Abstract

A one dimensional depressurization model based on heat and mass transfer phenomena coupled with kinetics is presented to describe the decomposition of a synthetic methane hydrate core. The model determines the amount of gas produced and the position of the interface as the hydrate core decomposes. The calculations reveal the relative importance of the mass and heat transfer resistances as well as the contribution of the intrinsic hydrate decomposition kinetics, In addition, the effect of variations in porosity, permeability and hydrate saturation are examined.

Introduction

There is evidence that enormous natural gas reserves exist in the form of gas-hydrate deposits in many regions of the world (Kvenvolden and McMenamin (I)). These deposits occur in suboceanic sediments as well as in rctic regions. Every volume of gas hydrate has the potential to contain 170 to 180 volumes of gas (standard conditions). This represents a huge source of energy.

Gas hydrates are relatively immobile and impermeable, In order to produce natural gas from hydrate reservoirs it is necessary to decompose the hydrate structure. Several methods for decomposing natural gas hydrates have been proposed. These include thermal recovery, In-situ combustion, depressurization, and chemical Injection Techniques.

McGuire (2) and Bayles et al.(3) report that thermal processes are efficient for hydrate decomposition. Holder et al. (4) reported that the thermal recovery technique is energy efficient from a thermodynamic view point. Kamath and Godbole(5) reported their simulation results of hot brine stimulation to produce gas from hydrate reservoirs. Selim and Sloan(6) presented a mathematical model for hydrate decomposition under thermal stimulation. Ullerich er al.(7) obtained experimental data on hydrate decomposition to verify the heat transfer rate odel of Selim and Sloan(6) Kim er al. (8) experimentally studied the kinetic rate of methane hydrate decomposition. Jamaluddin er al. (9) presented a thermal model for the decomposition of a methane ydrate block by coupling the intrinsic kinetics with the heat transfer rates.

Although, several laboratory and theoretical schemes are reported in the literature for the determination to gas recovery from hydrate reservoirs, the only instance of actual production from a hydrate reservoir has been the Messoyaka field in U.S.S.R. by depressurization technique. Holder and Angert(10) presented a mathematical model of the depressurization technique. Recently, Yousif el al. (II) reported experimental data on the gas production from a synthetic sandstone core containing methane hydrate using a depressurization technique. They also presented an approximate analytical model based on isothermal mass transfer phenomena. Thus, their model assumes that the decomposition rate is mass transfer controlled only. Hence, the kinetics of decomposition as well as the heat transfer effects are not taken into account.

In the present work, a one dimensional depressurization model based on heat and mass transfer phenomena coupled with kinetics is presented to describe the decomposition to a synthetic methane hydrate core. The calculations reveal the relative importance of the mass and heat transfer resistances as well as the contribution of kinetics of hydrate decomposition.

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