Volumetric analysis in oil and gas is a method used to estimate the amount of recoverable hydrocarbons present in a reservoir. It involves integrating geological and engineering data to calculate the volume of oil or gas in place.
Purpose of Volumetric Calculations: The data collected is used to estimate the volume of subsurface rock that contains hydrocarbons, determine the weighted average of the effective porosity, obtain a water resistivity value and calculate water saturation, and estimate if the reservoir is economical. (SEG Wiki)
Determine the Gross Rock Volume (GRV): This is the total volume of the reservoir rock including both the porous and non-porous components. It can be calculated based on geological and geophysical data, using structural maps and depth conversion of seismic data, and well logs.
Calculate the Net-to-Gross Ratio (NTG): This is the proportion of the gross rock volume that is made up of the reservoir rock. It is the ratio of net reservoir rock thickness to gross reservoir rock thickness. It is a measure of the proportion of porous rock within the total rock volume. It can be determined from core data, well logs, and seismic data.
Net Rock Volume (NRV): Net rock volume is the volume of porous rock, and it is calculated by multiplying the gross rock volume by the net to gross ratio.
NRV = GRV * NTG
Estimate Porosity: Porosity is the ratio of the void space (pore volume) in the rock to the total volume of the rock. It is obtained from well logs and core samples. Different rocks have different porosities, influencing their ability to store hydrocarbons.
Estimate Hydrocarbon Saturation: Hydrocarbon saturation represents the fraction of the pore space filled with hydrocarbons. It is determined through well logs, core analysis, and laboratory experiments. Speak with your Friendly Neighbourhood Reservoir Engineer.
Both OOIP and OGIP are estimated using similar methods, such as volumetric analysis, which involves integrating geological and engineering data to calculate the volume of oil or gas in place. However, due to the different physical properties of oil and gas, there are differences in how these quantities are measured and produced. For example, gas at the surface occupies more space than it does in the subsurface because of expansion. Conversely, oil at the surface occupies less space than it does in the subsurface, mainly due to gas evolving from the oil as pressure and temperature are decreased.