Article pubs.acs.org/EF
Gas Production from Methane Hydrate: A Laboratory Simulation of the Multistage Depressurization Test in Mallik, Northwest Territories, Canada Katja U. Heeschen,* Sven Abendroth, Mike Priegnitz, Erik Spangenberg, Jan Thaler, and Judith M. Schicks GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany ABSTRACT: Gas hydrate production is still in the test phase. It is only now that numerical models are being developed to describe data and production scenarios. Laboratory experiments are carried out to test the rationale of the conceptual models and deliver input data. Major experimental challenges include (I) the simulation of a natural three-phase system of sand−hydrate− liquid with known and high hydrate saturations and (II) the simulation of transport behavior as deduced from field data. The large-scale reservoir simulator (LARS; 210 L sample) at the GFZ has met these challenges and allowed for the first simulation of the gas production test from permafrost hydrates at the Mallik drill site (Canada) via multistage depressurization. At the starting position, hydrate saturation was as high as 90%, formed from dissolved methane only. Whereas gas hydrate dissociation determined the flow patterns in the early pressure stages, the importance of different transport behaviors increased at lower pressure stages and increasing water content. Gas flow patterns as observed in Mallik were recorded. While the conceptual model for the experimental data does agree with the model proposed for Mallik at moderate and low gas production, it is different at high gas production rates.
1. INTRODUCTION Gas hydrates are ice-like crystalline solids forming from water and guest molecules, such as methane (CH4), the dominant guest molecule in natural gas hydrates.1−3 They are stable at elevated pressure, low temperatures, and in the presence of sufficient amounts of water and gas, all of which are often found within the sediment along the continental margins4−7 and in permafrost regions.1,8 Despite large variations in the total estimate of carbon bound in gas hydrates of 100−400 000 Gt,9−12 there is a strong consensus that this natural gas reservoir is a large source of energy and worthwhile exploring.13 The gas hydrates in permafrost regions account for only ∼3% of this inventory14 but are easily accessible and have become a target of the first field tests for methane production from gas hydrates at the Mallik site, Northwest Territories, Canada.15−17 The tests used thermal stimulation (2001) and depressurization via hydraulic stimulation technics (2002 and 2007/2008). Another test took place in the Nankai Trough, Japan (March 2013), applying depressurization over 6 days and producing a total of 120 000 m3 of gas.18 In the gas hydrate field test at Igik Sikumi on the Alaska North Slope, carbon dioxide was injected to convert methane hydrates to carbon dioxide hydrates, followed by a subsequent depressurization.19 The focus of this paper is on the gas and water flow patterns observed at Mallik in 2008, which were described and modeled, e.g., by Uddin et al.20−22 Because the history match using the original version of the numerical reservoir simulator CMGSTARS failed to fit the data, Uddin et al.21,22 developed a new set of equations to model the unconventional flow observed during the Mallik production test 2008. Tunable parameters for gas hydrate dissociation and different gas bubble transport models were added to achieve a reasonable match with measured gas flow rates. Their conceptual and numerical model is inspired by the “foamy oil flow”21 developed in the oil © XXXX American Chemical Society
industry for solution gas drive reservoirs without an initial continuous gas phase.23,24 In the early stage of depressurization, small gas bubbles are transported within the oil/water in a “two-phase” or rather “pseudo-single-phase” flow before the size of the single bubbles as a result of pressure decrease or coalescence exceeds those of the pore throats. At this point, bubbles become trapped and gas production declines rapidly before progressive growth forms a continuous gas phase that allows for increasing gas transport. While a number of one-stage depressurization tests have been carried out in smaller (
