International Conference on Gas Hydrates (ICGH) (6th : 2008)


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 DEVELOPMENTS IN GEOPHYSICAL WELL LOG ACQUISITION AND INTERPRETATION IN GAS HYDRATE SATURATED RESERVOIRS   Doug Murray∗ Schlumberger Oilfield Services 6th floor, Block A2, Lido Office Tower, JiChang Road, Beijing, CHINA  Tetsuya Fujii Technology & Research Center, Japan Oil, Gas and Metals National Corporation 1-2-2 Hamada, Mihama-ku, Chiba, 261-0025, JAPAN  Scott Dallimore Geological Survey of Canada, Natural Resources Canada 9860 West Saanich Road, Sidney, BC V8L 4B2, CANADA  ABSTRACT There has been a dramatic increase in both the amount and type of geophysical well log data acquired in gas hydrate saturated rocks. Data has been acquired in both offshore and Arctic environments; its availability has shed light on the applicability of current tools and the potential usefulness of recently developed and developing technologies. Some of the more interesting areas of interest are related to the usefulness of nuclear elemental spectroscopy data and the comparison of thermal and epithermal neutron porosity measurements, the measurement of in-situ permeability, the interpretation of electrical borehole image and borehole sonic data. A key parameter for reservoir characterization and simulation is formation permeability. A reasonable understanding of this property is key to the development of future gas hydrate production. Typical applications of borehole image data are an appreciation of a reservoir’s geological environment. In hydrate saturated reservoirs, borehole images can also be used to assist in the understanding of the gas migratory path to the hydrate bearing formation. This paper presents a review of some of the current state of the art geophysical log measurements and their application in hydrate saturated reservoirs..  Keywords: geochemical spectroscopy, epithermal neutron, thermal neutron, anisotropy, permeability, clay volume.         _______________________ * Corresponding author: Phone +86 10 6474 6699 Fax: +86 10 6436 7532 ∗∗  Present address: Schlumberger Oilfield Services 6th floor, Block A2, Lido Office Tower, JiChang Road, Beijing, CHINA Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.   NOMENCLATURE Sw Water saturation [%] Shydrate Hydrate saturation [%] φ  Porosity [pu] φNma Thermal Neutron matrix [pu] φEpiNma Epithermal Neutron matrix [pu] Σma Sigma matrix [cu] ρma Matrix density [g/cm3] Vcl Volume Clay [%] k permeability [mD]  INTRODUCTION The importance of the acquisition of key petrophysical parameters like porosity (φ), water saturation (Sw), clay volume (Vcl) and permeability (k) is well understood by the exploration and production industry. Derivation of  these important parameters from geophysical well log investigations has evolved considerably since Archie first developed his empirical approaches over 60 years ago [1]. In certain environments, estimates of these parameters have been reasonably ascertained but for others environmental and geological conditions complicate the measurement and make the derivation of these petrophysical parameters problematic. In methane hydrate saturated reservoirs, with the use of traditional formation evaluation measurements; it can be difficult to establish accurate values for the petrophysical parameters mentioned above. More recent technologies such as magnetic resonance, nuclear spectroscopy and advanced borehole sonic measurements have vastly improved their derivation and are shedding light on other items of interest to methane hydrate production, providing geoscientists with more accurate reserve estimates and enhanced understanding of production viability. The following summarizes some of the newer and not so new technologies with a focus on nuclear spectroscopy, magnetic resonance, epithermal neutron and thermal neutron, borehole imaging and borehole sonic, and their applications to open hole methane hydrate reservoir characterization. Examples are drawn from the JOGMEC/NRCan/Aurora Mallik gas hydrate production research program recently conducted in the Mackenzie Delta and from the 2004 Field investigations in the Nankai trough. More detailed papers on the research and development studies ongoing at Mallik include a description of field operations [2], well log characteristics [3], geophysical monitoring techniques employed [4], and numerical modeling scenarios [5].  NUCLEAR ELEMENTAL SPECTROSCOPY An accurate understanding of mineralogy is important in porosity evaluation for matrix density (ρma), neutron porosity matrix (φNma), sigma matrix (Σma) and to estimate the clay mineral fraction to correct resistivity-based saturations for the effects of excess clay conductivity. Vcl estimates from traditional clay indicators such as gamma ray, SP and thermal neutron - formation density porosity separation are often misleading. In formations having light hydrocarbons such as gas or condensate, density- neutron logs under-estimate Vcl due to light hydrocarbon effects. Also, if realistic values of matrix density and neutron matrix porosity are not known then the computed neutron and density porosities may not show the signature light hydrocarbon cross-over effect and potential pay can be overlooked [6]. Conversely for methane hydrate reservoirs, the presence of methane hydrate causes a slight increase in both the neutron and density porosities. As for the case of light hydrocarbons this fluid affect needs to be considered when attempting to estimate Vcl. With traditional methods, the mineralogy in the hydrate bearing formations in Mallik complicate the estimation of Vcl, In this environment, nuclear spectroscopy data allows for more precise estimates of Vcl, ρma, φNma φEpiNma and Σma. Figure 1 shows the computed nuclear spectroscopy result over hydrate and non-hydrate saturated intervals in a well in Mallik. Track 1 contains the nuclear spectroscopy computed lithology, percent of clay (grey), combination of quartz-feldspar-mica (yellow), carbonate (blue), pyrite (orange) and coal (black). Track2 has the nuclear spectroscopy computed estimates for ρma, φNma  and φEpiNma while Track 3 has the computed estimate of Σma. The presence of hydrate cannot be identified from this log as the measurement responds only to reservoir lithology, not to fluid content. However one can observe that the hydrate saturated interval contains less clay than the surrounding zones.    Figure 1  Well log from Mallik showing interpreted mineralogy based on  nuclear spectroscopy. Also shown are estimates of ρma, φNma φEpiNma and Σma. The nuclear spectroscopy measurement responds to matrix lithology only, the presence of hydrate has no affect. However one can observe that the hydrate saturated interval has less clay volume than the zones immediately above and below.  EPITHERMAL NEUTRON A combination of formation density and magnetic resonance (MR) measurements, allow estimation of reservoir porosity. In methane hydrate reservoirs it is particularly useful to compute both accurate estimates of porosity (φ) and methane hydrate saturation (Shydrate) [7] and [8]. Similarly, in-situ gas hydrate saturated reservoir porosity and saturation can be estimated from a combination of epithermal neutron and MR log data. The key advantages of estimating gas hydrate reservoir porosity and saturation from the epithermal neutron - MR combination is reduced rugose borehole measurement affects, and in the case of pulsed neutron generator (PNG) tools, the elimination of a nuclear chemical source. Pulsed neutron tools generate neutrons on demand and eliminate the need for an americium beryllium (Am241 Be) chemical source, substantially reducing operational and transportation risk. Similar to MR measurements, neutron porosity measurements respond primarily to a formation’s hydrogen index. For gas reservoirs the use of a combination of neutron and MR measurements to estimate formation porosity is not practical as both tools respond primarily to formation hydrogen index (HI). In gas saturated reservoirs HI decreases and as such porosity responses of both tools are similar. In the case of methane hydrate saturated reservoirs neutron porosity measurements show a slight increase as the HI of methane hydrates is 1.05, or slightly greater than water at 1.0. As for the density porosity, the presence of methane hydrate causes the neutron porosity to read slightly higher than that for a 100% water saturated reservoir. Due to gas hydrates relatively fast MR relaxation time, gas hydrate volumes are invisible to MR formation porosity measurements. Hence, in gas hydrate saturated reservoirs, neutron and MR measurements can be combined to accurately quantify reservoir porosity and gas hydrate saturation. There are additional affects on the thermal neutron measurement due to the presence of thermal neutron absorbers such as boron, chlorine, gadolinium, and formation density. These affects cause the neutron porosity measurement to read higher than formation porosity and need to be considered. In order to account for these affects an epithermal neutron porosity measurement is advised. The Schlumberger Accelerator Porosity Sonde (APS) tool has an epithermal neutron measurement. It contains a high energy PNG source with detector configurations that minimize neutron absorber and density affects [9], [10] and [11]. The accuracy of the porosity and saturation computations is further improved with the acquisition of nuclear elemental spectroscopy data as described previously. Accurate values of φEpiNma can be derived from the nuclear spectroscopy measurement. Figure 2 shows the open hole logs over the same hydrate and non-hydrate intervals as Figure 1. Track 1 contains the gamma ray (green) and caliper curves. Track 2 has the shallow (green) and deep resistivity (red) curves. Track 3 has the thermal neutron (green), epithermal neutron (blue), formation density (red) and MR porosity (black) while Track 4 has the computed formation volumetrics.  Hydrate Saturated Zone Hydrate Free Zone  One can observe the resistivity increase in the presence of hydrates, that all four porosity curves have different values, and that there is a dramatic reduction in MR porosity when hydrate is present. The porosity values presented are corrected for environment conditions, the lithology is assumed to be 100% sandstone. Notice that the epithermal neutron records a similar porosity, but not the same as the density porosity. The thermal neutron for reasons mentioned above records a much higher porosity. Figure 2  Open hole logs from Mallik over hydrate and non-hydrate intervals. All four porosity curves have different values; there is a dramatic reduction in MR porosity when hydrate is present.  Figure 3 shows the thermal neutron, epithermal neutron and density porosities corrected for lithology effects via inputs from the nuclear spectroscopy log. There is a large reduction in the thermal neutron porosity. Also, as one could expect, the epithermal neutron and formation density porosities are almost identical. This suggests that the combination of lithology corrected epithermal neutron and MR porosity measurements could be used to estimate methane hydrate saturated reservoir porosity and saturation similarly to the density-MR (DMR) approach [12]. Figure 3  Open hole logs over hydrate and non- hydrate intervals. Compared to Figure 2 the thermal neutron porosity is dramatically reduced. The epithermal neutron and density porosity values overlay.  METHANE HYDRATE RESERVOIR PERMEABILITY In-situ values of gas hydrate saturated rock intrinsic permeability are critical input parameters for reservoir characterization, reservoir simulation, the understanding of hydrate production and the determination of the most economic method of production. Multiple studies have shown that original in-situ intrinsic permeability can be reasonably estimated from a derivation of magnetic resonance log data. [7] and [13]. As hydrate is a near impermeable solid, hydrate reservoirs have the unique property in that their permeability is heavily dependent on hydrate saturation. As hydrate is produced, less hydrate fills the pore space and as such overall reservoir permeability increases. To more fully understand hydrate reservoir behavior with production one needs to characterize the relationship between reservoir intrinsic permeability and hydrate saturation. Laboratory attempts at this Hydrate Saturated Zone Hydrate Free Zone Hydrate Saturated Zone Hydrate Free Zone  characterization have been made with sand samples and synthetically generated hydrate [14] and [15]. A useful in-situ approach to assist with the understanding of how in-situ permeability may change with hydrate saturation is to establish minimum and maximum intrinsic permeability endpoints. MR can used to estimate the minimum permeability [13] and the geochemical nuclear spectroscopy lithology measurement can be used to estimate the reservoir rock’s maximum permeability for the case of no hydrate (Sw =100% and/or Shydrate  = 0%) [16]. Figure 4 shows the results of this computation on the Mallik well for the interval shown previously. As expected the computed intrinsic permeabilities for both the lithology (black) and MR (red) based approaches are similar in the non- hydrate interval but diverge noticeably in the hydrate saturated rock. The different permeability estimates in the hydrate saturated rock establish the intrinsic permeability upper and lower bounds.  Figure 4  Comparison of the computed intrinsic permeabilities for both the lithology (black) and MR (red) based approaches. The different permeability estimates in the hydrate saturated rock establish the intrinsic permeability upper and lower bounds.  BOREHOLE IMAGES The use of electrical borehole image logs for geological interpretation is well known. In addition to the traditional applications such as structural and sedimentological interpretation, the evidence of fractures and faults from borehole image logs in the presence of hydrate saturated rocks can be used to highlight potential migratory paths. Kleinberg suggested that an accumulation of free gas can open a fracture in sediment above it and would occur when the free gas pressure exceeded the strength of the overlying sediment [17], [18], [19] and [20]. When the flux of free gas is substantial, gas conduits could be expected to remain open and move gas significant distances. Hydrate will form rapidly at fracture surfaces, stiffening the channel and allowing gas to flow through it without contacting liquid water. The fracture will propagate upwards as long as the gas pressure remains high enough to overcome the cohesion and the stress normal to the fracture plane [20]. It is instructive to review borehole image logs for possible evidence of fractures. Figure 5 contains an image example from the Nankai Trough, offshore Japan. It suggests that a high angle event occurs in the middle of a hydrate saturated zone. This is consistent with the hypothesis that the gas migratory path for some gas hydrate deposits is due to high angle fractures.  Hydrate Saturated Zone Hydrate Free Zone   Figure 5  Borehole image from the Nankai Trough, offshore Japan. High angle event occurring in middle of a hydrate saturated zone. Track 1 has the GR, Track 2 the resistivity and Track 3 the borehole image. Track 4 has the dip angle computation for the sinusoids overlaid on the image displayed in Track 3.  BOREHOLE SONIC Acoustic borehole measurements have been available to the oilfield since the 1950's. The first measurements were used to convert and/or calibrate surface seismic velocities to depth. Until recently, most of the interpretative and processing techniques were limited to the slowness, time and amplitude domains. Lately, the practice of presenting the data in the slowness versus frequency domain has gained acceptance and has been referred to as sonic dispersion analysis. It is particularly useful for analyzing and interpreting dispersive borehole modes for estimating both near-wellbore and far-field formation parameters [21]. These new approaches were used previously in a hydrate research well off the east coast of Japan. Reservoir far-field stresses were estimated from sonic dipole data, information that is of critical importance to wellbore design and optimal reservoir production [22]. The borehole sonic response in a hydrate saturated interval in Mallik was previously described by Plona [23]. A complete suite of sonic data was acquired including compressional-wave, Stoneley-wave, and four-component shear-wave with cross-dipole. Plona suggested that the hydrate bearing sandstone had low compressional-wave and shear-wave slownesses (fast sound speeds) and behaved in a homogeneous, isotropic manner. And that in contrast, the water-bearing sandstone section exhibited higher compressional-wave and shear-wave slownesses (slow sound speeds), stress-induced anisotropy, and mechanical damage around the borehole. The example described by Plona referred to data acquired with a previous generation sonic tool, the Schlumberger dipole shear sonic imager (DSI). The following describes the information obtained in a similar hydrate saturated interval with the more advanced Schlumberger Sonic Scanner tool. The key conclusions are that stress- induced shear anisotropy exists in both the hydrate and non- hydrate saturated intervals and that radial variation of slowness is strongest in the non-hydrate bearing intervals. The Schlumberger Sonic Scanner measurement offers a much improved signal noise ratio (SNR) and thus is better able to measure small amounts of acoustic anisotropy. Figure 6 highlights the recent Sonic Scanner log acquired in Mallik. Hydrate Saturated Zone   Figure 6  Recent Anisotropy log computed from Sonic Scanner cross dipole data. Track 1 illustrates the minimum and maximum cross energies, which are indicators of waveform energy anisotropy. Both are small in isotropic zones, but in an anisotropic zone the maximum cross energy increases. Track 2 plots the gamma ray and borehole caliper log. Track 3 shows the direction of the fast shear azimuth. Track 4 contains the fast- and slow-shear slownesses, and Track 5 displays the fast- and slow-shear waveforms.  In the non-hydrate saturated zone one can clearly see acoustic anisotropy – large amount of cross-energy, slow shear slowness is noticeably slower than the fast shear. In the hydrate saturated interval the acoustic anisotropy exists but is substantially reduced. This observation appears contradictory to Plona’s description of hydrate saturated intervals being isotropic. This difference is entirely due to differences in measurement accuracy between different generations of borehole sonic tools. Unequal stresses in the cross- sectional plane of the wellbore cause stress- induced anisotropy that exhibits a characteristic dipole dispersion crossover. This stress-induced crossover phenomenon is a result of near-wellbore stress concentrations [24], [25] and [26]. At low frequencies, the dipole flexural waves probe deep into the formation and sense the far- field stress. The dipole polarization along the maximum stress direction senses the higher stress and has the lower slowness (fast speed). At higher frequencies, the flexural waves probe the near- wellbore region and are mostly influenced by stress concentrations close to the borehole. In this case, the dipole polarization along the maximum far-field stress direction senses a lower stress and hence has the higher slowness (slow speed) [27]. Figures 7a and 7b show the dipole sonic dispersion data for two points from Figure 6; i.) in the hydrate saturated interval and ii.) in the non- hydrate saturated interval. In the hydrate saturated interval it is clear from the dipole dispersion curves shown in the left hand side of Figure 7a that a small amount of acoustic anisotropy exists and due to cross-over that the source of this anisotropy is unequal horizontal stresses [21]. The right hand side of Figure 7a shows the slowness change as a function of radial depth from the borehole into the formation. This slowness versus radial depth display is also consistent with stress-induced anisotropy. The slownesses are slow near to the wellbore and become faster with increases in radial depth. Also near to the wellbore the fast and slow slownesses cross-over.  Figure 7a  Slowness dispersion and radial slowness curves from hydrate saturated interval depicted in Figure 6.  Figure 7b is taken from the non-hydrate saturated interval. Here, due to the absence of hydrate (which effectively strengthens the rock), the stress induced anisotropy (fast and slow slowness cross-over) is significantly larger.  Hydrate Saturated Zone Hydrate Free Zone  Figure 7b  Slowness dispersion and radial slowness curves for non-hydrate saturated interval, as depicted in Figure 6.  A clearer picture of radial shear slownesses variation with well depth is shown in Figure 8. Track 2 plots the gamma ray and borehole caliper log. Track 3 the slowness variation with radial depth away from the borehole in the fast shear direction, Track 4, the petrophysical volumetric analysis and Track 5 the slowness variation with radial depth away from the borehole in the slow shear direction. In Tracks 3 and 5 the red shading indicates that the shallow DT-Shear slowness is 25% higher (slower velocity) than the deep. Most probably the sands have undergone mechanical alteration caused by the placement of the well [28] and [29].  Figure 8  Slowness Radial Variation Log – The red shading indicates that the shallow depth of investigation DT-Shear is 25 % higher (slower velocity) than the deep (green shading). The sands have become weaker near the well.  CONCLUSIONS Many existing and new state of the art formation evaluation technologies have application to the evaluation of methane hydrate reservoirs. It is envisioned that the continual development of these and other technologies will enhance methane hydrate formation evaluation.  ACKNOWLEDGEMENTS Many of the figures used in this paper have been published with the permission of the JOGMEC/NRCan/Aurora Mallik Gas Hydrate Production Research Program. This research was conducted under the MH21 research consortium funded by the Japan Ministry of Economy, Trade and Industry (METI). The author thanks the MH21 members: Japan Oil, Gas and Metals National Corporation (JOGMEC), the National Agency of Advanced Industrial Science and Technology (AIST), the Engineering Advancement Association (ENAA), and their personnel for permission to publish this paper. Parts of the study were carried out with the cooperation of Schlumberger Oilfield Services.  REFERENCES [1] Archie, G.E., The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics, Petroleum Transactions of the AIME 146, pp 54-62, 1942. [2] Numasawa, M., Dallimore, S.R., Yamamoto, K., Yasuda, M., Imasato, Y., Mizuta, T., Kurihara, M., Masuda, Y., Fujii, T., Fujii, K., Wright, J. F., Nixon, F.M, Cho, B., Ikegami, T., Sugiyama, H., Objectives and Operation Overview of the JOGMEC/NRCan/Aurora Mallik Gas Hydrate Production Test, in Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, B.C. July 6-11, 2008.. [3] Fujii T., Takayama T., Dallimore S.R., Nakamizu M., Mwenifumbo J., Kurihara M., Yamamoto K., Wright J.F. and Al-Jubori A.  Wire- line Logging Analysis of the JOGMEC/NRCan/Aurora Mallik Gas Hydrate Production Test; in Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, B.C. July 6-11, 2008. [4] Fujii, K., Cho, B., Ikegami, T., Sugiyama, H., Imasato, Y., Dallimore, S.R. and Yasuda, M., Development of a monitoring system for the JOGMEC/NRCan/Aurora Mallik Gas Hydrate Production Test Program, in Proceedings of the Hydrate Saturated Zone Hydrate Free Zone  6th International Conference on Gas Hydrates, Vancouver, B.C. July 6-11, 2008.. 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Collett (Editors), Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well, Mackenzie Delta, Northwest Territories, Canada, Bulletin 585. Geological Survey of Canada, Ottawa, 2005. [8] Murray, D., Kleinberg, R., Sinha, B., Fukuhara, M., Osawa, O., Endo, T. and Namikawa., T., Formation Evaluation of Gas Hydrate Reservoirs, 46th Annual Logging Symposium, New Orleans, USA. 2005. [9] Scott, H. D., Wraight, P. D., Thornton, J. L, Olesen, J-R., Hertzog, R. C., McKeon, D. C., DasGupta, T., and Albertin I. J., Response of a Multidetector Pulsed Neutron Porosity Tool: 35th Annual SPWLA Symposium, 1994. [10] Darling, H., Scott, H., and Toufaily, A. Applications of an Epithermal Neutron Measurement in Formation Evaluation: 38th Annual SPWLA Symposium, 1997. [11] Casu, P. A., Andreani, M. and Klopf, W., Consonant Measurement Sensors for a More Accurate Log Interpretation: 39th Annual SPWLA Symposium, 1996. 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Collett (Editors), Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well, Mackenzie Delta, Northwest Territories, Canada, Bulletin 585. Geological Survey of Canada, Ottawa, 2005. [16] Herron, M.M., Johnson, D.L. and Schwartz, L.M., A robust permeability estimator for siliciclastics, SPE paper 49301 Annual Technical Conference and Exhibition, New Orleans, 27-30 September, 1997. [17] Grauls, D., Blanche, J.-P. and Poudre, J.-L., Hydrate sealing efficiency from seismic AVO and hydromechanical approaches, in Proceedings of the International Symposium on Methane Hydrates: Resources in the Near Future?, JNOC- TRC Chiba, 20-22 October 1998. [18] Flemings, P.B., Liu, X., and Winters, W.J., Critical pressure and multiphase flow in Blake Ridge gas hydrates, Geology 31, 1057-1060, 2003. [19] Hornbach, M.J., Saffer, D.M. and Holbrook, W.S., Critically pressured free gas reservoirs below gas hydrate provinces, Nature 427, 142- 144, 2004. 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[24] Sinha, B.K., Kane, M.R., and Borland, W.H., Analysis of sonic data in an Indonesian well for formation damage, stresses, and bedding. SPE/ISRM 78232, SPE/ISRM Rock Mechanics Conference, 2002. [25] Sinha, B.K., Inversion of borehole dispersions for formation stresses, IEEE International Ultrasonics Symposium, IEEE Catalog No. 97CH36118, pp. 781-786, 1997. [26] Sinha, B.K. and Kostek, S., Stress-induced azimuthal anisotropy in borehole flexural waves, Geophysics, Vol. 61, pp. 1899-1907, 1996. [27] Plona TJ, Kane MR, Sinha BK, Walsh J and Viloria O. Using acoustic anisotropy. Paper RR, presented at 41st SPWLA Symposium, 2000. [28] Sinha, B., K. and Asvadurov, S., Dispersion and radial depth of investigation of borehole modes, Geophysical Prospecting, vol. 52, pp. 271- 286, 2004. [29] B. K. Sinha, Vissapragada, B., Renlie, L., and Tysse, S., Radial profiling of the three formation shear moduli and its application to well completions, Geophysics, vol. 71, No. 6, pp. E65- E77, November-December 2006. (Annual Meeting Selections).


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