely but is always less than 100 percent under real field conditions.
Original Oil in Place (OOIP) - This is the total amount of oil present in the reservoir before any production starts.
Remaining Oil in Place (ROIP) - The amount of oil left in the reservoir after some production has occurred.
Oil in place is a measure of the total hydrocarbon endowment of a reservoir, forming the basis for reservoir engineering analysis, field development planning, and economic evaluation, but it is not itself a direct indicator of producible volume.
