Page 148 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 117

Connate water saturation is the fraction of pore volume in an undisturbed
formation filled with connate water.

volume of water
s, = (5-84)
volume of pores

Flushed zone saturation (Sxo) is the fraction of the pore volume filled with
flushing agent (normally drilling fluid).

volume of flushing agent
sxo = volume of pores (5-85)

Prior to penetration by a drill bit, only two fluids are assumed to be present in
the formation-water and hydrocarbons. Therefore, all pore space that is not
occupied by water is occupied by hydrocarbons. With this assumption hydro-
carbon saturation can be calculated

s, = 1 - sw (5-86)

where S, = hydrocarbon saturation.
Pay Thickness. The thickness of a hydrocarbon-bearing formation (hp,) is easily
determined from well logs once 4, and Sw cutoffs are established. The Sw cutoff
is the maximum value for Sw for a given rock type. The 4, cutoff is the minimum
value for 4, below which hydrocarbons cannot be produced. For example:

Depth $ sw Comment
3,668-3,670 1% 53% $ too low
3,666-3,668 2% 50% $ too low
3,664-3,666 6% 38% possible hydrocarbons
3,662-3,664 6% 36% possible hydrocarbons
3,660-3,662 8% 31% possible hydrocarbons
3,658-3,660 7% 74% too wet
3,656-3,658 8% 100% too wet

In this case, the water saturation cutoff is a maximum of 60% and the porosity
cutoff is a minimum of 31, so this well will have 6 feet of pay (hpy = 6 ft).
Other factors that may reduce hpay include shaliness, shale streaks, low perme-
ability, and low reservoir pressure. Porosity and water saturation cutoffs are
usually established for specific regions or reservoirs based on detailed pro-
duction and geologic information.
Lithology. It is often necessary to know the rock type in order to properly design
downhole assemblies, casing programs, and completion techniques. Data from
well logs can provide the geologist or engineer with an estimate of the lithologic
makeup of any formation. The accuracy of this estimate is a function of the
complexity of the formation (mineralogic makeup and fluid types) and the kinds
of tools used to investigate the rocks. More tools are needed to accurately
determine compositions of complex formations. Simple lithologies (three or less