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IRREDUCIBLE WATER SATURATION
Saturation - BUCKLE'S METHOD
Capillary pressure curves define irreducible water saturation SWir (vertical dashed line near left edge of graph). Irreducible water saturation varies inversely with porosity: Sw = Constant / Porosity, but the Constant can vary with pore geometry. A reservoir with Sw > SWir will produce some water with the hydrocarbons.
The difference between Sw and SWir, and relative permeability of water and hydrocarbon, determine the water cut. These concepts are best described by the capillary pressure curve and relative permeability curves illustrated above.
Irreducible water saturation is a necessary value for water cut and permeability calculations.
SWir1 - Irreducible Water Saturation from Buckle's Number
1: KBUCKL = PHIe * Sw (in a CLEAN zone that produced initially with no water, or from core data)
STEP 2: Solve for irreducible water saturation in each zone.
2: IF zone is obviously hydrocarbon bearing
3: THEN SWir = Sw
4: OTHERWISE SWir = KBUCKL / PHIe / (1 – Vsh)
5: IF SWir > Sw
6: THEN SWir = Sw
but equivalent, model is:
· Use always in preparation for permeability calculations.
· Buckles Number can be found by observing the porosity times water saturation product in pay zones where RW@FT is known, or where a water zone can be used to calibrate RW@FT. Data can also be found from capillary pressure data.
· If Sw is greater than SWir, then the zone will produce with some water cut (if it produces anything at all).
· If Sw is less than SWir, then the Buckles number for the layer is wrong.
· The (1 – Vsh) term can be replaced by (1 – Vsh^2) if needed.
· Calibrate water saturation to core by preparing a porosity vs SWir graph from capillary pressure data. Adjust KBUCKL, Vsh, PHIe until a satisfactory match is achieved.
The illustrations below demonstrate the difference between actual and irreducible water saturation in a partially depleted or long transition zone.
The NMR transform is illustrated below. The matrix and dry clay terms of NMR response are zero. An NMR log run today can display clay bound water (CBW), irreducible water (capillary bound water, BVI), and mobile fluids (hydrocarbon plus water, BVM), also called free fluids or free fluid index (FFI). On older logs, only free fluids (FFI) is recorded and some subtractions, based on other open hole logs, are required.
Nuclear Magnetic Resonance Response to Fluids
SWir2 - Irreducible Water Saturation from NMR LOGS
7: PHIt = CBW + BVI + BVM
8: PHIe = BVI + BVM
9: SWir = BVI /
(BVI + BVM)
Some or all of the sums defined above may be displayed on the delivered log. Log presentation is far from standard for NMR logs. PHIt and PHIe from NMR do not always agree with that derived from density neutron methods, which see much larger volumes of rock.
For older logs, the BVI measurement was not
In higher permeability rock, the cap pressure curve quickly reaches an asymptote and the minimum saturation usually represents the irreducible water saturation in an undepleted hydrocarbon reservoir above the transition zone. In tight rock, the asymptote is seldom reached, so we pick saturation values from the cap pressure curves at two heights (or equivalent) Pc values) to represent two extremes of reservoir condition.
Only sample 1 in the above table behaves close to asymptotically, as in curve A in the schematic illustration at the right. All other samples behave like curves B and C (or worse). The real cap pressure curves for samples 1 and 2 are shown below.
The summary table shows wetting phase saturation
selected by observation of the cap pressure graphs at two different
heights above free water, namely 100 meters and 425 meters in this
example. In this case, the 100 meter data gives water saturations
that we commonly see in petrophysical analysis of well logs in
hydrocarbon bearing Bakken reservoirs in Saskatchewan. This is a
pragmatic way to indicate the water saturation to be expected when a
Bakken reservoir is at or near irreducible water saturation. The
data for the 450 meter case is considerably lower and probably does
not represent reservoir conditions in this region of the Williston