Sinai well logging compares TDT, CHFR applications

May 28, 2007
Logs from wells in a Sinai, Egypt, oil field provide a comparison between the use of thermal decay time (TDT) and cased-hole formation resistivity (CHFR) techniques for monitoring reservoir water saturation

Logs from wells in a Sinai, Egypt, oil field provide a comparison between the use of thermal decay time (TDT) and cased-hole formation resistivity (CHFR) techniques for monitoring reservoir water saturation

Low water saturation suggests the zones may contain bypassed hydrocarbons.

The comparison of the two techniques showed that water saturations calculated from CHFR logs were more accurate than from TDT logs in most cases in these low-salinity reservoirs. The quick-look CHFR logs also always agreed with its quantitative interpretation, while the interpretation of the quick-look TDT log was difficult in most cases and often did not agree with the quantitative interpretation.

TDT logs also indicated water saturations greater than the CHFR log for the same intervals.

Interpretation of CHFR logs is easier than for TDT logs because the two factors used in the CHFR analysis have the same origin and are comparable with each others. These two factors are open and cased-hole resistivities. It is, therefore, easy to compare them and detect any differences.

In the case of TDT logs, the two factors compared with each other have different origins. These factors are TDT sigma and openhole resistivity, making a comparison difficult.

Monitoring saturation

Continuous reservoir saturation monitoring, through cased wells, is the key for proper reservoir management and recovery optimization, especially for large and mature oil fields.

A TDT log has been the main technique for monitoring reservoir water saturations, but the log has problems in formations with low water salinity. This problem was obvious in some wells in the Sinai oil field that produce from sandstone formations.

Some of these reservoirs have naturally low 20,000-30,000 ppm water salinity. Also low salinity occurs in waterflooded reservoirs that use formation water diluted with less saline injected water. Water injection projects in the field use Red Sea water with a 40,000-ppm salinity mixed with 150,000-ppm formation water.

TDT measurements depend on the chlorine (NaCl) content in the formation water; therefore, in zones with water salinity less than 60,000 ppm, a TDT cannot distinguish between water and oil. This problem was solved with the introduction of the carbon-oxygen (C/O) technique, which was combined with TDT technique into the RST tool.1-3

Other problems with TDT logs occur in wells with high pressure or wells that need to be killed before a workover.

Running TDT logs in wells completed with electrical submersible pumps (ESP) also in problematic. About 88 % of wells in the Sinai field have ESPs. The ESP string because it is close-ended prevents the TDT tool from reaching the producing formation. In this case, the production string must be pulled from the hole first, which requires killing the well with a heavier fluid that may invade the producing zone and influence the reading of the TDT log.

This problem may also be encountered in wells that produce with a high water cut and are shut-in for extended periods. In these wells water may invade the hydrocarbon-perforated zones. The TDT because of its short depth of investigation, about 1 ft, will be influenced by the invaded water in the near wellbore region. The RST tool did not solve this problem because it has a short 6-8 in. depth of investigation, even shorter than the TDT.

For these reasons, the Sinai wells need anther type of cased-hole log for monitoring water saturation, although the field experience indicates that both the TDT and CHFR logs have advantages and limitations under certain conditions.

TDTs

Interpreting TDTs based on a time-lapse technique is the most useful and practical approach. This technique requires the running of a reference log soon after completing the well. Equation 1 (see equation box) then can be used to compute the change in water saturation.1 4 This technique has the disadvantage that many wells do not have a reference log or base TDT because these runs must be made a few weeks after the initial completion. For wells such as subsea completion, recording a TDT base run a few weeks after completing the well is impractical.

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The reliability of a TDT quantitative evaluation generally increases with greater formation porosity and water salinity. This evaluation is based on the summation of the cross-section of all material multiplied by their respective volume proportions. In the general case of a shaly porous formation containing water and hydrocarbons, Equation 2 calculates the water saturation, Sw from a TDT log.2 4

The TDT technique is limited by the following:

  • Not for formations that have low formation-water salinities because measurement depends on the capturing of neutrons of the chlorine nuclei of NaCl dissolved in formation brine.
  • Not for formations with different salinity brines.
  • Not for formations with near wellbore effects, formation damage, or washed-out holes because of the logs short depth of investigation.
  • Not for formations with low porosity (<15%).
  • Not for acidized wells.
  • Difficult to interpret the TDT log with a cross plot if the well has no logged aquifer or water zone along with the hydrocarbon zone.
  • Not for wells producing with ESPs because of the closed end in the tubing and the requirements to kill the well with fluids that may invade the formation.

CHFRs

The CHFR resistivity tool has a much greater investigation depth then the nuclear logging tools used for through-casing evaluation. The CHFR evaluation is also possible in low porosity and low formation-salinity zones.5-9

Because the CHFR log provides cased-hole resistivity values that resemble openhole resistivity values (ROH), the quantitative interpretation of CHFR logs is similar to openhole log quantitative interpretation with the Archie equation.

Equation 3 shows the Archie equation for determining water saturation (Sw) from true formation-resistivity logs for clean formation.5

The depletion-indicator (DI) s the ratio of the reference openhole to new cased-hole Archie water-saturation values (Equation 4). DI can be plotted vs. depth for a formation, taking the value 1 as a base line, below which the formation can be considered as it is depleted: RCHFR less than ROH.5 10

Low-resistivity cements typically found in oil wells do not degrade the measurement, but highly resistive cements (>8 Ω-m.) will require a correction.

Another problem is that casing scales may inhibit contact between the electrodes and the casing, necessitating a scraper run before running the CHFR tool. The tool also may not make good contact at casing collars and thus may lose 4-6 ft of data. Casing collars can cause distortions up to 10-20% of formation resistivity. The current CHFR tool cannot be run inside tubing.

Tool contact with the casing may be impaired if the casing has a non-conductive coating. In other instances, the electrodes may contact the casing by scratching away the coating, meaning that the CHFR measurement would be unaffected but that corrosion may be induced.

The CHFR is not designed to operate in dual casing strings because these installations have too many variables to enable quantifying formation resistivity.11-14

CHFR field examples

The first field example is from Well A, the first well in the field logged with a CHFR tool. The log was recorded after running of an openhole resistivity log and setting the casing. Fig. 1 shows that the CHFR and the openhole LLD data have a good match. This logging data comparison is an important step for testing and evaluating CHFR data to monitor water-saturation changes.

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The next example, Well B, shows the use of CHFR logging technique based on determining the difference between CHFR resistivity and openhole resistivity. These logs are from a well that has been on production for the years and produces 2,700 b/d of total fluid from a 90-m shaly sand layer. Fig. 2 shows the old perforations from which the well started producing a 49.5% water cut.

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This well is completed in a reservoir with a mixed, 90,000-ppm water salinity. The aquifer water has a 150,000 ppm while the injected water has a 40,000-ppm salinity.

This current formation-water salinity complicates use of a TDT to monitor changes in reservoir water saturation; therefore, a CHFR was used to evaluate the water cut problem. The CHFR was recorded over the interval 2,495-2,523 m.

The comparison of openhole LLD and CHFR logs (Fig. 2) showed that the CHFR resistivity is generally lower than the openhole resistivity in intervals R2A and R2B, indicating that these are depleted and flooded zones.

A squeeze job on the 2,513-43 m interval and reperforating selectively the total interval (Fig. 2) to avoid the flooded out zones reduced the water cut. The work reduced the total production to 1,500 b/d with the water cut decreasing to 0.8%.

A third example, Well C, initially produced 820 b/d total fluid, 0.8% water cut, with an ESP from sandstone reservoirs. After the water cut increased and reached 72.3%, the operator decided to make a logging run to determine the reason for the high water cut. Because the formation has a low, 22,000-ppm water salinity, the CHFR was selected instead of the TDT.

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Fig. 3 compares the CHFR resistivity and openhole resistivity. The logging data clearly show that the middle of the perforated interval, 2,589-2,615 m, is the source of the water.

After squeezing of the waterflooded interval 2,595-2,611 m, water cut decreased to about 35%.

TDT field examples

Well D is an oil producer with an ESP. Initial production was 1,260 b/d of fluid with a 1.6% water cut. Fig. 4 shows openhole log of the producing sand and shaly sand intervals (2,804-12, 2,816.5-32, and 2,832.5-48 m).

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After 2 years of production, water cut reached 62.3%. Formation-water salinity was about 178,000 ppm; therefore, the operator decided to run TDT for detecting the waterflooded zone.

Fig. 5 shows the TDT data for the same intervals. The TDT interpretation indicated that waterflood intervals were 2,816.5-32 and 2,832.5-48 m. Based on this interpretation, the operator isolated the lower intervals, but this did not change the water cut and the work decreased oil production.

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Another example is the lower section of Well D that initially produced 600 b/d. Producing intervals were mainly sand and shaly sand at 2,933-41 and 2,946-51.

Fig. 4 shows the openhole data, neutron-density logs indicating clearly the producing sections. Fig. 5 illustrates the TDT for the same intervals with the TDT sigma curve clearly showing that the waterflooding of the producing intervals. The interpretation of the TDT indicates that the water is coming form Zone IVa.

TDT, CHFR comparison

A CHFR may replace a TDT log in cases where reservoir conditions or well completions are unsuitable for running a TDT log. Also CHFR logs can clarify any confusion in the TDT interpretation.

One major problem with the TDT is the difference between quick-look interpretation of the TDT log and the quantitative interpretation of the TDT with a water saturation model.

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Fig. 6 compares the TDT and CHFR for Zone IV, Well D, and Fig. 7 compares the TDT and CHFR for Zone IVA, Well D.

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The quick-look interpretation of the TDT indicates that the lower sections of Zone IV and IVa were waterflooded; whereas the CHFR indicates that only the upper section of IV and lIVa were waterflooded.

Fig. 8 shows a quantitative interpretation of openhole data, TDT, and CHFR for Zone IV. The TDT interpretation shows that the entire section was flooded, which disagrees with the TDT quick-look interpretation.

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The CHFR interpretation indicated that only the top section was waterflooded, which agrees with the CHFR quick interpretation.

Fig. 8 also shows the quantitative interpretation of openhole log, TDT, and CHFR for Zone IVa. Both the TDT and CHFR indicated that this is a flooded zone.

Worth noting is that the TDT shows more water saturation difference with reference openhole water saturation, indicating more flooding than the CHFR.

The difference between openhole saturation profile and CHFR or TDT saturation profiles indicates that Zone IVa and the top section of Zone IV were producing zones, but now water has swept these zones.

Based on TDT data interpretation, the operator isolated Zone IVa, but the water cut was not reduced.

But based on the CHFR data interpretation, the operator isolated Zone IVa and the upper section of Zone IV, thereby reducing the water cut to 38% from 62.3%. The water cut has continued to decrease to 2%.

Acknowledgment

The authors thank Petrobel Co. for permitting publication of well logging data and thank King Fahd University of Petroleum & Minerals for supporting this work.

References

  1. Morris, F., et al., “Applications of Pulsed Neutron Capture Logs in Reservoir Management,” Paper No. SPE 93889, SPE Western Regional Meeting, Irvine, Calif., Mar. 30-Apr. 1, 2005.
  2. Hamada, G.M., and Hiekal, S.A., “Gas Detection Model Applied to Heterogeneous Reservoirs Using TDT,” Paper No. JJJ, SPWLA 39th Annual Logging Symposium, Keystone, Colo., May 26-29, 1998.
  3. Aldred, R.D., “Interpretation of Pulsed-Neutron Data in Low-Salinity Environments Without Base Logs,” Paper No. SPE 25374, SPE Asia Pacific Oil & Gas Conference & Exhibition, Singapore, Feb. 8-10, 1993.
  4. Rinehart, C.E., and Weber, H.J., “Measuring Thermal Neutron Absorption Cross Sections of Formation Brines,” Paper G, SPWLA 17th Annual Logging Symposium, New Orleans, June 9-12, 1975.
  5. Dubourg, I., et al., “Resistivity Behind Casing.” OilField Review, Schlumberger, Spring 2001, pp. 2-25.
  6. Hunka, J.F., et al. “A New Resistivity Measurement System for Deep Formation Imaging and High-Resolution Formation Evaluation,” Paper No. SPE 20559, SPE ATCE, New Orleans, Sept. 23-26, 1990.
  7. Singer, B.S., et al., “Measurement of Formation Resistivity Through Steel Casing,” Paper No. SPE 30628, SPE ATCE, Dallas, Oct. 22-25, 1995.
  8. Maurer, H.M., and Henniker, J., “Early Results of through Casing Resistivity Field Tests,” Paper No. DD, SPWLA 41st Annual Logging Symposium, Dallas, June 4-7, 2000.
  9. Beguine, P., et al., “Recent Progress on Formation Resistivity Measurement Through Casing,” Paper No. CC, SPWLA 41st Annual Logging Symposium, Dallas, June 4-7, 2000.
  10. Ferguson, R., et al., “Direct Measurement of Formation Resistivity Through Steel Casing Solves a North Sea Production Question,” Paper No. SPE 71715, SPE ATCE, New Orleans, Sept. 30-Oct. 3, 2001.
  11. Hupp, D., et al., “Cased Hole Formation Resistivity Application in Alaska,” Paper No. SPE 76715, SPE Western Regional/AAPG Pacific Section Joint Meeting, Alaska, May 20-22, 2002.
  12. Starcher, M., et al., “Next Generation Waterflood Surveillance: Behind Casing Resistivity Measurement Successfully Applied in the ‘A3-A6’ Waterflood at Elk Hills Field,” Paper No. SPE 76730, SPE Western Regional/AAPG Pacific Section Joint Meeting, Alaska, May 20-22, 2002.
  13. Ma, S.M., et al., “Looking Behind Casing: Evaluation and Application of Cased Hole Resistivity in Saudi Arabia,” Paper No. SPE 88467, SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, Oct. 18-20, 2004.
  14. Murty, C.R.K., et al., “Analysis Behind Casing: A Window for Improved Reservoir Management of a Mature Bahrain Oil Field,” Paper No. SPE 93582, 14th SPE Middle East Oil & Gas Show and Conference, Bahrain, May 12-15, 2005.

The authors

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Gharib M. Hamada ([email protected]) is professor of well logging and applied geophysics at King Fahd University, Saudi Arabia. Previously he was with Cairo University, Technical University of Denmark, Sultan Qaboos University, and King Saud University. His main research interests are well logging technology, formation evaluation, and seismic data analysis. Hamada holds a BS and MS in petroleum engineering from Cairo University, and a DEA and. D’Ing from Bordeaux University, France. He is a member of SPE and SPWLA.

Ahmed El.M. Hegazy is a petroleum engineer at Petrol Belyem Petroleum Co. (Petrobel), Cairo. Hegazy holds a BS and an MS in petroleum engineering, from Cairo University, Egypt.