Abstract
Gas Hydrates - Occurance, Production and Economics

Gas hydrates are crystalline compounds stable under particular conditions of low temperature and high pressure. Relatively large quantities of methane can be trapped in small volumes of hydrates, which represent a substantial in-situ natural reserve and resource in the shallow crust.

In order to recover natural gas from this resource, it is necessary to dissociate the water structure and remove the gas. This is the reason why a new method is put forward:

Hot Solvent Stimulation after Hydraulic Fracturing, with a method called S.T.S.F. The technique resembles Acid Fracturing but replaces Acid by Hot Solvent for the hydrate dissociation. Moreover, the method can be improved by drilling a second production well in order to control the solvent flow in the fracture and the gas production. The shape of the fracture can be either horizontal or vertical depending on the depth and the in-situ stresses.

The following paper describes the development and applications of a heat transfer model, as well as the flow through the dissociated zone by leak-of f from the fracture. The analytical solution, obtained through the introduction of some simplifying assumptions, has two main advantages: it can be used without any digital computing equipment and can be easily handled by field engineers.

Graphical results indicate that with S.T.S.F. method gas production and the energy efficiency ratio are three times as high as with any method.

The paper ends with an attempt to analyse the economics of natural gas production from gas hydrates, assuming a satisfactory method of hydrates recovery.

Introduction

Occurrences of gas hydrates are found at shallow depths of 200-1500m., where low temperatures 100-400K, and moderate pressures, 10 E.-2 to 2 -10 E.3 MPA, prevail. Gas hydrates are ice like crystalline inclusion substances composed of dodecahedral and tetracahedral water molecules cages, where methane and other light weight natural gas are trapped. Relatively large quantities of methane can be stored in small volumes of hydrates (1 cubic metre of 90% saturated hydrate contains about 180 standard cubic meters of methane).

Since 1985 Soviet scientist have reported the existence of large deposits of natural gas hydrates in sediments under permafrost in Siberia (MARAGON, 1985). Later, gas hydrates were found under the permafrost in Alaska and Canada. More recently, extensive occurrences of hydrate deposits in ocean bottom sediments have been found along the North American and Mexican continental margin (KVENVOLDEN, 1988).

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