Conventional Wireline Logging Operations in the
 Gulf of Mexico Gas Hydrate JIP Drilling Program

Conventional wireline (CWL) logging operations in the Gulf of Mexico Gas Hydrate JIP Drilling Program (GOM-JIP) was scheduled to include the deployment of a signal logging string (Figure 1) and a vertical seismic profiling (VSP) tool (Figure 2) in several of the Atwater Valley and Keathley Canyon drill sites. The only wireline logging tool scheduled to be deployed was the FMS-sonic tool string, which consisted of the Formation MicroScanner (FMS), a general purpose inclinometer tool (GPIT), and scintillation gamma ray tool (SGT), and the dipole shear sonic imager tool (DSI). The vertical seismic imager tool (VSI) will also be deployed during the GOM-JIP drilling program. The wireline logging tools were provided by Schlumberger wireline services.

Early in the planning phase for the GOM-JIP drilling project, considerable effort was made to assess the use of existing Logging While Drilling (LWD) acoustic logging technology for logging near-surface, relatively acoustically “slow”, formations. It was decided that emerging quadrapole acoustic LWD logging technology may theoretically yield both compressional- and shear-wave data form these slow formations, it could not be conclusively proven that we would acquire the needed acoustic data. Thus, it was decided to move ahead with plans for a conventional wireline logging program with the DSI, which has been used in the past to obtain both compressional- and shear-wave acoustic log data in very slow formations during ODP and IODP operations.

Dipole Shear Sonic Imager Tool
The DSI tool employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of formations. In addition to a robust and high-quality measurement of compressional wave velocity, the DSI excites a flexural mode in the borehole that can be used to estimate shear-wave velocity even in highly unconsolidated formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in un-consolidated sediments, the flexural wave travels at the shear-wave velocity and is the most reliable way to estimate a shear velocity log. Meanwhile, the omni-directional source generates compressional, shear, and Stoneley waves into hard formations. The configuration of the DSI also allows recording of both in-line and cross-line dipole waveforms. In many cases the dipole sources can yield estimates of shear wave velocity in hard rocks better than or equivalent to the monopole source. These combined modes can be used to estimate shear-wave splitting caused by preferred mineral and/or structural orientation in consolidated formations. A low-frequency (80 Hz) source enables Stoneley waveforms to be acquired as well.

DSI measures the transit times between sonic transmitters and an array of eight receiver groups with 15-cm spacing, each consisting of four orthogonal elements that are aligned with the dipole transmitters. During acquisition, the output from these 32 individual elements are differenced or summed appropriately to produce in-line and cross-line dipole signals or monopole-equivalent (compressional and Stoneley) waveforms, depending on the operation modes. In the GOM-JIP drilling program we followed standard GOM practices in ran the DSI in BCR (Expert) mode.

Formation MicroScanner Tool
The FMS produces high-resolution images of borehole wall micro-resistivity that can be used for detailed sedimentologic or structural interpretation. This tool has four orthogonally oriented pads, each with 16 button electrodes that are pressed against the borehole walls. Good contact with the borehole wall is necessary for acquiring good-quality data. Approximately 30% of a borehole with a diameter of 25 cm is imaged during a single pass. The vertical resolution of FMS images is ~5 mm, allowing features such as burrows, thin beds, fractures, veins, and vesicles to be imaged. The resistivity measurements are converted to color or grayscale images for display. In site chapters in this volume local contrasts in FMS images were improved by applying dynamic normalization to the FMS data. A linear gain is applied, which keeps a constant mean and standard deviation within a sliding window of 1 m. FMS images are oriented to magnetic north using the GPIT (General Purpose Inclinometer Tool). This allows the dip and strike of geological features intersecting the hole to be measured from processed FMS images. FMS images can be used to visually compare logs with the core to ascertain the orientations of bedding, fracture patterns, and sedimentary structures and to identify stacking patterns, and in some cases to identify gas-hydrate-bearing sedimentary sections.

General Purpose Inclinometer Tool
The GPIT is included in the FMS-sonic tool string to calculate tool acceleration and orientation during logging. The GPIT contains a triple-axis accelerometer and a triple-axis magnetometer. The GPIT records the orientation of the FMS images and allows more precise determination of log depths than can be determined from cable length, which may experience stretching and/or be affected by ship heave.

Hostile Environment Spectral Gamma Ray Sonde and Scintillation Gamma Ray Tool
The HNGS measures the natural gamma radiation from isotopes of potassium (K), thorium (Th), and uranium (U) and a five-window spectroscopy to determine concentrations of radioactive K (in weight percent), Th (in parts per million), and U (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The spectral analysis filters out gamma ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy. Corrections to the HNGS account for variability in borehole size and borehole potassium concentrations. The HNGS also provides a measure of the total gamma ray emission SGR (API units), and the uranium-free or computed gamma ray CGR (API units). All of these effects will be corrected for during processing of HNGS data at LDEO-BRG.

The SGT uses a sodium-iodide (NaI) scintillation detector to measure the total natural gamma ray emission, combining the spectral contributions of K, U, and Th concentrations in the formation. The SGT is not a spectral tool but provides high-resolution total gamma ray data for depth correlation between logging strings. It is included in all tool strings (except the triple combo, where the HNGS is used) to provide a reference log to correlate and depth between different logging runs. With the FMS-DSI tool string, the SGT is placed between the two tools, providing correlation data to a deeper level in the hole.

Vertical Seismic Imager (VSI)
The Vertical Seismic Imager (VSI-4) is a borehole seismic wireline tool optimized for obtaining vertical and walkaway seismic profiles (VSP; W-VSP) in both cased hole and open hole, vertical, and deviated wells. The VSI consists of mulitple three-axis geophones in series separated by "hard wired", acoustically-isolating spacers. A schematic illustration of the tool is given in Figure 2. The tool diameter is 3 3/8 inches, with temperature and pressure ratings to 175 °C and 20,000 psi, respectively.

During the GOM-JIP drilling program, the VSI was configured using three geophone shuttles (15 m spacing with rigid interconnections) and combined with the SGT natural gamma tool. Only vertical incident or zero-offset VSP experiments were conducted during the GOM-JIP drilling program. For vertical incidence VSP operations, the shuttle was mechanically clamped against the borehole wall and sources (1520 cubic inch guns in a Dual Itaga Air Gun Array) on the Uncle John were fired between 5 and 15 times by control hardware in the Schlumberger logging unit. The VSI tool was then unclamped and pulled 7.06 m uphole, maintaining a 7.06 m receiver station depth spacing throughout the hole. The VSI records the full seismic waveform for each firing. These waveform data are stacked by the Schlumberger recording software and output in both LDF (internal Schlumberger format) and SEG-Y formats.

Logging Data Flow and Processing

Data for each logging run will be recorded and stored digitally and monitored in real time as the data was acquired. After logging was completed, the data were transferred first to Schlumberger wireline services for compilation and data quality check. The final and complete field data sets will be transferred to the LDEO-BRG for processing.

Logging data quality may be seriously degraded by changes in the hole diameter and in sections where the borehole diameter greatly decreases or is washed out. Deep-investigation measurements such as resistivity and sonic velocity are least sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data in order to reduce these effects. The effects of very large washouts, however, cannot be corrected. Logs from the LWD and CWL tool strings will have minor depth mismatches caused by that fact that the data was obtained in two different holes at each site surveyed. A gamma ray log has been included in each tool run to correlate the log data between each at hole within a drill site. Because of technical difficulties, the CWL surveys were conducted without heave compensation. In the case of the Atwater Valley LWD holes, the drill-string heave compensator was not used during LWD operations.

Gas Hydrate Detection and Evaluation

With growing interest in natural gas hydrate, it is becoming increasingly important to be able to identify the occurrence of in-situ gas hydrate and accurately assess the volume of gas hydrate and included free gas within gas-hydrate accumulations. Numerous publications (Mathews, 1986; Collett, 1993, 1998a, 1998b, 2001; Goldberg, 1997; Guerin et al., 1999; Goldberg et al., 2000; Helgerud et al., 2000) have shown that downhole geophysical logs can yield information about the occurrence of gas hydrate.

Since gas hydrates are characterized by unique chemical compositions and distinct electrical resistivities, physical and acoustic properties, it is possible to obtain gas-hydrate saturation (percent of pore space occupied by gas hydrate) and sediment porosity data by characterizing the electrical resistivity, acoustic properties, and chemical composition of the pore-filling constituents within gas-hydrate-bearing reservoirs. Two of the most difficult reservoir parameters to determine are porosity and the degree of gas-hydrate saturation. Downhole logs often serve as a source of porosity and hydrocarbon saturation data. Most of the existing gas hydrate log evaluation techniques are qualitative in nature and have been developed by the extrapolation of untested petroleum industry log evaluation procedures. To adequately test the utility of standard petroleum log evaluation techniques in gas-hydrate-bearing reservoirs would require numerous laboratory and field measurements. However, only a limited number of gas hydrate occurrences have been sampled and surveyed with open-hole logging devices.

Reviewed below are downhole log measurements that yield useful gas hydrate reservoir information. The downhole measurements considered include gamma-gamma density, neutron porosity, electrical resistivity, acoustic transit-time, and nuclear magnetic resonance.

Gamma-Gamma Density Logs
Density logs are primarily used to assess sediment porosities. The theoretical bulk-density of a Structure-I methane hydrate is about 0.9 g/cm3 (Sloan, 1998). Gas hydrate can cause a small but measurable effect on density-derived porosities. At relatively high porosity (>40%) and high gas-hydrate saturation (>50%), the density-log-derived porosities need to be corrected for the presence of gas hydrate (Collett, 1998b).

Neutron Porosity Logs
Neutron logs are also used to determine sediment porosities. Since Structure-I methane hydrate and pure water have similar hydrogen concentrations it can be generally assumed that neutron porosity logs, which are calibrated to pure water, are not significantly affected by the presence of gas hydrates. At high reservoir porosities, however, the neutron porosity log could overestimate porosities (Collett, 1998b).

Electrical Resistivity
Water content and pore-water salinity are the most significant factors controlling the electrical resistivity of a formation. Other factors influencing resistivity of a formation include the concentration of hydrous and metallic minerals, volume of hydrocarbons including gas hydrates, and pore structure geometry. Gas-hydrate-bearing sediments exhibit relatively high electrical-resistivities in comparison to water-saturated units, which suggests that a downhole resistivity log can be used to identify and assess the concentration of gas hydrates in a sedimentary section. The relation between rock and pore-fluid resistivity has been studied in numerous laboratory and field experiments. From these studies, relations among porosity, pore-fluid resistivity, and rock resistivity have been found. Among these findings is the empirical relation established by Archie (Archie, 1942), which is used to estimate water saturations in gas-oil-water-matrix systems. Research has shown that the Archie relation also appears to yield useful gas-hydrate saturation data (reviewed by Collett, 2001).

Acoustic transit-time
The velocity of compressional and shear acoustic waves in a solid medium, such as gas-hydrate-bearing sediment, is usually several times greater than the velocity of compressional and shear acoustic waves in water or gas-bearing sediments. Studies of downhole acoustic log data from both marine and permafrost associated has hydrate accumulations have shown that the volume of gas hydrate in sediment can also be estimated by measuring interval velocities (Guerin et al., 1999; Helgerud et al., 2000; Collett, 2001; Guerin and Goldberg, 2002).

Nuclear Magnetic Resonance Logs
Nuclear magnetic resonance (NMR) logs use the electromagnetic properties of hydrogen molecules to analyze the nature of the chemical bonds within pore-fluids. Relative to other pore-filling constituents, gas hydrates exhibit unique chemical structures and hydrogen concentrations. In theory, therefore, it should be possible to develop NMR well-log evaluation techniques that would yield accurate reservoir porosities and water saturations in gas-hydrate-bearing sediments. Because of tool design limitations, gas hydrates cannot be directly detected with today's downhole NMR technology; however, they can be useful to yield very accurate gas-hydrate saturation estimates. Due to the short transverse magnetization relaxation times (T2) of the water molecules in the clathrate, gas hydrates are not "seen" by the NMR tool and may be assumed to be part of the solid matrix. Thus, the NMR-calculated total porosity in a gas-hydrate-bearing sediment should be lower than the actual porosity. With an independent source of accurate total porosity, such as density- or neutron-porosity-log measurements, it should be possible to accurately estimate gas-hydrate saturations by comparing the apparent NMR-derived porosity to the total density-derived porosity.

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Conventional Wireline and Vertical Seismic Profiling Tools

Signal Logging String	Vertical Seismic Profiling (VSP) Tool

References

Archie, G.E., The electrical resistivity log as an aid in determining some reservoir characteristics. Journal of Petroleum Technology, v. 5, p. 1-8, 1942.

Collett, T.S., Natural gas hydrates of the Prudhoe Bay and Kuparuk River area, North Slope, Alaska: American Association of Petroleum Geologists Bulletin, v. 77, no. 5, p. 793-812, 1993.

Collett, T.S., Well log evaluation of gas hydrate saturations. Transactions of the Society of Professional Well Log Analysts, Thirty-Ninth Annual Logging Symposium, May 26-29, 1998, Keystone, Colorado, USA, Paper MM, 1998a.

Collett, T.S., Well log characterization of sediment porosities in gas-hydrate-bearing reservoirs. Proceedings of the 1998 Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, September 27-30, 1998, New Orleans, Louisiana, USA, 12 p. (CD-ROM), 1998b.

Collett, T.S., A review of well-log analysis techniques used to assess gas-hydrate-bearing reservoirs: In Natural Gas Hydrates: Occurrence, Distribution, and Detection, American Geophysical Union, Geophysical Monograph 124, p. 189-210, 2001.

Goldberg, D., The role of downhole measurements in marine geology and geophysics. Review of Geophysics, v. 35, no. 3, p. 315-342, 1997.

Goldberg, D., Collett. T.S., and Hyndman, R.D., Ground truth: in-situ properties of hydrate. in Max, M.D., ed., Natural Gas Hydrate in Oceanic and Permafrost Environments, Kluwer Academic Publishers, The Netherlands, p. 295-310, 2000.

Guerin, G., Goldberg, D., and Melster, A., Characterization of in situ elastic properties of gas hydrate-bearing sediments on the Blake Ridge. Journal of Geophysical Research, v. 104, 17,781-17,795, 1999.

Guerin, G., and D. Goldberg, Sonic attenuation measurements in the Mallik 2L-38 gas hydrates research well, MacKenzie Delta, NWT Canada, Journal of Geophysical Research, v. 107, 2002.

Helgerud, M.B., Dvorkin, J., and Nur, A., Rock physics characterization for gas hydrate reservoirs, elastic properties. In Holder, G.D., and Bishnoi, P.R., eds., Gas Hydrates, Challenges for the Future, Annals of the New York Academy of Sciences, v. 912, p. 116-125, 2000.

Mathews, M., Logging characteristics of methane hydrate. The Log Analyst, v. 27, no. 3, p. 26-63, 1986.

Sloan, E.D., 1998. Clathrate hydrates of natural gases. Marcel Dekker Inc. Pub., New York, pp. 641.