The estimation for the resources of methane hydrate ...

The estimation for the resources of methane hydrate in the Nankai Trough, offshore Japan Koji Ochiai, Japan National Oil Corporation, Technology Research Center Masao Hayashi, Japan National Oil Corporation, Technology Research Center Nobutaka Oikawa, Japan National Oil Corporation, Technology Research Center Shoshiro Shimizu, Japan National Oil Corporation, Technology Research Center Masaru Nakamizu, Japan National Oil Corporation, Technology Research Center Tetsuo Yonezawa, Japan National Oil Corporation, Technology Research Center Kuniaki Takayama, Japan National Oil Corporation, Technical Department Masami Hato, Japan Petroleum Exploration Co., Ltd. Kei Baba, Japan Petroleum Exploration Co., Ltd. Research Center Sumito Morita National Institute of Advanced Industrial Science and Technology

1. INTRODUCTION 1.1. Methane Hydrate Methane hydrates are ice-like materials formed from a framework of molecules (H2O) that contain large intra-lattice voids [1]. Methane hydrates are transparent to translucent white to gray crystals. 1m3 of fully saturated methane hydrate contains 172m3 of methane [2]. Hydrate has its stability zone (HSZ) controlled by high pressure and low temperature in the natural condition. Methane hydrates were recognized in the permafrost areas in Siberia and Alaska in early stage and their developments have been confirmed even in the sea region in the later stage. 1.2. Distribution of Methane Hydrate in Japan Seismic data acquired in the ocean deeper than about 500m show wide distribution of strong impedance contrast reflectors which are in general parallel to the sea bottom, therefore, they are called as bottom simulating reflectors (BSR). The scientific investigation in offshore Japan has revealed the development of BSRs in many areas and the total size of its distribution is assumed to be 60,000Km2 [3]. As BSR is interpreted to be a possible indication of boundary between hydrate bearing zone and underlying sedimentary sequence, a delineation of BSR in the territorial waters of Japan is considered to be important for the country’s energy security. Available scientific data indicates that the relatively wide distribution of BSR is expected in the Nankai Trough, offshore Honshu Island of Japan. 1.3. Characteristics of Methane Hydrate As mentioned above, BSR is considered to indicate the base of hydrate because a stronger impedance contrast will be generated there than in other areas without hydrate zones. This phenomenon is caused by impedance contrast occurred at the contact between high interval velocity

such as methane hydrate bearing zone and low velocity zone. If the pressure-temperature condition is within the hydrate stability zone (HSZ), methane hydrate is formed and preserved in the subsurface sediments. The burial depth of HSZ in the Nankai Trough with water depth of deeper than 1,000m is calculated as 250m to 300m from mud line based on a well-known phase boundary curve. Seismic survey with high frequency energy source is very effective to obtain a detailed geological information for the shallower section where BSR is expected to develop. This is called high resolution seismic survey and the method has been established through “site survey” in the petroleum industries. As burial depth of hydrate is relatively shallow, the required time of drilling a well to confirm hydrate-bearing zones is considered to be remarkably shorter than that for conventional hydrocarbon. Because of this, for the exploration of methane hydrate more reliable data can be collected through drilling of many wells comparing to that of reduction of

conventional hydrocarbon, which will lead a positive

uncertainties for the calculation of resources.

2. EXPLORATION HISTORY IN THE NANKAI TROUGH 2.1. Seismic Survey, offshore Tokai, eastern Nankai Trough

JNOC (Japan National Oil Corporation) conducted seismic survey composed of high resolution and conventional in offshore Tokai in 1996 (Fig.1). The specification of the survey is shown in Table 1. Shot length for the high resolution survey was 500km and that for the conventional was 500km.

Fig. 1 2D Seismic survey line map.

Survey HR* Conv.**

Sampling rate msec 1 2

Shot interval m 25 25

Airgun volume ci 1330 2660

Shot depth m 4 6

No. of Channel channels interval m 474 6.25 282 12.5

Cable depth m 8 12

Table 1 Specification of seismic surveys at offshore Tokai in 1996 *High Resolution, ** Conventional

Filter Hz 3-250 3-218

CMP interval m 3.125 12.5

2.2. DTAGS, offshore Tokai Provisional survey of DTAGS (Deep Towed Acoustics-Geophysics System) was conducted in the same but limited offshore area as above in 1996 and very high resolution data for the shallower portion of sedimentary sequence was successfully obtained. The frequency for the acoustic source was too high to penetrate the shallower sedimentary layers and the information of BSR was not delivered by this survey. It is expected that the results shall be reflected for the design of energy source of DTAGS when the second generation equipment will be manufactured. 2.3. Drilling of Stratigraphic Test Well “MITI Nankai Trough”, offshore Tokai To confirm the geological characteristics of the BSR delineated by the seismic survey in offshore Tokai, a stratigraphic test well, named “MITI Nankai Trough”, was drilled by JNOC in the end of 1999 (Fig.1). The water depth of the location is 945m, and 50km away from the nearest shoreline. Total depth of the well was 3,300m since possible gas bearing zones in the deeper section were another targets for this well. To detect potential geohazards and to make a preliminary confirmation of methane hydrate layers, two pilot wells were drilled before the operation of “MITI Nankai Trough”. Three post survey wells were drilled after “MITI Nankai Trough” for the data gathering purposes. BSR delineated by a seismic survey was confirmed to be the lowest boundary of consecutive and multiple methane hydrate bearing zones for the first time through the drilling of this stratigraphic test well. PTCS (Pressure Temperature Core Sampler) was used to obtain in-situ sample of hydrate bearing sediments as well as conventional coring equipment for the petrophysical analysis. Various logging data were obtained for the calibration of them with core and seismic data. 2.4. Seismic Survey, offshore Tokai and Kumano, Nankai Trough Though the main area of BSR development is in offshore Tokai, science oriented multiple surveys indicate that the considerable methane hydrates may exist even in the Shima Spur and Kumano Trough, Nankai Trough 100km south west of the offshore Tokai. The seismic survey with the same specification as previous one in 1996 was conducted by JNOC in September to October 2001 for the delineation of wide development of BSR in the Nankai Trough region (Fig.1). This seismic survey with wide frequency range for acoustic energy source gathered 2,800km of good quality data in the entire region especially for the shallow interval where methane hydrate is expected to develop. BSR is recognized in the wide area covered by this survey and based on the result of interpretation, the objective areas for the three dimensional seismic survey were selected. 2.5. Three Dimensional (3D) Seismic Survey in the Nankai Trough Accurate subsurface information is important for the selection of reliable drilling locations, which will give serious influence to the calculation of resources and establish of development program of methane hydrate in the future. This subsurface data is essential too, to reduce risks by geohazards

when drilling operation will take place and offshore structures for production will be installed. 3D seismic survey was planned to cover the areas where the accumulation potential of methane hydrate was evaluated to be relatively high based on the previous surveys, namely Offshore Tokai (1,125Km2), Atsumi Knoll (625Km2) and Kumano Trough (210Km2) in the Nankai Trough (Fig.2). The first 3D seismic operation for methane hydrate survey in Japan started on June 2002 and completed on December with several interruptions by storms. Interpretation of the 3D seismic data has been progressed and comprehensive evaluation report will be published in the future.

Fig. 2 Distribution map of BSR interpreted from 2D seismic survey data. Closed rectangle areas show area of 3D seismic survey.

3. RESULTS OF GEOLOGICAL INVESTIGATION 3.1. Evaluation of Seismic Data in 1996 The detailed distribution of BSR in that particular region was delineated for the first time and four areas are recognized as distinguished BSR regions (Fig.3). Based on the result of BSR interpretation, the location for the said test well “MITI Nankai Trough” was selected [4]. The water depths for the areas are from 500m to 1,500m and it is reported that active fluxes of cold water (cold seepage) distribute in this area [5]. The flux is considered to be related to dissociation of methane hydrate underground, therefore the flux in this area will be an indirect evidence that BSRs are closely related to methane hydrates.

Fig. 3 Interpretation result of 1996 2D seismic survey data. Solid lines are conventional survey line and dashed lines shows high resolution survey line.

3.2. Analysis of Well Data Drilling of the historic stratigraphic test well “MITI Nankai Trough” was begun on November 1999 and completed on December 1999. The well penetrated the deepest methane hydrate zone at around 1,240m which corresponds to BSR appeared in the seismic section crossing the location and reached the TD of 3,300m to finish the operation. The reason of its great depth is that another target of the well was conventional gas bearing zones in the deeper section of Miocene. For the research purposes, three additional Post Survey Wells were drilled to methane hydrate bearing zones down to 1,250 to 1,300m and invaluable information was collected through LWD, coring and wireline logging (Fig.4).

Fig. 4 Well logging data of “MITI Nankai Trough” Post Survey Well 1 closed areas of dashed line indicate the methane hydrate bearing zone. Four remarkable hydrate bearing zone have been observed. Total net thickness of hydrate zone is approximately 20m from 1,141m to 1,213m.

Not only conventional wireline coring system, but specially designed and manufactured PTCS (pressure temperature core sampler) for this operation was used to obtain in-situ core sample of hydrate bearing sediments from 1,110m to 1,272m and recovered core samples with natural hydrate at 1,152m. These wells revealed, for the first time, the occurrence of methane hydrates in the Nankai Trough to be not massive in the silty or muddy layers but intergranular in sandy layers. Analysis of various logging data by multiple wells suggest that a lateral distribution of methane hydrates is expected. Detailed studies indicate that the average porosity of hydrate bearing sandy zone is 36% with thickness of 20m and the hydrate saturation is calculated to be 75% [6]. These figures will be fundamental parameters when the calculation of resources for the methane hydrates in the Nankai Trough will be made. 3.3. Confirmation of Additional Distribution of Methane Hydrate in the Nankai Trough As mentioned above, very wide distribution of BSR was confirmed and delineated for the first time in the Nankai Trough. Based on the detailed investigation of BSRs recorded in the seismic data of 2001, it is recognized that there are several types in their appearance. 3.3.1. Clear BSR BSRs with high amplitude are easy to recognize in seismic sections (Fig.5) and they can be confirmed by checking its existence in the crossing lines. This is the same procedure when certain geological markers are interpreted in the seismic sections. Based on the distribution of this “Clear BSR” (Fig.2), several objective areas for 3D seismic survey were selected as explained and candidate

locations for test wells planned in 2004 will be also discussed. 3.3.2. Possible BSR Some BSR appears in one seismic line is difficult to recognize in the crossing line with this because of amplitude attenuation. In some areas, though clear BSR disappears suddenly, continuation of very weak amplitude anomaly parallel to the sea bottom is observed in certain extent (Fig.5). These anomalies are categorized as “Possible BSR” and mapped for the further investigation (Fig,2). Drilling of several wells into this “Possible BSR” is planned to confirm the existence of methane hydrate in it.

Fig. 5 Typical example of Possible BSR At the northern and southern part of the seismic section, remarkable BSR is observed, but center of the section, amplitude of BSR drastically weaken. Sometime it is difficult to recognize this kind of BSR as common BSR.

3.3.3. Masked BSR It is difficult to distinguish BSR from a strong signal of stratification, if the recorded bedding plane is parallel to the sea bottom especially the topography of bottom is flat (Fig.6). BSR distinguished in the dipping area is hard to track in the said stratigraphic condition even if the BSR continues longer than clearly observed. This is categorized as “Masked BSR” (Fig.2) and preliminary study indicates that a drastic change of seismic velocity occurs in the vicinity of it and further investigation is in progress.

Fig. 6 Typical example of Masked BSR.

At the western part (left side) of the section Clear BSR is

observed but at the eastern part (right side), BSR is hardly distinguished from a strong signal of stratification.

4. ESTIMATION OF METHANE HYDRATE RESOURCES 4.1. General Understanding The Geological Survey of Japan made an assessment on the resources and disclosed the figure of 4.65 x 1012m3 for the volume of dissociated gas from methane hydrate in all offshore Japan [3]. Without subsurface geological data, this estimation was made based on the area of BSR and certain necessary parameters experienced in West Siberia and Alaska. After completion of extensive seismic surveys and drilling of stratigraphic test wells for methane hydrate in the Nankai Trough area, it seems to be very important to reassess the resources for the selection of test field in the next production stage. 4.2. Assessment Method The established method for a calculation of conventional gas reserves will be applied in general to the estimation of methane hydrate resources as follows: Gi = A x ΔZ x Φx Sh x E While,

Gi

: Total amount of dissociated gas from methane hydrate

A

: Total area of the distribution of methane hydrate

ΔZ

: Net thickness of methane hydrate bearing zone

Φ

: Porosity (ratio of effective pore space)

Sh

: Methane hydrate saturation

E

: Methane gas yield (Volume factor x occupation ratio of methane gas in hydrate cage)

4.3. Determination of parameters with uncertainties All these parameters mentioned above are independent each other and different from area to area. To obtain reliable figures for them is very important to calculate resources. Generally speaking, uncertainties for the area of hydrate distribution and thickness of it are big, on the other hand, the range of saturation for methane hydrate is relatively small. Porosity will be in the range of 30 to 50% if the experience in the “MITI Nankai Trough” is applied and the effect of uncertainty seems to be limited. 4.3.1. Area Uncertainties for area of hydrate distribution are remarkable and influential. BSR is understood to represent the base of methane hydrate bearing zone in many cases, therefore, if the BSR of particular region is generated by existence of methane hydrate, the figure “A” derived from seismic data in the above equation has high reliability. If methane hydrate is developed without BSR like in offshore Guatemala, the resources will be underestimated. Whether BSR with subtle amplitude anomaly represents low saturation of hydrate or thin layer of hydrate is not clarified yet. This is another uncertainty to be studied. 4.3.2. Thickness The thickness of methane hydrate is determined by drilling of a well but only at its location. Seismic correlation with the well data may furnish answer to this question with less reliability. If detailed velocity analysis can detect the boundary of upper limit of hydrate bearing zone, this method gives the breakthrough to this problem and reliable ”ΔZ” will be given. 4.3.3. Saturation Saturation “Sh” is relatively high as 60 to 80% from experience in wells “MITI Nankai Trough” and “Mallik 2L-38” of Canadian Mackenzie Delta. This range in high value shall be regarded as standard since hydrate zone with lower saturation than this range is not practical as objective for development and commercial production. 4.3.4. Methane gas yield “E” value means how much methane gas is captured in the cage of hydrate. As explained previously, 1m3 of methane hydrate contains 172m3 of methane gas at standard temperature and pressure condition. Based on the laboratory analysis, approximately 90% of gas is likely to be stored with methane hydrate [7], therefore, “E” has no serious uncertainty for the resource calculation.

5. DISCUSSION The major goal of present research is to delineate the distribution of prospective areas for Methane Hydrate offshore Japan and estimate the resources in the said area, and finally select the

suitable site for production test scheduled in Phase 2 (FY2007-FY2011). To reach the goal, precise estimation of the Methane Hydrate resources in Nankai Trough enough for future development must be made. Therefore, the data and sample collection must be done effectively and efficiently for the estimation during Ministry of Economy, Trade and Industry (METI) drilling campaign at the Nankai Trough area in 2004. 5.1. Distribution Area of Methane Hydrate Figure.7 is one of examples that the clear BSR with high amplitude disappears suddenly. Remarkable BSRs are recognized at the eastern and western parts of the hill, called Daini-Atsumi Knoll, however no-BSR is observed around top of the Knoll. No difference in sedimentary facies for both areas with BSR and without BSR is observed, but based on the detailed interpretation, the sedimentary interval without BSR is separated from older and younger sediments with BSR by two distinct unconformities .

Fig. 7. Seismic section crossing Daini Atsumi Knoll. The sudden disappearance of high amplitude BSR around top of the Knoll is remarkably observed.

Two distinct unconformities

are observed in the sedimentary interval without BSR.

Sudden disappearance of clear BSR attributes to several reasons such as 1) amount of hydrate is too small because the sediments containing small hydrate crystal are too fine grained such as silt and clay, 2) no methane hydrate exists because of no effective source of Methane gas and 3) insufficient acoustic impedance between hydrate bearing zone and underlying strata without hydrate This

sudden disappearance and discontinuation

of clear BSR with

leaving some indication of

amplitude anomaly there, is observed in some areas in Nankai Trough (Fig. 2)and it is defined as Possible BSR. To confirm these phenomena and verify whether Methane Hydrate exists there or not, some wells should be drilled in appropriate area. The areas characterized by “Masked BSR” (see Fig. 6)shall not be

considered as the area of

Methane Hydrate because it is impossible to distinguish BSR from strong reflected signal of stratification. Approximately, 10% of whole area of BSR is recognized as “Masked BSR”, especially at the western part of surveyed area, approximately half area of BSR is categorized as “Masked BSR” (Fig. 2). Therefore overestimation of hydrate distribution area taking this type of BSR into account will mislead

the future development scenario.

Detailed seismic velocity analysis which will be

discussed bellow, could be a key to distinguish the area of Methane Hydrate from “Masked BSR”. Drilling of some exploratory wells will verify the existence of Methane Hydrate in the area and reduce the uncertainty of its distribution 5.2. Occurrence of Methane Hydrate in Sediment The result of detailed interpretation of seismic data shows that the intensity of amplitude for the BSR is not uniform but has some variations (Fig.8). Amplitude anomaly of the BSR represents the lateral impedance changes , and is thought to be dependent on the saturation of the methane hydrate in the layer and occurrence of the hydrate in sediments [8]. If Methane Hydrate is preserved in the pore space of sediments, the differential of the P-wave velocity between sediments with Methane Hydrate and without Methane Hydrate will be big because P-wave velocity of the hydrate is much faster than formation fluid containing subtle amount of gas. However, since density of hydrate is same as that of formation fluid, the differential of the density between them would be constant. Amplitude depends on acoustic impedance, therefore, it is thought that intensity of amplitude for the BSR is highly depending on the amount of hydrate in the pore space of sediment . If

pure and massive methane hydrate is generated in subsurface,

impedance contrast between the said pure hydrate and the formation fluid in underlying layer is too small to bring BSR at the boundary. Most of methane hydrate is developed in the pore space of sandy sediment at “MITI Nankai Trough” wells. Pure and massive hydrate was not observed in the drilled area and the thickness of it seems to be in not sufficient level for the future development. Therefore, it is necessary to apply the BSR amplitude for identification of effective methane hydrate.

Fig.8 Distribution map of BSR amplitude from 2D seismic data. White colored area shows relatively high amplitude.

If the variation of BSR amplitude represents the occurrence of the Methane Hydrate in the sediment and/or thickness, it means that hydrate has different character from area to area. The estimation of hydrate resources would have uncertainty due to this difference. If the amplitude of BSR corresponds

with occurrence of hydrate quantitatively, amplitude analysis

will be effective and

efficient in reduction of the uncertainty when resources estimation will be made. Drilling of some wells in appropriate area will be necessary for the confirmation of this theory. 5.3. Thickness of Methane Hydrate Bearing Zone BSR probably represents the horizontal extent of the Methane Hydrate but it does not give the information t on the upper limit of Methane Hydrate . For the estimation of resources of Methane Hydrate, vertical distribution of Methane Hydrate is one of critical parameters as well as its horizontal extent. Therefore it is important to give

the thickness

of Methane Hydrate

from the seismic

data . 5.3.1. Interval Velocity Anomaly Since Methane Hydrate

adheres loose sediments, the elastic velocity of sediments

containing methane hydrate should be relatively faster than unconsolidated sediments. Laboratory analyses for elastic velocity measurement of core sample obtained from Mallik 2L-38 in Northern Canada revealed that the P wave velocity of sandstone containing methane hydrate was more than 3000m/sec. while that of non hydrate and unconsolidated sandstone represents 1500-1800m/sec.[9] . Logging data of “MITI Nankai Trough” also indicates the velocity of

2000-2500m/sec. at hydrate

concentration interval and 1600-1800m/sec. at non hydrate layer [6]. Such obvious variation of elastic

velocity between Methane Hydrate layer and other layer is possibly extracted from extensive velocity analysis of seismic data. The detailed velocity analysis for 2D seismic data offshore Tokai and Kumano was made to verify the possibility of application of this technique to identify the Methane Hydrate layer. Detailed velocity analysis of 2D and 3D seismic data is able to give the interval velocity of relatively thinner layer and makes the delineation of hydrate layer easier and more efficient [10]. Figure. 9 represents the result of PSDM Velocity Analysis of 2D seismic data intersecting “MITI Nankai Trough”[11]. Relatively higher velocity zone related to Methane Hydrate is detected clearly just above BSR. This zone is likely correlated with confirmed Methane Hydrate zone by well logging data. This result suggests that the velocity analysis of seismic

data is one of effective and efficient

techniques for estimating thickness of Methane Hydrate.

Fig. 9 Result of PSDM velocity analysis of 2D seismic survey data where across the wells of “MITI Nankai Trough “ and “MITI Omaezaki Oki” [11]. White colored areas indicate the relative high velocity zone gray colored areas indicate lower velocity zone. Higher velocity zone is observed just above BSR. At “MITI Nankai Trough” Methane Hydrate was observed approximately 70m interval just above BSR but at “MITI Omaezaki Oki” no BSR exists due to beyond the Hydrate Stability condition.

This technique has limitation to distinguish the detailed vertical distribution of Methane Hydrate layer which consists of thinner layers as observed at “MITI Nankai Trough”. Identification

of each

thinner sedimentary layer containing Methane Hydrate is difficult, too due to relatively lower resolution than well logging data. Therefore velocity analysis should be applied to the detection of Methane Hydrate zone by correlation of it

with

logging data. Some

located at the appropriate point for verification of them. 5.3.2. Seismic Attribute Analysis

wells drilled in the Nankai Trough

Analysis of seismic attribute which is obtained from signal of seismic trace could give the quantitative and qualitative property of the materials and widely applied to evaluation of reservoir facies and geological structure analysis in oil and gas industries. Since Methane Hydrate has unique physical property, this technique could be applied to extraction of interval of hydrate bearing zone and/or delineation of occurrence of the hydrate in sediments. Preliminary result of this analysis applying to 2D seismic survey data and logging data in the Nankai Trough shows that some of l hydrate bearing layers are probably extracted. Consequently the possibility and effectiveness for evaluation of thickness and occurrence of Methane Hydrate have been supposedly demonstrated. To proceed this analysis more accurately, well logging data must be corrected as many as possible by the drilling campaign in the Nankai Trough in 2004. 5.3.3. Other techniques Application of other geophysical surveys such as “4 component seismic survey” which enables to obtain shear wave and “Electoromagnetic survey” which corrects the resistivity data of subsurface commonly used in mineral mining is under investigation in order to identify the top of hydrate to know the thickness of it . If the seismic data itself is not sufficient for the understanding of parameters for resources estimation, , though the contribution of these techniques might be subsidiary. Acknowledgment This paper is the latest results of joint study by researchers from JNOC, JAPEX, TOC, JGI and AIST. The authors would like to express their sincere appreciation to the Agency of Natural Resources and Energy of METI, JAPEX, JNOC and JNOC-TRC for permitting the publication of invaluable data and interpretation during the preparation of this paper for the World Gas Congress.

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6. Takahashi, H., T. Yonezawa and Y. Takedomi, (2001) Exploration for natural hydrate in Nankai-Trough wells offshore Japan. Jour. Japanese Assoc. Petroleum Technology, Vol.66 No.4 652-665. 7. Davidson, D.W., El-Defrawy,M.K., Fuglem,M.O. and Judge,O.S., (1978), Natural gas hydrates in northern Canada. In Natural Research Council of Canada. Proc. 3rd Int. Conf. On Pernafrost, 1978,1.938-943. 8. Kuramoto, S., (1996) Geophysical investigation for methane hydrates and the significance of BSR. Jour. Geol. Soc. Japan, Vol.102, No.11, 951-958 9. Winters, W. J., I.A.Pecher, J.S.Booth, D.H.Mason, M.K.Relle and W.P.Dillon, (1999) Properties of samples containing natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate well, dtermined gas hydrate and sediment test laboratory instrument. Geological Survey of Canada Bull 544, 241-250 10. Hato, M. and T. Inamori, (2002) Delineation of methane hydrate zone using seismic methods on Nankai Trough. Butsuri-Tansa, Vol.55, No.5, 435-445 11. Takanashi, M., Saito, H. and Nakajima, Y. (2002) Detailed Velocity Analysis using PSDM., Annual report of JNOC TRC’s Activities for the Year 2001, 198-201