THE UNIVERSITY OF TRINIDAD & TOBAGO

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1 THE UNIVERSITY OF TRINIDAD & TOBAGO B. A. SC. PETROLEUM ENGINEERING ALTERNATE 2012 FINAL EXAMINATIONS Course Code and Title: Programme: DATE: DURATION: FMEV BASIC WELL LOGGING BASc B.A.SC. Petroleum Engineering TIME: 3 HOURS Read all instructions carefully before you begin this examination. Materials Supplied: 1. A copy of a log section for question Copy of Schlumberger Chart Gen 9 - Resistivity of NaCl Solutions. 3. Copy of Schlumberger Chart CP Hydrocarbon Density Estimation. 4. Copy of Schlumberger Chart s CP- 1c - CNL* Compensated Neutron Log and Litho-Density* Tool. 5. Copy of Schlumberger Charts CP 16 - Porosity and Lithology Determination Instructions to Candidates 1. This paper has 11 pages and 17 questions. 2. You are required to answer and submit ALL questions on the question paper. 3. All charts required for these questions have been supplied. 4. Please note that some questions are worth more marks than others so you should allocate your time accordingly. 5. Submit your answers on the question paper. 6. Material that is foreign to that allowable or provided in the examination room is prohibited. 7. You must return the question paper along with other writing paper to the Invigilator at the end of the examination. (FMEV 2002) Page 1

2 Key Examination Protocol 1. Students please note that academic dishonesty (or cheating) includes plagiarism, collusion, falsification, replication, taking unauthorised notes or devices into an exam, obtaining an unauthorised copy of the exam paper, communicating or trying to communicate with another student during the exam and being a party to impersonation in relation to an exam. 2. These and any other actions which compromise the integrity of the academic evaluation process will be fully investigated and dealt with in accordance with UTT s academic regulations. 3. Please be reminded that looking into another candidate s script is strictly prohibited. 4. Speaking without the Invigilator s permission is NOT allowed. There are three sections as follows: Section 1 Questions 1 10 (20 marks) Section 2 Questions (40 marks) Section 3 Questions 17 (40 marks) Show all working to gain full marks. Use additional sheets as necessary. Your student number and the question number(s) must be shown clearly on all additional sheets which should be stapled to the question paper. WHEN SUBMITTING YOUR PAPER PLEASE PLACE THE QUESTIONS IN NUMERICAL ORDER FROM 1 TO 17. (FMEV 2002) Page 2

3 Section 1 Well Logging Concepts (TOTAL 20 marks) You should not spend more than 20 minutes on this section. Circle the correct answer(s). 1. Which type of resistivity tool can be used in a well which has been drilled with oil-base mud? (Circle one) Laterolog Induction Either Laterolog or Induction 2. Circle two of the following which can cause the resistivity of a rock to increase: (a) (b) (c) An increase in connate water resistivity A decrease in tortuosity A decrease in formation porosity (d) An increase in water saturation 3. Generally with resistivity tools the greater the spacing between the measuring electrodes the greater the depth of investigation. True or False. True False 4. Which of the following indicate that a rock is permeable? (Circle all that apply). Low gamma-ray Low resistivity SP deflection High porosity 5. The density porosity tool responds to the total formation porosity not effective porosity. (Circle one) a) True b) False c) It Depends on the depth of invasion (FMEV 2002) Page 3

4 6. Which of the following is correct with respect to the effect of the presence of shale and gas on the Compensated Neutron tool? a) They have the similar effects, each causing the tool to read high porosities. b) They have similar effects, each causing the tool to read low porosities. c) They have opposite effects, shale causing the tool to read high and gas causing it to read low porosities 7. When gas replaces oil in a clean sandstone, Density-Neutron log separation: a) Increases b) Decreases c) Remains the same. 8. You are given the Gamma-Ray log on a well which has been drilled through a siliciclastic sequence. The Gamma-Ray reading of a clean sand is 20 API units and the shale baseline has a reading of 120 API units. The Gamma Ray Shale Index of a formation is The Gamma Ray reading across f this formation is? 25 API units 35 API units 45 API units 9. The response of the gamma ray tool is unaffected by the presence of hydrocarbons. (Circle One) True False It depends on the depth of invasion 10. You are working with porosity logs the header indicates that the logs are recorded on a limestone matrix and fluid density is 1.0 g/cc. The density porosity reading is 15% and the neutron porosity reading is 8%. What is the most likely lithology of the formation? The only choices are: Sandstone, Limestone, Shale, Dolomite. (FMEV 2002) Page 4

5 Section 2 Well Logging Computations (TOTAL 40 marks) All working is to be shown below each question. 11. The resistivity of a water-bearing sand was found to be 0.8 Ohm-m. If the porosity of the formation is 0.31 and for this formation the relationship between the formation factor, F, 0.81 and porosity has been established as F. What is the salinity of the formation water? 2 (4 Marks) Show working below: Formation water salinity = 12. A well is drilled through a hydrocarbon-bearing formation which is at its irreducible water saturation and it has a porosity of 28%. Rock core data indicates that (a= 0.81, m=2, and n=2). The mud used for drilling is water-base, with a salt concentration (NaCl) of 50,000 ppm. The connate water has a salt concentration (NaCl) of 20,000 ppm. The shallow and deep resistivity logs readings are 12 ohm-m and 50 ohm-m respectively. You should assume that the deep resistivity tool will be sensing the virgin zone, which does not experience any mud-filtrate invasion, and that the shallow resistivity tool will be sensing the flushed zone. The formation temperature is 145 deg F. What is the hydrocarbon saturation of the formation and what is the water saturation in the flushed zone? (10 marks) Show working below: Hydrocarbon saturation of the formation = (FMEV 2002) Page 5

6 Water saturation in flushed zone = 13. In a clean hydrocarbon-bearing sandstone formation the neutron and density logs read 10 and 38 sandstone porosity units, respectively. The shallowest resistivity reading is 10 ohm-m across the hydrocarbon-bearing formation and the resistivity of mud filtrate at the temperature of the formation is Ohm-m. The residual hydrocarbon saturation in the flushed zone is What is the in situ hydrocarbon density? Estimate the effective porosity of the formation. Assume that a=0.81, m and n = 2 in Archie s equation. (8 marks) Show working below: In situ hydrocarbon density = Effective porosity of formation = 14. An interval transit time of 90 µsec/ft was measured in a sandstone reservoir. The acoustic velocity of the matrix was ft/sec. (6 Marks) a. What is the interval transit time of the matrix? Show working below Interval transit time of the matrix= (FMEV 2002) Page 6

7 b. Assume a fluid transit time of 189 µsec/ft. Calculate the porosity in the sandstone reservoir using Wyllie s time average equation. Show working below: Formation porosity= 15. The following P e and ρ b values were recorded at various depths in a well drilled with fresh mud. Determine the lithology and the porosity for each set of figures. (6 marks) P e (b/e) ρ b (gm/cc) Lithology Porosity Show working below: 16. The table below shows the porosity values in limestone porosity units which were obtained across two formations. Identify the lithology and the effective porosity of each formation. (8 Marks) Formation Density Porosity Neutron Porosity A B Show working below: Lithology Effective Porosity (FMEV 2002) Page 7

8 Section 3 Well Log Analysis Question 17 (Marks for each part as indicated) (TOTAL 40 marks) You have been asked to evaluate a recommendation to perforate the interval on the attached log. You can make the top six feet of the interval i.e the focus of your evaluation. Note that you have only been provided with sections of the well log so that all depths are not continuous. The questions below are the basis of your evaluation You can assume that all necessary environmental corrections have been done to the logs. Assume the simplest linear relationship between Gamma-Ray Shale Index and V Sh. Also, a=0.81; m= 2 and n=2 in Archie s equation. Show all working were possible to gain full marks. a. Based on a review of the SP log indicate which of the following is correct. (Select one). Rmf > Rw Rmf = Rw Rmf < Rw b. Indicate the depth of the top of the hydrocarbon bearing interval in the well and provide the gross thickness of the hydrocarbon bearing interval in the well. (6 marks) Top of hydrocarbon bearing interval Gross thickness of hydrocarbon bearing interval Depth of hydrocarbon / water contact, if any (FMEV 2002) Page 8

9 c. What is the salinity of the mud filtrate in the well? (3 marks) Salinity of mud filtrate = d. Calculate the resistivity of the formation water in the well, Rw, using the Assume that the value of Vsh in the interval = The porosity logs should be corrected for shale in determining the effective porosity (5 marks) Show all work below Formation water resistivity, Rw = e. What is the salinity of the formation water in the well? (3 marks) Salinity of the formation water in the well = (FMEV 2002) Page 9

10 f. Calculate the value of Vsh at 5910 given that the value of Vsh at 6053 is You should assume a simple linear relationship between Gamma Ray Shale Index and Vsh. (5 marks) Show all work below Vsh at 5910 = g. Calculate the hydrocarbon saturation at (6 marks) You should use the average of the corrected neutron and density porosity values as the porosity value in calculating the hydrocarbon saturation. Show all work below Hydrocarbon saturation at 5910 = (FMEV 2002) Page 10

11 h. Calculate the hydrocarbon saturation in the flushed zone at (4 marks) Show all work below Hydrocarbon saturation in flushed zone at 5910 = i. Determine the in-situ hydrocarbon density at 5910 and based on this determine whether the hydrocarbon bearing interval contains oil or gas. (3 marks) Hydrocarbon-bearing interval contains (state whether oil or gas) (FMEV 2002) Page 11

12 j. Calculate the effective porosity at 5910 (3 marks) Show all work below Effective porosity at 5910 = END OF EXAMINATION (FMEV 2002) Page 12