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Tristan ReillySenior Technical Researcher, E&P Fields – IHS Markit Ltd

Deep Marine Gas Hydrates An Answer to India’s Growing Energy Requirements?

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MARINE GAS HYDRATE FORMATIONGas hydrate is a crystalline solid which is the product of natu-ral gases, such as methane, coming into contact with water in low temperature (4 – 10 °C) and high pressure (10 - 30 MPa) conditions. These are the prevailing conditions in the upper stra-tigraphy of deepwater sediments (>400 m water depth) where un-trapped gaseous hydrocarbons react with in-situ connate water. The reaction causes gas hydrates to crystallise within the pore spaces of the sediments in clathrate compounds whereby water forms a lattice or cage surrounding gas molecules creat-ing a substance similar in texture and appearance to ice. Vast amounts of hydrocarbons can be trapped in this manner with every m3 of solid gas hydrate containing up to 164 m3 of gase-ous methane.

Gas hydrates form within unlithified deep sea sediments in a stability zone which is defined by the temperature with respect to depth below the ocean floor. Temperature within the sedi-mentary layers generally increases with depth in a relatively con-stant manner meaning that at a certain depth below the ocean floor (usually 150 - 400 m depth) gas hydrate will not form and will remain in a gas saturated water solution. Above this critical depth, gas hydrate is stable and forms a zone of relatively uni-form thickness following the bathymetry of the ocean floor and not necessarily conforming to the orientation of sedimentary layers and structures. The hydrate also forms a seal to any free-gas beneath the stability zone as it fills the porosity of the sedi-ment blocking upward gas migration. The bottom of the stability zone can be identified in seismic surveys as a high-amplitude Bottom Simulating Reflector (BSR) which mimics the topology of the ocean floor reflector but has the opposite polarity and can cut across dipping strata.

INDIAN EXPLORATION EFFORTSIn 1997, the Indian Ministry of Petroleum & Natural Gas (MoPNG) set up the National Gas Hydrate Program (NGHP) to work closely with the Directorate General of Hydrocarbons (DGH), national research institutions (NIO, NGRI and NIOT) and national E&P companies (ONGC, OIL, GAIL and IOC) with the

India is expected to become the world’s third largest energy consumer by 2020 due to its expanding economy and population. Energy security is vital for India’s ambitions for continued development but the country is becoming increasingly dependent on foreign gas imports to meet the country’s gas demand. Imported LNG has risen from 22.7% of gas consumption in 2009-10 to 45.5% in 2014-15 and is expected to increase further (IHS Markit Ltd., 2016).

In recent years there has been a greater impetus on domestic conventional gas resource identification and extraction with the government attempting to reduce the amount of regulations on the petroleum industry and boost investment. Longer term, one area which has been identified as a potential way of increasing production is by unlocking gas hydrates from under the ocean floor. India is estimated to have as much as 66,900 Tcfg located in India’s deep water regions (AK Jha, SPE, 2012). This volume is 420 times the estimated 157 Tcfg of India’s in place conventional gas (IHS Markit, 2016). Despite these extraordinary volumes and with gas hydrates being a relatively clean fossil fuel, the technology to produce gas hydrate remains at the theoretical and testing stages. This article will look at gas hydrates located in deep marine conditions; how they are formed, how they are imaged and how they could potentially be extracted. The other main type of location in which gas hydrate is found is in extremely low temperature conditions, predominantly inside the permafrost of the Arctic Circle, and is outside the scope of this article.

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desired aim of understanding gas hydrate exploration techniques and possible methods of safe and cost ef-fective extraction. Seismic surveys off the East and West Coasts of India and the waters around the Andaman Islands, during the late 1990s and early 2000s, indicated large expanses of gas hydrates in the Krishna-Godavari, Andaman, Mahanadi and Kerala-Konkan Basins with sig-nificant deposits in other areas (Figure 1).

The NGHP, in partnership with the USGS, undertook an exploration program, NGHP-01, between April - August 2006, at a cost of USD 36 million, focusing on the four main prospective basins which had been previously identified in seismic studies. A total of 21 drill sites were established; 1 site in the Kerela-Konkan, 15 sites in the Krishna Godavari, 4 sites in the Mahanadi and 1 site in the Andaman. Utilising the drillship ‘Joides Resolution’, a total of 39 wells were drilled, 27 with cored holes, 13 with wireline logged holes and 12 with LWD-MWD holes

and a total of 6 VSP surveys shot. More than 9,250 m of sedimentary records were taken with 2,850 m of core recovered. Gas hydrate was found to be predominantly located in coarse grained (mostly sand rich) sediments as well as sub-vertical fracture sets (mostly in low porosity clays/shales). The calculated depth based on the BSRs imaged in seismic studies also correlated well with the base of the gas hydrate stability zone (BGHSZ) derived from recorded temperature profiles within the wells. The highlight of the expedition included the identification of one of the world’s richest gas hydrate deposits at well-site NGHP-01-10 in the Krishna Godavari Basin where deposits were found in the fracture sets of a shale domi-nated area. Furthermore, one of the world’s deepest and thickest occurring deposits worldwide was discovered at wellsite NGHP-01-17 in the Andaman Basin where gas hydrate was recorded at depths over 600 m below the ocean floor.

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Figure 1 - Map detailing gas hydrate thickness concentrations in the Indian Ocean. Adapted from DGH Presentation, 2011

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At the beginning of March 2015, the NGHP con-ducted another expedition, NGHP-02, which was undertaken off the east coast, also in conjunc-tion with the USGS, at a cost of USD 92 million. The expedition’s aim was the targeting of deeper water, toe-of-slope, coarse, sand rich dominated strata that were deemed most suitable for future gas production. A total of 25 drill sites were select-ed in the Krishna-Godavari and Mahanadi Basins based on data recorded during the NGHP-01 ex-pedition and additional seismic surveys. The sites were the locale for 42 exploratory holes drilled from the drillship, ‘Chikyu’, in water depths ranging between 1,519 – 2,815 m, with subsea comple-tion depths ranging between 239 – 567 m. A total of 6,659 m of sedimentary section was logged by LWD and wireline methods, in 25 and 10 of the holes respectively. A total of 2,271 m of core was also recovered from 16 of the holes with the for-mation temperature measured ahead of the drill bit. Significantly, 156 m of pressurised core was recovered, preserving the in-situ conditions of the gas hydrate for either mechanical triaxial testing or to be quantitatively degassed for hydrate concen-tration analysis. A Modular Dynamic Tester (MDT) was also employed; successfully flow testing gas in 2 holes. The expedition confirmed the presence of large, highly saturated gas hydrate deposits within the coarse grained sand-rich sediments as expected, helping to validate the NGHP’s depo-sitional models. Effective permeabilities were also shown to be significantly higher than previously interpreted laboratory and field studies.

It became apparent that reservoirs within the Mahanadi Basin were limited by the availability of gas to charge the depositional systems. However, Krishna Godavari deposits showed a high degree of charging, especially within two study areas. The study area drilled by the wells NGHP-02-16, 17, 20 & 21 targeted a regional anticline with a well-defined BSR (Figure 2). The prospect contained two significant reservoirs with high levels of gas hydrate saturation through a combination of frac-ture filling and pore filling gas hydrates.

An even more significant accumulation was en-countered by the wells NGHP-02-08 & 09 which intersected a large channel levee complex with a reasonably well developed BSR (Figure 3). The wells had discovered a fully developed gas hydrate depositional system after encountering a 50 m

Deep Marine Gas Hydrates An Answer to India’s Growing Energy Requirements? (cont.)

Figure 2 - Krishna Godavari Basin seismic section through regional anticlinal prospect drilled by the NGHP-16, 17, 20 & 21 wells. NGHP-02 Expedition, 2015

Figure 3 - Krishna Godavari Basin seismic section through channel levee complex drilled by the NGHP-08 & 09 wells. NGHP-02 Expedition, 2015

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For further information please contact:

Anthony Jaep, Field Researcher – Europe, IHS MarkitAnthony.Jaep@IHSMarkit.com

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section which showed very high gas hydrate saturation and high porosity. ONGC sources estimated the find to be as large as 134 Tcfg GIIP.

POTENTIAL EXTRACTION TECHNIQUESThe solid state of gas hydrate makes it very difficult to extract from be-neath the ocean floor compared to fluid gas extraction from convention-al reservoirs. A number of ways to initiate the production process have been theorised including depressuri-sation, thermal stimulation, inhibitor injection and CO2/N2 replacement.

1) Depressurisation involves pro-ducing any free gas from beneath the gas hydrate stability zone, reduc-ing the pressure below the pressure stability limit of the hydrate, thus lib-erating gas.

2) Thermal stimulation is a pro-cess whereby the reservoir is heated by processes including hot water/steam injection, internal combustion or applying a voltage. The extra heat within the reservoir would ‘thaw’ the hydrate, releasing gas.

3) Inhibitor injection involves the injection of chemicals such as meth-anol and glycol into the reservoir which shifts the chemical equilibrium of the system, reducing the freezing point of the hydrate, making it desta-bilise and liberate gas.

4) CO2/N2 replacement is a pro-cess whereby CO2 and N2 are injected into the reservoir and the molecules preferentially replace the hydrocarbon molecules locked within the water lattice structure. This carbon sequestration process also has major incentives as an environ-mentally friendly technique.

There has been no commercial production of gas hydrates to date but the largest step towards pro-duction was taken in March 2013

by a Japanese collaboration. The Japanese Ministry of Economy, Trade and Industry (METI) and the Japan Oil, Gas and Metals National Corporation (JOGMEC), amongst others, undertook a test produc-tion study at the AT1-P wellsite at the Daini-Atsumi Knoll in the east-ern Nankai Trough, Pacific Ocean. Gas was produced at ~700,000 cfg/d for 6 days before major sand ingress halted the operation. The process followed the ‘depressurisa-tion’ model and demonstrated the production potential from marine gas hydrates even though only a small amount was produced.

Gas hydrate extraction requires major environmental and safety considerations. During production, the unlithified sedimentary pile may destabilise as the solid hydrate be-comes liquid and gaseous. This is of particular concern as the wellbore in-tegrity may be compromised as well as causing ocean floor slumping and subsidence. Additionally, methane is a much more potent greenhouse gas than CO2 and if the gas liberation process becomes uncontrolled, any escaped gas may further compound the greenhouse effect. The pump-ing of methanol, glycol, CO2 and N2 into the ocean floor also poses major technical challenges requiring mitiga-tion against leakage into the ocean ecosystem. It is widely understood that a combination of the different extraction techniques would be op-timal to minimise ocean floor insta-bility and to mitigate environmental damage.

THE FUTURE OF GAS HYDRATEGas hydrate is one of the largest sources of hydrocarbons on earth and has the potential to become an important clean energy alterna-tive of the future. It is expected that

gas hydrates will have a large effect on the world gas market when it is harnessed, especially if countries like India and Japan, which import large amounts of LNG, can develop domestic gas hydrate industries of their own. However, the technology for gas hydrate extraction is still in its infancy and there are many challeng-es still to overcome. In 2014, the Oil and Gas Financial Journal estimated the current cost to produce gas hy-drates was USD 30 – 50 /MMBtu which is much higher than current LNG landed price of around USD 3 – 6 /MMBtu. The International Energy Agency has estimated that it will be 2030 before gas hydrate becomes commercially viable, with the cost projected to have reduced to USD 4.70 – 8.60 /MMBtu.

The NGHP has provided an excel-lent opportunity for industry, aca-demia and government to increase the understanding of marine gas hy-drates in India. The two expeditions carried out over the last decade, coupled with laboratory research and seismic surveying have been instrumental in creating advanced depositional models and the devel-opment of innovative exploration techniques. The expeditions also provided an unprecedented amount of core, wireline and sample data which will prove invaluable in the organisation’s ongoing research. In 2017, the NGHP intend to under-take production testing at the chan-nel levee complex discovered by the NGHP-02-08 & 09 wells in the Krishna Godavari Basin. The tests aim to build upon the success of the 2013 Japanese test production study and will undoubtedly lead to a more comprehensive understanding of gas hydrate extraction marking the next major milestone in the story of gas hydrates.