The present invention relates to the recovery of natural gas hydrate to produce large volumes of useable hydrocarbon fuels, particularly methane (CH4). In particular, the present invention involves the use of an inexpensive and commercially available acid catalyst that allows natural gas hydrate and liquid CO2 to be converted into CO2 hydrate and natural gas in a form that can then be readily recovered using conventional means. The present invention has the added benefit of providing a cost-effective and environmentally safe method for disposing of CO2 used during conversion of the hydrate.
Natural gas hydrate is a methane-bearing, ice-like material that occurs in abundance in marine and arctic sediments. The hydrate contains large amounts of methane in a xe2x80x9ccagexe2x80x9d of water ice molecules surrounding the gas, typically over 160 volumes of gas per unit volume of water (at nominal pressures and temperatures). Methane hydrates are known to form and remain stable at moderately high pressures and low temperatures, i.e., the conditions found on land in permafrost regions and within ocean floor sediments at water depths greater than about 500 meters. The increasing heat encountered in deeper sediments tends to preclude hydrate formation toward the bottom of the hydrate zone. Although free natural gas can be found in sediments below the hydrate zone, the hydrate deposits themselves, which may be several hundred meters thick, remain a vast and relatively untapped natural resource.
Methane hydrates have been detected around most continental margins, for example in Alaska, the west coast from California to Washington state, the east coast, including the Blake Ridge offshore the Carolinas, and in the Gulf of Mexico. Most scientists believe that at least 200,000 tcf of methane exists in hydrate systems located in the U.S. permafrost regions and surrounding waters. That amount is hundreds of times larger than the total estimated U.S. natural gas reserves. Most estimates of U.S. hydrate resources range between 112,000 tcf and 676,000 tcf (at 0.95 and 0.05 probability levels respectively, with the mean value being approximately 200,000 tcf). The amount of offshore hydrate is estimated to be about 99% of that total. Thus, if only 1% of the estimated 200,000 tcf methane hydrate resource became economically recoverable, the domestic U.S. natural gas resource could at least double.
It is well known that the U.S. will consume increasing volumes of natural gas in the 21st century. Thus, methane hydrates could become a reliable, low cost domestic supply, particularly when used for power generation, transportation or even fuel cells due to the increasing pressure for cleaner fuels with reduced CO2, particulates, sulfur oxides, and nitrogen oxides.
Although the estimated reserves of natural gas hydrate are larger than the reserves of all other fossil fuels combined, the technology necessary to recover large amounts of natural gas from hydrate has never proven to be economically feasible. Thus, with one exception in Russia (discussed below), natural gas hydrate has not been used to generate recoverable fuel in commercially significant amounts.
A Department of Energy report (xe2x80x9cDraft Methane Hydrate program Plan, April 1998xe2x80x9d), available on the internet at www.fe.doe.gov discusses various proposed methods of producing natural gas from hydrate deposits. One method proposes that hot water be pumped down into the deposit to melt the solid hydrate into liquid water and gaseous methane. The heat necessary to do this, however, approaches that necessary to melt an equivalent amount of ice. Since hydrate deposits are relatively xe2x80x9cthinxe2x80x9d geological structures spread out over large areas, supplying such a large amount of heat in the form of hot water would be very expensive and logistically difficult. Furthermore, converting the massive amounts of solid hydrate into liquid water may have undesirable environmental effects. For example, a concern exists that converting solid hydrate to water may cause geological instability at or near the deposit. The possibility also exists that large amounts of methane liberated during the conversion process might escape into the atmosphere. In addition to posing an immediate health risk to persons living or working in the area, such uncontrolled emissions could add to the greenhouse effect.
The DOE report also discusses using indirect heating techniques to convert hydrate deposits into natural gas and liquid water. Unfortunately, changing the way the heat is supplied does not change the amount of heat that must be supplied. Thus, the same environmental concerns exist with the conversion of large volumes of solid hydrate into liquid to liberate methane gas, regardless of the heat source.
Recently, scientists have recognized that carbon dioxide hydrate, i.e., CO2 encased within water-ice molecules, is more stable than natural gas hydrate at the same depth, i.e., at a given pressure and temperature. Thus, geologists have now postulated that it may be possible to use carbon dioxide hydrate to liberate the methane in the natural gas hydrate. However, even though carbon dioxide hydrate is known to be more stable than methane hydrate with certain pressure and temperature ranges, one will not readily convert to the other. Because natural gas hydrate does not readily absorb the carbon dioxide (and vice versa) only the hydrate close to the surface of the ice will reject one absorbed material to accept another. Thus, the inside of the hydrate remains essentially unchanged.
One known prior art technique for recovering methane developed in Japan, Komai et al, xe2x80x9cPreprints Div. of Fuel Chemistry, ACS national Meeting 1997, San Francisco, 568-572) has suggested injecting liquid CO2 directly into the hydrate zone to convert methane hydrate into methane and CO2 hydrate. Since the reaction is very nearly thermoneutral, this technique eliminates the need to supply heat to the hydrate. The Komai et al method also avoids some of the environmental concerns relating to the conversion of one solid hydrate into a liquid because a solid hydrate is converted into another solid hydrate of different composition and form.
The Komai et al methodology suffers from a number of critical limitations primarily because methane hydrate is thermodynamically stable under only a limited range of temperatures and pressure. As noted above, hydrate deposits tend to exist as geologically xe2x80x9cthinxe2x80x9d layers compared to other fossil fuel resources. The deposits also tend to be spread out over large geographic areas, making conventional recovery techniques less economically viable. The hydrate also tends to form under impermeable geological formations (xe2x80x9ccapsxe2x80x9d). As the process of converting liquid water (and ice) to solid hydrate goes to completion over many thousands of years, the conversion process leaves behind a dry, nonporous mixture of sand and hydrate known as overburden.
When liquid CO2 (or some other liquid) is pumped into the ground at a pressure that exceeds the weight of the overburden (as suggested in Komai et al), after an extended time period the substratum may fracture, with the cracks spreading horizontally. Thus, even though it may be technically feasible to inject liquid CO2 into a hydrate formation to allow the CO2 to contact a large area of the hydrate deposit, the Komai et al process is not economically practical on a large scale unless the reaction of liquid CO2 and methane hydrate to form CO2 hydrate and gaseous methane could be made to occur at a much higher rate. Generally speaking, if the hydrate of gas A is in direct contact with molecules of gas B, then the molecules of gas A on the surface of the hydrate will diffuse out to be replaced by the molecules of gas B which diffuse into the surface openings. If gas B is the more stable hydrate, the surface of the first hydrate particle will readily be converted from A to B, with the rate of conversion of the bulk hydrate from A to B being controlled by diffusion through the solid phase (normally a very slow process). When the reaction of liquid CO2 with the methane hydrate to form CO2 hydrate and gaseous methane is slow, the rate of production of natural gas will likely be too slow to be economically viable. In addition, the amount of liquid CO2 that accumulates underground over an extended time period of time during the conversion could become large and create additional environmental and geological concerns. Unfortunately, the Komai et al reference provides no teaching as to how an acceptably high rate of hydrate conversion may be achieved.
The present invention solves the problem of CO2 disposal while producing natural gas from hydrate by providing a new and more efficient method for converting liquid CO2 and natural gas hydrate into natural gas and CO2 hydrate, and thereafter recovering the natural gas product using conventional means. In particular, the invention provides a means whereby the conversion of CO2 and methane hydrate to CO2 hydrate and gaseous methane occurs much more rapidly than Komai et al, thereby reducing the cost of producing natural gas while avoiding any serious environmental concerns.
The invention takes advantage of the relative differences in density between liquid CO2, methane hydrate and CO2 hydrate at a given field depth. That is, the method according to the invention uses the predicted differential in density to facilitate the conversion of methane hydrate to CO2 hydrate, with the added beneficial result being the release of methane gas from the hydrate.
It has now been discovered that the use of SO3 or HCl or other reagents which are strongly water-soluble and soluble in liquid CO2 in combination with liquid CO2 injected into the hydrate zone will cause the methane hydrate ice crystals to melt and form a layer of water. On one side, this aqueous phase contacts the methane hydrate ice and, on the other side, contacts the, liquid carbon dioxide. The melting of the ice dilutes the HCl or the sulfuric acid, which in turn tends to become lower in concentration until it no longer melts the methane hydrate. Conversely the removal of water from the solution by the formation of CO2 hydrate tends to increase the concentration and restore its ability to melt the methane hydrate. The system spontaneously adjusts so that the aqueous acid solution remains strong enough to melt methane ice, but not strong enough to prevent the formation of the carbon dioxide ice, which is more thermodynamically stable at the same depth. Thus, on one side of the aqueous solution the methane ice melts, and on the other side carbon dioxide ice forms. This catalyst xe2x80x9ctrickxe2x80x9d results in dramatically better methane formation and recovery rates, primarily because the liberation of methane is no longer limited to amount of methane which was initially at or near the surface of the hydrate. Instead the continual melting of the methane hydrate and freezing of carbon dioxide hydrate provide a facile mechanism for freeing the methane.