GAS HYDRATE QUANTIFICATION BY COMBINING PRESSURE CORING AND IN-SITU PORE WATER SAMPLING TOOLS

Transcription

1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, GAS HYDRATE QUANTIFICATION BY COMBINING PRESSURE CORING AND IN-SITU PORE WATER SAMPLING TOOLS Edwin Tervoort, Joek Peuchen Fugro Veurse Achterweg 10, 2264 SG Leidschendam THE NETHERLANDS Gary Humphrey Fugro 6100 Hillcroft, Houston, TX77081 U.S.A. ABSTRACT Scientific investigations of marine gas hydrates have utilised the HYACINTH pressure coring and core transfer and analysis system since The latest developments in pressure core processing include non-destructive testing, sub-sampling and core storage. Sub-sampling allows part of a pressure core to be depressurised for quantifying the natural gas hydrates content in sediments while retaining other sections of the same core for further laboratory studies. Depressurising pressure cores results in gas hydrate dissociation. The amount of gas hydrate in a core can be determined by either the amount of excess gas or by pore water freshening. The amount of excess gas is determined by comparing the total gas concentration and the gas solubility at in-situ conditions. Pore water freshening relies on dissociation of gas hydrates releasing freshwater and diluting pore water salinity. Pore water freshening has become a common measure for determining the gas hydrate content in pressure cores. Gas hydrate quantification from both excess gas and pore water freshening requires measures for the in-situ pore water conditions. The introduction of an improved in-situ pore water sampler (FPWS) validates assumptions on the in-situ pore water conditions, i.e. pore pressure, temperature, salinity and gas saturation. These in-situ pore water conditions improve the quantification of gas hydrates from pressure cores. Keywords: pressure coring, in-situ pore water sampling INTRODUCTION Pressure cores and pressure core analysis have become standard for scientists, exploration geologists, and geohazard engineers for groundtruthing gas-rich formations. This is not limited to gas-hydrate-bearing marine sediments, but also applies to coalbed methane formations and shale gas reservoirs. The argument for taking pressure cores in comparison with conventional (non-pressurised) coring techniques includes recovery of gashydrate-bearing sediments with minimal disturbance. Non-pressurised coring techniques cause inevitable disturbance to samples due to gas hydrate dissociation and exsolution of dissolved Corresponding author: Phone: Fax E.tervoort@fugro.nl

2 gas from pore water. Pressure coring allows for non-destructive testing, including gamma-density and X-ray imaging. The latter reveals the morphological structure of gas hydrates, which is important to understand the mechanism of formation and the nature of dissociation [1]. The gamma-density is a measure for the total pore volume in a core, which determines the amount of pore water (i.e. pore water saturation). Depressurisation of pressure cores represents current best practice for accurately quantifying the total gas content and hence the gas hydrate content in a core [1]. This paper evaluates gas hydrate quantification from depressurisation of pressure cores and demonstrates improvement of these analyses by determining the in-situ pore water conditions from pore water sampling. The Fugro Pore Water Sampler (FPWS) is an in-situ pore water sampler with additional sensors to measure the in-situ pore pressure and temperature (Figure 1). methane in pore water at in-situ conditions, i.e. beyond methane saturation. When the methane is over-saturated at in-situ conditions, the excess gas is assumed to exist only as gas hydrate within the GHSZ i.e. the free gas phase does not to exist next to the gas hydrate phase. Therefore, the amount of excess gas is a measure for the volume of gas hydrate, knowing the molar weight and specific density of the gas hydrates present. Below the GHSZ, excess gas will exist only as free gas. When the methane concentration is under-saturated, no excess gas will exist and the methane will be dissolved. Figure 2 Methane occurrence defined by methane concentration, temperature and pressure. Figure 1 Fugro Pore Water Sampler (FPWS). GAS HYDRATE QUANTIFICATION FROM DEPRESSURISATION Calculations for quantifying gas hydrates from pressure cores assume that the dissolved gas, free gas and gas hydrate phases are in thermodynamic equilibrium [1]. Figure 2 shows that the gas hydrate stability zone (GHSZ) is determined by the in-situ conditions (i.e. in-situ pressure and temperature). The base of the GHSZ is defined by the geothermal gradient. Note that increasing the geothermal gradient results in shallowing of the base GHSZ and decreasing the geothermal gradient results in deepening of the base GHSZ. Gas hydrate will only occur when the methane concentration is higher than the solubility of the The amount of excess gas is based on the assumption that the in-situ pore water is fully saturated with gas. This assumption should be verified by taking in-situ pore water samples and determining the gas concentration. When a pressure core is fully depressurised, only the free gas phase and water will result. Figure 3 shows schematically how gas hydrate is quantified from depressurising pressure core. The total gas content is the sum of the amount of dissolved gas (i.e. gas saturation) at in-situ conditions and the amount of gas from hydrate dissociation. The total water content is the sum of the amount of pore water at in-situ conditions and the amount of water from hydrate dissociation. The total amount of pore water in the pressure core is determined from either comparison of the wet and dry density of the

3 core sample, or from porosity measurements (i.e. gamma-density) and pore water saturation. This requires that baseline pore water salinity at in-situ conditions is known. In-situ salinity can be determined from water samples taken from (pressure) cores without gas hydrates, i.e. (pressure) cores taken from soil outside the gas hydrate stability zone or (pressure) cores taken from soil inside the gas hydrate stability zone, but without gas hydrates. Baseline salinity is not necessarily the in-situ salinity for cores containing gas hydrates. The in-situ salinity can also be calculated from pressure cores containing gas hydrates by determining the gas hydrate content from the total gas concentration (i.e. excess gas). The amount of fresh water released from the known gas hydrate content can be compared with the diluted pore water salinity to determine the in-situ pore water salinity. Calculating several of these pore water salinity values from pressure core samples containing gas hydrates results in a baseline measure of salinity for samples containing gas hydrates. The calculated salinity values should be verified by taking in-situ pore water samples from gas-hydrate-bearing formations and determining the in-situ salinity. Figure 3 Gas hydrate quantification from depressurising pressure core. Depressurising at isothermal conditions is a time consuming process. Therefore, in practice, the pressure cores are not depressurised isothermally [1]. Depressurising the core at standard conditions until no gas is released is generally sufficient to determine the total gas content. The remaining gas dissolved in the pore water can be determined from the solubility of the gas at the depressurising conditions. The dissolved gas content has to be added to the amount of gas released during depressurising to determine the total gas content of the core. The dilution of pore water by hydrate dissociation can be determined from the concentration of the chloride ion, because chloride is a conservative element in pore water [1]. Comparing the chlorinity (i.e. salinity) of the water from the pressure core at in-situ conditions and after depressurising provides a measure for dilution. The accuracy of the quantitative assessment of gas hydrate from pore water freshening is largely dominated by the accuracy and confidence in the background chlorinity values, i.e. the in-situ values of chlorinity [1]. Therefore it should be a routine procedure to take in-situ pore water samples to determine the in-situ pore water chemistry. Accurate and independent measurements from pressure cores and pore water samples enable calculations on gas saturation of the gas hydrates (i.e. percentage of cages occupied by gas). The calculations are based on the amount of water and the amount of gas released from dissociation of gas hydrates (Figure 3). When the (main) gas hydrate type (Type I, II or H) is determined (e.g. by composition of the gas released from dissociation), the maximum number of cages associated with that gas hydrate type predicts the maximum amount of gas that could be occupied by the amount of water released during dissociation. The actual amount of gas released from the gas hydrate is then a measure for the gas saturation of the gas hydrate.

4 For the in-situ conditions, it is generally assumed that the pore pressure is similar to the hydrostatic pressure. However, the in-situ pore pressure might deviate from hydrostatic (e.g. [3] and [4]). The cementing properties of gas hydrates in sediments may result in less consolidation than sediments without gas hydrates, which might result in underpressure. The in-situ temperature is generally based on the geothermal gradient from the interpolation between some discrete temperature measurements versus depth. This interpolation might be an oversimplification, because it has been observed from downhole geophysical logs that the temperature was constant over some depth intervals within the gas hydrate stability zone [5]. COMBINING PRESSURE CORES AND IN- SITU PORE WATER SAMPLER TOOLS The HYACINTH pressure coring and core analysis system has been utilised since 2002 to determine nature, distribution and concentration of marine gas hydrate throughout the gas stability zone at a number of sites [1]. These include Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, Oregon Margin [6]; the Chevron/DOE (US Department of Energy) Naturally-Occurring Hydrates JIP (Joint Industry Project), Gulf of Mexico [7]; Integrated Ocean Drilling Program (IODP) Expedition 311, Cascadia Margin [8]; the Shell Gumusut-Kakap project, offshore Sabah [9]; the Indian National Gas Hydrate Program Expedition I, Bay of Bengal [10]; the Chinese Guangzhou Marine Geological Survey Expedition I, South China Sea [11]; the Korean Ulleung Basin Gas Hydrate Expedition I, East Sea [12]; and the Korean Ulleung Basin Gas Hydrate Expedition II, East Sea (fall of 2010). The HYACINTH system involved techniques for analysing pressure cores in a non-destructive way and storing pressure cores for further laboratory studies. Since then, these techniques have been refined. The latest developments in core processing include sub-sampling. Sub-sampling allows part of a pressure core to be depressurised (i.e. destructive testing) for quantifying the natural gas hydrate content in sediments while still retaining other sections of the same core for further laboratory studies. The HYACE/HYACINTH programs consists of two types of wireline pressure coring tools that have been operational since: the Fugro Pressure Corer (FPC) and the Fugro Rotary Pressure Corer (FRPC), previously referred to as HYACE Rotary Corer (HRC) [13]. The FPC is a percussion corer that was designed for weaker formations and the FRPC is a rotary corer that was designed for more competent gas-hydrate-bearing formations. Both coring systems were developed to cut and recover core in a wide range of lithologies where gashydrate-bearing formations might exist. The FPC and FRPC penetrate the formation using downhole driving mechanisms powered by fluid circulation rather than by top-driven rotation of the drill string. This downhole operation allows the heave-compensated drill string to remain stationary in the hole while core is being cut. This improves the core quality. The coring portion of these pressure corers moves relative to the main bit during the coring process. This also improves core quality. The use of flapper valve sealing mechanisms on both pressure corers maximise the diameter of the recovered core. The recovered cores are in plastic liners, which facilitate transfer into other chambers for analysis, storage and transportation under full pressure. The FPWS [14] was successfully introduced in The FPWS utilises additional sensors to determine the in-situ pore pressure and temperature (Figure 1). These in-situ measurements are important to verify and validate assumptions on the in-situ conditions. The FPWS operates in weak formations (i.e. soils) only and was not designed to operate in more competent formations (e.g. rock). The FPWS has been successfully deployed to determine the in-situ gas saturation and in-situ salinity for gas hydrate identification and quantification from HYACINTH pressure corers, particularly the Chinese Guangzhou Marine Geological Survey Expedition I, South China Sea [11]; the Korean Ulleung Basin Gas Hydrate Expedition I, East Sea [12]; and the Shell Gumusut-Kakap project, offshore Sabah [9]. The FPWS system also includes analytical equipment to depressurise pore water samples for the exsolution of dissolved gas. Figure 4 shows a schematic diagram of the setup for analysing dissolved gas from water samples. The pore water

5 is collected in a sample vessel within the FPWS. After retrieval of the FPWS, this sample vessel is removed from the probe and installed in a decompression apparatus. Volume (V) is increased by a piston to reduce the pressure (P) at constant temperature (T) which promotes exsolution of dissolved gas from the water sample. During decompression of the water sample, the pressure (P) and temperature (T) are recorded by a logger connected to a personal computer (PC). The gas is analysed for composition and concentration applying a portable gas chromatograph (GC). The in-situ pore water chemistry (including chlorinity) is determined from the water sample. CONCLUSION Gas hydrate quantification can be determined by depressurising pressure cores. Depressurising pressure cores with gas hydrates results in dissociation of the gas hydrates and hence in excess gas and in pore water freshening. Quantification of gas hydrates based on excess gas and/or pore water freshening relies on assumptions for in-situ gas saturation and pore water salinity. Sampling pore water at in-situ conditions validates these assumptions. The introduction of an in-situ pore water sampler (FPWS) with additional sensors to determine the in-situ pore pressure and temperature further improves gas hydrate quantification. The accuracy of the quantitative assessment of gas hydrate from pressure cores is largely dominated by the accuracy of the in-situ measurements. Therefore it should be a routine procedure to combine pressure coring with in-situ pore water sampling. Track records are available for the required tools and analysis methods. ACKNOWLEDGEMENTS The authors gratefully acknowledge Fugro s commitment and support to improving practice. The opinions expressed in this paper are those of the authors. They are not necessarily shared by Fugro. Figure 4 Schematic diagram of setup for analysis of (dissolved) gas from water samples. REFERENCES [1] Schultheiss P, Holland M, Roberts J, Humphrey G. Pressure core analysis: the keystone of a gas hydrate investigation. In: Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, Canada, July 6-10, [2] Zhang Y. Methane escape from gas hydrate systems in marine environment, and methanedriven ocean eruptions. Geophysical Research Letters 2003;30(7): [3] Dugan B, Sheahan TC, Thibault JM, Evans TG. Offshore sediment overpressures: overview of mechanisms, measurement and modeling. In: Gourvenec S, White D, editors. Frontiers in Offshore Geotechnics II: Proceedings of the 2 nd International Symposium on Frontiers in Offshore Geotechnics, Perth, Australia, 8-10 November 2010, p [4] Peuchen J and Klein M. Prediction of formation pore pressures for tophole well integrity. In: OTC2011: 2011 Offshore

6 Technology Conference, 2-5 May, Houston, Texas: Proceedings, OTC Paper [5] Fukuhara M, Fujii K, Igarashi J, Tertychnyi V, Shandrygin A, Matsubayashi O, Fujii T. Thermal regime long-term monitoring for marine gas hydrate-bearing sediments. In: Proceedings of the Fifth International Conference on Gas Hydrates, June 12-16, 2005, Trondheim, Norway (ICGH 2005). [6] Tréhu AM, Bohrmann G, Rack FR, Torres ME, et al. Drilling gas hydrates on Hydrate Ridge, Cascadia continental margin: Proceedings of the Ocean Drilling Program Volume 204 Initial Reports. College Station: Integrated Ocean Drilling Program, [7] Claypool G. Joint industry project (JIP) Gulf of Mexico gas hydrate coring update. Fire in the Ice 2005;Summer:9-13. [8] Riedel M, Collett TS, Malone MJ, Expedition 311 Scientists. Cascadia Margin Gas Hydrates: Proceedings of the Integrated Ocean Drilling Program Volume 311. Washington: Integrated Ocean Drilling Program Management International, Inc., [9] Hadley C, Peters D, Vaughan A, Bean D. Gumusut-Kakap project: geohazard characterisation and impact on field development plans. In: International Petroleum Technology Conference, 3-5 December 2008, Kuala Lumpur, Malaysia, Paper IPTC [10] Collett T, Riedel M, Cochran J, Boswell R, Presley J, Kumar P, Sathe A, Sethi A, Lall M, Sibal V, NGHP Expedition 01 Scientists. Indian National Gas Hydrate Program Expedition 01 Initial Reports. New Delhi: Indian Directorate General of Hydrocarbons, [11] Zhang H, Yang S, Wu N, Su X, Holland M, Schultheiss P, Rose K, Butler H, Humphrey G, GMGS-1 Science Team. Successful and surprising results for China s first gas hydrate drilling expedition. Fire in the Ice 2007;Fall:6-9. [12] Park KP, Bahk JJ, Kwon Y, Kim GY, Riedel M, Holland M, Schultheiss P, Rose K, UBGH-1 Scientific Party. Korean National Program expedition confirms rich gas hydrate deposits in the Ulleung basin, East Sea. Fire In the Ice 2008;Spring:6-9. [13] Schultheiss PJ, Francis TJG, Holland M, Roberts JA, Amann H, Thjunjoto, Parkes RJ, Martin D, Rothfuss M, Tuynder F, Jackson PD. Pressure coring, logging and subsampling with the HYACINTH system. In: Rothwell RG, editor. New Techniques in Sediment Core Analysis. London: The Geological Society, p [14] Tervoort EPP, Peuchen J. An improved tool for in situ pore water and gas sampling. In: Offshore Site Investigation and Geotechnics: Confronting New Challenges and Sharing Knowledge: Proceedings of the 6 th International Conference, September 2007, London, UK, p