Methane hydrates, which are ice like materials and constructed with methane and water molecules, are spread widely under permafrost and deep water areas. It is thought that methane hydrates are ones of occurrence type of methane and can be future energy resources. BSR (bottom simulating reflector) occurs at the basement of GHSZ (gas hydrate stability zone) and therefore it is used to detect occurrences of methane hydrates in deep waters using reflection seismic data. At the same time, BSR is only the first step for methane hydrate exploration because BSR does not provide information about thickness, saturation and type of methane hydrate bearing zone. From the viewpoint of resource exploration, practical methods to analyze details of methane hydrate resource fields have been required. We constructed an effective strategy to delineate methane hydrate concentrated zones, of which reservoir are turbidite sand bodies, utilizing four following indicators in 3D seismic data: (A) BSR, (B) turbidite sequence (above BSR), (C) strong seismic reflectors and (D) relatively higher interval velocity, suggesting methane hydrate concentrated sand layers. It enabled us to delineate methane hydrate concentrated zones and evaluate their rock volume successfully in the eastern Nankai Trough as similar as prospects and leads in conventional oil and gas fields are delineated by geological and geophysical approaches.
The Japanese Government has acquired regional reconnaissance seismic data in territorial waters since 1969 to evaluate the country's hydrocarbon potential. Total length of seismic lines is about 100,000 km and many surveys cover relatively deep water areas where gas hydrate stability zone (GHSZ) is expected to exist. BSRs related to methane hydrates in the offshore areas surrounding Japan were studied jointly by the group comprised of JNOC (at present, JOGMEC) and 10 private sectors based on these archived seismic data, and the areal extent of BSRs was reported to be 44,000 km2 in 2000. The country's research into methane hydrates has been accelerated since 2001 by establishment of the research consortium (MH21) under administrative guidance of METI. To investigate into methane hydrate, extensive 3D seismic surveys were conducted in the eastern Nankai Trough and many LWD wells were drilled there. Through the exploration campaign, certain new knowledge on seismic attributes related to the concentration of methane hydrates was obtained. In the light of this advanced knowledge about the appearance of BSRs, MH21 has decided that the archived seismic data should be investigated for the comprehensive understanding of methane hydrates as the potential future energy resources. Old seismic data processed suitably for loading in a “work station” was interpreted in the sophisticated system, though the volume of them was very limited. A method of high density velocity analysis was applied to the seismic data which recorded clear BSR appearing at abnormally shallow depth as 200msec below the sea floor in the Sea of Japan and in the Sea of Okhotsk, comparing to 500 to 700 msec in the Pacific side. This may represent that the geotectonic setting of the Japanese Islands controls GHSZ and this understanding seems to be important for the future exploration of MH. Present study has revealed that the areal extent of BSRs in offshore Japan is 122,000 km2.
In the methane hydrate research project by METI/MH21 consortium, some unconventional data analysis techniques are developed and various types of geophysical surveys have been carried out to estimate the distribution and characteristics of methane hydrate. In this paper, we describe the continuous velocity analysis technique, 2D multi-component seismic survey and 2D deep-towed seismic survey which could give promising results not only for methane hydrates but for conventional oil and gas exploration. The continuous velocity analysis is applied to PSTM gather of 2D/3D seismic data in order to estimate the detailed velocity structure around methane hydrate bearing layer. Especially the high velocity anomaly above BSR is recognized as one of the indication which implies existence of methane hydrate concentrated zone. This velocity analysis technique has been applied for monitoring the oil-sand production with SAGD and it gave the successful results. A 2D multi-component seismic survey was carried out using the RSCS in 2006. It is an ocean bottom cable system employing a series of 3-component geophones connected with submarine optical cable. The resulting PP reflection PSTM sections with the vertical component showed excellent agreement with the 3D survey migrated volume. The PS-converted wave sections with inline horizontal component showed amplitude anomalies around BSR. This fact indicates S-wave velocity anomaly and gives information to estimate the rock physics model. A 2D deep-towed seismic survey was carried out in 1996. Both the source and the hydrophone cable are towed close to the seabed to obtain higher resolution section and velocity profiles compared with 3D marine seismic in the very shallow part. But, we can not recognize BSR clearly on the section because of its insufficient source energy. The high resolution section of the survey has great potential for finding active faults or hydrothermal deposits.
An integrated sedimentological workflow was applied to the methane-hydrate exploration project in the eastern Nankai Trough area, central Japan, as a useful tool to investigate the distributions and actual volume of methane hydrates. Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of submarine-fan turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the occurrence of methane hydrates. Accordingly, this study conducted a) core facies analysis, b) log facies analysis mainly using FMI, c) depositional sequence division, d) seismic facies analysis on the 2D/3D seismic sections, and e) seismic geomorphological analysis on the 3D seismic survey data to map submarine-fan turbidite facies distributions for 17 depositional sequence horizons in the targeted Pleistocene interval. The obtained facies maps reveal that submarine-fan depositional styles changed throughout Pleistocene from a braided channel type, through small radial fan, trough-fill fan, and muddy sheet fan types, to a channel-levee system type. As the next step, the facies maps of each depositional sequence were overlaid with bottom simulating reflector (BSR) distributions as a proxy of methane hydrates. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. The facies maps were also used for calculation of average net-to-gross ratio of the methane hydrate occurrence zones, as the maps provide information on the spatial distributions of seismic facies class, which has individual value of net-to-gross ratio. Finally, this study analyzed reservoir-scale geologic bodies, such as depositional lobes and braided channels of a submarine fan, using 3D seismic survey data, and constructed a detailed geologic model for simulations of methane-hydrate generation and production.
It is well known that the methane hydrate exists below the sea floor over 500 m or the polar region under the frozen ground in the Earth. We made the cross-plot of the methane hydrate and Vp, Vs, or Vp/Vs from the well log data at the eastern Nankai Trough in Japan and the Mackenzie Delta in Canada. We found that as the methane hydrate saturation increases, Vp and Vs increase, and Vp/Vs decreases from these plots. We discussed the rock physics model of methane hydrate bearing layers. We calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation, based on four imaginary rock physics model of methane hydrate bearing layers. From the comparison between the real well sonic data and the calculated data from four models, we inferred that the rock physics model of methane hydrate bearing layers was matrix-supporting model at both the eastern Nankai Trough and the Mackenzie Delta areas. However, there were some error and difference with the Vp, Vs and Vp/Vs from the matrix-supporting model and real well sonic data between the eastern Nankai Trough and the Mackenzie Delta. We estimated the effect of Vp, Vs, and Vp/Vs for the change of clay content of matrix-supporting model. As the clay content decreases, the Vp and Vs increase, and Vp/Vs decreases. The clay content from well core data corresponds with the calculated Vp, Vs, and Vp/Vs as the function of the methane hydrate saturation and the clay content from the matrix supporting model. In case of the estimation of methane hydrate saturation from Vp, Vs, and Vp/Vs, we need to consider the geology, especially sand/clay ratio of the methane hydrate bearing layers.
MH 21 Research Consortium in Japan is planning to implement a gas production test targeting a methane hydrate (MH) reservoir located in MH concentrated zone in the Eastern Nankai Trough (ENT). Two areas surrounding the exploration wells drilled in different MH concentrated zones in ENT were selected as candidates of the production test area because of the availability of well log analysis and seismic interpretation results. The evaluation studies for these areas have been conducted from the various points of view to implement the test successfully. As one of these studies we constructed the 3D reservoir models for the areas consulting the well log analysis and the seismic interpretation results, and predicted the production test performances through numerical simulation. This paper shows how we constructed the 3D models and what results we obtained thorough numerical simulation. First, the frames of the 3D reservoir models for the vicinity of the candidate wells were constructed by consulting seismic interpretation results. The flames were then divided into multiple layers to reproduce the alternating beds structure of sand and mud reflecting the well log interpretation results. The distributions of reservoir properties of each grid layer were estimated by geostatistical techniques using well log interpretation results as hard data and seismic attributes as soft data. Finally the reservoir models were completed by specifying initial pressure and temperature. The well production test performances were predicted through numerical simulation assuming the application of depressurization method. In addition, case studies were conducted to investigate the effect of various scale heterogeneities on the production test performances. The followings were reveled through these studies : (1) Average gas production rate during 30 day test would be over 50,000 m3/d. (2) Large scale heterogeneities such as faults and reservoir dip would affect production test performances significantly.
The Bingham plastic model and the power law model have been widely used to describe the rheological property of drilling mud. However, the Herschel-Bulkley model, also referred to as the yield-power law model or the modified-power law model, is being recognized to more accurately characterize the mud rheological property in low shear rate range. The author has developed a transient cuttings transport simulator in previous studies, where the power law model was used as a mud rheological model. In this study, theoretical discussion is made about modification of generalized Reynolds number presented in the constitutive equations of the cuttings transport model to adopt the Herschel-Bulkley rheological model. Then using the modified cuttings transport simulator, the author conducted steady-state simulations of cuttings transport in vertical and horizontal wells to validate the adoption of the Herschel-Bulkley rheological model. The obtained results were (1) cuttings transport was more effective with mud having larger yield stress of the Herschel-Bulkley model and (2) the power law model and the dual power law model in API RP 13D respectively underestimated and overestimated the cuttings transport efficiency and were inadequate models for mud whose rheology was best described by the Herschel-Bulkley model.
Permeability of sediments is the most effective factor to estimate methane gas from methane hydrate-bearing layers. Although, it is very difficult to sample, because methane hydrate which is contained in un-/semi- consolidated sediments is not stable at atmospheric conditions, moreover, to measure their permeability need techniques. We had developed the pressure temperature core sampler (PTCS) and it brought up sediment samples from methane hydrate-bearing zone of the eastern Nankai Trough, successfully. Thus, several core tests for measuring permeability were carried out. From the results of permeability measurements, we could grasp the relationships between permeability of sand/mud strata and pore space volume in samples. It was considered that these relationships were able to estimate in-situ permeability of sand/mud strata by calculation using the wire-line logging data. Applying the relationships on cored well for calculation were very suitable not only for the values of laboratory measurements but also very close to the relative permeability curves calculated by CMR analysis, especially Timur-Cotes method for permeability calculation. Finally, we could calibrate relative permeability by the measurement relationships.
During Integrated Ocean Drilling Program (IODP) Expedition 314 at Nankai Trough, located on the sudbuction margin offshore Kii Peninsula, five sites were drilled and logged with logging-while-drilling (LWD). A 935-m-thick basin fill was penetrated and logged in Kumano Forearc Basin at Site C0002. The basin fill consists of repeated turbidite sequences intercalated with background hemipelagic mud. A gas hydrate-bearing zone was identified in the interval of 218-401 mbsf above a bottom-simulating reflector (BSR) by indirect evidences of electrical resistivities and P-wave velocities. More than sixty sharp resistivity spikes were identified and most of them are developed in sand layers. Thicker hydrate layers are localized near the BSR and frequency of hydrate occurrence is decreasing upward. Below the BSR, a number of turbidite sequences were identified and the thickest turbidite zone occurs at 482-547 mbsf as a possible gas/fluid conduit to produce hydrates. Distribution of turbidite sequence plays an essential role as potential conduits and containers for gas hydrate generation in Kumano Basin. Detailed observations of the LWD logs demonstrate a possible lithological control on gas migration and systematic distribution of gas hydrates in turbidite-dominant marginal basins.