SS: Hydrates: Experimental results for long term CO2 injection near methane hydrate formations
Authors: Ikegawa Yojiro , Miyakawa Kimio , Suzuki Koichi , Masuda Yoshihiro , Narita Hideo , Ebinuma Takao 
Published: July 2010

Abstract
CO2 has been focused for improving recovery factor of methane hydrate. The most of the principle is replacing guest molecular. It is also known that CO2 hydrate formation generates a large amount of exothermic heat. This heat can warm sediments up to 10 degrees Celsius as one of heating method. But a problem is how to inject CO2 at near fields of methane hydrate formations. CO2 hydrate formed in sediment blocks the pore of sediments. Then CO2 cannot inject into sediments for about ten-year term. But it is possible to inject CO2 for long term by a thermo-dynamic principle of the stability zone of CO2 hydrate. We will present experimental results to prove the principle of CO2 injection into formations.

1. Introduction
Methane hydrate is solid, and it doesn't flush and flow from production well. Methane hydrate must be dissolved into gas and water in sediments by lowering the water pressure of the sediments or by heating the sediments to flow out from production well.
Research consortium for methane hydrate resources in Japan succeeded to onshore depressurization test at permafrost in Canada for 6 days, and the consortium estimated the resources in the east part of Nankai trough[1]. The first trial of offshore test in Nankai trough is planed in 2012.
On the other hand, CO2-enhanced oil recovery is commercialized by injecting CO2 into oil wells in the U.S. to enlarge oil production [2]. This method mitigates CO2 release into atmosphere. A research development is therefore necessary for an enhanced recovery of methane hydrate by using CO2.
There is a problem of productivity by dropping of temperature for methane hydrate production by depressurization, because dissolving methane hydrate is endothermic reaction. It absorbs heat. Combining a heating method as an option to the depressurization to warm sediments, the productivity would be kept for long term. The recovery factor can also be expected to enlarge by the heating method. The productivity and the recovery factor are very important for commercialization of methane hydrate production.
However, if we use fossil energy for the heating method, production rate, which is produced energy over input energy, become small. Then new technical developments are necessary for heating sediments for commercialization.
We focused that CO2 hydrate formation is exothermic reaction, and we have proposed a heating method using heat of CO2 hydrate formation [4,5]. Sediments can be warmed up to 10 degrees Celsius when the pressure is more than or equal to 4.5 MPa to accelerate dissolving methane hydrate. CO2 works as heating energy of sediments in this case.
Blockage of gas transportation pipeline cased by gas hydrate was reported in 1934. If CO2 is injected in to sediments, CO2 hydrate is formed and it blocks the pore of sediments in the same manner. Then CO2 can't inject into sediments.
A subject for applying the heating method using exothermic heat of CO2 hydrate formation is to show a method to inject CO2 in to sediments continuously for long terms. The point of this method is using equilibrium state. No CO2 hydrate formation and dissolving are occurred. It balanced on the boundary of CO2 hydrate stability zone. Then the pore is kept opening for flow. Also the temperature of the formations is kept at 10 degrees Celsius by formation and dissolving of CO2 hydrate naturally. Then CO2 can inject into sediments continuously for long terms by using the equilibrium state. When the pressure is more than or equal to 4.5 MPa, the temperature of the equilibrium state is 10 degrees Celsius.
In the following chapter, the principle of CO2 injection into formation continuously for long terms is verified by experiments are presented.