Offshore India Research Articles

Numerical analysis of gas production potential from a gas-hydrate reservoir at Site NGHP-02-16, the Krishna–Godavari Basin, offshore India–Feasibility of depressurization method for ultra-deepwater environment

Sandy gas-hydrate reservoirs are considered to be promising deposits for gas recovery, but the feasibility of the depressurization method in ultra-deepwater environments remains unclear because high pressure gradient induced by depressurization may increase the water production and reduce the gas–water ratio of produced fluid. In numerical simulations, this study predicts the gas production behavior of a hydrate reservoir (Site NGHP-02-16 in the Krishna–Godavari Basin, offshore India) located in ultra-deepwater (depth 2546.5 m). The reservoir properties such as the hydrate saturation and initial effective (in-situ) permeability were determined from log data and pressure-core analysis. By comparing the hydrate saturations calculated from resistivity log with the estimations from P-wave velocities of pressure-cores, the model parameters a , m , and n of Archie's equation were determined as 1.8, 2.0, and 2.9, respectively. The initial effective permeability of the hydrate-bearing layers (with up to 78% hydrate saturation) was estimated to range from approximately 0.01 to 1.0 millidarcy (mD). At day 180 after the start of depressurization in a single vertical well, the gas production rates ranged from 320 to 9500 Sm 3 /d, depending on the bottom-hole pressure. Lowering the bottom-hole pressure increased the gas production rate and gas–water ratio of the produced fluid. The bottom-hole pressure of 10 MPa or less is recommended for the production test condition in this site; however, aquifer below the gas-hydrate-bearing zone and poor hydraulic sealing of the over- and under-burden increased the water production. Hydraulic isolation of the gas-hydrate-bearing layers is essential when applying depressurization in an ultra-deepwater environment. • Site NGHP-02-16 was modeled by combining pressure-core analysis and log data. • Empirical parameters of Archie's equation were determined. • A bottom-hole pressure of 10 MPa or less is a possible field-test scenario at this site.

Permeability anisotropy and relative permeability in sediments from the National Gas Hydrate Program Expedition 02, offshore India

Gas and water permeability through hydrate-bearing sediments essentially governs the economic feasibility of gas production from gas hydrate deposits. Characterizing a reservoir's permeability can be difficult because even collocated permeability measurements can vary by 4–5 orders of magnitude, due partly to differences between how various testing methods inherently measure permeability in different directions and at different scales. This study uses a customized flow anisotropy cell to investigate geomechanical and hydrological properties of hydrate-bearing sediments focusing on permeability anisotropy (i.e., horizontal, kh, to vertical, kv, permeability ratio) and relative permeability. Two cores recovered during India's National Gas Hydrate Program Expedition 02 (NGHP-02) are tested in this study. Near in situ effective vertical stress, ∼ 2 MPa, the permeability anisotropy is approximately kh/kv = 1.86 for the “seal core” (from a fine-grained non-reservoir overburden sedimentary section) and kh/kv = 4.24 for the gas hydrate reservoir core with tetrahydrofuran (THF) hydrate saturation Sh = 0.8. Permeability anisotropy increases exponentially with effective vertical stress, as described by kh/kv = α(σv/MPa)β, with α = 1.6, β = 0.22 for seal sediment and α = 3, β = 0.5 for THF hydrate-bearing sediment. Results imply the measured permeability from permeameter tests with vertical flow may underestimate the reservoir's flow performance, which is mainly horizontal (radial) toward a vertical well. Hydrate in sediment increases the gas-entry pressure and residual water saturation, but decreases the water retention curve's shape factor (m), resulting in a steeper curve. Distributions of available pore space sizes for flow in sediment with and without THF hydrate (Sh = 0.8) follow a log-normal distribution. Hydrate formation decreases the apparent mean pore size from ∼10 μm to ∼2 μm, without evidently changing the pore size distribution's standard deviation. Gas hydrate dissociation increases effective permeability and relative permeability to gas.

Gas hydrate saturation and morphology from analysis of pressure cores acquired in the Bay of Bengal during expedition NGHP-02, offshore India

India's National Gas Hydrate Program Expedition 2 (NGHP-02) completed an ambitious pressure coring program as part of their effort to evaluate gas hydrate in coarse-grained reservoirs in the Bay of Bengal. The 75 successful pressure cores were used to measure gas hydrate saturation, examine the morphology of the gas hydrate at in situ conditions, and to provide samples for further laboratory work. Pressure core quality was assessed via pressure-temperature records and X-ray computed tomographic reconstructions of cores. Gas hydrate saturations were measured by using the gas from quantitative degassing experiments in methane mass balance calculations. Gas hydrate morphology was determined from X-ray computed tomographic reconstructions, P-wave velocity, and gamma density measured under pressure using PCATS, the Pressure Core Analysis and Transfer System. Gas hydrate saturations reached 65–85% in the pores of sandy reservoir sediments, and were near full saturation in gravelly sediments. Clayey interlayers between reservoir sediments had gas hydrate saturations near 10%, with no evidence of gas hydrate veins in X-ray computed tomographic (CT) reconstructions. Veins of gas hydrate were visible in X-ray images of clayey non-reservoir sediments; at some locations the full complement of gas hydrate was contained in X-ray-resolveable veins, but in others the veins only accounted for part of the total gas hydrate burden in the sediment, implying that some gas hydrate was finely distributed in the pore space or in veins below the resolution of the X-ray CT data. At one location low-density, biosilica-rich silty clays hosted pore-filling gas hydrate at saturations of 45%, with only slightly elevated P-wave velocities. Gas hydrate saturation and morphology from all the pressure cores are presented here and placed into geological context. • Pressure core quality must be critically examined before using data. • Gas hydrate saturations reached 65–85% in the pores of sandy reservoir sediments. • Clayey interlayers between reservoir sediments had hydrate saturations near 10%. • Visible veins did not contain all the gas hydrate in many clayey samples. • Low-density biosilica-rich silty clays contained 45% pore-filling gas hydrate.

Multi-measurement approach for establishing the base of gas hydrate occurrence in the Krishna-Godavari Basin for sites cored during expedition NGHP-02 in the offshore of India

The 2015 National Gas Hydrate Program of India's second expedition, NGHP-02, acquired logging and coring datasets for constraining the base of the gas hydrate occurrence zone (deepest GH) and the theoretical base of gas hydrate stability zone (BGHS). These data are used here for two primary goals: to constrain the deepest occurrence of gas hydrate relative to predicted stability limits and the observed BSR, and to characterize the nature of the contact between gas hydrate-bearing sediment and the underlying gas hydrate-free sediment. A consensus depth for the deepest GH is derived for each NGHP-02 coring site from downhole indicators of gas hydrate occurrence obtained from well-log electrical resistivity and acoustic data, pressure core compressional wave velocity measurements, and conventional core measurements of anomalously low temperatures. To establish the theoretical BGHS, models of gas hydrate phase stability with depth are compared with downhole temperature profiles derived from: 1) assuming a constant geothermal gradient consistent with downhole temperature measurements, and 2) assuming constant heat flow using a geotherm through the downhole temperature measurements and incorporating thermal conductivity calculated from borehole logging data. Although the deepest NGHP-02 GH occurrences are controlled at several sites by a lithologic boundary, most sites have deepest GH occurrences within a single coarse-grained lithology. Cutoffs within a single coarse-grained lithology, which occur for the primary NGHP-02 Area B gas hydrate reservoir, will inhibit pore-pressure drawdowns used to extract methane from gas hydrate as an energy resource.

Permeability variation and anisotropy of gas hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin, offshore India

Abstract Permeability measurements were conducted on gas hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin during India's National Gas Hydrate Program Expedition 02. Pressure cores collected in the deep seabed of the Indian Ocean were cut and stored under high pressure and low temperature on the D/V Chikyu using pressure core analyzing tools. A total of 25, 1.2-m storage chambers were transferred to Japan from India for pressure core studies using pressure-core analysis tools at the National Institute of Advanced Industrial Science and Technology. The sediment core's pressure and temperature were maintained within the hydrate stability conditions during the entire process of transfer and loading into a triaxial testing apparatus called TACTT to preserve the hydrate, which can simulate the in-situ effective stress state under triaxial conditions. The hydrate saturation of the samples was in the range of 50–90%. It was found that the initial effective water permeability was in range of 0.01 mD to tens of mD, depending on the hydrate saturation and the mean particle size of the host sediment. The hydrate pore space morphology is also a critical factor, and results suggest hydrates are forming in the pore centers of sand grains as pore filling, but also as a sediment grain coating. In a first for gas hydrate-bearing pressure cores, the permeability anisotropy was established via vertical and horizontal flow tests combined with a pore fluid flow simulation. The horizontal/vertical permeability ratio was found to be 4. Additionally, the effective stress dependency of permeability was investigated by performing flow tests at different effective stresses. Due to decreasing porosity and particle crushing, the permeability dropped by approximately 90% for an effective stress increase of 10 MPa.

Compressibility and particle crushing of Krishna-Godavari Basin sediments from offshore India: Implications for gas production from deep-water gas hydrate deposits

Abstract Depressurizing a gas hydrate reservoir to extract methane induces high effective stresses that act to compress the reservoir. Predicting whether a gas hydrate reservoir is viable as an energy resource requires enhanced understanding of the reservoir's compressibility and susceptibility to particle crushing in response to elevated effective stress because of their impact on the long-term permeability and geomechanical stability of the reservoir. This study investigates physical and geomechanical properties of natural sediments with and without tetrahydrofuran (THF) hydrate subjected to high effective stresses of up to 25 MPa. Experimental results show the stiffness of hydrate-free sediments is mainly governed by the stress state and history, while the stiffness of hydrate-bearing sediments reflects both the grain supporting nature of the interconnected hydrate phase and stress effects. The Poisson's ratio of hydrate-bearing sediments at low stresses is dominated by the Poisson's ratio of the interconnected pore-filling phases, and dominated at high stresses by elastic properties of both the skeleton and pore-filling phases. The stress-void ratio responses of hydrate-bearing sediments above the pre-consolidation stress yields a slightly convex-downward trend, suggesting compressibility is influenced by the stiffness of THF hydrate and sediment grains rather than only by void space reduction. The shape of the compression index (Cc) trend may be attributed to an increasing effective gas hydrate saturation as the total pore volume decreases under loading. The results also show that the presence of THF hydrate in sediments can mitigate particle crushing by suppressing particle rearrangement and supporting a portion of the load that would otherwise have to be carried by the sediment. Therefore, the loss of hydrate crystals during gas production may exacerbate sand crushing.

Characterizing Seismo-stratigraphic and Structural Framework of Late Cretaceous-Recent succession of offshore Indus Pakistan

Abstract Regional seismic reflection profiles and deep exploratory wells have been used to characterize the subsurface structural trends and seismo-stratigraphic architecture of the sedimentary successions in offshore Indus Pakistan. To improve the data quality, we have reprocessed the seismic data by applying signal processing scheme to enhance the reflection continuity for obtaining better results. Synthetic seismograms have been used to identify and tie the seismic reflections to the well data. The seismic data revealed tectonically controlled, distinct episodes of normal faulting representing rifting during Mesozoic and transpression at Late Eocene time. A SW-NE oriented anticlinal type push up structure is observed resulted from the basement reactivation and recent transpression along Indian Plate margin. The structural growth of this particular pushup geometry was computed. Six mappable seismic sequences have been identified on seismic records. In general, geological formations are at shallow depths towards northwest due to basement blocks uplift. A paleoshelf is also identified on seismic records overlain by Cretaceous sediments, which is indicative of Indian-African Plates rifting at Jurassic time. The seismic interpretation reveals that the structural styles and stratigraphy of the region were significantly affected by the northward drift of the Indian Plate, post-rifting, and sedimentation along its western margin during Middle Cenozoic. A considerable structural growth along the push up geometry indicates present day transpression in the margin sediments. The present comprehensive interpretation can help in understanding the complex structures in passive continental margins worldwide that display similar characteristics but are considered to be dominated by rifting and drifting tectonics.

National Gas Hydrate Program Expedition 01 offshore India; gas hydrate systems as revealed by hydrocarbon gas geochemistry

Abstract The National Gas Hydrate Program Expedition 01 (NGHP-01) targeted gas hydrate accumulations offshore of the Indian Peninsula and along the Andaman convergent margin. The primary objectives of coring were to understand the geologic and geochemical controls on the accumulation of methane hydrate and their linkages to underlying petroleum systems. Four areas were investigated: 1) the Kerala-Konkan Basin in the eastern Arabian Sea, 2) the Mahanadi and 3) Krishna-Godavari Basins in the western Bay of Bengal, and 4) the Andaman forearc Basin in the Andaman Sea. Upward flux of methane at three of the four of the sites cored during NGHP-01 is apparent from the presence of seafloor mounds, seismic evidence for upward gas migration, shallow sub-seafloor geochemical evidence of methane oxidation, and near-seafloor gas composition that resembles gas from depth. The Kerala-Konkan Basin well contained only CO2 with no detectable hydrocarbons suggesting there is no gas hydrate system here. Gas and gas hydrate from the Krishna-Godavari Basin is mainly microbial methane with δ13C values ranging from −58.9 to −78.9‰, with small contributions from microbial ethane (−52.1‰) and CO2. Gas from the Mahanadi Basin was mainly methane with lower concentrations of C2-C5 hydrocarbons (C1/C2 ratios typically >1000) and CO2. Carbon isotopic compositions that ranged from −70.7 to −86.6‰ for methane and −62.9 to −63.7‰ for ethane are consistent with a microbial gas source; however deeper cores contained higher molecular weight hydrocarbon gases suggesting a small contribution from a thermogenic gas source. Gas composition in the Andaman Basin was mainly methane with lower concentrations of ethane to isopentane and CO2, C1/C2 ratios were mainly >1000 although deeper samples were

Comparative Study of Pore Pressure in Wells of Indus Offshore, Southwest Pakistan

Hydrocarbon exploration in frontier areas or offshore requires significant considerations regarding drilling and engineering for the purpose of lowering cost without compromising safety and the environment, and managing the risk associated with very expensive drilling operations. Clastics strata encountered in the wells drilled in the Offshore Indus Basin of Pakistan (Indus Marine-1A, Indus Marine-1C) are characterized by the development of abnormal formation pressures. In this study, an estimation of abnormal pressures using offshore well data is carried out, using Eaton’s resistivity model, to obtain better pressure estimates. The study uses well log data to derive the overburden gradient (OBGrad), pore pressure gradient (PPG), and fracture gradients (FGs) in studied wells, based on Eaton’s resistivity model. The gradient and pressure cross-plots generated for these wells show the relationship of pressure with increasing depth. The study concludes that Indus Marine-1C is characterized by a higher over pressure than Pak Can-01 and Indus Marine-1A. Findings from the study confirm that the Indus Marine-1C shows high FG, while in Pak Can-01 and Indus Marine-1A, the FG is below the maximum limit. The study concludes that rapid burial and high rates of sedimentation during the Miocene Epoch (23.03–5.3 million years ago) have led to the development of overpressure by disequilibrium compaction.

Qualitative analysis of seismic amplitudes for characterization of Pliocene hydrocarbon sands, eastern offshore India

A comparative study of amplitudes of near- and far- stacks with prestack can be a simple and effective way for characterization of Pliocene hydrocarbon channel sands in offshore India. Synergetic studies of amplitude analysis constrained with well logs and testing data make it possible to predict quality of reservoir sands, the pore fluid content and their production potential. The amplitude studies can also help in taking decisions for testing doubtful zones in drilled wells. An example of a seemingly genuine high amplitude seismic anomaly is authenticated by AVO but testing water is also discussed as an example of seismic pitfall.

Growth and demise of a Paleogene isolated carbonate platform of the Offshore Indus Basin, Pakistan: effects of regional and local controlling factors

Based on high-resolution seismic and well datasets, this paper examines the evolution and drowning history of a Paleocene–Eocene carbonate platform in the Offshore Indus Basin of Pakistan. This study uses the internal seismic architecture, well log data as well as the microfauna to reconstruct factors that governed the carbonate platform growth and demise. Carbonates dominated by larger benthic foraminifera assemblages permit constraining the ages of the major evolutionary steps and show that the depositional environment was tropical within oligotrophic conditions. With the aid of seismic stratigraphy, the carbonate platform edifice is resolved into seven seismic units which in turn are grouped into three packages that reflect its evolution from platform initiation, aggradation with escarpment formation and platform drowning. The carbonate factory initiated as mounds and patches on a Cretaceous–Paleocene volcanic complex. Further, the growth history of the platform includes distinct phases of intraplatform progradation, aggradation, backstepping and partial drownings. The youngest succession as late-stage buildup records a shift from benthic to pelagic deposition and marks the final drowning in the Early Eocene. The depositional trend of the platform, controlled by the continuing thermal subsidence associated with the cooling of volcanic margin lithosphere, was the major contributor of the accommodation space which supported the vertical accumulation of shallow water carbonate succession. Other factors such as eustatic changes and changes in the carbonate producers as a response to the Paleogene climatic perturbations played secondary roles in the development and drowning of these buildups.

Seismic attenuation in Jati area, Lower Indus Basin, Pakistan

Reduced amplitude and distorted dispersion of seismic waves caused by attenuation, especially strong attenuation, always degrade the resolution of migrated images. To improve seismic imaging, attenuation must be compensated. This study addresses the factors causing seismic attenuation in Jati Block. Jati Block lies in Lower Indus Basin, Pakistan, approximately 25 mi north of the offshore Indus. Method used for Q factor is empirical equation method. Q factor correlation indicates that there are three major zones of attenuation in Jati block, i.e., zone I (surface to top Khadro), zone II (Upper Goru Formation), and zone III (Lower Goru Formation). Lowest Q value is in zone I, followed by zone II and zone III, respectively. Commonly, Khadro Formation (Basalt) of Paleocene and saucer-shaped igneous intrusion is considered as sources of attenuation. However, surprisingly, Khadro Formation of Paleocene and saucer-shaped igneous intrusion is zone of minimum attenuation and causes minimal transmission loss. Anisotropy analysis is performed to determine cause of attenuation within these zones. Thomson anisotropic parameters are computed for vertical wells using Backus averaging algorithm. These parameters are calibrated using sonic scanner data available for one well. Correlation of Q factor curve with mud log suggests that loose, unconsolidated sands and sand-shale layering are sources of attenuation within zone I. Attenuation in this zone is mostly due to fluid motion relative to the framework of loosely packed grains. Major lithology of Upper Goru Formation is marl. It is a slow formation, and in this formation, P wave loses energy to the formation in what is known as leaky P mode (sonic logging) and is dispersive (seismic). Epsilon (Ɛ) value is greater than almost 0.02 throughout Lower Goru Formations, indicating that this formation is strongly anisotropic. Fluctuation of epsilon (Ɛ) in Lower Goru Formation also suggests that this formation is causing layer-induced anisotropy. This layer-induced anisotropy in Lower Goru Formation added by dispersive and slow nature of Upper Goru Formation causes rapid attenuation.

Controls on evolution of gas‐hydrate system in the Krishna‐Godavari basin, offshore India

AbstractIn this study, we integrate environmental magnetic, sedimentological, and geochemical records of sediment core of Hole NGHP‐01‐10D overlying methane hydrate deposits to decipher the controls on the evolution of fracture‐filled gas‐hydrate system in the Krishna‐Godavari (K‐G) basin. Four distinct sedimentary units have been identified, based on the sediment magnetic signatures. An anomalous zone of enhanced magnetic susceptibility (Unit III: 51.9–160.4 mbsf) coinciding with the gas hydrate bearing intervals is due to the presence of magnetite‐rich detrital minerals brought‐in by the river systems as a result of higher sedimentation events in K‐G basin and has no influence over hydrate formation. A strong to moderate correlation between magnetite concentration and chromium reducible sulfur (CRS) content indicates significant influence of sulfidization on the magnetic record and could be further exploited as a proxy to decipher paleo‐H2S seepage events. Analysis of high‐resolution seismic, bathymetry, and sub‐bottom profiler data reveals the existence of a regional fault system in K‐G basin. The opening and closing dynamics of the faults facilitated the migration and trapping of required gas concentrations resulting in accumulation of gas hydrates at the studied site. The seismic data provides support to the rock‐magnetic interpretations. The observed variations in magnetic and geochemical properties have resulted from the episodic flow of methane and sulfide‐enriched fluids through the fracture‐filled network formed as a result of shale‐tectonism. Our study demonstrated the potential of using an enviro‐magnetic approach in combination with other proxies to constrain the evolution of gas‐hydrate system in marine environments.

Sequence stratigraphic analysis of Eocene Rock Strata, Offshore Indus, southwest Pakistan

In this study, seismic data from two wells (Pak G2-1 and Indus Marine-1C) and age diagnostic larger benthic foraminifera (LBF) within drill cuttings has been used for the first time to identify depositional sequences within the carbonates in the Offshore Indus Basin, Pakistan. The Offshore Indus is tectonically categorized as a passive continental margin where carbonates occur as shelf carbonates in the near offshore and on volcanic seamounts in deeper waters. Seismic data analysis has indicated the presence of minor faults and carbonate buildups above the igneous basement in the south. Patterns of the seismic reflections enabled definition of three seismic facies units identified as: Unit 1 basement, represented by chaotic, moderate amplitude reflection configuration; while parallel bedding and the drape of overlying strata is typical character of Unit 2, carbonate mound facies. The younger Miocene channels represent Unit 3. The diagnosis of Alveolina vredenburgi/cucumiformis biozone confirmed the Ilerdian (55–52 Ma) stage constituting a second order cycle of deposition for the Eocene carbonates (identified as Unit 2). The carbonate succession has been mainly attributed to an early highstand system tract (HST). The environmental conditions remained favorable leading to the development of keep-up carbonates similar to pinnacle buildups as a result of aggradation during late transgressive system tract and an early HST. The carbonate sequence in the south (Pak G2-1) is thicker and fossiliferous representing inner to middle shelf depths based on fauna compared to the Indus Marine-1C in the north, which is devoid of fossils. Three biozones (SBZ 5, SBZ 6 and SBZ 8) were identified based on the occurrence of LBF. The base of the SBZ 5 zone marks the larger foraminifera turnover and the Paleocene–Eocene (P–E) boundary. The LBF encountered in this study coincides with earlier findings for the P–E boundary. Our findings indicate that the entire Ilerdian stage ranges from 55.5 to 52 Ma that was the episode of warmer water conditions on the carbonate shelves leading to the diversification of K-strategist larger foraminifera. The larger foraminiferal assemblage encountered in this study confirms the findings. The possible indication of stratigraphic-combination traps, revealed as reflection terminations, make carbonate mounds in the south a potential exploration target.

Estimating a starting model for full-waveform inversion using a global optimization method

Full-waveform inversion (FWI) has become a popular method to estimate elastic earth properties from seismograms. It is formulated as a data-fitting least-squares minimization problem that iteratively updates an initial velocity model with the scaled gradient of the misfit until a satisfactory match between the real and synthetic data is obtained. However, such a local optimization approach can converge to a local minimum if the starting model used is not close enough to an optimal model. We have developed a two-step process in which we first estimate a starting model using a global optimization method. Unlike local optimization methods, a global optimization method starts with a random starting model and is not generally susceptible to be trapped in a local minimum. The starting model for FWI that we aim to estimate is sparsely parameterized and contains a set of interfaces and velocities that are used to represent the entire velocity model. We have obtained the depth of the interfaces and the velocities by minimizing the data misfit in the least-squares sense using a global optimization method called very fast simulated annealing (VFSA). Once the sparse velocity model was obtained from VFSA, we used that as a starting model in a conventional gradient-based FWI to obtain the final model. We have applied the proposed method to one synthetic data set and two field data sets from offshore India. The proposed method was able to estimate a velocity model that was not cycle skipped for realistic frequency bands. We have demonstrated that with the proper choice of model parameterization and optimization parameters, the global and gradient optimization algorithms converge in a finite number of iterations. We have determined that the resulting algorithm is computationally feasible in two dimensions and accurate for practical implementation of FWI.

Offshore Propagation of Coastal Precipitation

Abstract This work focuses on the seaward propagation of coastal precipitation with and without mountainous terrain nearby. Offshore of India, diurnal propagation of precipitation is observed over the Bay of Bengal. On the eastern side of the bay, a diurnal but nonpropagating signal is observed near the west coast of Burma. This asymmetry is consistent with the inertio-gravity wave mechanism. Perturbations generated by diurnal heating over the coastal mountains of India propagate offshore, amplify in the upwind direction, and dissipate in the downwind direction relative to the steering wind, owing to critical-level considerations. A linear model is applied to evaluate sensitivity to gravity waves, as these affect deep moist convection and propagation. Analyses are performed for various heating depths, mountain widths, stability, Coriolis effect, background mean wind, and friction. Calculations reveal how these factors affect the amplitude, dissipation, initiation phase, and propagation speed of the diurnal disturbance. The propagation of precipitation triggered by land–sea breezes is distinguishable from that triggered by a mountain–plains circulation. Convection resulting purely from mountain heating begins earlier, propagates slower, and damps faster than that of the land–sea breeze. For mountains near a coast, slower propagation and stronger earlier convection result from a resonance-like combination of two dynamical mechanisms. The propagation of precipitation is initially triggered by the mountain breeze near the coastal mountain. Over the open ocean, the dominant signal propagates as that of the land breeze but with stronger convection.

Projected impact of climate change on waves at Mumbai High

Structures built in the sea are traditionally designed according to historical climate observations or hindcasts. In geographical locations typical of India, such designs do not take the effect of future climate change into account. For structural safety, considerations of such effects are highly desirable. Many recent studies have demonstrated likely changes in the extreme wave conditions at different offshore locations. This paper reports similar results in the Mumbai High region, where the majority of India's offshore oil platforms are located. Using historical wind data provided by the National Centre for Environmental Prediction/National Centre for Atmospheric Research and a Canadian general circulation model for future data (CMIP5-RCP8.5), a numerical wave model of the past and future wave conditions was simulated over a 40-year period. A statistical analysis of both past and projected datasets obtained significant wave heights with a 100-year return. The comparison of wave heights derived from past and future datasets showed that wave magnitudes would increase at almost all locations within the Mumbai High area. The northern locations may undergo a significant rise from 10% to 28%, while the southern sites may experience a smaller increase, from almost zero to 10%, under the highest projected pathway of global warming.

Exploration application of seismic amplitude analysis in the Krishna-Godavari Basin, east coast of India

Seismic amplitude anomalies are attractive exploration targets in the Krishna-Godavari Basin offshore India. These bright spots mostly have very high amplitudes, so confident interpretations have been possible. We distinguished between hydrocarbon-bearing sands, water-bearing sands, and high-impedance nonreservoir bodies. Also, we mapped channel architecture and accurately predicted reservoir thickness. Strong amplitude anomalies, prospective seismic character based on an understanding of data phase and polarity, flat spots, and amplitude versus offset have all provided valuable evidence.

Comparison of the physical and geotechnical properties of gas-hydrate-bearing sediments from offshore India and other gas-hydrate-reservoir systems

The sediment characteristics of hydrate-bearing reservoirs profoundly affect the formation, distribution, and morphology of gas hydrate. The presence and type of gas, porewater chemistry, fluid migration, and subbottom temperature may govern the hydrate formation process, but it is the host sediment that commonly dictates final hydrate habit, and whether hydrate may be economically developed.In this paper, the physical properties of hydrate-bearing regions offshore eastern India (Krishna-Godavari and Mahanadi Basins) and the Andaman Islands, determined from Expedition NGHP-01 cores, are compared to each other, well logs, and published results of other hydrate reservoirs. Properties from the hydrate-free Kerala-Konkan basin off the west coast of India are also presented. Coarser-grained reservoirs (permafrost-related and marine) may contain high gas-hydrate-pore saturations, while finer-grained reservoirs may contain low-saturation disseminated or more complex gas-hydrates, including nodules, layers, and high-angle planar and rotational veins. However, even in these fine-grained sediments, gas hydrate preferentially forms in coarser sediment or fractures, when present. The presence of hydrate in conjunction with other geologic processes may be responsible for sediment porosity being nearly uniform for almost 500 m off the Andaman Islands.Properties of individual NGHP-01 wells and regional trends are discussed in detail. However, comparison of marine and permafrost-related Arctic reservoirs provides insight into the inter-relationships and common traits between physical properties and the morphology of gas-hydrate reservoirs regardless of location. Extrapolation of properties from one location to another also enhances our understanding of gas-hydrate reservoir systems. Grain size and porosity effects on permeability are critical, both locally to trap gas and regionally to provide fluid flow to hydrate reservoirs. Index properties corroborate more advanced consolidation and triaxial strength test results and can be used for predicting behavior in other NGHP-01 regions. Pseudo-overconsolidation is present near the seafloor and is underlain by underconsolidation at depth at some NGHP-01 locations.

Geologic implications of gas hydrates in the offshore of India: Krishna–Godavari Basin, Mahanadi Basin, Andaman Sea, Kerala–Konkan Basin

Gas hydrate resource assessments that indicate enormous global volumes of gas present within hydrate accumulations have been one of the primary driving forces behind the growing interest in gas hydrates. Gas hydrate volumetric estimates in recent years have focused on documenting the geologic parameters in the “gas hydrate petroleum system” that control the occurrence of gas hydrates in nature. The primary goals of this report are to review our present understanding of the geologic controls on the occurrence of gas hydrate in the offshore of India and to document the application of the petroleum system approach to the study of gas hydrates.National Gas Hydrate Program of India executed the National Gas Hydrate Program Expedition 01 (NGHP-01) in 2006 in four areas located on the eastern and western margins of the Indian Peninsula and in the Andaman Sea. These areas have experienced very different tectonic and depositional histories. The peninsular margins are passive continental margins resulting from a series of rifting episodes during the breakup and dispersion of Gondwanaland to form the present Indian Ocean. The Andaman Sea is bounded on its western side by a convergent margin where the Indian plate lithosphere is being subducted beneath southeast Asia.NGHP-01 drilled, logged, and/or cored 15 sites (31 holes) in the Krishna–Godavari Basin, 4 sites (5 holes) in the Mahanadi Basin, 1 site (2 holes) in the Andaman Sea, and 1 site (1 hole) in the Kerala–Konkan Basin. Holes were drilled using standard drilling methods for the purpose of logging-while-drilling and dedicated wireline logging; as well as through the use of a variety of standard coring systems and specialized pressure coring systems.NGHP-01 yielded evidence of gas hydrate from downhole log and core data obtained from all the sites in the Krishna–Godavari Basin, the Mahanadi Basin, and in the Andaman Sea. The site drilled in the Kerala–Konkan Basin during NGHP-01 did not yield any evidence of gas hydrate. Most of the downhole log-inferred gas hydrate and core-recovered gas hydrate were characterized as either fracture-filling in clay-dominated sediments or as pore-filling or grain-displacement particles disseminated in both fine- and coarse-grained sediments. Geochemical analyses of gases obtained from sediment cores recovered during NGHP-01 indicated that the gas in most all of the hydrates in the offshore of India is derived from microbial sources; only one site in the Andaman Sea exhibited limited evidence of a thermogenic gas source. The gas hydrate petroleum system concept has been used to effectively characterize the geologic controls on the occurrence of gas hydrates in the offshore of India.