Homework Assignment No. 4

February 24, 2015 Due on Tuesday, March 3, 2015@ 2:00 PM

PGE368 Spring Semester 2015

Fundamentals of Well Logging

Instructor: Carlos T. Verdn, Ph.D.

INTERPRETATION OF WELL LOGS IN THE PRESENCE OF MUD-

FILTRATE INVASION DESCRIPTION: The objective of this homework is to exercise the interpretation of well logs in the presence of mud-filtrate invasion. Students will be asked to differentiate changes in water saturation with respect to time of invasion and radial distance away from the wellbore due to mud-filtrate invasion. GUIDELINES FOR THE PREPARATION OF HOMEWORK REPORTS:

(A) YOUR REPORT SHOULD BE CLEAN, NEAT, AND WELL ORGANIZED. DESCRIPTIONS SHOULD BE LEGIBLE AND READABLE.

(B) ALL FIGURES AND TABLES SHOULD BE LABELED AND PROPERLY ANNOTATED WITH A CAPTION. PLOTS SHOULD BE DISPLAYED WITH THEIR AXES, VARIABLES, AND MEASUREMENT UNITS.

(C) ALL RELEVANT RESULTS AND QUANTITIES SHOULD BE WRITTEN WITH THE CORRESPONDING MEASUREMENT UNITS.

(D) ATTENTION SHOULD BE PAID TO NUMBER OF DECIMAL FIGURES USED TO DISPLAY RESULTS.

(E) SPREADSHEETS ARE NOT SELF-EXPLANATORY NOR ARE LOOSE FIGURES. (F) CLEARLY INDICATE THE NUMBER OF THE PROBLEM THAT YOU ARE

ANSWERING EACH TIME. (G) CIRCLE YOUR NUMERICAL ANSWERS. (H) ANSWER QUESTIONS IN ORDER (IF THE ANSWERS ARE ON LOGS, KEEP THE

ORDER AND GIVE THE REFERENCE TO THE PAGE NUMBER WITH THE PLOT), (I) IF YOU DID NOT HAVE TIME TO FINISH THE HOMEWORK, YOU SHOULD CLEARLY

INDICATE WHICH QUESTIONS YOU DID NOT ANSWER. POINTS WILL BE DEDUCTED FROM HOMEWORK THAT DOES NOT ADHERE TO THE ABOVE PRESENTATION RULES.

LEGIBLYWRITEYOURNAMEINTHEBOXABOVEANDCHOOSEONEOFTHETHREEOPTIONSBELOW: Undergraduate PGE Graduate PGE Other

QUESTIONS: (NB: all of the questions below have been drawn from actual field

examples; you may need to consult your geology books or references to answer some of them).

1. Which of the following resistivity measurements provides the best approximation of Rxo?

SFL LLS AHT90

2. Which of the following petrophysical conditions will cause relatively thick mudcake? (check all that apply)

Large differences in salt concentration between mud and connate water Low shale concentration Large formation mobility Large residual hydrocarbon saturation Low hydrocarbon density

3. Which of the following statements is true? (check all that apply)

Effective porosity is lower in sandstones than in carbonates For equal values of total porosity, permeability decreases with a

decrease of effective porosity For equal values of total porosity, smaller grain sizes will cause a

decrease of effective porosity Effective porosity is largely controlled by pore pressure

4. The following conditions are generally observed across a pure shale

(check all that apply) High sonic travel times Low pore pressure Negligible permeability Negligible mudcake thickness Negligible separation between resistivity logs Substantial non-connected porosity Fresh connate water Negligible electrical conductivity

CASE:1 CASE:2

5. For equal values of total porosity, in which case will radial length of invasion be shorter? (Assume oil-base mud-filtrate invasion into a water-saturated sandstone; check only one) High irreducible water saturation Low irreducible water saturation

6. Two cross-bedded sandstones exhibit the same total porosity, they are

both saturated with gas and produce with no water influx, but their deep electrical resistivities are not the same; which of the two rocks will you recommend for perforation? Low electrical resistivity sandstone High electrical resistivity sandstone

7. Which of the following two rocks will (in general) tend to exhibit the largest

residual water saturation? Assume equal-size grains. Calcarenite Sandstone with microporous chert grains

8. Which of the two following rocks will (in general) tend to exhibit the largest residual water saturation? Assume equal-size grains. Sandstone with glauconite-coated grains Sandstone with siderite grains

9. Consider the attached set of well logs shown in Figure 4-1. These are idealized log examples. There are six cases. No case should be chosen more than once in the answers below.

Figure 4-1

CASE:3 CASE:4

CASE:5 CASE:6

(a) Which case corresponds to the specific situation of a oil-saturated sandstone at irreducible water-saturation invaded with oil-base mud? Check only one. Case 1 Case 2 Case 3 Case 4 Case 5 Case 6

(b) Which case corresponds to the specific situation of a hydrocarbon-saturated sandstone invaded with fresh water-base mud and producing a resistivity annulus? Check only one. Case 1 Case 2 Case 3 Case 4 Case 5 Case 6

(c) Which case corresponds to the specific situation of a gas-saturated

sandstone invaded with fresh water-base mud? Check only one. Case 1 Case 2 Case 3 Case 4 Case 5 Case 6

Figure 4-1 (continued)

2a

(d) Which case corresponds to the specific situation of a gas-saturated

sandstone invaded with oil-base mud? Check only one. Case 1 Case 2 Case 3 Case 4 Case 5 Case 6

(e) Which case corresponds to the specific situation of a water-saturated, low-porosity sandstone invaded with oil-base mud? Check only one. Case 1 Case 2 Case 3 Case 4 Case 5 Case 6

(f) Which case corresponds to the specific situation of a gas-saturated

sandstone invaded with water-base mud of similar salinity to that of connate water? Check only one. Case 1 Case 2 Case 3 Case 4 Case 5 Case 6

10. Consider the sets of well logs shown in Figure 4-2. This is an idealized log example. Well logs were acquired 2 hours (Set No. 4-2a) and 5 days (Set No. 4-2b) after drilling. Clean (shale free) formations in the sequence exhibit approximately the same total porosity. Additionally, mud filtrate and formation water exhibit approximately the same salinity. Based on the time-lapse differences of well logs due to mud-filtrate invasion, which of the following rock units exhibits the largest permeability? Check only one!

Figure 4-2a

2b

Rock unit centered at 5050 ft MD

Rock unit centered at 5120 ft MD Rock unit centered at 5180 ft MD

Rock unit centered at 5250 ft MD

11. Consider the well logs shown on page 15 of PGE368s Well-Log

Compendium. Depths are given in ft MD from the KB. Neutron and density logs are expressed in apparent sandstone porosity units. The well was drilled with water-base mud. This is an example of an onshore clastic sedimentary sequence. Archies parameters are a=0.9, m=2.1, and n=2.0. (11.a) What is the dominant in-situ (pre-drill) saturating fluid in the depth section between 10,320 and 10,460 ft MD? salty water fresh water hydrocarbon uncertain (11.b) What is the dominant in-situ (pre-drill) saturating fluid in the depth section between 10,280 and 10,300 ft MD? salty water fresh water hydrocarbon uncertain

Figure 4-2b

(11.c) What is the total porosity at 10,323 ft MD? (11.d) What is Rw (electrical resistivity of formation water) at 10,323 ft MD? (11.e) If formation temperature is 140 degF, what is the concentration of NaCl of formation water at 10,323 ft MD? (11.f) What is the concentration of NaCl in mud filtrate at 10,323 ft MD? (11.g) What is the in-situ water saturation at 10,294 ft MD? Assume that Rw is the same as that of item (11.d).

12. Consider the example on page 27 of PGE368s Well-Log Compendium. Depths are given in m MD from the KB. Neutron and density logs are plotted on an equivalent scale of apparent sandstone porosity units. The well was drilled with water-base mud. This is an example of turbidite sedimentary sequence. Archies parameters are a=0.95, m=2.2, and n=1.9. Assuming that all sandstone units have a constant porosity equal to 31%, construct of plot of water saturation as a function of radial distance from the wellbore at 2,104 m MD. The deepest and shallowest sensing resistivity logs are M2RX and M2R1, respectively. Assume that that the radial length of investigation of M2RX and M2R1 is 7 ft and 0.5 ft, respectively, with all other logs evenly spaced in between. (Hint: first calculate Rw in the fully water saturated rock unit and assume it constant within a depth interval of 200 ft).

13. Consider the example on page 69 of PGE368s Well-Log Compendium. Depths are given in ft TVD from the KB. Neutron and density logs are expressed in apparent sandstone porosity units. The well was drilled with oil-base mud. This is an example of a turbidite sedimentary sequence in the deepwater Gulf of Mexico. Archies parameters are a=0.87, m=1.7, and n=1.6. (13.a) What is your best approximation for the location of the free hydrocarbon-water contact? Explain your answer. (13.b) What is the most likely hydrocarbon type in this sedimentary sequence? (Check only one) Explain your answer. gas condensate oil uncertain (13.c) Are the major sand units here shaly or clean? Explain your answer. shaly clean uncertain

(13.d) Do all shale sections in this well indicate constant shale properties? Explain your answer. yes no uncertain (13.e) What is your best approximation for the porosity of sand units in this well? Explain your answer. (13.f) Is there evidence for non-uniform grain sizes in the sand units penetrated by this well? Explain your answer. (13.g) What is your best approximation for Rw in this well? Show your calculations. (13.h) What is Sxo (water saturation in the invaded zone) at 22,802 ft MD? (Hint: assume that Rw is constant regardless of depth). Show your calculations. (13.i) What is your best approximation of residual water saturation in this reservoir?

14. Consider the two idealized set of logs shown below. One set was acquired

before mud-filtrate invasion and another one after invasion. Based on the differences between before- and after-invasion well logs, what type of mud was used in the drilling process (in the case of water-base mud, be specific about the degree of salinity of mud filtrate)? Explain your answer to receive credit.