Modeling of Natural Gas Production From Oceanic Hydrate Accumulations
Authors: De Bel Ruben , He Yuanyuan , Rudolph-Floter Susanne Eva Johanne , Golombok Michael , Zitha Pacelli Lidio Jose 
Published: May 2009

Research in the field of methane gas hydrates is one of the approaches to meet future energy demands. As an unconventional resource these accumulations represent large amounts of methane. Future production schemes and approaches rely on experimental studies and reservoir modeling. The objective of this study is to improve insight in the reservoir behavior of a typical oceanic hydrate accumulation.

This study aims to investigate the reservoir behavior and methane production potential of a homogeneous oceanic hydrate layer subject to a constant production rate. The hydrate dissociation method applied is depressurization. The model consists of a radial system with a single vertical well at its center. Initial conditions include 2-phase equilibrium. The response of the reservoir in terms of pressure, temperature, hydrate saturation, water saturation and gas saturation has been simulated. Furthermore a production analysis and a sensitivity analysis have been performed.

Results indicate that the pressure drop occurs in the same fashion as in conventional hydrocarbon reservoirs. The temperature significantly drops due to the endothermic nature of the hydrate dissociation and is found to be a strong function of the production rate. The results for the different initial hydrate saturations show that during the largest part of the simulation a process of water-gas segregation takes place leading to a steady-state situation. The production analysis performed gives a produced methane volume of 1200MMscf. The associated water production is 1,8MMbbl. Based on the sensitivity analysis it was possible to determine 4 key parameters that have significant influence on the reservoir behavior. These parameters are the production rate, the absolute rock permeability, the initial hydrate saturation and the presence of an aquifer. One of the main conclusions is that production rate must be high enough to sufficiently depressurize the reservoir but low enough to prevent the formation of ice.

Developing economically feasible production schemes for methane exploitation from hydrate accumulations is vital for the success of future projects. This study contributes to the realization of this objective.

Introduction

Gas hydrates are crystalline compounds that consist of water and gas. Globally the most common natural form is methane gas hydrate. The gas molecules in these gas hydrate structures are trapped within the lattices of ice crystals. They are thermodynamically stable only under specific conditions characterized by relatively high pressures and relatively low temperatures. Gas hydrates are compact carriers of gas. One volume of hydrate contains about 164 volumes of gas. The geological occurrence of natural hydrates can be divided into two main regions:the permafrost andthe deep ocean sediments.

Oceanic accumulations form the largest part of global gas hydrate accumulations. These accumulations are subdivided into four classes [Moridis, G.J. et al, 2007] depending on geological settings of the hydrate bearing layers (HBL). Class 1 deposits consist of the HBL and a mobile gas layer lying on top of the hydrate layer. Class 2 deposits involve a HBL on top of a water zone whereas a Class 3 accumulation is characterized by a single HBL and the absence of zones with mobile fluids around it. Class 4 is defined as disperse, low saturation deposits.