RST* Reservoir Saturation Tool
Schlumberger
RST* Reservoir Saturation Tool
© Schlu Schlumber mberger ger 1993 Schlumberger Wireline & Testing P.O. Box 2175 Houston, Texas 77252-2175 All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transcribed in any form or by any means, electronic or mechanical, including photocopying and recording, without prior written permission of the publisher. SMP-9250
An asterisk (*) is used throughout this document to denote a mark of Schlumberger.
Contents Introduction. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 1
Processing the spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Applications . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 3
Carbon/oxygen ratio interpretation. . . . . . . . . . . . . 11
Tool description . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . 4
Alpha processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Tool specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Sigma processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Tool features . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . 6
Logging speed . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . 16
Dual-detector system . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Examples. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . 17
Neutron generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Compact high-speed electronics . . . . . . . . . . . . 7
RST operation through tubing with the well flowing and shut in . . . . . . . . . . . . . . . 17
Data acquisition . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . 8
Reservoir monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Inelastic-capture mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Detecting injection water breakthrough . . . . . . . 21
Capture-sigma mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Monitoring fluid contacts . . . . . . . . . . . . . . . . . . . . . . . . 22
Sigma mode . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . 9
Summary . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . 23 References and recommended reading . . . . . . . . . 24 Nomenclature . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . 26 Log and plot mnemonics . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
RST Reservoir Saturation Tool
Introduction Reservoir evaluation and saturation monitoring through casing are generally performed in two ways. One measures the decay of thermal neutron populations (TDT* Thermal Decay Time principle), and the other determines the relative amounts of carbon and oxygen in the formation by inelastic gamma ray spectrometry, as used in the GST* Induced Gamma Ray Spectrometry Tool. Because chlorine has a large neutron capture cross section, the TDT technique provides good results in areas with highly saline formation waters. When the formation water is not sufficiently saline or when its salinity is unknown, the carbon-oxygen method usually provides a more reliable answer, and the
TDT data may not be interpretable. A combination of the two methods may sometimes provide the best results and yield additional information. For carbon-oxygen measurements, a gamma ray spectrometer measures the energy spectrum of gamma rays produced by neutrons from a pulsed neutron source. Responses of carbon and oxygen are obtained from analysis of gamma ray spectra produced during a short burst of high-energy neutrons that are scattered inelastically by formation and borehole elements. Figure 1 compares the fast neutron inelastic scattering process with the capture process that occurs predominantly after the neutrons have been slowed to thermal energies.
Neutron Capture
Inelastic Scattering Excited nucleus
Slow neutron
Nucleus
Excited nucleus Fast neutron
Nucleus
g -ray g -ray
Figure 1. Neutron capture reactions are the basis for sigma measurements. Inelastic scattering reactions are used for the carbon-oxygen measurement.
RST Reservoir Saturation Tool
1
Large tool sizes, slow logging speeds and significant sensitivity to the borehole fluid limit the application of existing carbon-oxygen (C/O) tools. To accommodate the large tool and to provide a known borehole fluid, it is usually necessary to kill the well and pull the production tubing. The new smaller-size RST Reservoir Saturation Tools (111 ⁄ 16- and 2 1 ⁄ 2-in. diameters) allow logging below tubing, eliminating the need to kill the well and pull tubing. Dual detectors bring improvements in measurement precision and, in the case 1 of the 2 ⁄ 2-in. RST tool, provide the means to compensate the measurement for borehole fluid composition. With the 2 1 ⁄ 2-in. tool, it is now possible to measure formation oil saturation with the well flowing and to simultaneously determine the borehole oil/water fraction, or holdup.
2
Introduction
The dual-detector system and a new interpretation model with a superior characterization data base can provide a more precise answer in less time than existing C/O tools. Since the RST tool also measures the thermal neutron decay time with techniques similar to those used with the Dual-Burst* TDT tool—and can be combined with production logging sensors—a complete reservoir monitoring operation can be performed during one trip in the well. The MAXIS 500* wellsite unit provides total control of the field operation—from downhole tool functions to acquisition and data processing.
Applications The RST tool provides a vastly improved carbonoxygen measurement. The dual detector measurement can provide new answers, and the precision of the measurement exceeds the precision achieved with previous tools. The tool is also capable of measuring formation capture cross section with better accuracy and precision than the TDT tool. The carbon-oxygen and sigma measurements can be made during the same trip in the well. RST tools can be run on small-diameter coaxial cables and are combinable with production logging sensors for a complete reservoir monitoring program with only one trip in the well. The carbon-oxygen measurements are extremely useful in medium- to high-porosity oil reservoirs under the following conditions: • Formation waters of any salinity This is the basic application for inelastic spectrometry tools since water salinity does not affect carbon-oxygen measurements. Thermal neutron capture cross sections of fresh water and oil are so similar that the two fluids cannot be distinguished with TDT-type logs. C/O tools should be used in this case.
• Variable or unknown formation water salinity When the formation water salinity changes between reservoirs crossed by the same well, analysis of TDT-type logs is difficult. If the salinity is unknown, changes in oil saturation cannot be directly inferred from changes in the sigma measurement. Combining carbon-oxygen and sigma measurements is useful in this case. • Horizontal wells Due to the detector configuration, the 2 1 ⁄ 2-in. RST tool provides values of oil holdup even in horizontal wells where standard production logging differential pressure sensors do not work. In lower-porosity reservoirs, carbon-oxygen measurements can be used for monitoring gas-oil and oil-water contact movement but not for saturation evaluation.
• Injection fluid of different salinity than the formation water When water of different salinity is injected into the reservoir—through surface-fed injection wells or by dump flooding—the analysis of TDT-type logs becomes complex, and the results may be misleading. A combination of carbon-oxygen and sigma measurements provides the best solution since both the oil saturation and the formation water salinity can be quantified.
RST Reservoir Saturation Tool
3
Tool description The new RST tool overcomes the tool-size limitation of existing large-diameter carbon-oxygen tools with a 1 11 ⁄ 16-in. version for induced gamma ray spectrometry and TDT logging below 2 3 ⁄ 8-in. 1 tubing, and a 2 ⁄ 2-in. version for operations below 1 3 ⁄ 2-in. tubing. These sizes eliminate the need to kill the well and pull tubing—saving time and reducing lost production. Both tools use dual detectors, giving improved precision with the 111 ⁄ 16-in. tool and allowing compensation of the measured response for borehole fluid composition 1 with the 2 ⁄ 2-in. tool. Figure 2 shows the major components of the two RST tools. Optionally, a gamma ray and production logging sensors can be included in the tool string. • Telemetry cartridge Both the 111 ⁄ 16- and 2 1 ⁄ 2-in. RST versions use the same telemetry cartridge. This cartridge is the interface between the logging cable and the tool, and includes a casing collar locator. • Acquisition cartridge Both tools use the same acquisition cartridge but with different pressure housings. This cartridge acquires gamma ray spectral and time information from the detectors in the sonde.
21 ⁄ 2-in. RST Tool
111 ⁄ 16-in. RST Tool
Gamma ray (optional)
Telemetry, casing collar locator (CCL)
Acquisition cartridge
Sonde
Accelerator cartridge
Figure 2. The RST tool string.
4
Tool description
111 ⁄ 16 -in. RST Sonde • Dual-detector sonde As shown in Fig. 3, both sondes have two detectors. The actual detectors are the same in both versions of the RST tool, but the physical arrangement of the detectors is different in each version. The arrangement in the 1 11 ⁄ 16-in. tool resembles that of the Dual-Burst TDT tool. Both detectors are on the tool axis, separated by neutron and gamma ray shielding. With this arrangement, borehole-to-formation contrast is insufficient in most cases to allow simultaneous determination of borehole fluid and formation fluid compositions. Since the near detector is also sensitive to the formation fluid, it contributes significantly to reducing the statistical variations of the measurement. • Accelerator control cartridge The accelerator control cartridges of both tools are identical—containing the power supplies and control circuitry for the pulsed neutron generator. In the 2 1 ⁄ 2-in. tool, the detectors are offset from the tool axis. The near detector faces the borehole and is shielded from the formation. The far detector faces the formation and is shielded from the borehole. A bow spring must be used with this sonde in order to orient the detectors properly. The strong signal contrast provided by this arrangement facilitates simultaneous determination of the borehole fluid and formation fluid compositions. Contrast is improved, however, at the expense of 1 2-in. logging speed. The logging speed of the 2 ⁄ RST tool is typically less than 50 percent of the speed of the 1 11 ⁄ 16-in. tool.
1 2 ⁄ 2-in. RST Sonde
Electronics Photomultiplier tube GSO detector (far) Electronics Photomultiplier tube GSO detector (near) Shielding
*
Neutron generator
*
Far
Near
Figure 3. RST detector configuration and shielding.
RST Reservoir Saturation Tool
5
Tool specifications
• good energy resolution
Table 1 lists RST tool specifications.
• high density for improved gamma ray detection sensitivity, particularly at high energies
Tool features
• fast decay constant for a significantly higher instantaneous counting rate during the highintensity neutron burst than other types of detector
Dual-detector system
A key feature of the Reservoir Saturation Tool is the dual-detector spectroscopy system that includes a new type of scintillation crystal and a new photomultiplier tube. The detector crystal is cerium-doped gadolinium oxy-ortho silicate (GSO). Several properties make this material attractive for use in carbon-oxygen logging:
RST tool
111 ⁄ 16 in.
1 2 ⁄ 2 in.
Pressure rating
15 kpsi
15 kpsi
Temperature rating
300°F [150°C]
300°F [150°C]
Maximum tool diameter
1.710 in.
2.505 in.
Minimum tubing size
3 8-in. API 2 ⁄
1 2-in. API 3 ⁄
Minimum restriction
1.813 in.
2.625 in.
Maximum recommended casing size
7 5 ⁄ 8-in. API
9 5 ⁄ 8-in. API
Maximum recommended borehole size
10 in.
12 1 ⁄ 4 in.
Tool length
33.6 ft
32.7 ft
Tool weight
143 lbm
250 lbm
Table 1. RST tool specifications.
6
• performs well at temperatures up to 150°C and therefore does not require a Dewar flask for temperature stabilization (no limit to the logging time at rated temperatures).
Tool description
Table 2 shows a comparison of properties of GSO with sodium iodide (NaI) and bismuth germanate (BGO). The new, highly sensitive photomultiplier tube helps compensate for the lower light output of the GSO crystal. Neutron generator
An improved neutron generator provides a stable and controlled neutron burst. The almost perfectly square burst shape (Fig. 4) improves separation of the carbon and oxygen gamma rays produced during the neutron burst from capture gamma rays produced during and after each burst. Compact high-speed electronics
Crystal
NaI
BGO
GSO
Relative light output
100
13
20
Energy resolution (at 662 keV for a 1-cm 3 crystal)
6.5%
9.3%
8.0%
Density (g/cm 3)
3.67
7.13
6.71
Effective atomic number
51
75
59
Primary decay constant (nsec)
230
300
56
Fragile?
Yes
No
Slightly
Hygroscopic?
Yes
No
No
Dewar system required?
No
Yes
No
Table 2. Properties of different scintillation crystals.
New compact high-speed gamma ray pulse processing techniques significantly boost the counting rate for increased precision and logging speed.
s t n u o C
0
10
20
30
40
50
60
Time (µsec)
Figure 4. RST neutron burst profile taken with a fast neutron monitor showing the nearly square shape.
RST Reservoir Saturation Tool
7
Data acquisition The RST tool has three software-selectable logging modes: inelastic-capture mode, capturesigma mode and sigma mode. Each uses an optimum timing sequence for pulsing neutrons and acquiring the resulting gamma ray energy spectra and counting rates as a function of time. Spectra are recorded with 256 channels covering the energy range from 0.1 to 8 MeV.
Time (µsec) Burst
100
80 C
Inelastic-capture mode This mode records gamma ray spectra produced by inelastic neutron scattering from formation and borehole elements. Analysis of these spectra provides the carbon and oxygen yields used to determine formation oil saturation and borehole oil fraction. In addition, thermal neutron capture gamma ray spectra are recorded after the neutron burst. Element yields from these spectra provide lithology, porosity and apparent water salinity information. The tool timing is shown in Fig. 5. Timing gate A records inelastic spectra during the neutron burst. Timing gates B and C record capture gamma ray spectra after the neutron burst. A fraction ß of gate B spectrum is subtracted from gate A spectrum to remove capture background, resulting in the net inelastic spectrum. To obtain sufficient precision, several passes over the region of interest are usually required.
8
Data acquisition
C
60
B
Net inelastic = A – bB
40 Net Inelastic B
20 Burst
A
A 0
Figure 5. RST timing for inelastic-capture mode. The net inelastic spectrum is formed by subtracting a fraction ß of spectrum B from spectrum A acquired during the neutron burst.
Capture-sigma mode The capture-sigma mode simultaneously records capture gamma ray spectra and thermal neutron decay time distributions. Elemental yields from the capture spectra provide lithology, porosity and apparent water salinity information as in the inelastic-capture mode. Decay time distributions are used to determine the formation thermal neutron capture cross section (sigma). The timing sequence is similar to that of the Dual-Burst TDT tool with a short neutron burst followed by a longer burst (Fig. 6). It produces time-decay distributions optimized for the determination of both borehole and formation sigma with
Short burst
low statistical variations. The count rate spectrum is recorded in 126 time gates of varying width, covering the entire sequence, including the burst and the “burst-off” background.
Sigma mode The sigma mode provides capture cross-section data in a fast logging pass. This mode uses timing identical to the capture-sigma mode but records only the time-decay data, burst-off background gamma ray spectra and associated quality curves.
Long burst
Figure 6. RST timing for capture-sigma and sigma modes. s t n u o C
0
500
1000
1500
Time (µsec)
RST Reservoir Saturation Tool
9
Processing the spectra Each element has a characteristic gamma ray energy spectrum; therefore, particular elements can be identified by their signature within the total spectrum. In addition, the magnitude of the gamma ray contribution is related to the amount of the element in the formation or borehole. A large number of laboratory measurements characterize the tool response by systematically exploring different downhole conditions. The main parameters of interest are
Each point characterized consists of four sets of data. The large data base covers more than 100 different downhole situations and consists of at least 500 laboratory measurements for each tool. Gamma ray spectra recorded by the near and far detectors are analyzed with a full-spectrum procedure based on a least-squares fitting technique. The procedure uses a set of standard elemental response spectra to determine the contribution of each element to the measured spectrum. Elements from the formation and borehole fluid contributing to spectra measured during the neutron burst period include carbon, oxygen, silicon, calcium and iron. Figure 7 shows the standard spectra of these elements and the tool background 1 2-in. tool. for the far detector of the 2 ⁄
• formation fluid • borehole fluid • formation porosity • lithology • borehole size • casing size • casing weight.
Oxygen Silicon Tool background
Calcium Iron Carbon
Figure 7. Standard spectra for the far 1 2-in. detector of the 2 ⁄ RST tool.
1
2
3
4
5
Energy (MeV)
10
Processing the spectra
6
7
8
Figure 8 displays typical spectra obtained during the neutron burst with the far detector in a water and an oil tank. The carbon signature in the oil spectrum and the oxygen signature in the water spectrum are easily identified. The spectrum analysis procedure provides a correction of the recorded spectra for slight gain and offset shifts and changes in detector energy resolution. After subtraction of the capture background component, a net inelastic spectrum is obtained to determine the elemental contributions—expressed as elemental yields—and their associated statistical errors.
The dual-detector RST interpretation model is an extension of the single-detector model used for GST interpretations. With the dual-detector system 1 2-in. tool, both the formation carbon/ of the 2 ⁄ oxygen ratio and the borehole w ater fraction, or holdup, can be determined. The model includes the environmental response plus the partitioning of the formation response and borehole response. A fan chart shows the response of the tool by plotting the ratio of carbon and oxygen yields versus porosity for S o = 1 (100 percent) and S o = 0. Figure 9 1 shows fan charts for the 2 ⁄ 2-in. tool in limestone formations for carbon/oxygen ratio processing. The charts are for 5-in., 18-lbm/ft casing set in a 6-in. borehole. Data for oil- and water-filled boreholes are given. Figure 10 shows the fan charts for the 111 ⁄ 16-in. tool for the same conditions.
Carbon/oxygen ratio interpretation In addition to inelastic-capture data, carbonoxygen interpretation requires information on lithology, porosity, borehole diameter, casing size, casing weight and downhole fluid carbon density.
Hydrogen
s t n u o C
Figure 8. Comparison of the inelastic burst spectra obtained with the far detector of the 2 1 ⁄ 2-in RST tool in tanks of oil and water.
Carbon
Oxygen
0
2
4
6
8
Energy (MeV)
RST Reservoir Saturation Tool
11
Oil in borehole 0.6
Figure 9. Fan charts for carbon/oxygen ratio processing 1 2-in. RST near and for the 2 ⁄ far detectors in a limestone formation with a 6-in. borehole, 5-in., 18-lbm/ft casing, with oil and water in the borehole.
Near Carbon/Oxygen Ratio
Water in borehole
S o = 1
o i t 0.4 a r n e g y x o / n o b r 0.2 a C
S o = 0
S o = 1 S o = 0
0.0 0
10
20
30
40
50
60
Porosity (%) 0.8
Oil in borehole Water in borehole
Far Carbon/Oxygen Ratio S o = 1
0.6
S o = 1
o i t a r n 0.4 e g y x o / n o b r a C 0.2
S o = 0
S o = 0 0.0 0
10
20
30
Porosity (%)
12
Processing the spectra
40
50
60
0.9
0.8
Oil in borehole
Near Carbon/Oxygen Ratio
Water in borehole
0.7
S o = 1
0.6
Figure 10. Fan charts for carbon/oxygen ratio processing for the 111 ⁄ 16 -in. RST near and far detectors in a limestone formation with a 6-in. borehole, 5-in., 18-lbm/ft casing, with oil and water in the borehole.
o i t 0.5 a r n e g 0.4 y x o / n o 0.3 b r a C
S o = 0
0.1
S o = 0
S o = 1
0.2
0 0
10
20
30
40
50
60
Porosity (%) 1.0
Oil in borehole Water in borehole
0.8
Far Carbon/Oxygen Ratio S o = 1
o i t a r 0.6 n e g y x o / n o 0.4 b r a C
S o = 1
S o = 0
0.2
S o = 0
0 0
10
20
30
40
50
60
Porosity (%)
RST Reservoir Saturation Tool
13
1.2
The interpretation process ensures that the measured data are consistent with the interpretation model. Known formation and borehole data are used to compute the expected values of carbon/oxygen ratio for each detector using water saturation and borehole holdup values ranging from 0 to 1. Figure 11 shows a plot for a 43-p.u. 1 limestone formation with an 8 ⁄ 2-in. borehole and 7-in. casing. All data should fall statistically within the bounded region. After transforming the C/O data to oil saturation and borehole holdup, the data are plotted for each level with porosity, φ > 10 p.u., on a template. Ideally, the data should lie within the box bounded by the limits on S o and yo. The RST data base provides a method for determining the sensitivity of oil saturation and oil holdup to changes in the borehole and formation parameters. For example, Table 3 lists the change in each model parameter to produce an increase of 10 s.u. in S o, or an increase in oil holdup yo of 0.10 for the 2 1 ⁄ 2-in. RST tool. This tabulation is for a standard condition of 7-in., 23-lbm/ft casing cen1 2-in. diameter boretered and cemented in an 8 ⁄ hole. The formation is a 30-p.u. limestone with an oil density of 0.85 g/cm 3 and S o = 50 s.u. These data can be used to decide how to reconcile data falling outside normal limits on the crossplot.
Dual Detector COR Model for 111 ⁄ 16-in. RST Tool
1 0.8
l i o e l h o r e o B
Far carbon/ oxygen 0.6 ratio 0.4
o-w
o-o
l i o n i o t a r m F o
w-o 0.2
w-w
0 0
0.2
0.4
0.6
0.8
1
Near carbon/oxygen ratio 1.2
Dual Detector COR Model for 21 ⁄ 2-in. RST Tool
1 0.8
Far carbon/ oxygen 0.6 ratio
o-o
o i l o l e h e r B o
w-o
0.4
y o
S o
0.2
o-w
l i o n i o t a m o r F
w-w
0 0
0.2
0.4
0.6
0.8
1
Near carbon/oxygen ratio w -w:
w at er in bo re ho le
w at er in fo rm at io n
o-w:
oil in borehole
water in formation
o-o:
oil in borehole
oil in formation
w-o:
water in borehole
oil in formation
Figure 11. Plots used for RST interpretation. Each pair of near-far C/O ratios represents a formation oil saturation and borehole oil holdup. These plots are also used to check that the measured data are consistent with the i nterpretation model.
14
Processing the spectra
Parameter change needed S o = +0.10
Parameter Formation
Borehole
Formation and borehole
yo = +0.10
Porosity
+3.5 p.u.
–15 p.u.
Volume (lime)
–0.10
–0.50
Volume (sand)
+0.10
+0.50
Borehole diameter
–0.5 in.
< –1.5 in.
Casing OD
–0.5 in.
n.a.
Casing ID
–0.4 in.
–0.3 in.
Casing center moved closer to formation
0.5 in.
> 0.75 in.
Oil density
+0.09 g/cm3
+0.09 g/cm3
Table 3. Effects of parameter changes for the 2 1 ⁄ 2-in. RST tool in 30-p.u. limestone, with 1 2-in. borehole. S o = 0.50, 7-in., 23-lbm/ft casing, and 8 ⁄
Alpha processing Results show that the spectral deconvolution technique may give large statistical uncertainties on the spectral yields. A technique that produces a significantly lower statistical uncertainty consists of setting windows on broad regions of the spectrum that contain the main contributions from carbon and oxygen. Figure 8 shows a typical set of carbon and oxygen windows for the inelastic spectra. The windows technique is appealing because of its simplicity and statistical robustness. However, even in the simplest case with the tool surrounded by water, the carbon window contains signals that are largely due to oxygen. In a real environment, additional contributions come from other surrounding elements. Therefore, the ratios are dependent on environmental effects—especially porosity and near-wellbore effects such as washouts and cement quality. The use of windows can yield a precise, repeatable answer, but the technique is prone to large systematic errors or inaccuracies. Alpha processing combines the advantages of both methods—the accuracy of the COR model and the precision of the windows approach. This
method calculates the volume of oil (VUOI = φ S o) from both COR and windows. Typically, the volume of oil from the windows shows the correct structure over a limited depth range, although its absolute value may not be correct. Therefore, determining a linear relationship between the two VUOIs requires averaging or fitting over a large depth interval (normally 10.5 ft): VUOIavg (COR) = α 0 + α1 VUOIavg (windows). Coefficients α 0 and α1 allow the determination of VUOI from the windows ratio: VUOI (alpha) = α 0 + α1 VUOI (windows). This approach results in a VUOI precision that is very close to the windows precision, and an accuracy that matches VUOI from COR. The present model uses α0 only; α1 is set to 1.
Sigma processing A new interpretation model provides an analysis of the decay of a burst of fast neutrons. This technique provides improved accuracy over existing TDT-type tools with faster operating times.
RST Reservoir Saturation Tool
15
Logging speed Table 4 gives a comparison of logging speeds for the GST and RST tools in various formations and conditions. Despite the much smaller size of the 111 ⁄ 16-in. RST tool, its logging speed exceeds the speed of the GST tool by factors varying from 1.5 to 4. The logging speeds are for a 10-s.u. statistical uncertainty (±1 standard deviation, or 67 percent confidence) in the computed oil or water saturation. A 95 percent confidence (±2 standard
Lithology
Porosity (p.u.)
deviations) reduces the logging speed by a factor of 4. The 2 1 ⁄ 2-in. RST tool column shows logging speeds for flowing (or unknown) borehole fluid conditions. Very slow logging speeds are difficult to attain (< 60 ft/hr) and require multiple log passes. For example, 4 passes at 120 ft/hr provide an effective logging speed of 30 ft/hr.
Logging speed (ft/hr) 111 ⁄ 16-in. RST tool (shut in)
1 2 ⁄ 2-in. RST tool (flowing)
5 3 ⁄ 8-in. GST tool (shut in)
Sand
16
30
8
18
Sand
33
250
110
160
Lime
16
30
10
11
Lime
41
250
70
60
Table 4. Summary of logging speeds to achieve 10-s.u. precision for S o and S w in a 10-in. borehole with 7-in., 23-lbm/ft casing, at 100°C. Computations for the RST tool use a 21-level (10.5-ft) alpha processing and a 5-level (2.5-ft) filter.
16
Logging speed
Examples RST operation through tubing with the well flowing and shut in The well in this example produces from a carbonate reservoir with porosity varying from 5 to 30 p.u. Production is from a 6-in. diameter openhole completion. The well is vertical, and when flowing it produces oil with a water cut of about 20 percent. The objective was to determine the oil saturation in the reservoir and to identify the producing intervals. Data include seven logging passes in inelastic-capture mode with the well shut in, and five passes with the well flowing. The carbon/oxygen ratio curves shown in Fig. 12 are the average of all logging passes, with the width of the line showing ±1 standard deviation from the average. The sharp increase in both the near and far detector ratios at X851 ft, with the well shut in, indicates an oil-water interface in the borehole. The shut-in and flowing data overlay below that depth, indicating no oil production from that interval. Above X850 ft, the carbon/oxygen ratios from both detectors increase steadily, showing the depths at which oil is produced. Figure 13 shows a crossplot of the near and far carbon-oxygen data compared with the laboratory data for limestone saturated with either water or oil having a density of 0.85 g/cm 3. The outer bounded area shows the dynamic range for 43-p.u. limestone, and the inner bound area is for 17-p.u. limestone. Data recorded with the well shut in match the dynamic range for the near detector and are consistent with the data recorded with the well flowing. Some of the points fall outside the boundaries because of statistical variations, a borehole slightly larger than 6 in. in diameter, and a low oil density of 0.715 g/cm 3 at reservoir conditions. Figure 14 shows the result of processing the data using the dual-detector interpretation model. The porosity information came from an ELAN* Elemental Log Analysis of openhole logs. The plot includes all limestone and dolomite points with porosity greater than 10 p.u. The model matches the log data.
Figure 12. Comparison of near and far detector carbon/ oxygen ratio logs with the well shut in and flowing.
RST Reservoir Saturation Tool
17
The final interpretation presentation (Fig. 15) shows the borehole holdup in the left-hand track for the well shut in and flowing. The separations of the saturation curves in the central track, through the interval from X770 to X850 ft, indicate that oil from the borehole reinvaded the formation while the well was shut in. During the subsequent flowing period, the formation water flushed out the oil—as shown by the increased water saturation
and verified by the flowing borehole holdup. The volumetric analysis, presented in the right-hand track, uses formation lithology from the openhole ELAN interpretation. The green shading indicates substantial oil saturation in the upper half of the reservoir. The log of borehole holdup indicates that most of the oil is produced from the interval from X728 to X750 ft.
Figure 15. Final interpretation shows the borehole holdup in the l eft track, the flowing and shut-in saturations in the center track, and the volumetric analysis in the right track.
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Reservoir monitoring Figure 16 shows the 1 11 ⁄ 16-in. RST log results combined with openhole log data from a well with 4 1 ⁄ 2-in., 9.5-lbm/ft casing set in a 6 1 ⁄ 2-in. borehole. The interval shown, logged with the well shut in, consists of a clean limestone reservoir with a shaly limestone and sandstone section below.
The near and far COR curves, displayed on the left, read between 0.16 and 0.3 in the limestone section but drop rapidly when the tool is in the shale or sand zones. The fluid analysis in the center track shows some depletion in the limestone reservoir from X290 to X350 ft. This example confirms that the interpretation handles the mixed and changing lithology without exhibiting a bias in the saturation answers.
Figure 16. RST log results from the inelastic-capture operating mode, combined with openhole data, in a reservoir monitoring project.
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Examples
Detecting injection water breakthrough Figure 17 presents the RST log result from carbon-oxygen and sigma analysis, combined with openhole data, to monitor the progress of a flood project. The high formation water salinity, compared with the fresher injection water, provides a good contrast. The first track shows the fluid analysis. With porosity from openhole logs, oil saturation from carbon-oxygen measurements, and a known formation water salinity, the sigma measurements and the salinity indicator from capture yields provide sufficient information for determining volumes of both injection and formation water. Track 2 displays the uncorrected sigma from the RST
dual-burst sigma mode and a salinity indicator ratio (Cl/Cl+H) curve. Track 3 displays the bulk volume analysis. The log results indicate that the sweep of injection water has virtually depleted the oil in the zone from X320 to X375 ft. The interval above X320 ft shows little change in oil saturation but some replacement of formation water with injection water. This full diagnosis would not be possible with either carbon-oxygen measurements or thermal decay time measurements alone. The RST tool, however, can make both measurements with only one trip in the well.
Figure 17. RST presentation of carbon-oxygen and sigma analysis, combined with openhole data, to determine injection water breakthrough.
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Monitoring fluid contacts Figure 18 displays both the GST and RST log results in a cased monitor well in a sandstone formation. The well has 5-in., 18-lbm/ft casing set 1 2-in. borehole. The fluid analysis from the in a 6 ⁄ GST log appears in the second track and the results from the RST measurements in the third track. Both logs show the oil-water contact at X560 ft, but the improved precision with the RST tool is evident.
The original interpretation from openhole logs appears in the fourth track. The change in the depth of the gas-oil contact—between the time of the openhole logs and the RST monitor log—is obvious. The left track shows the CNL neutron porosity and the near-far inelastic count rate ratio (CRRA) from the RST measurement. The CRRA curve tracks a recent NPHI curve and provides an excellent gas indicator. The acquisition of CRRA and C/O data on one trip in the well makes additional logging runs to locate the gas cap unnecessary.
Figure 18. Comparison of a GST and an RST log in a sandstone reservoir. The oil-water contact and the gas cap are clearly visible from the fluid analysis and from the CRRA curve.
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Examples
Summary The new through-tubing RST tool uses a dualdetector spectrometry system to record carbonoxygen measurements for simultaneous evaluation of the oil saturation in the formation and, with the 1 2 ⁄ 2-in. tool, the oil/water fraction in the borehole. This evaluation is particularly important in cased wells where the formation water salinity is very low or unknown—conditions where TDT interpretation methods are unreliable. The measurement is also very useful for monitoring reservoirs in waterfloods of contrasting salinity, measuring residual oil saturation and monitoring the success of enhanced recovery methods. The slim RST tool size eliminates the need to kill the well and pull tubing, minimizing the associated risks and lost production. The interpretation is more reliable because the effect of invasion by the kill fluids is eliminated, reducing the need for special monitoring wells. Two RST tool sizes are available for inelasticcapture and sigma measurements—a 1 11 ⁄ 16-in. ver1 2-in. sion for logging below 2 3 ⁄ 8-in. tubing and a 2 ⁄ 1 tool for operations below 3 ⁄ 2-in. and larger tubing. The larger tool has special detector shielding that permits its use not only in static but also in flowing wells. Wells can be logged under dynamic conditions, reducing the production lost during the operation. The RST tools can be run on small-diameter coaxial cables and are combinable with production logging sensors for a complete reservoir monitoring program with only one trip in the well. These capabilities result in time savings, minimum lost production, reduced operational risks and better, more extensive answers.
New GSO gamma ray detectors, together with new photomultiplier tube technology and advanced processing techniques, allow logging speeds comparable to or better than those of large-diameter carbon-oxygen tools that cannot run through tubing. The detector system does not require a Dewar flask, so there is no limit to the logging time up to rated temperatures. With the 1 2-in. tool, the dual detectors are arranged to 2 ⁄ allow compensation for borehole fluid, which eliminates the need for an independent measurement of borehole fluid composition. An improved neutron generator provides a stable and controlled neutron burst. Its almost perfectly square output improves separation of the carbon and oxygen gamma rays produced during the neutron burst from capture gamma rays produced after each burst. New compact, high-speed electronics significantly improve the counting rate capabilities for increased precision. Gamma ray spectra recorded by the near and far detectors are processed with a full-spectrum analysis procedure based on a least-squares technique. Standard response spectra are used to determine the contribution of each element to the measured spectrum. The dual-detector system and interpretation model of the RST tool—together with the very large characterization data base—provide a more accurate answer than can be obtained with any other carbon-oxygen logging tool.
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References and recommended reading 1.
2.
3.
4.
5.
6.
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McGuire JA, Rogers LT and Watson JT: “Improved Lithology and Hydrocarbon Saturation Determination Using the Gamma Spectrometry Log,” paper SPE 14465, presented at the 60th SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada, September 22–25, 1985. Hull RL: “The Muldoon Field: An Evaluation of Behind-Casing Pay Zones in a Freshwater Environment,” paper SPE 14464, presented at the 60th SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada, September 22–25, 1985. Morgan WD and Hertzog RC: “The Application of Induced Gamma-Ray Spectroscopy in Cased Hole Formation Evaluation in Sumatra, Indonesia,” presented at the 12th Annual Convention of the Indonesian Petroleum Association, June 7–8, 1983. “Cased Hole Reservoir Evaluation,” Schlumberger Well Evaluation Conference Nigeria 1985, Paris, France: Schlumberger (1985): 215–230. Woodhouse R and Kerr SA: “The Evaluation of Oil Saturation Through Casing Using Carbon/Oxygen Logs,” paper SPE 17610, presented at the SPE International Meeting on Petroleum Engineering, Tianjin, China, November 1– 4, 1988. Westaway P, Hertzog R and Plasek RE: “The Gamma Spectrometer Tool Inelastic and Capture Gamma-Ray Spectroscopy for Reservoir Analysis,” paper SPE 9461, presented at the 55th SPE Annual Technical Conference and Exhibition, Dallas, Texas, September 21–24, 1980.
References and recommended reading
7. Hertzog RC: “Laboratory and Field Evaluation of an Inelastic-Neutron-Scattering and Capture Gamma Ray Spectroscopy Tool,” SPE Journal 20 (October 1980): 327–333. 8. Freeman DW and Fenn CJ: “An Evaluation of Various Logging Methods for the Determination of Remaining Oil Saturation in a Mixed Salinity Environment,” paper SPE 17976, presented at the SPE Middle East Oil Technical Conference and Exhibition, Manama, Bahrain, March 11–14, 1989. 9. Johnston J and Hook P: “The Gamma to Sigma of Saturation,” Middle East Well Evaluation Review 6 (1989): 24–33. 10. Gilchrist WA Jr, Rogers LT and Watson JT: “Carbon/Oxygen Interpretation—A Theoretical Model,” Transactions of the SPWLA 24th Annual Logging Symposium, Calgary, Alberta, June 27–30, 1983, paper FF. 11. Cannon DE and LaVigne JA: “ThroughCasing Reservoir Evaluation,” SPE Formation Evaluation 2 (June 1987): 201–208. 12. Felder RD and Hoyer WA: “The Use of Well Logs to Monitor a Surfactant Flood Pilot Test,” Journal of Petroleum Technology 36 (August 1984): 1379–1391. 13. Neuman CH and Oden AL: “Cased-Hole Measurement of Residual Oil—San Joaquin Valley, California,” paper SPE 9918, presented at the SPE California Regional Meeting, Bakersfield, California, March 25–26, 1981. 14. Roscoe BA, Stoller C, Adolph RA, Boutemy Y, Cheeseborough JC III, Hall JS, McKeon DC, Pittman D, Seeman B and Thomas SR: “A New Through-Tubing Oil-Saturation Measurement System,” paper SPE 21413, presented at the SPE Middle East Oil Show, Bahrain, November 16–19, 1991.
15. Melcher CL, Schweitzer JS, Manente RA and Peterson CA: “Applicability of GSO Scintillators for Well Logging,” IEEE Transactions on Nuclear Science 38, no. 2 (April 1991): 506–509. 16. Roscoe BA, Grau JA and Wraight PD: “Statistical Precision of the Neutron-Induced Gamma-Ray Spectroscopy Measurements,” Transactions of the SPWLA 27th Annual Logging Symposium, Houston, Texas, June 9–13, 1986, paper CC. 17. Roscoe BA and Grau JA: “Response of the Carbon/Oxygen Measurement for an Inelastic Gamma Ray Spectroscopy Tool,” SPE Formation Evaluation 3 (March 1988): 76–80. 18. Scott HD, Stoller C, Roscoe BA, Plasek RE and Adolph RA: “A New Compensated Through-Tubing Carbon/Oxygen Tool for Use in Flowing Wells,” Transactions of the SPWLA 32nd Annual Logging Symposium, Midland, Texas, June 16 –19, 1991, paper MM.
19. Steinman DK, Adolph RA, Mahdavi M, Marienbach E, Preeg WE and Wraight PD: “Dual-Burst Thermal Decay Time Logging Principles,” paper 15437, presented at the 61st SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, October 5–8, 1986. 20. Olesen J-R, Mahdavi M, Steinman DK and Yver J-P: “Dual-Burst Thermal Decay Time Logging Overview and Examples,” paper 15716, presented at the 5th SPE Middle East Oil Show, Manama, Bahrain, March 7–10, 1987. 21. Stoller C, Scott HD, Plasek RE, Lucas AJ and Adolph RA: “Field Tests of a Slim Carbon/Oxygen Tool for Reservoir Saturation Monitoring,” paper 25375, presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Singapore, February 8–10, 1993. 22. Audah T and Chardac J-L: “Reservoir Fluid Monitoring Using Through-Tubing CarbonOxygen Tools,” Transactions of the SPWLA 34th Annual Logging Symposium, Calgary, Alberta, June 13–16, 1993, paper LL.
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Nomenclature
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BGO
bismuth germanate
MeV
million electron volts
cm
centimeter
NaI
sodium iodide
cm3
cubic centimeter
nsec
nanosecond
ft
foot
φ
porosity
g
gram
p.u.
porosity unit
g/cm3
gram per cubic centimeter
S o
oil saturation
GSO
gadolinium oxy-ortho silicate
s.u.
saturation unit
in.
inch
yo
oil holdup
KeV
thousand electron volts
yw
water holdup
lbm
pound (mass)
Nomenclature
µsec
microsecond
