Catalyst allowing conversion of natural gas hydrate and liquid CO2 to CO2 hydrate and natural gas

[0002] 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 CO₂to be converted into CO₂hydrate 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 CO₂used during conversion of the hydrate.

[0003] 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 “cage” 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.

[0004] 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.

[0005] It is well known that the U.S. will consume increasing volumes of natural gas in the 21^stcentury. 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.

[0006] 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.

[0007] A Department of Energy report (“Draft Methane Hydrate program Plan, April 1998”), 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 “thin” 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.

[0008] 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.

[0009] Recently, scientists have recognized that carbon dioxide hydrate, i.e., CO₂encased 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.

[0010] One known prior art technique for recovering methane developed in Japan, Komai et al, “Preprints Div. of Fuel Chemistry, ACS national Meeting 1997, San Francisco, 568-572) has suggested injecting liquid CO₂directly into the hydrate zone to convert methane hydrate into methane and CO₂hydrate. 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.

[0011] 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 “thin” 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 (“caps”). 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.

[0012] 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 CO₂into a hydrate formation to allow the CO₂to 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 CO₂and methane hydrate to form CO₂hydrate 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 CO₂with the methane hydrate to form CO₂hydrate 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 CO₂that 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.

[0013] The present invention solves the problem of CO₂disposal while producing natural gas from hydrate by providing a new and more efficient method for converting liquid CO₂and natural gas hydrate into natural gas and CO₂hydrate, and thereafter recovering the natural gas product using conventional means. In particular, the invention provides a means whereby the conversion of CO₂and methane hydrate to CO₂hydrate 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.

[0014] The invention takes advantage of the relative differences in density between liquid CO₂, methane hydrate and CO₂hydrate 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 CO₂hydrate, with the added beneficial result being the release of methane gas from the hydrate.

[0015] It has now been discovered that the use of SO₃or HCl or other reagents which are strongly water-soluble and soluble in liquid CO₂in combination with liquid CO₂injected 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 CO₂hydrate 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 “trick” 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.

[0016] FIG. 1 is a graphical representation of the relative stabilities of methane and carbon dioxide hydrates at various geological depths plotted against temperature;

[0017] FIG. 2 shows the density of liquid carbon dioxide, CO₂hydrate and CH₄hydrate in grams per cc plotted as a function of depth below the earth's surface in meters;

[0018] FIG. 3aillustrates the condition of a methane hydrate deposit prior to injection of liquid CO₂;

[0019] FIG. 3bshows the condition of a methane hydrate deposit after injection of liquid CO₂alone; and

[0020] FIG. 3cdepicts the condition of a methane hydrate deposit after injection of liquid CO₂and SO₃.

[0021] As noted above, it has now been found that the use of certain catalyst materials, preferably sulfur trioxide or HCl, can cause the methane hydrate to melt (thereby liberating the natural gas), while forming carbon dioxide hydrate as a product of the reaction. FIG. 1 shows the results of thermodynamic calculations of the stability of methane and CO₂hydrate as a function of depth using the computer program described in “Clathrate Hydrates of Natural Gases” by E. Dendy Sloan, Jr., Marcel Dekker, Inc., New York 1998. The calculations were done for a typical hydrate deposit. The deposit was assumed to be in a region with an average temperature of −10° C., with a permafrost depth of 610 m, a geothermal temperature gradient of 3.2° C./100 meters and a pore pressure gradient of 11.311 kpa per meter. In practical terms, the deposit would be located in the far north (or the far south) where the average temperature is about −10° C. and the ground is frozen to a depth of 610 meters. Under the permafrost, the temperature increases at a nominal rate of 3.2° C. for every 100 meters increase in depth; any underground liquids or gases are under a pressure that increases by about 11.311 kpa per meter.

[0022] As FIG. 1 illustrates, hydrate tends to form in three geological zones—one with only CO₂hydrate, one in which only methane hydrate can exist, and one in which both hydrates exist but with the CO₂hydrate being more thermodynamically stable. This last zone is particularly useful in practicing the recovery method according to the invention.

[0023] FIG. 2 shows the calculated density of methane hydrate, CO₂hydrate, and liquid CO₂as a function of depth in a typical hydrate formations. Although a wide range of temperature/pressure conditions exist under which CO₂hydrate is more stable than methane hydrate, direct contact between the liquid CO₂and the solid hydrate will only convert the surface of the hydrate, i.e., a trivially small fraction of the total hydrate. The use of SO₃or HCl catalysts as discussed above changes the entire dynamic of the conversion process by allowing methane far below the original hydrate surface to eventually be liberated as the conversion process from methane hydrate to CO₂hydrate progresses. That is, as the ice melts, it continuously forms a new aqueous layer containing the acid catalyst and continues to liberate more methane within the lower recesses of the hydrate as the melting proceeds and CO₂hydrate continues to form. The exact amount of SO₃and HCl necessary to create the aqueous layer is not critical, provided the amount is not so high that it impedes the formation of the CO₂hydrate layer.

[0024] Another aspect of the subject invention relies on the known fact that CO₂recovered from fossil fuel combustion typically contains at least a few hundred ppm SO₂. The SO₂can easily be converted into SO₃by various processes known in the art, e.g., by reaction with H₂O₂. Thus, an alternative embodiment of the present invention eliminates the step of adding SO₃and instead injects H₂O₂which then reacts with the SO₂already present in the liquid CO₂.

[0025] Under either process described above, when the liquid CO₂contacts the methane hydrate, it converts the surface of the hydrate to CO₂hydrate. Again, as the SO₃diffuses into the methane hydrate ice surface, it reacts to form dilute sulfuric acid. Since sulfuric acid is strongly hygroscopic, it tends to pull water out of the hydrate. This removes the surface layer of CO₂hydrate and leaves the surface coated with a thin layer of dilute sulfuric acid. Additional methane hydrate can then be dissolved and additional CO₂hydrate precipitates. The process continues until much of the methane hydrate in the formation has been converted. The methane gas is then removed by conventional means.

[0026] As shown in FIG. 2 above, liquid CO₂is less dense than CO₂hydrate but more dense than methane hydrate. Thus, as the conversion of the hydrate deposit proceeds, the liquid CO₂will tend to remain under the methane hydrate while floating on the CO₂hydrate. As a result, relatively good conversion of the hydrate deposit may be achieved at a much higher rate, particularly as compared to the prior art Komai et al method. FIG. 3 shows the reaction sequence that occurs when generating free methane in accordance with the invention. The recovery of natural gas from the hydrate occurs as the methane is continuously liberated during the conversion reaction forming CO₂hydrate.

[0027] FIGS. 3a, 3b, and 3cdepict the general reaction sequence that occurs when liquid carbon dioxide alone is injected into the methane hydrate (FIG. 3b) and in the presence of SO₃or HCl or some other substance which readily forms an aqueous solution (FIG. 3c). FIG. 3adepicts the methane hydrate prior to the injection of liquid CO₂. The methane hydrate is depicted as a layer resting on the sand. FIG. 3bdepicts the situation which occurs when liquid CO₂alone is injected. The liquid CO₂is depicted as a layer between the methane hydrate and the sand. The CO₂hydrate (depicted in a darker gray shade in FIG. 3b) forms only as a thin layer between the methane hydrate and the liquid CO₂. FIG. 3cdepicts the situation which occurs when liquid CO₂is injected along with SO₃. A layer of aqueous H₂SO₄forms between the methane hydrate and the liquid CO₂. This layer is shown in white in FIG. 3c. At the interface between the aqueous H₂SO₄and the CH₄hydrate, water and natural gas are liberated with the natural gas escaping. At the interface between the aqueous H₂SO₄and the liquid CO₂water and CO₂are converted into CO₂hydrate. The CO₂hydrate is, however, more dense than liquid CO₂and hence instead of accumulating at the interface falls to the bottom of the liquid CO₂layer and accumulates there.

[0028] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.