Estimating Hydrocarbon Volumes for Oil & Gas Fields

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)

Below I discuss some of the basic steps involved in volumetric calculations:

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.

Apply the Formation Volume Factor (FVF - Bo for oil, Bg for gas): Volumetric factors are used to convert the volume of hydrocarbons at reservoir conditions to standard conditions (usually at surface conditions). These factors correct for the differences in pressure and temperature. The formation volume factor is the ratio of the volume of hydrocarbons at reservoir conditions to the volume at surface conditions. It accounts for the expansion of hydrocarbons as they move from reservoir to surface. It is obtained through laboratory experiments and reservoir simulation. It can be determined from reservoir fluid samples. Again speak with your Friendly Neighbourhood Reservoir Engineer, for typical values for the Formation Volume Factor and Bo and Bg for your area. https://www.e-education.psu.edu/png520/m18_p7.html

Recovery Factor (RF): The recovery factor is the ratio of recoverable reserves to the initial hydrocarbon in place. It is influenced by reservoir properties, fluid properties, and production techniques. Recovery factor can be estimated based on historical data, reservoir simulation, and engineering analysis.

Initial Hydrocarbon in Place (OOIP - Original Oil in Place or OGIP - Original Gas in Place): The initial hydrocarbon in place is the total amount of hydrocarbons present in the reservoir. The original oil or gas in place can then be calculated using the following formula:

For oil: Original Oil-in-Place (OOIP) = (GRV * NTG * Porosity * Oil Saturation) / Formation Volume Factor

For gas: Original Gas-in-Place (OGIP) = (GRV * NTG * Porosity * Gas Saturation) / Formation Volume Factor

Please note that these are extremely oversimplified equations and actual calculations will involve more complex factors and require more detailed data.

Example:

Volumetric calculations in oil and gas are used to estimate the amount of recoverable hydrocarbons in a reservoir. Here’s a typical but overly simplified example of how it might be done:

Let’s say we have the following data for a hypothetical oil reservoir:

Gross Rock Volume (GRV): 2,221,000 cubic meters

Net-to-Gross Ratio (NTG): 0.45 (45% of the rock volume is reservoir rock)

Porosity: 0.25 (25% of the reservoir rock volume is pore space)

Oil Saturation: 0.7 (70% of the pore space is filled with oil)

Formation Volume Factor (FVF): 1.1 (accounts for the expansion of oil when brought to surface conditions)

We can then calculate the Original Oil-in-Place (OOIP) using the formula:

OOIP = (GRV NTG Porosity * Oil Saturation) / FVF

Substituting in the values:

OOIP = (2,221,000 m3 * 0.45 * 0.25 * 0.7) / 1.1 = 159,000 cubic meters

So, the estimated volume of recoverable oil in this hypothetical reservoir would be approximately 159,000 cubic meters.

Please note again that this is an oversimplified example and actual calculations will involve more complex factors and require more detailed data.

So, the estimated volume of recoverable oil in this hypothetical reservoir would be approximately 159,000 cubic meters. What is this in Barrels of Oil?

The volume of recoverable oil in the hypothetical reservoir is approximately 159,000 cubic meters. Therefore to convert this volume to barrels of oil, we can use the conversion factor of 1 cubic meter being approximately equal to 6.2898107704 barrels of oil.

So, 159,000 cubic meters * 6.2898107704 (barrels of oil/cubic meter) = approximately 1,000,000 barrels of oil.

Therefore, the estimated volume of recoverable oil in this hypothetical reservoir would be approximately 1,000,000 barrels of oil.

IF GAS:

The conversion from oil to gas can vary depending on the specific properties of the oil and gas in question. However, a commonly used conversion is that 1 barrel of oil is approximately equivalent to 5,614 cubic feet of natural gas.

So, if we have 1,000,000 barrels of oil and want to convert this to natural gas, we would multiply 1,000,000 by 5,614:

1,000,000 barrels * 5,614 cubic feet/barrel = approximately 5,614,600,000 cubic feet of natural gas.

Therefore, the estimated volume of recoverable gas in this hypothetical reservoir would be approximately 5,614,600,000 cubic feet, or 5.6Bcf (Bcf = Billions of cubic feet).

Please note that this is a rough estimate and actual conversions can vary based on the specific properties of the oil and gas.

Original Oil in Place (OOIP) and recoverable reserves are two different but related concepts in the field of oil and gas exploration:

Original Oil in Place (OOIP) refers to the total volume of hydrocarbon (in this case, oil) that is stored in a reservoir prior to any production. This is essentially the total amount of oil that exists in the reservoir.

Recoverable Reserves, on the other hand, refer to the volume of hydrocarbons that can be profitably extracted from a reservoir using existing technology. Not all the oil in the reservoir (OOIP) can be extracted due to various factors such as technological limitations, economic viability, and regulatory constraints. Current recovery factors for oil fields around the world typically range between 10 and 60 percent; some are over 80%.

So, while OOIP gives us an estimate of how much oil is present in the reservoir, the recoverable reserves tell us how much of that oil can actually be economically extracted and produced.

Original Oil in Place (OOIP) and Original Gas in Place (OGIP) are terms used in the oil and gas industry to refer to the total volume of hydrocarbon (oil or gas, respectively) stored in a reservoir prior to any production.

The difference between OOIP and OGIP primarily lies in the state of the hydrocarbon:

OOIP: Refers to the total volume of oil stored in the reservoir. Oil is in a liquid state.

OGIP: Refers to the total volume of gas stored in the reservoir. Gas is in a gaseous state.

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.

It’s important to note that OOIP and OGIP should not be confused with recoverable reserves, which refer to the volume of hydrocarbons that can be profitably extracted from a reservoir using existing technology.

Estimating recoverable reserves in oil and gas involves integrating geological and engineering data. Depending on the amount and quality of data available, one or more of the following methods may be used to estimate reserves:

Volumetric Method: This is the most basic technique for reserve estimation. It is used to indirectly estimate recoverable volumes from estimates of reservoir volume, porosity, oil saturation, and recovery efficiency.

Material Balance Method: This method is used in a mature field with abundant geological, petrophysical, and engineering data. It is highly dependent on the quality of reservoir description and the amount of production data available.

Production History Method: This method is used after a moderate amount of production data is available. It is dependent on the amount of production history available.

Analogy Method: This method is used early in exploration and initial field development. It is highly dependent on the similarity of reservoir characteristics.

The basic equation used to calculate recoverable oil reserves and recoverable gas reserves is:

Recoverable oil reserve (STB) = OOIP * RF

Recoverable gas reserve (SCF) = OGIP * RF

Where:

STB stands for Stock Tank Barrels

SCF stands for Standard Cubic Feet

OOIP is Original Oil in Place

OGIP is Original Gas in Place

RF is the Recovery Factor, which depends on the efficiency of the reservoir drive mechanism.

Please note that these are very oversimplified equations and actual calculations may involve more complex factors and require more detailed data.

There are software packages that are out there that can help you quickly work through volume calculations, and give P10 - P50 - P90 results: ie: 斯伦贝谢 (https://www.slb.com/products-and-services/delivering-digital-at-scale/software/geox/geox-software-play-and-prospect-assessment) or CrystalBall from 甲骨文 (https://www.oracle.com/ca-en/applications/crystalball/)

Here are some very useful videos from Alan Foum , who explains this topic and others in a series of YouTube videos

https://www.youtube.com/watch?v=Bz5pKzmqMP0

https://www.youtube.com/watch?v=YaE_-4CX5pk

https://www.youtube.com/watch?v=R3PG8XzrOHE

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