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