Improvement of Near Wellbore Permeability by Methanol Stimulation in a Methane Hydrate Production Well
Authors: Masuda Yoshihiro , Konno Yoshihiro , Iwama Hiroki , Kawamura Taro , Kurihara Masanori , Ouchi Hisanao 
Published: October 2008

Abstract
Depressurization is a promising method for recovering gas from methane hydrate reservoirs, but its effectiveness is much influenced by near-wellbore formation permeability. For example, it is not effective when a production well is completed in the formation of high methane hydrate saturation, because pressure reduction is not transmitted into the reservoir due to its initial low permeability. In such a case, the stimulation to improve permeability near the wellbore is a key factor for assuring the success of methane-hydrate gas production. This paper proposes the methanol huff-and-puff stimulation as a possible way for improving gas productivity of such a methane hydrate well and presents our numerical studies to demonstrate that the huff-and- puff stimulation before depressurization works well in improving gas productivity.
The methanol huff-and-puff stimulation we propose consists of the two-process cycles: injection of a methanol-solution slug into the well, and flowback with depressurization. Based on calculation using our numerical simulator, MH21-HYDRES (MH21 Hydrate Reservoir Simulator), we analyzed hydrate dissociation behavior during the huff-and-puff stimulation and successive gas production performance for a hypothetical methane hydrate well. Our calculations lead the following conclusions:Huff-and-puff stimulation dissociates hydrates near the wellbore for the formation to produce gas at much higher rates than the ones without stimulation.Formation temperature goes down below zero along the hydrate equilibrium curve during the huff-and-puff cycles and the decreased temperature helps the surrounding formation to supply heat for promoting hydrate dissociation.In some cases, hydrate regenerates in the region where hydrate-dissociated water dilutes methanol, but its influence is little.Huff-and-puff schedule like huff/puff time ratio and methanol-slug concentration affects the stimulation results.
More studies on maximizing the stimulation effect are necessary, but the authors recommend the methanol huff-and-puff stimulation before depressurization in testing methane hydrate wells.

Introduction
Current research on methane hydrate as an energy resource makes a remarkable advance under the national research projects in Japan, Canada, and U.S. and several field production tests have been carried out for demonstrating methane hydrate gas production1. Among the methods proposed for methane-hydrate gas production, the depressurization is a promising and environmentally sound method because it uses natural geothermal energy for dissociating hydrates. The first step for future hydrate-gas production is to demonstrate and assure a long-term gas production with depressurization. However, the depressurization test in methane hydrate wells has a unique problem arising from the nature of hydrate. Since hydrate exists as a solid state in sediments, gas productivity depends on rate of hydrate dissociation. When a production well is located in the formation of high methane hydrate saturation, pressure in the wellbore is not easy to travel into the reservoir due to initial low permeability. In such a case, hydrate-dissociated region is limited to the vicinity of wellbore and there is little hope of continuous gas production with depressurization. This paper proposes the methanol huff-and-puff stimulation as a possible way for improving gas productivity of such a low-permeability methane hydrate well, and presents our numerical studies to demonstrate that the huff-and-puff stimulation before depressurization works well in improving gas productivity.