Prediction of Gas Productivity From Eastern Nankai Trough Methane-Hydrate Reservoirs

Abstract

Summary The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) has been evaluating methane-hydrate (MH) reservoirs located in the eastern Nankai trough from the viewpoints of geology, geophysics, petrophysics, and reservoir/ production engineering. As one of these studies, we have been predicting gas/water production performances from these MH reservoirs showing diverse characteristics. This paper presents the results of our examinations on the applicability of a variety of MH dissociation/production methods to these MH reservoirs and on the feasibility of future development in terms of gas production and economics. Eastern Nankai trough MH reservoirs, which are composed of alternating beds of sand, silt, and clay in turbidite sediments, have various conditions of clay distribution and of initial pressure, temperature, permeability, and MH saturation. Some of these reservoirs contain MH of high saturation at a certain interval (MH concentrated reservoir), while in the others, MH is deposited sparsely (MH-nonconcentrated reservoir). Detailed numerical reservoir models were constructed for both MH-concentrated and -nonconcentrated reservoirs, consulting the well-log and seismic-interpretation results. MH-dissociation/-production performances were then predicted through numerical simulation, assuming the application of various MH-dissociation methods (such as depressurization, well-bore heating, hot-water huff'n'puff, and hot waterflooding. The simulation studies clarified the difference in the gas production between MH-concentrated and -nonconcentrated reservoirs. These studies also revealed that the permeability not only of sand layers but also of clay layers has a significant effect on the gas productivity from MH-concentrated reservoirs. Furthermore, it was suggested that the hot-water injection was effective when it was applied as a secondary-recovery method after depressurization. Simple economic analyses on the basis of these simulation results exhibited the promise that some MH reservoirs in the Eastern Nankai trough could be developed economically if the well spacing and MH-dissociation/-production methods were designed appropriately.

Evaluation of the Gas Production Potential of Some Particularly Challenging Types of Oceanic Hydrate Deposits

“…If these conditions are met, hydrate deposits can yield methane at high rates (well in excess of 2.8 × 10 5 ST m 3 /day = 10 MMSCFD) for long periods using conventional production technology (Moridis and Reagan 2007a,b;Kurihara et al 2009). If one or more of these Fig.…”

Evaluation of gas production from methane hydrates using depressurization, thermal stimulation and combined methods

“…In addition, the formation of ice and the reformation of hydrate during the decomposition process also have an impact on the gas production [23]. Macrocosmic numerical results also show that for depressurization of gas hydrate sediment in the South China Sea, the hydrate deposit exploitation by depressurization only is not a good method because of the secondary hydrate formation and ice formation at the wellhead during the hydrate dissociation [17,24,25]. Therefore, based on depressurization, the application of thermal stimulation in certain stages of depressurization is one of the effective methods worthy of study [8,25,26].…”

“…The thermal injection method could effectively improve the problems that emerged in the depressurization process, such as the low rate of gas production, the secondary hydrate formation or ice formation, the blocking effect, and the depletion of sensible heat [13,25,27,28]. In the field of thermal stimulation or heating studies, numerical simulations and experiments have been researched.…”

“…As you can easily imagine, the energy efficiency of these approaches is poor. Kurihara et al [25] compared the theoretical energy efficiency of different production methods, such as depressurizing, depressurizing and heating the wellbore, hot-water huff and puff, hot-water and methanol huff and puff, and hot-water flooding by numerical simulations. They also concluded that, on the basis of the boundary conditions of the eastern Nankai Trough, the economics of additional wellbore heating at a constant 50°C are almost the same as for depressurization only.…”

“…They also concluded that, on the basis of the boundary conditions of the eastern Nankai Trough, the economics of additional wellbore heating at a constant 50°C are almost the same as for depressurization only. Overall, the problems of poor energy efficiency, the unclear relationships between the hydrate saturations and the production methods, and a shortage of measurements using large scale reactors mean that investment in laboratory-based experiments is still necessary [8,25].…”