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

Reservoir Simulators Comparison

Simulator	Developer	Finite Element	Streamline	BlackOil	Compositional	Chemical EOR	Geo-mechanics	Thermal
E100	Schlumberger	Yes	No	Yes	No	No	No	No
E300	Schlumberger	Yes	No	No	Yes	No	No	No
INTERSECT	Schlumberger	Yes	No	Yes	Yes	No	?	?
tNavigator	RFD	Yes	No	No	Yes	No	No	No
Nexus	Landmark Graphics / Halliburton	Yes	No	Yes	No	No	No	No
UTCHEM	U. of Austin	Yes	No	Yes	No	Yes	No	No
MoRes	Shell	Yes	No	Yes	No	No	No	No
EMPOWER	ExxonMobil	Yes	No	Yes	No	No	No	No
STARS	CGM	Yes	No	Yes	No	No	No	No
IMAX	CGM	Yes	No	Yes	No	No	No	No
GEM	CGM	Yes	No	Yes	No	No	No	No
REVEAL	Petroleum Experts	Yes	No	Yes	No	No	No	No

ASP Flooding - Various Lab Tests Required

Polymer
- Aqueous stability test
- Filtration test
- Viscosity @ various shear rate

Surfactant
- Salinity scan test
- IFT (may be Solubility Test) - done with O/W=1:1 ratio
- Oil Scan Test ( IFT test donet at different O/W ratio)

Core Flood Test
- To see if significant oil recovery could be achieved.
- To see the reaction of rock mineralogy with the proposed chemical slug.

ASP Flooding - Reservoir Selection Criteria

A] Formation / Rock Mineralogy Sandstone Reservoirs are preferred and "Anionic Surfactant" can be used. In Carbonate Reservoirs, "Anionic Surfactant" are highly absorbed, hence cannot be used. On the other hand, "Cationic Surfactant", which might be used in Carbonate reservoirs are expensive and economically not viable. Moreover, there is also a rock of Anhydrite formation in such reservoirs. Clay also absorbs surfactant. If the reservoir has high clay contents, it might not be suitable for ASP flooding. 

B] Oil Composition and Acid Number/Acidity These are important specially for alkali & surfactant. The Acid Number of Crude should be high (min. 0.3mg KOH/g of oil or higher) so that Alkali reacts with it and produce enough soap. 

C] Oil Viscosity This is particularly important for polymer component of the flooding. It determines the required Mobility Control and hence the design Polymer concentration, slug size etc. Usually crude oil of lower viscosity of 35cP or less is preferred. 

D] The Reservoir Temperature is expected to be less than 90 C, and preferably between 40-50C. Higher temperature degrades polymer and impacts the optimal salinity of surfactant.

E] Formation Permeability is expected to be high from 500mD to Darcies for easy movement of high molecular weight chemicals, especially polymers.