ASSESSMENT OF GAS PRODUCTIVITY OF NATURAL METHANE HYDRATES USING MH21 RESERVOIR SIMULATOR

Masanori Kurihara¹, Hisanao Ouchi¹, Yoshihiro Masuda², Hideo Narita³, Yo Okada¹
¹ Japan Oil Engineering Company, 1-7-3 Kachidoki, Chuo-ku, Tokyo, 104-0054, Japan;
² University of Tokyo, School of Engineering, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan;
³ National Institute of Advanced Industrial Science and Technology (AIST),
2-17-2-1 Tsukisamu-Higashi, Toyohira-ku, Sapporo, 062-8517, Japan

Introduction

The Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), which was organized to attain the exploration and exploitation of methane hydrate offshore Japan, has been implementing a variety of research projects towards the assessment of MH resources, establishment of MH production methods and examination of impact of MH development on the environment. As part of such research projects, we have been developing and improving the state-of-the-art numerical simulator for rigorously predicting MH dissociation and production behaviors both at core scale experiments and at field scale implementation.

This simulator has the capability to deal with 3-D, 4-phase, 4-component problems and has the following features (Figure 1):

3-D Cartesian and 2-D radial coordinates applied with local grid refinement
four-components of methane, water, methanol and salt available
four phases of gas (mobile), water (mobile), ice (immobile) and MH (immobile) available
Darcy’s law and relative permeability curves applied to gas and water flows
endothermic dissociation of MH and ice and exothermic formation of MH and ice accounted
Kim-Bishnoi equation used as MH dissociation kinetics and similar kinetics applied to MH formation as well as ice-water phase behavior
V-H-L and V-H-I equilibrium pressure estimated as a function of temperature and methanol/salt concentration.

Simulation Studies

In this study, using the up to date numerical simulator thus developed, we conducted the case studies to examine the capabilities of MH dissociation and production, assuming diverse MH reservoir characteristics and production methods. First, MH reservoirs were categorized into three types, namely confined reservoirs, reservoirs with free water and those with free gas, in accordance with their configurations. As for MH dissociation and production schemes, three major methods of depressurization, thermal stimulation and inhibitor injection as well as combinations of these methods were taken into consideration.

Vicinities of well(s) were modeled based on the above mentioned reservoir types and development schemes. Simulation studies were then conducted for the cases of a variety of reservoir properties and operation conditions such as absolute permeability, initial hydrate saturation, reservoir temperature and bottomhole flowing pressure.

Results & Discussion

The case simulation studies for confined reservoirs revealed the following.

In producing gas by depressurization, gas productivities are analyzed in terms of initial fluid effective permeability (k*). In reservoirs with k* less than 0.01 mD, area of MH dissociation is restricted to the well vicinity. In reservoirs with k* greater than 0.01 mD, area of MH dissociation and hence gas productivities are significantly affected by intrinsic absolute permeability.
In producing gas by depressurization, gas productivities are affected by reservoir characteristics as follows:

Gas production may be doubled with increase in initial reservoir temperature by 2 degree C.
Effects of initial pressure and thermal conductivities are relatively small.

Effects of thermal methods on MH dissociation and production are envisaged as follows:

Hot water circulation with high BHFP increases gas production only from reservoirs with extremely small k*, while it has an adverse effect on gas production from the other reservoirs.
Hot water huff & puff and combination of depressurization and wellbore heating significantly increases gas production from reservoirs with small k*, while gas production increases only slightly by these methods in reservoirs with medium-large k*.
Hot water injection directly into a MH zone is difficult and less efficient.

Confined MH reservoirs can be categorized into three classes in terms of reservoir absolute permeability, initial effective permeability and temperature (Figure 2). In reservoirs with large enough absolute permeability, promising gas production can be expected if reservoirs have sufficient initial effective permeability and temperature. However, no practical gas production is expected in reservoirs with small absolute permeability.

Furthermore, the studies targeting the reservoirs with free water suggested the advantages of the MH dissociation and production from this type of reservoir with the following features.

More gas production is expected from a reservoir with free water by producing free water together with gas. Heat supplied into MH zone associated with water coning enables the expansion of the area of MH dissociation.
The gas production volume is expected to increase with increase in thickness and permeability of water zone.
Even if a reservoir has free water, large volume of gas production is not expected from MH zone when water zone permeability or thickness is small.

The studies for the reservoirs with free gas manifested that the MH dissociation and production from a reservoir with free gas was the most promising for commercial gas production with the following observations.

Along with the depletion of the pressure of a free gas zone, MH located in the vicinity of MH-free gas contact is dissociated, which induces huge gas production from a MH zone if a reservoir has sufficient initial effective permeability.
Even if a reservoir has free gas, large volume of gas production is not expected from a MH zone when initial effective permeability is small.
Gas hydrate may be re-formed in a well vicinity in the course of gas production due to adiabatic expansion of gas, which is adverse to sound gas production.

Acknowledgments

This work was financially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) on the National Methane Hydrate Exploitation Program by the Ministry of Economy, Trade and Industry (METI).

Figure 1. Mass and Heat Transport in Each Phase Equilibrium Region Calculated in the Simulator.