Oil Properties

Reservoir Engineering 1

CGE 567

Department Oil & Gas, Faculty of Chemical Engineering

Oil Properties


CGE 567

Reservoir Engineering 1

Oil Properties

• An accurate description of physical properties of crude oils is of a considerable importance in the fields of both applied and theoretical science and especially in the solution of petroleum reservoir engineering problems.

• Data on most of these fluid properties are usually determined by laboratory experiments performed on samples of actual reservoir fluids.

• In the absence of experimentally measured properties of crude oils, it is necessary for the petroleum engineer to determine the properties from empirically derived correlations.


Crude Oil Gravity

• The crude oil density is defined as the mass of a unit volume of the crude at a cubic foot. The specific gravity of a crude oil is defined as the ratio of density of the oil to that of water. Both densities are measured at 60°F and atmospheric pressure;

• The density of water is approximately 62.4 lb/ft³


API Gravity

• The API gravity is the preferred gravity scale

• The gravity scale is precisely related to the specific gravity by the following expression:

• The API gravities of crude oils usually range from 46° API for the lighter crude oils to 10° API for the heavier asphaltic crude oils


API Gravity


API Gravity

• In general, crude oil is sold through a variety of contract arrangements and in spot transactions. Oil is also traded on futures markets but not generally to supply physical volumes of oil, more as a mechanism to distribute risk. These mechanisms play an important role in providing pricing information to markets.

• In fact, the pricing of crude oils has become increasingly transparent from the 1990s onwards through the use of benchmark crudes such as:

• West Texas Intermediate (WTI – USA)

• Brent (Europe, Africa and Asia)

• Dubai and Oman (Middle East)

• Dubai, Tapis and Dated Brent (in Asia-Pacific)

AIP (Australian Institute of Petroleum)


API Gravity

• In Asia there is no futures exchange where crude oil is traded and which would provide pricing information to the same extent as WTI and Brent. In Asia the pricing mechanism for say Tapis, a benchmark for light sweet crudes in the region, is based on an independent panel approach where producers, refiners and traders are asked for information on Tapis crude trades.

• There are details assessment done by Platts, McgGraw Hill Financial as for the benchmarking. Generally this is based on a formula approach where a benchmark crude is used as the base and then a quality differential (premium/discount) as well as a demand/supply (premium/discount) is added depending on the crude being purchased.

AIP (Australian Institute of Petroleum) Department Oil & Gas, Faculty of Chemical Engineering

API Gravity

• Prices of crude oil markers and petroleum product markers are affected by a myriad of factors including:

• overall supply/demand for crude

• supply/demand for petroleum products

• freight rates

• competition in the crude markets

• competition in the regional and domestic markets for petroleum products.

AIP (Australian Institute of Petroleum)


API Gravity

• West Texas Intermediate (WTI) is the benchmark oil prices most often used in North America

• Also known as Texas light sweet.

• WTI is light crude oil with API gravity of around 39.6 , specific gravity 0.827 (lighter than Brent crude) and sulfur content of 0.25%


API Gravity

• Brent Crude is a major trading classification of sweet light crude oil that serves as a major benchmark price for puchase of oil worlwide.

•Brent Blend crude oil has an API gravity of 37.9 degrees and 0.45 percent sulfur.

•Brent is the leading global price benchmark for Atlantic basin crude oils


API Gravity

•The Tapis field, off the coast of Terengganu in Peninsular Malaysia, was discovered in 1969 and started production in 1978.

•Tapis, among Asia’s premium grades along with Australia’s Cossack and Griffin, is valued because it enables refiners to produce a greater amount of gasoline without increased investments. Most types of crude oil from the Middle East contain a greater proportion of sulfur and other pollutants.

•Tapis has an API gravity of 46 degrees and contain 0.0343 % sulfur by weight

• “Malaysia’s Tapis crude oil, once the world’s most expensive grade, is losing its status as a benchmark for Asian refiners buying grades from Australia, Indonesia and Vietnam because of declining output.”

Christian Schmollinger, Blooberg; December 15, 2008 Department Oil & Gas, Faculty of Chemical Engineering

API Gravity


Specific Gravity of the Solution Gas • The specific gravity of the solution gas γg is describe by the

weighted average of the specific gravities of the separated gas from each separator.

• This weigthed-average approach is based on the separator gas-oil ratio;


Specific Gravity of the Solution Gas


Bubble point pressure

• The bubble-point pressure pb of a hydrocarbon system is defined as the highest pressure at which a bubble of gas is first liberated from the oil.

• Pressure at which the first gas bubble is liberated from the reservoir oil

• Upon isothermal pressure reduction

• This is important property can be measured experimentally for a crude oil system by conducting a constant-composition expansion test.


Gas Solubility

• The gas solubility Rs is defined as the number of standard cubic feet of gas that will dissolve in one stock tank barrel of crude oil at certain pressure and temperature

• Know as solution gas-oil ratio (the volume of gas in SCF dissolved in one STB of oil at a specific reservoir pressure)


Gas Solubility

• The solubility of a natural gas in a crude oil is a strong function of the pressure, temperature, API gravity and gas gravity.

• For a particular gas and crude oil to exist at a constant temperature, the solubility increases with pressure until the saturation pressure is reached.

• At the saturation pressure (bubble-point pressure) all the available gases are dissolved in the oil and the gas solubility reaches its maximum value.

• Rather than measuring the amount of gas that will dissolve in a given stock-tank crude oil as the pressure is increased, it is customary to determine the amount of gas that will come out of a sample of reservoir crude oil as pressure decreases.


Gas Solubility

• As the pressure is reduced from the initial reservoir pressure pi, to the bubble-point pressure pb, no gas evolves from the oil and consequently the gas solubility remains constant at its maximum value of Rsb.

• Below the bubble-point pressure, the solution gas is liberated and the value of Rs decreases with pressure.


Oil Formation Volume Factor, Bo • The reservoir volume in barrels (RB) that is occupied by one

stock tank barrel (STB) of oil and its dissolved gas

• The oil formation volume factor can be expressed mathematically as;

@


Oil Formation Volume Factor, Bo • As the pressure is reduced below the initial reservoir pressure

pi, the oil volume increases due to the oil expansion (oil compressibility).

• This behavior results in an increase in the oil formation volume factor and will continue until the bubble-point pressure is reach.

• At pb, the oil reaches its maximum expansion and consequently attains a maximum value of Bob for the oil formation volume factor.

• As the pressure is reduced below pb, volume of the oil and Bo are decreased as the solution gas liberated.

• When the pressure is reduced to atmospheric pressure and temperature to 60°F, the value of Bo is equal to 1


Oil Formation Volume Factor, Bo


• Oil and gas production rates, Qo (STB/D) and Qg (SCF/D), are measured at the surface at a time when the reservoir pressure is P. This pressure is below the bubble point pressure Pb. What are the corresponding underground withdrawal rates (RB/D)?



The producing gas-oil ratio is; R = Qg/Qo (SCF/STB)

This means that for every STB of oil which is produced at this pressure, we produce R SCF of gas at the surface

Since every STB of oil produced to the surface Rs SCF of solution gas, the volume of free gas produced from the reservoirs (R - Rs) (SCF/STB)

Each STB of oil produced to the surface requires the withdrawal of Bo RB of oil and is accompanied by (R - Rs)Bg RB of free gas

If we produce Qo STB of oil, the total underground withdrawal is Qo(Bo + (R - Rs)Bg)

Oil Compressibility, Co

•The fractional reduction in oil volume that results from a pressure increase of one psi.

•The volume changes of oil above the bubble point are very significant in the context of recovery of undersaturated oil.

•The oil formation volume factor variations above the bubble point reflect these changes.

•The equation for oil compressibility :


Oil Compressibility, Co

•In term of formation volume factors this equation yields :

•Assuming that the compressibility does not change with pressure, the above equation can b integrated to yield :

•Where P1 & P2 and V1 & V2 represent the pressure and the volume at condition 1 & 2.


Oil Properties

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