Saturation - BUCKLE'S METHOD
SWir is equivalent to the minimum water saturation found from capillary pressure curves determined from special core analysis. Typical capillary pressure curve relationships are shown below.
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.
STEP 1: Find Buckles number from special core analysis or from log analysis in a known clean pay zone that produced initially with zero water cut.
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:
· 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.
Sandstones Carbonates KBUCKL
Very fine grain Chalky 0.120
Fine grain Cryptocrystalline 0.060
Medium grain Intercrystalline 0.030
Coarse grain Sucrosic 0.020
Conglomerate Fine vuggy 0.010
Unconsolidated Coarse vuggy 0.005
Fractured Fractured 0.001
Irreducible Water Saturation from Nuclear Magnetic Log
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
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
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