A Thermodynamic Evaluation of Thermal Recovery of Gas From Hydrates in the Earth (includes associated papers 11863 and 11924 )

Abstract

Introduction Gas hydrates are crystalline ice-like compounds composed of water and natural gas. Until recently studieson hydrates were directed toward preventing their formationin natural gas pipelines. The apparent discovery of natural gas hydrates in arctic regions, and under these a floor, has generated interest in their study as apotential source of clean energy. Hydrates also have been discovered in oil-beating reservoirs. In such reservoirs, the presence of hydrates is important not only as a potential source of free gas, but also in the way they affect oil production. Hydrates will block reservoir poresand will selectively remove high-vapor-pressure components such as methane, thus increasing oil viscosity and reducing the driving force for oil production. The existence of hydrates in the earth, therefore, has diverse implications. Recent estimates indicate that as much as 10(18) m3 of natural gas could exist as in-situhydrates. While there is no certainty that hydrated gascan be produced economically, the potential of thisresource clearly demands evaluation. This paper examines the potential for recovering gasfrom naturally occurring hydrates. Factors to be considered in such a study arelocation of the hydratefields,purity of hydrates in the reservoir,types of media in which hydrates form,thermodynamic conditions of temperature, pressure, and composition, andthermal properties of the reservoir. Based on these considerations, calculations were made to determine the energy needed to dissociate hydrates and the amount of gas recovered per gmol of hydrate dissociated. Nature of Gas Hydrates Two strictures of gas hydrates called structures 1 and 2 are known to form from mixtures of water and light gases. Each of these structures has two approximatelyspherical cavities of different diameters (Table 1). Not all the cavities need be occupied by gas molecules toproduce a stable hydrate, but a completely unoccupied lattice phase is metastable and does not exist. The thermodynanmic behavior of gas hydrates isillustrated by the phase diagram for methane/propane/water hydrate-forming mixtures(Fig. 1). A gas of anyindicated composition will form hydrates at pressure/ temperature points above the corresponding curve. Below the curve, hydrates will decompose, high pressure and low temperature favor hydrate formation. The enthalpy of formation of gas hydrates from waterand free gas can be approximated by a modification ofthe Clapeyron equation. dlnpH =Z RT2 -----...............................(1)Diss dT The derivative dlnp/dT is the slope of the semilogarithmic p - T graph shown in Fig. 1. Calculation of the enthalpy of formation is important because it gives the energy required to dissociate the hydrates. The hydrates that form in the earth are likely to be Structure 1 hydrates only when pure methane is present. Structure 2 hydrates generally will form in the presence of even small quantities of heavier gas constituents, suchas propane. The amount of gas in the hydrate does notgenerally depend on the structure; Structure 1 has one cavity for every five and three-fourths water molecules and structure 2 has a cavity for every five and two-thirds water molecules. Occupation of all the cavities of either structure results in a maximum gas concentration, 15%, in the hydrate phase. Table 2 shows the calculated composition of the hydrate that would be in equilibrium witha natural gas containing 97% methane, 2% ethane and 1 % propane. As this table indicates, the hydrate phase gas composition depends on the formation temperature. JPT P. 1127^