5- Given the following well log and based on the information that is provided to
ID: 108584 • Letter: 5
Question
5- Given the following well log and based on the information that is provided to you in the log header, what is the formation water resistivity at the reservoir temperature? What is shale volume at reservoir depth? Why you did not use the SP relationship with R. instantly to calculate water resistivity (20 pts) Show your work on the graphs and calculation steps to get partial credit. HIGH RES INDUCTION HALLIBURTON SPECTRAL DENSITY DUAL SPACED NEUTRON COMPANY GO FOR IT WELL FIELD TRAVIS COUNTY TRAVI STATE TEXAS Me as Tempe 200 7500 MEAS MAS NA NA NA NA BHTExplanation / Answer
All conventional logging runs carry a maximum temperature recording device whose value, T, is recorded on the log heading and corresponds to the temperature at the deepest point of the log run, D (generally bottom hole). A linear temperature gradient is assumed as a first approximation between the bottom of the hole and the topographic surface. The mean annual surface temperature, S, is used to establish the temperature at approximately zero depth. Then the temperature of the formation: t = S + d(T-S / D).
The procedure is a simple linear interpolation where the quantity in parentheses represents an estimate of the temperature gradient. A map of mean surface temperature enables the selection of an appropriate value for any well location.
The formation water resistivity may be corrected from its value at laboratory temperature to formation temperature either by use of a chart found in most logging manuals or by Arp’s empirical formula, for Fahrenheit: RW2 = RW1 (T1 + 6.77) / (T2 + 6.77) and for Centigrade:
RW2 = RW1 (T1 + 21.5) / (T2 + 21.5).
Where RW1 and RW2 are formation water resistivities at temperatures T1 and T2.
Therefore the Mean annual surface temperature = 57 degrees F
Zone formation temperature = 57 + 4838 * ((118 - 57) /5398) = 112 degrees F.
The resistivity of a water sample was measured to be 0.05, ohm-m at a laboratory temperature of 75°F. Therefore the resistivity in the subsurface zone at the well would be:
Rw = 0.05 * (75 +7) / (112 + 7) = 0.0345 ohm-m.
A triangle neutron-density porosity (N Vs. D) cross-plot is used to determine shale type and volume, and effective porosity (Fig. 1). Three distinct points (F, M, Sh) are shown in this cross-plot; Point F represents fluid or water point where D= N=100%. Point M represents matrix point; if density and neutron tools are calibrated in terms of the existing matrix, then N= D=0. Point SH represents shale point; the coordinate of point SH [NSh , DSh] must be determined for shaliest portion of well. The laminar, dispersed and structural shale points areas fall on or around LS-Sh, DIS and STR lines, respectively. For each point within triangle VSh is estimated on M-Sh line parallel to clean formation line, and also, e is determined on clean formation line parallel to M-Sh line. For example, point A in Fig. 1 represents a shaly formation that has values of e = 9% and VSh = 23% with dispersed shale content. If formation contains hydrocarbons, neutron and density porosities have to be corrected before points are plotted since calculated shale volumes will be too low in gas-bearing intervals. The procedure of hydrocarbon correction is as follows:
Ncorr = N – N and dcorr = d –d
Therefore Vsh = IGR / 3-2IGR. Therefore Depth interval 2730.5-2731.5m (dispersed shale).
Calculation from knowledge of the SP value in a clean zone has been a traditional method for finding RW. It works best in clean water bearing zones, but the RWa or R0 method would be better in this case. Shale content and hydrocarbon content reduce the SP value and cause RW to too high, giving very pessimistic saturation results. This algorithm should only be used IF the SP log has sufficient character, the zone of interest is a clean water bearing sandstone, and the result is reasonable for the area. Use caution since many SP logs are not calibrated, and RMF or RW can be measured carelessly. Water resistivity data can be sparse or overwhelming, depending on where you are working at the moment. The usual sources in order of preference are:
1.Produced water from the zone being analyzed in the same well or nearby offset wells, analyzed for Rw in the lab.
2. Drill stem test or perf test water from the zone being analyzed in the same well or nearby offset wells, analyzed for Rw in the lab. The test should produce at least 1000 ft (300 m) of water before using the data, to prevent mud filtrate contamination from causing errors. The sample should be from the bottom of the test.
3. Produced or DST water from a nearby zone in the same geologic horizon (do not cross erosional boundaries), analyzed as above.
4. Water catalogues produced by local well log societies or government agencies.
5. Back calculated from log data in clean water bearing zone in the same well or nearby offset well (Rwa or Ro method).
6. Back calculated from nearby water bearing zone in same geologic horizon.
7. Calculated from SP in clean water bearing zone in same or nearby zone in same well or nearby offset well.
8. If no water has ever been produced in the area, back calculated from a laboratory measured or assumed PHIxSW product.
9. Local rule of thumb for water resistivity versus depth or versus geologic horizon.
Do not use if:
1. Water from a DST or perf test that recovered mostly filtrate water (check water chemistry) or recovered only a small amount of water.
2. SP or Rwa in a shaly zone.
3. SP or Rwa in a hydrocarbon bearing zone.
4. SP in a carbonate or evaporite sequence.
5. SP in a low porosity zone.
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