Homework_Assignment_4_Solution

Homework_Assignment_4_Solution - Homework Assignment No. 4...

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Homework Assignment No. 4 February 16, 2010 Due on February 23, 2010 @ 2:00 PM PGE368 Spring Semester 2010 Fundamentals of Well Logging Instructor: Carlos T. Verdín, Ph.D. INTERPRETATION OF SPONTANEOUS POTENTIAL (SP) LOGS SOLUTION SET DESCRIPTION: The objective of this homework project is to make use of spectral gamma-ray, SP, and resistivity logs to qualitatively and quantitatively assess both lithology and potential hydrocarbon-bearing reservoir units. TASKS/QUESTIONS : 1. (a) Using the SP log shown in Figure 1 (also shown on page 20 of PGE368’s Well-Log Compendium), estimate the electrical resistivity of connate water in each of the two existing water-saturated sand units. (b) Determine the corresponding salinity of connate water in ppm. (c) Determine the salinity of mud filtrate in ppm. (d) Explain why the SP log is not null within hydrocarbon-bearing sands. Note the header of this log was included in Homework Assignment No. 3. Perform your calculations of water resistivity and salt concentration using both the formulae included in the Appendix and Schlumberger’s GEN-6 interpretation chart (available on Blackboard). [15 POINTS] Answer:
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Figure 1 . Log display.
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The water-saturated sands to be analyzed are located between 5,680 ft and 5,740 ft and between 6,075 ft and 6,100 ft, here designated sand A and sand B, respectively as shown in Figure 1. In order to calculate water resistivity, we need to know the temperature for each sand. From the header shown in HW 3, we read the surface temperature, which is equal to 75 o F, with R MF =1.32 .m, and 162 o F at 6,884ft as the maximum recorded temperature. We can estimate the geothermal gradient in the well: 162 - 75 1 . 2 6 6,884 100 o F GG ft == The temperature is therefore: Sand A: 75 5,700 0.0126 146.8 o F TF =+ = Sand B: 75 6,090 0.0126 151.7 o F = Selection of SSP requires reading the maximum SP amplitude deflection (mV) in a given clean or shale-free water-bearing permeable zone and its difference in mV with respect to the shale baseline. The SP can be used to verify the value of w R or to compute it when it is unavailable: log( ) w mf C SSP K C =− × (2) where SSP = Static Spontaneous Potential in mV mf C = mud filtrate salt concentration in ppm w C = salt concentration in connate water in ppm K is a constant independent of temperature and is equal to: 71 K = mV 1. Select the depth zone and determine both formation temperature and SSP values. From Figure 1, we read the maximum SP deflection for sand A as -75.2 mV. The value of SP in the nearby shale is approximately 3.1 mV, so that the Static Spontaneous Potential is SSP A =-78.8 mV . For sand B, the deflection value is equal to -72.5 mV and the nearby shale SP is equal to +6.5 mV. Therefore, SSP B = -79.0 mV 2 . Calculate mf C based on mf R using the following equation: 3647.5 1.0471 log 6.77 0.0123 81.77 [] 1 0 F w T R NaCl ⎛⎞ ⎜⎟ + ⋅− ⎝⎠ = (3)
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In this question, 1.32 mf R ohm m = (we replace w R by mf R and [] NaCl by mf C ), therefore: 4053.64 mf Cp p m = 3. Use equation (2) to calculate w C : For sand unit A we have: 52204.033 w C ppm = For sand unit B we have: 52543.737 w C ppm = 4. Use the following equation to calculate R w at formation temperature: 0.955 3647.5 81.77 0.0123 6 . 7 7 w F R NaCl T ⎛⎞ =+
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This note was uploaded on 02/13/2012 for the course PGE 368 taught by Professor Staff during the Spring '08 term at University of Texas at Austin.

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Homework_Assignment_4_Solution - Homework Assignment No. 4...

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