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       Porosity - Water Saturation (Buckle's Number) Method   

Water Saturation From The Porosity Water Saturation Product
A handy rule of thumb can be used to estimate water saturation in many zones. The rule is based on the observation that the product of porosity and water saturation is constant for a particular rock type, which is confirmed by core analysis studies. We therefore do not require knowledge of water resistivity, which is one of the most difficult interpretation parameters to find.

The technique is often called the Buckle's Number method, named after the man who first published the concept in 1950. In his original paper, Buckle proposed that SW = Constant / Porosity. In this Handbook, we have named the constant KBUCKL and extended the model to shaly sands by adding a (1 - Vsh) term. In shaly sands the KBUCKL is held constant at the value for a relatively clean sand and the shale term effectively varies KBUCKL in proportion to the shaliness.

From capillary pressure data:
      1: KBUCKL = SWmin * PHIcore

If capillary curves are not available from the well or nearby offsets, we can use calibrated log analysis results to estimate KBUCL::
      2: KBUCKL = Swa * PHIe

Where:
  KBUCKL = Buckle's Number (unitless)
  SWmin = minimum water saturation on a core plug with porosity PHIcore (fractional)
  Swa = Archie water saturation in a relatively clean zone with porosity PHIe (fractional)

For equation 2 to give reasonable results, the zone must be known or presumed to produce hydrocarbons with little or no water cut on initial completion. KBUCKL will vary with changes in grain size, sorting, and overall pore geometry, so a particular value is often associated with a particular rock type.

A table of porosity water saturation products (KBUCKL values) is listed below in the Recommended Parameters section. Your own data should be added to the table shown later on this page. Such data can be found in technical publications and trade journals, from good log analyses where RW is known, as well as from special core studies (capillary pressure curves).

Constant KBUCKL lines form hyperbolic lines on a porosity vs wirreducible water saturation crossplot. Permeabilyu usuall varies with both PHIe and SWir, shown by diagonal lines on this plot ==>

Buckle's Number, porosity, irreducible water saturation, and permeability are intimately linked by the texture and pore geometry of the rocks, as shown in the crossplot at the right.

Swp - Water Saturation from Porosity Saturation Product (Buckle's Model)
The relationship to use is:
      3: IF PHIe <= 0.0
      4: OR IF Vsh >= 0.9
      5: OR IF FLUID$ = “WET”
      6: THEN Swp = 1.0
      7: OTHERWISE Swp = KBUCKL / PHIe / (1 - Vsh)
In all cases:
      8: SWir = MIN (1, Swa, Swp)

WHERE:
  PHIe = effective porosity (fractional)
  KBUCKL = porosity saturation product (fractional)
  Swa = actual water saturation from Archie-type log analysis (fractional)
  Swp = water saturation from porosity saturation product (fractional)
  Vsh = shale volume (fractional)

COMMENTS:
Use this method when RW is unknown and zone is KNOWN to be hydrocarbon bearing.

Do not use in water zones.

The product of porosity and water saturation, PHI * SW, in many rocks is a constant, and the product is called Buckle’s Number, after the man who first described this factor.

KBUCKL is found in a clean hydrocarbon bearing zone with a known RW and is used in other zones of similar rock types where RW is unknown.

It can also be found by plotting core porosity vs wetting phase saturation at an arbitrary capillary pressure from special core analysis data.

If regression is used to determine SW from PHI, the relationship is usually hyperbolic (KBUCKL = constant) or a skewed hyperbola (KBUCKL varies with porosity).

The shale term has been added by the author to raise KBUCKL and Swp automatically for the finer grained nature of shaly sands.

Water saturation from porosity-saturation
product (Buckles Method) ==>

Hydrocarbon zones with water saturation (Sw) above irreducible saturation (SWir) will produce some water along with hydrocarbons. This can occur in transition zones between the oil and water leg, or after water influx into a reservoir due to production of oil or gas.

NUMERICAL EXAMPLE:
1. For example, if a deep vuggy carbonate has a porosity of 0.10 (10%) and a PHIxSW product of 0.005, then:
Swp = 0.005 / 0.10 = 0.05 (5%)

2. The shaly sands of Example 1 will have:

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