Separator Test

Separator Tests are conducted to determine the changes in the volumetric behavior of the reservoir fluid as it passes through the separators and then into the stock tank. The resulting volumetric behavior is influenced to a large extent by the operating conditions, i.e., pressure and temperature of the surface separation facilities.

Objectives

• To provide the essential laboratory information for determining the optimum surface conditions, which in turn will maximize the Stock-Tank oil production.
• To obtain the PVT parameters (Bo, Rs and Bt) in combination with the DL test.

The SEP tests are performed only on the original oil at Pb.

The Test

• Place a HC sample at Pb and Tres. Let the vol be `V_(sat)`.
• Sample is displaced and flashed through laboratory multistage separator system
• Commonly 1 to 3 stages
• The P & T are set to represent actual durface facilities
• Gas liberated in each stage is removed and sp gr & vol at std condition meadured.
• The vol of remaining oil in  the last stage is (representaing stock -tank condition) is measured and recorded as `(V_(o))_(st)`
• The experimentally measured data are used to calculate FVF & Rs:
• Oil FVF @ `P_(b)` as measured by flash lib., `B_(ofb)=V_(sat)/(V_(o))_(st)` [bbl/STB]
• Solution GOR`P_(b)` as measured by flash lib, `R_(s fb) = (V_(g))_(sc)/(V_(o))_(st)` [scf/STB]
• `(V_(g))_(sc)` = total volume of gas removed from separators, [scf]

Determination of the optimum separator pressure

The above lab procedure is repeated at a series of different sep. P and at a fixed Temp.

• Recommended for at least four tests.
• The optimum set of Pressures is the one that gives the minumum oil FVF, `B_(ofb)`

At the same time, 

• Stock-tank Oil gravity must be maximum
• Total gas removed, `(V_(g))_(sc)` must be mnimum.

Notes:

By definition, Oil FVF (Bo) is the volume of oil at the reservoir pressure and temperature divided by the resulting stock-tank oil volume after it passes through the surface separations. Hence, its value is very much dependent on the surface operations. 

The Differential Liberation (DL) test is considered as a multiple series of flashes at the elevated reservoir temperature, whereas Separator Test (SEP) is a one or two-stage flash experiment at low pressure and low temperature as expected in actual field surface facilities.

Adjustment of `B_(o)` and `R_(s)` from Separator Test

Amyx et al. (1960) and Dake (1978) adjustment of `B_(o)` and `R_(s)`

Amyx et al. (1960) and Dake (1978) proposed a procedure for constructing the oil formation volume factor and gas solubility curves by using the DL test data in conjunction with the experimental SEP (flash) test data for a given set of separator conditions:

Step 1: Calculate the differential shrinkage factor `S_(od)` (bbl/bbl) at various pressures by

`S_(od) = B_(od)/B_(odb)`
where,
`B_(od)` = Differential relative oil volume factor at pressure p, bbl/STB
`B_(odb)` = Differential relative oil volume factor at `P_(b)`, bbl/STB

* `S_(od)` will be 1 (one) at `P_(b)` and less than one at p < `P_(b)`

Step 2: Adjust the relative volume data

`B_(o) = B_(ofb) * S_(od)`
where, `B_(o)` = Oil FVF bbl/STB
`B_(ofd)` = Oil FVF at `P_(b)` as obtained from SEP (flash) test = `V_(sat)/(V_(o))_(st)`

Step 3: Calculate Oil FVF above `P_(b)` by

`B_(o) = V_(rel) / B_(ofb)`
where,
`V_(rel)` = relative oil volume as generated CCE test.

Step 4: Adjust the differential gas solubility data `R_(sd)` to give the required gas solubility factor `R_(s)`

`R_(s) = R_(s fb) - (R_(sdb)-R_(sd))*B_(ofb)/B_(obd)`
`R_(s)` = gas solubility, scf/STB
`R_(s fb)` = bubble-point solution gas-oil-ration from SEP test , scrf/STB
`R_(sdb)` = solution gas-oil-ratio at bubble point pressure as measured by the DL test, scf/STB
`R_(sd)` = solution gas-oil-ratio at various pressure levels as measured by the DL test, scf/STB

Notes

These adjustments will typically,

• Lower Bo (FVF) as Rs as compared to the DL test data
• May produce Bo < 1.0 and Rs < 0. These needed to be manually corrected as Bo=1 and Rs=0.

Step 5: Calculate the two phase (total) FVF, `B_(t)` by

`B_(t) = B_(ofb)/V_(rel)`
where,
`V_(rel)` = Relative oil volume below `P_(b)`
Similar values can be obtained from DL test by
`B_(t) = B_(td) * B_(ofb)/B_(odb)`
where, `B_(td)` = relative total volume

Hydrocarbon Reservoir Types

Hydrocarbon Reservoirs can be classified in terms of how the fluid present changes due to change in pressure and temperature as it is brought out to the surface. 

• Dry Gas Reservoir: The fluid present in the reservoir is in gaseous form and it remains in gaseous form as it is brought out to the surface.

• Retrograde-Condensate Gas Reservoir: The fluid present in the reservoir is in gaseous form. However, part of it condenses into liquid form either in the reservoir itself or on it's way to the surface. What is produced is primarily gas with smaller quantities of liquid hydrocarbon.

• Wet Gas Reservoir: The fluid present in the reservoir is primarily gas, but contains small amount of liquid. When produced, gas is produced along with small amount of liquid. 

• Volatile Oil Reservoir: The fluid present in the reservoir is very light oil. Gas forms as it is brought to the surface. If produced oil is kept in an open container, it evaporates away.

• Black Oil Reservoir: The fluid present in the reservoir is heavier oil. Some dissolved gas might be present. However, the produced oil is "dead", i.e., does not evaporates away if kept in an open container.

Sources:

• http://www.informit.com/articles/article.aspx?p=2241145&seqNum=4
• http://petrowiki.org/Natural_gas_properties

What is Viscosity ?

In laymen's language, viscosity is the measure of how thick a fluid is. A more viscous material will flow slowly compared to a less viscous material. The following statements makes the idea of viscosity even clearer.

• Water is less viscous than molasses.

The common unit used for viscosity is centipoise, denoted by "cp".

Viscosity is the result of molecules of the system interacting with each other. More strongly they interact, less freely they can move, causing higher viscosity. The amount of molecular interactions, in turn, is caused by types of molecules, energy they possess and proximity of the molecules.

For a given temperature, viscosity of crude oil varies with pressure. Above bubble point pressure, viscosity increases as pressure increases. Due to pressure the molecules tend to come closer increasing interactions among them. This results in higher viscosity.

Packed Hall = High Pressure  = Difficult to move around = High Viscosity	Sparse Hall = Low Pressure = Easy to move around  = Low Viscosity

One the other hand, below bubble point pressure, viscosity increases as pressure decreases. At lower pressure, the lighter gas molecules escapes from the system, allowing the more heavier molecules to interact with each. This results in higher viscosity.

What is GOR ?

GOR stands for Gas-Oil-Ratio.

It is very common way of expressing amount of gas produced while producing an oil well. Common unit is MSCF/bbl.

Just like we have dissolved gas in a pressurized coke bottle, some amount of gas  remains in solution in the crude oil at reservoir conditions. The ratio of dissolved gas or "solution gas" per unit volume of oil is called Solution Gas-Oil-Ratio. It is often denoted by Rs.

However, the well produces not only solution gas, but also some amount of free gas. The total gas produced per unit volume of oil is called Produced Gas-Oil-Ratio and is denoted by Rp.

Mathematically,
Rp = Rs + Free Gas

What is FVF ?

FVF stands for formation volume factor. It is the amount of oil at reservoir condition that can produce 1 unit volume of oil in stock tank or surface conditions. Mathematically,

FVF = Reservoir Volume of Oil / Stock Tank Volume of Oil.

In barrels, the unit is RB/STB ( Reservoir Barrels / Stock Tank Barrels)

It is often denoted by Bo.

What is PVT Analysis?

An important task that the Reservoir Engineers carries out is Fluid Characterization or PVT (Pressure-Volume-Temperature) Analysis. The objective is to define the hydrocarbon fluid system present in a given reservoir and model it a computer program. A number of experiments are conducted on carefully collected fluid samples and a mathematical equation is calibrated to represent them. The equations known as "Equation of State" (EOS), are then used to predict the behavior of the fluid in different reservoir and surface pressure-temperature conditions.