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A Review of Gas Hydrates and Formation Evaluation of Hydrate-Bearing Reservoirs

Stephen Prensky

[Originally published in 1995, SPWLA 36th Annual Logging Symposium Transactions, paper GGG]

Gas hydrates occur within a limited range of pressure and temperature that is found in ocean sediments and in association with permafrost in the Arctic. Gas hydrates represent an enormous potential energy resource, estimates of the methane gas contained in worldwide methane hydrate accumulations exceed estimates of the combined reserves of oil and gas that can be produced by conventional methods. The Messoyakha field, in the west Siberian basin, has produced natural gas from gas hydrates since the early 1970s. Gas hydrates and hydrate-bearing rocks and sediments are characterized by high electrical resistivities, high acoustic velocities, and low bulk densities (in zones of massive hydrates) relative to water or gas-bearing rocks or sediments. In oceanic settings or below the base of ice-bearing permafrost, well-log measurements of these properties can readily identify gas-hydrate zones. However, the presence of ice (which has similar physical properties to gas hydrates) within a permafrost interval complicates the use of logs for qualitative identification and quantitative evaluation. Identification of gas hydrates requires a combination of well logs in conjunction with gas shows on drilling or mud logs. Thermal invasion during drilling or coring can dissociate hydrates and introduce additional complicating factors.

Although the physical properties of gas hydrates are well known, little is known about those of hydrate-bearing rocks and sediments. Additional laboratory data are required for calibrating quantitative well-log and seismic techniques for evaluating gas-hydrate-bearing reservoirs and assessing the their methane content. Some of the unique physical properties of gas hydrates can be measured by new wireline logging devices (e.g., array resistivity, dipole acoustic, dielectric, inelastic and capture gamma-ray spectrometry, and borehole imaging) whose application may help distinguish ice from hydrates in permafrost regions, provide improved determinations of porosity, identify type of hydrates present and their distribution within a sediment or rock matrix, and they enable improved determination of the degree of gas-hydrate saturation in hydrate-bearing zones. New and improved measurement-while-drilling (MWD) logging devices offer the potential for detecting and quantifying gas hydrates prior to any decomposition and may provide early warning of potential drilling hazards.

The U.S. Geological Survey, through its Gas Hydrate Research Program, is developing advanced gas-hydrate well-log and seismic models based on (1) laboratory modeling of tool response using actual gas hydrates as well as synthetic samples, and (2) analysis of well-logging data from the Alaskan North Slope and Ocean Drilling Program. This report summarizes the current state of gas-hydrate formation evaluation and discusses work in progress.

Gas hydrates are naturally occurring clathrates (inclusion compounds) in which gas molecules are contained within a rigid cage-like structure of water molecules. The most common occupying gas is methane, primarily biogenic although thermogenic gas from a deeper source may be mixed in, and the clathrate arrangement results in 150 to 180 volumes of free methane gas per volume hydrate (see Kvenvolden, 1993a and Sloan, 1990 for a review of gas-hydrates and their properties). Gas hydrates occur naturally within a limited range of temperature and pressure known as the gas-hydrate stability zone (Figure 1) and can exist at conditions where pure ice would be unstable. Hydrates are found in clastic marine sediments, siltstones, unconsolidated sands and poorly cemented sandstones, and conglomerates at depths ranging from 150 to 2000 m below the surface in permafrost regions of the Arctic, and below the sea floor where water depths exceed 300 m (primarily along continental slopes and rises and in deep inland lakes and seas)(Figure 2). Their distribution is further restricted to regions where there is sufficient organic richness in rocks and ocean sediments to generate the large volumes of methane needed to form hydrates. Gas hydrates have the appearance of wet snow or ice and can form as disseminated interstitial pore filling (and may act as a cementing agent), layers, nodules, or in a massive form. Matrix porosity in these reservoirs is generally high and matrix permeability moderate to high, however, the net effect of hydrate formation is to reduce porosity and permeability by blocking pore space (Bily and Dick, 1974, Collett, 1983; Krason and Finley, 1993).

The current interest in gas hydrates stems from the large amount of methane contained in gas hydrates as a potential energy resource, estimates of the volume of methane gas contained in known and inferred gas-hydrate accumulations is double that of known fossil-fuel resources (Table 1, Figure 3), and is also a potential factor in climate change and global warming as a "greenhouse" gas (Table 1) (see Kvenvolden, 1993a, for a summary; MacDonald, 1990a; Englezos and Hatzikiriakos, 1994). 2). Moreover, gas hydrates are recognized of as a hazard in hydrocarbon exploration; blowouts, fires, and collapsed casing have occurred when drilling through a hydrate zone due to the buildup of gas pressure resulting from hydrate dissociation (Yakushev and Collett, 1992). Gas-hydrate blockage of flow lines and gas pipelines occurs in Arctic regions (see discussions in Sloan, 1990; Sloan et al., 1994). Massive submarine slides and slumps on continental margins and slopes are a significant geologic hazard that may be caused by instability in underlying hydrate accumulations (McIver, 1977; Booth et al., 1994).

The U.S. Geological Survey, through its Gas Hydrate Research Program, has been investigating the origin and geologic relationships of gas hydrates for the past 15 years. The emphasis of this research has been on understanding the origin and formation of gas hydrates, characterizing the geology of gas-hydrate occurrences in order to assess their potential as an energy resource, studying their potential role in causing failure of submarine slopes, and evaluating their role as a factor in climate change. Summaries of this work are found in Collett (1992, 1993a), Collett et al. (1988), Dillon et al. (1993), Kvenvolden (1993a, b), Lee et al. (1992, 1994), and Winters et al. (1994).

The evidence for the presence and distribution of gas hydrates falls into two categories: direct and inferred. Direct evidence consists of actual recovery of samples, either through drill coring (terrestrial and marine) or piston coring (marine). The indirect evidence, summarized below, consists of the association of geologic, geophysical, and geochemical observations in sediments within the predicted gas hydrate stability zone (Figure 1).

The occurrence, distribution mapping, and estimates of the volume of marine gas-hydrate accumulations are inferred from, and are based primarily on the presence of bottom simulating reflectors (BSRs) on seismic sections. BSRs are anomalous seismic reflectors that parallel the seafloor topography but which may cut across other reflectors and which lie between 100 and 1,100 m below the seafloor (Figure 4). They are found in areas where water depths exceed 300 m. A BSR signifies a change in acoustic impedance (Stoll et al., 1971), primarily a velocity contrast. Although some BSRs may result from lithologic or temperature-controlled diagenetic changes, e.g., the opal transformation (Hein et al., 1978), when a BSR is found at a depth that lies near the base of the predicted hydrate stability zone, it is considered to be a reliable indicator of the presence of gas hydrates (MacDonald, 1990a). Not all oceanic gas-hydrate accumulations may have an associated BSR, the recovery of massive gas-hydrates on DSDP Leg 84 was not accompanied by a BSR (Kvenvolden and McDonald, 1985). Even in the absence of a BSR, the presence of anomalously fast intervals within the predicted gas-hydrate stability zone may server as an indicator of gas hydrates (Wood et al., 1994).

The magnitude of the BSR amplitude may be related to the thickness of an underlying free-gas zone and/or the amount of free-gas present (Miller, et al., 1991; Lee et al., 1992; McKay et al., 1994; Singh and Minshull, 1994). Paull and Dillon (1981) have suggested that seismic "blanking", the reduced interface reflectivity of sediments above a BSR compared with those below it, results from reduced impedance contrasts across the interfaces due to gas-hydrate cementation of sediments. Lee et al. (1992) have developed a technique that uses velocity as well as the observed degree of "blanking" (reduction in amplitude) (Figure 4), calibrated by interval velocity, to quantify the amount of hydrate per unit volume. After a regional calibration is performed, bulk-hydrate concentrations can be estimated using only reflection amplitudes. Because the majority of gas-hydrate occurrences are in marine environments, a primary objective of gas-hydrate research and formation evaluation is to support and refine these seismic techniques. One question in particular, that is being addressed is the relationship between BSR velocity changes and the degree of hydrate cementation above the BSR.

The only documented sample of gas hydrates recovered in a permafrost region was in the Arco-Exxon NW Eileen State No. 2 well, located in the west portion of the Prudhoe Bay field on North Slope of Alaska (see Collett, 1983; Mathews, 1985, for details). This well was designed specifically to log and test gas-hydrate intervals that had been inferred in an adjacent well. A pressure test of core recovered in a pressure core barrel (Figure 5) combined with drilling, well-log, mudlog data, and gas analysis from the cored interval confirmed the presence of gas hydrates and indicated a 9-m thick zone (Figure 6). Although there was no visual or laboratory determination of physical properties of recovered gas-hydrates in core samples, the interstitial (disseminated) form of hydrate is assumed. Gas hydrates have been recovered from only 14 oceanic locations (Kvenvolden et al., 1993). The best documented samples are from Site 570, Leg 84 of the Deep Sea Drilling Project, off of Guatemala (Kvenvolden and MacDonald, 1985; Mathews and von Huene, 1985; Mathews, 1985). A 1-m core of massive (pure ) hydrate was recovered and log response indicated a 4 m interval of massive hydrates within a 15-m thick hydrate zone (Figure 7). Only one recovered sample has been tested in the laboratory (Davidson et al., 1986); other laboratory results are from synthetic hydrates. The instability of gas hydrates at ambient conditions makes sample recovery difficult, and because of the lack of commercial interest in gas hydrates, DST or production tests of hydrate-bearing zones are rare (the Arco-Exxon NW Eileen State No. 2 being one).

The Messoyakha field in the west Siberian Basin in Russia has been producing from sub-permafrost methane hydrates since the 1970s (Figure 8) (Makogon, 1981, Krason and Finley, 1993). This is based on the unique pressure and production history of the field (Figure 9). Comprehensive studies of the geologic conditions of gas-hydrate occurrence in this field and the Prudhoe Bay and Kuparuk River fields of the Alaskan North Slope (Collett et al., 1988; Collett, 1991, 1993a, 1993b) indicate many geologic similarities. The Messoyakha field may serve as a production analog for the recovery of a gas-hydrate resource estimated at 37-44 Tcf in the Prudhoe Bay Kuparuk complex (Collett, 1991, 1993b).

The first test of the commercial viability of oceanic gas hydrates as an energy resource may come in 1999. A Japanese consortium, under the auspices of the Japanese National Oil Company, is planning to drill gas hydrates in the Nankai Trough.

Progressive freshening of pore waters, by as much as 50 percent, within the predicted gas-hydrate zone and associated with high methane concentrations, has been observed in a number of DSDP and ODP holes (Hesse and Harrison, 1981). The formation process of gas hydrates, like that of ice, results in a selective exclusion of salt ions and when the hydrates dissociate, fresh water is released and mixes with pore waters to create a trend of decreasing chlorinity through the gas-hydrate zone. This effect may be detectable by wireline and MWD resistivity logs.

Experimental data on synthetic hydrates and empirical data from in-situ measurement show that the change of phase from water as liquid, which is an electrical conductor, to rigid crystalline structures such as ice or clathrate which are electrical insulators, results in a marked change in some physical properties (Tables 2-3). In particular, because gas hydrates (and ice) are characterized by high electrical resistivities, high acoustic velocities, and low bulk densities, zones containing massive gas hydrates and gas hydrate-bearing rocks and sediments can be identified by the responses of well logs to these properties relative to water or gas-bearing rocks or sediments (Tables 2-4). These responses are reviewed by Collett et al. (1988) and Collett (1992) and presented in summary form in Figure 10. In oceanic settings or below the base of ice-bearing permafrost, well-log response can readily identify gas-hydrate or hydrate-bearing zones. However, the presence of ice within a permafrost interval, which has similar physical properties to gas hydrates and, therefore, similar log responses (Figure 11), complicates the application of log responses for identification and evaluation of gas-hydrate zones. Decomposition (thawing) of hydrates immediately around the borehole due to thermal invasion during drilling or logging, can result in well-log responses similar to those from intervals containing free gas, thereby complicating quantitative evaluation. In general, identification of gas hydrates and hydrate-bearing rocks or sediments is qualitative and based on a combination of well log responses in conjunction with gas shows on drilling or mud logs. Well-log responses similar to those in ice-bearing permafrost, but accompanied by strong gas mudlog shows provided the first indirect evidence for in-situ gas hydrates (Makogon et al., 1972; Bily and Dick, 1974; Davidson et al., 1978; Weaver and Stewart, 1982). Figures 6-8 illustrate examples of log response to hydrates in permafrost and oceanic environments.

Collett et al. (1984 and 1988), Trofimuk et al., (1984), and Mathews (1985) proposed crossplot techniques based on corrected resistivity-, acoustic-, density-, and neutron-log data for distinguishing hydrate-bearing zones and massive hydrates from ice, water- and gas-saturated zones (e.g., Figure 12A). Collett et al.(1988) and Mathews (1985) also developed quantitative evaluation methods for calculating average water and hydrate saturations in zones of mixed water and hydrates as (e.g., Figure 12B) well as methane content. Little is known of the physical and chemical conditions that result in different forms (disseminated, layered, nodular, massive) and distributions (uniform or heterogeneous) of gas hydrates. The quantitative techniques now being used assume uniform distribution of hydrates as interstitial deposits or cement. In this scenario, hydrates can be evaluated using techniques developed for a multi-component (matrix, water, gas, ice, gas hydrates) shaly-sand system. Similar techniques can be applied in the case of layered hydrates a situation somewhat analogous to the case of dispersed versus layered shales. Based on available information, the massive hydrates evaluated by Mathews (1985) appear to be the exception.

The primary objectives of well-logging in gas hydrates are gas-hydrate identification, characterization of gas-hydrate reservoirs (distinguishing gas-hydrate from permafrost ice, determining the level of homogeneity in hydrate distribution, the mode of occurrence within the reservoir and at the pore level, porosity determination, percent of pore space occupied by hydrates, and determination gas-hydrate saturation and methane content) and providing calibration to improve seismic techniques used for resource assessment. The latest wireline and MWD technology along with the improved type, range, resolution, and accuracy of logging devices can provide more accurate, and perhaps definitive, identification of gas-hydrate zones and improved quantitative methods for evaluating those zones.

Array resistivity tools have multiple depths of investigation and can "image" the extent of thermal invasion, if any, resulting from dissociation. Array sonic tools measure both compressional and shear velocities in the type of "soft" and unconsolidated formations that hydrates form in. Based on experimental data in permafrost (Zimmerman and King, 1986) it is believed that the presence of even small amounts of gas hydrates in high-porosity unconsolidated sands and marine sediments significantly affect compressional and shear velocities (Foley and Burns, 1992; Dvorkin and Nur, 1993). Seismic techniques developed to characterize gas hydrates use models based on either interval velocity, amplitude and reflection coefficients, or blanking effects (see summary in Foley and Burns, 1992). Variations in Vp/Vs ratios resulting from the presence of gas hydrates may be detectable by a dipole-type acoustic logging device. If so, well logs, together with additional laboratory data on acoustic properties, can provide porosity, free-gas saturation, and hydrate saturation data needed to calibrate seismic models for estimating hydrate saturation and methane content.

MWD devices have not been used to log gas-hydrate-bearing intervals. These devices obtain measurements very soon after bit penetration, prior to significant thermal invasion and hydrate dissociation and may provide the most accurate measurements of in-situ properties. The vertical resolution of the latest wireline tools and MWD devices is greater than that of previous devices.

Borehole imaging devices, combined with image-analysis techniques can provide information on the mode of gas-hydrate present and its distribution within a zone. This information is critical to the validation of quantitative evaluation.

New pulsed neutron-porosity devices can provide more accurate measurements of porosity than the older, compensated neutron devices because they are less affected by lithology and borehole environment. New and improved gamma-ray spectroscopy tools (including the pulsed neutron-porosity devices), operating in either inelastic and capture modes may provide important information concerning the percent of methane filling of the clathrate lattice which may range from 80 to 100 (a 90 percent fill is generally assumed). The carbon/oxygen and hydrogen/carbon ratios obtained by these tools are being investigated as a means for distinguishing gas hydrates from ice in permafrost intervals as well as improved determinations of gas-hydrate saturation (Collett, personal communication). These data are necessary for accurate assessment of the potential methane resource of both continental and oceanic accumulations.

Dielectric logs can exploit the significant difference between the dielectric constants of ice and gas hydrates (Table 2) and may provide a means for identifying gas hydrates that occur within permafrost intervals.

There are few data regarding the nuclear magnetic resonance (NMR) properties of gas hydrates and ice and the available data (Table 2) don’t indicate a significant difference. While NMR logging devices may not be helpful in identification of gas hydrates, they can provide a lithology-independent porosity measurement and estimates of permeability. These data can improve quantitative techniques for determination of hydrate saturation.

Dissociation of hydrates during drilling could, theoretically, result in an increase of pore pressure within the gas-hydrate bearing interval. The use of time-lapse resistivity and acoustic logs obtained by MWD devices may confirm this dissociation and, if present, provide another means for identifying gas-hydrate zones.

In theory, temperature anomalies resulting from the lower thermal conductivity of gas hydrates relative to ice (Stoll and Bryan, 1979) (Table 2) can be used to identify gas hydrates in permafrost intervals by means of multi-sensor or new fiber-optic devices. However, this may not be practical because these variations are probably small and thermal equilibrium in permafrost regions may take weeks, or longer. Similar thermal conductivities of water and hydrates suggest that this may not be a practical means of detecting oceanic hydrates: although the DSDP Site 570 hole was drilled with sea water, a temperature log run going in the hole did not show any variation in the previously cored hydrate interval.

An upcoming ODP cruise to The Blake Ridge Plateau, off the Bahamas, will test theories of gas-hydrate formation and evaluation. The research program, including seismic and logging phases, has been designed to address many of the unresolved issues discussed in this review.

To better understand the properties of gas-hydrate-bearing rocks and sediments, in general, and under in-situ conditions in particular, the USGS has designed and built a new computer-controlled system, the gas hydrate and sediment test laboratory instrument (GHASTLI). This system is being used to study the formation of gas hydrates and for testing the physical, electrical, acoustic, and chemical properties of synthetic gas hydrate samples in sediments (Booth et al., 1994; Winters et al., 1995). The data from these studies will be used to develop corrections and calibrations for new and current well-log and seismic-based hydrate-evaluation techniques.

The accurate assessment of gas-hydrate accumulations is important to developing strategies for energy planning and resource development and modeling global climate change. Additional information on the in-situ properties of hydrate-bearing rocks and sediments, especially the relationship between the mode of hydrate occurrence, distribution, and methane content, are needed to confirm and calibrate seismic models of gas-hydrate accumulation and evaluation techniques and to develop comprehensive well-log models for in-situ evaluation of gas hydrate. New computer-based laboratory testing equipment will be used to measure these parameters in simulated oceanic and permafrost conditions, on synthetic and (hopefully) recovered samples. New, and advanced wireline and MWD logging technologies that are currently available can measure the in-situ formation properties of hydrate-bearing rocks and sediments with improved accuracy and vertical resolution. The application of these well-logging tools, in future ODP holes exploration and production wells in Arctic regions, could provide data that would permit improved evaluation of hydrate-bearing rocks and sediments as well as definitive identification of hydrate-bearing zones within ice-bearing permafrost.

I wish to thank Tim Collett, Myung Lee, and John Miller of the U.S. Geological Survey, for reviewing the manuscript and suggesting improvements to it, and Keith Kvenvolden, also of the U.S. Geological Survey, for providing copies of many of the figures.

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