Fluid Saturation
Assignment Fluid Saturation
You are your company’s petrophysicist, responsible for evaluating the LWD logs recently acquired in a successful frontier exploration well. Full diameter cores have been acquired in the very shaly sandstone reservoir but have yet to be analyzed. Your management wants an immediate log analysis to be conducted in order to make well operational decisions. What equation will you use to determine the water saturation through the reservoir section?
Solution
The basic Archie water saturation equation would be unsuitable for use in this situation where the key reservoir formation is known to be a shaly sandstone. The Archie equation is only suitable for use with clean formations.
Until the core analysis is processed, there is uncertainty as to the actual distribution of shale and clay in the main reservoir formation. In this case, a good initial interpretation approach would be to use the following total shale relationship that is non-specific as to the type of shale model:
When the full diameter cores have been described to determine the type and distribution of shale and clay, and analyzed to establish the petrophysical parameters a, m and n, the well log evaluation can be updated by incorporating these core data and information, as well as by using a different, more applicable shaly sandstone water saturation equation if warranted.
Assessment Fluid Saturation
1. What is the industry standard term for the resistivity of the formation near an open wellbore which has been flushed by the invading drilling mud filtrate?
A .Rxo
B .Rt
C .Rw
D .Rmc
2. The uninvaded zone resistivity at the top of a very thick, clean formation with consistently 24% porosity is 11 ohm-m; SCAL results have shown a=1, m=2 and n=2. The formation water resistivity at this depth is 0.04 ohm-m. What is your initial estimate of the irreducible water saturation?
A .35%
B .25%
C .40%
D .30%
3. Structural shale, with a resistivity of 2 ohm-m, occupies an estimated 20% of that part of a formation logged which has an effective porosity of 20% and a reading of 4 ohm-m on the deepest reading resistivity log. SCAL data reports: a=1, m=2 and n=2. The produced water in this formation has a resistivity of 0.03 ohm-m at reservoir temperature. What is your estimate of the water saturation?
A .44%
B .22%
C .55%
D .33%
4. An oil well is successfully completed in a 32-foot-thick zone with 13% average porosity and 170 mD average permeability, which has oil saturation varying from 55% to 75%, with an average oil saturation of 65%. The irreducible water saturation is deemed to be 25%. What produced fluids do you expect? (Select all that apply.)
A .No produced fluids
B .Only dry gas
C .Water
D .Oil
5. In a clean, pure limestone interval of 20% porosity, the deepest reading resistivity measurement is 4 ohm-m. The formation water resistivity is 0.05 ohm-m at reservoir temperature. Making your best estimate assumptions about the Archie equation input parameters (a, m and n) for this typical limestone, what would you estimate the water saturation to be?
A .46%
B .56%
C .26%
D .36%
6. What additives are often used in water base drilling muds? (Select all that apply.)
A .Barite
B .Bentonite
C .Clays
D .Diesel
7. Why do many companies acquire full diameter cores from their oil reservoirs using an oil base mud?
A .Oil base mud is a less expensive alternative.
B .The core-derived water saturation measurements are more valid.
C .Filtrate from oil base drilling mud is safer for the environment.
D .Adding oil makes the field look bigger.
8. An oil reservoir has an average area of 12,500 acres, with a clean, continuous limestone reservoir section of 87 feet average net thickness and a weighted average porosity of 18%. The initial oil formation volume factor is 1.3. If the irreducible water saturation is 20% and the transition zone is very short, what is the approximate stock tank oil initially in place (STOIIIP) in barrels?
A .820,260,000
B .934,540,000
C .710,870,000
D .1,098,000,000
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