Method of gas production from geopressurized geothermal brines

In accordance with the method of the present invention, methane and other similar fuel components are separated from the brine found in geopressurized geothermal zones by immersing gas permeable membranes selective for these fuel components into the brine in these zones, and permitting these fuel components to permeate through the membranes while rejecting the brine. The permeated fuel components then are collected.

This invention relates to a method of recovering methane and other similar 
fuel components from underground fluid zones that exist naturally at high 
pressures and, in many cases, at both a high temperature and pressure as 
found, for example, in various geopressurized geothermal zones. 
Recent oil embargoes and subsequent price increases have prompted many in 
the United States, including many government agencies like the Energy 
Research and Development Administration, to look for and investigate new 
or heretofore untapped sources of energy. One such source of considerable 
interest is the enormous supply of methane, the chief constituent of the 
natural gas burned in homes and factories, estimated to be found and, in 
many cases, to be dissolved under pressure in hydro-pressurized zones or 
reservoirs of water or hot salt water deep within the earth. 
For example, such hydro-pressurized zones are found in Louisiana and one 
such geopressurized geothermal zone underlies the Gulf of Mexico and 
coastal areas of Texas and Louisiana. It has been estimated that this 
latter zone alone contains from 24,000 to 105,000 quads of methane. This 
is a stupendous figure considering the fact that one quad is roughly 
equivalent to the amount of energy found in a trillion cubic feet of 
natural gas. At the present time, the United States consumes about 70 
quads of energy a year. 
Obviously, if this dissolved methane could be economically recovered, this 
source of energy would be of tremendous value to the United States. The 
existence of these geopressurized geothermal zones has been known since 
the 1950's when wildcatters frequently encountered enormous water 
pressures and observed that natural gas was associated with the vast 
quantities of hot salt water that gushed out under pressure. 
Since then, even though the technology for drilling and completing the 
wells for tapping these geopressurized geothermal zones, producing the 
fluid, and separating the natural gas from the water has been developed, 
no one has ever attempted to withdraw the brine or to separate the methane 
from it. Several reasons for not having done so exist, among the major 
ones being the cost for drilling the wells and the environmental problems 
which exist in extracting the brine. It has been estimated that the cost 
of each well may be $2 million or more. 
Among the various environmental problems, the subsidence problem is the 
most serious one associated with tapping these zones. It is not presently 
known what effect withdrawing the large quantities of brine, possibly 
40,000 barrels or more per day, will have on the region. Another problem 
of withdrawing such large quantities of brine from each of the wells is 
how to dispose of the salt water. As to the latter, it has been proposed 
to re-inject this salt water back into the zone or, alternatively, to 
channel it to the sea. 
Recently, a new method has been devised for removing or recovering this 
methane from the brine in these geopressurized geothermal zones, in a 
fashion which eliminates the above-mentioned, as well as other, 
environmental problems. In addition, the cost of the wells and/or cost of 
the fuel is anticipated to be substantially less than present estimates, 
so that recovery is more economical and practical. The same method can be 
used to recover the methane found in hydro-pressurized fluid zones wherein 
the methane is dissolved in the water as a result of the high hydro-static 
pressures or heads encountered. Accordingly, the term "liquid" as 
hereinafter used is intended to encompass both the water found in 
hydro-pressurized zones as well as the brines found in geopressurized 
geothermal zones. 
It is generally well-known that methane and other similar fuel components 
can be separated from other gaseous components by utilizing gas permeable 
membranes which are selective for that or those fuel components. The 
technology associated with membrane utilization for purposes of separation 
of particular fuel components is extensive, but none of this technology 
has heretofore been directed or utilized to separate methane and other 
similar fuel components from liquids in underground fluid zones wherein 
this methane is dissolved under high pressures. Unexpectedly, 
methane-selective membranes, stable-to-high temperatures, were found to 
perform as well or better when immersed in liquid brines containing 
dissolved gases than when in gaseous mixtures. 
Accordingly, in accordance with the method of the present invention, 
methane and other similar fuel components are separated from the water in 
these hydro-pressurized zones, or from the brine found in these 
geopressurized geothermal zones, by immersing gas permeable membranes 
selective for these fuel components into the liquid in these zones, and 
permitting these fuel components to permeate through the membranes while 
rejecting the liquid. The permeated fuel components then are collected. By 
means of this simple expedient, numerous unexpected results are achieved, 
among which are the fact that these fuel components can be separated and 
recovered in situ, thus eliminating at least the above-mentioned 
environmental problems associated with withdrawing large quantities of 
brine, that of subsidence and disposal of the brine. Further still, by 
withdrawing and re-injecting the spent brine after the fuel components are 
separated from it at ground level, additional energy value can be 
extracted both from the pressure and heat of the brine. In either event, 
the cost of the fuel components recovered is substantially less than 
present estimates, and sufficiently economical to make recovery practical, 
whereas now it is questionable whether it would be practical to recover 
these fuel components using existing methods and technology. 
Accordingly, it is an object of the present invention to provide an 
improved method of removing methane from underground fluid zones, which 
eliminates the environmental problems associated with land subsidence on 
brine withdrawal and disposal, reduces the cost of fuel and optionally 
eliminates a need for withdrawing large quantities of liquid for surface 
processing. 
The above objectives are accomplished, in accordance with the present 
invention, by immersing gas permeable membranes selected for the fuel 
components in the underground fluid zones to separate these components 
from the liquids. The membranes are suitably supported to withstand high 
pressure differentials, are chemically resistant to hot brines and are 
incorporated in the gas production system so that the permeated gas is 
collected substantially free of contaminants. The membranes can be 
immersed in the liquids to separate the fuel components from the liquids 
in situ or, alternatively, the liquids can be withdrawn to the surface 
where the fuel components are separated with heat recovery, and with the 
liquid being rejected. 
In particular, in the case of recovering these fuel components from the 
brine in these geopressurized geothermal zones, the methods proposed by 
the prior art all generally consist of drilling a large number of wells, 
withdrawing large volumes of brine, stripping in some fashion the methane 
from the brine at ground level, and then disposing of the spent brines, 
either with or without heat recovery. None of these prior methods 
discloses or permits gas separation from the brines in situ to eliminate 
disposal and environmental problems, to yield a relatively pure gas, nor 
do they disclose stripping the gas from the brine at ground level using 
gas permeable membranes, as taught by the present invention. 
The method of the present invention, therefore, comprises the steps of 
incorporating one or more gas permeable membranes into a gas collection 
system; continuously or intermittently subjecting part or all of the 
membrane permeators to a liquid containing dissolved methane and other 
similar fuel components; permeating the methane and similar fuel 
components through the membrane; and collecting the permeated methane and 
similar fuel components while rejecting the liquid. 
The membranes which can be used in the invention are those having methane 
permeabilities, anisotropic, composite or homogeneous structure; and 
spirally wound, tubular, or hollow fiber configurations. Suitable chemical 
structures include polytetrafluorethylene, polyolefins, polypeptides, and 
other polymers that exhibit good high-temperature performance, low water 
permeability and high methane permeability.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to the drawings, in FIG. 1 there is generally illustrated the 
manner in which methane and other similar fuel components can be stripped 
from the brine, in situ. As there illustrated, a well is drilled in 
conventional fashion to the depth of the geopressurized zones containing 
brine with the methane and other fuel components dissolved in it. The well 
contains the usual well casing 10. 
A pipe, 12 including one or more gas permeable membranes 14 which are 
incorporated into it in spaced relation, extends through the well casing 
10, and extends beyond it into the brine so that the membranes 14 are 
immersed in the brine. The methane and other fuel components permeate 
through the membranes 14, while the brine is rejected. The methane and 
other fuel components then flow through the pipe 12, to a suitable gas 
collection system 16. 
In this embodiment of the invention, obviously the brine does not have to 
be withdrawn, thus the above-mentioned problems of brine disposal and land 
subsidence both are eliminated. The methane is simply stripped from the 
brine as it permeates through the membranes 14 and the brine is rejected 
and remains in situ. 
In FIG. 2, there is generally illustrated a method of recovering the 
methane and other fuel components from the brine above ground. In this 
case, a well is drilled in conventional fashion, with the well casing 18 
extending well into the brine in the geopressurized zone. A module 20 
containing one or more gas permeable membranes is incorporated into the 
piping 22 coupled to the well casing, such that the brine withdrawn from 
the fuel pressurized zone flows through the module 20 in contact with the 
membranes therein so that methane and other fuel components are stripped 
from the brine. This methane and other fuel components flow through the 
pipeline 24 to the gas collection system 26. 
Also, preferably and advantageously, the hot brine is conveyed to and 
utilized in a power production unit 28 of a type which extracts the heat 
from the brine and, for example, generates electricity. The spent brine 
then preferably is reinjected into the geopressurized zone, via a second 
well 30. 
As indicated above, the membranes are those having methane permeabilities; 
may be anisotropic, composite or homogeneous structures; and may have 
spirally wound, tubular or hollow fiber configurations. These all are 
typical configurations which are well-known in the art, and the manner in 
which the membrane is supported so that it does not collapse under a 
pressure differential depends on the configuration. 
An obvious configuration is a membrane sheet supported by a frame. However, 
a membrane case in the form of a tube is more practical because the 
membrane area-to-volume ratio is much higher, and if the membrane is thick 
enough for the particular pressure differential conditions, it will not 
collapse. Porous supporting material can be placed within the tube to 
increase the strength, or the active membrane layer can be cast on a 
physical support layer having high gas permeability. The anisotropic and 
composite configurations can be supported by this technique. The very thin 
capillary tubes often do not need any additional support, and a bundle of 
tubes can be used as illustrated in FIG. 3. 
For example, as can there be seen, a number of the capillary tubes 40 are 
bundled and potted by means of a resin or the like, into a pair of end 
caps 42 and 44. An outlet 46 is provided for removing the permeated gas, 
while the end caps 42 and 44 have pipe stems 47 and 48 for injecting and 
withdrawing brine. In other words, brine is pumped or otherwise caused to 
flow through the pipe stem 47, through the bundle of capillary tubes 40, 
and out through the pipe stem 48. The methane and other fuel components 
which permeate through the capillary tubes 40 is collected at the outlet 
46. Alternatively, the brine can be permitted to pass around the capillary 
tubes, with the methane permeating through the tubes for collection. 
Another membrane assembly is illustrated in FIGS. 4 and 5. In this case, it 
is of a spirally wound configuration, having two membrane sheets 52 and 
54, each of which is slightly shorter than the completed module attached 
to a backing 56 for permeate collection. The resulting sandwhich is then 
attached to a core 58 with holes (not shown) that serve as a manifold for 
the permeated gas. An intersandwich mesh 62 is placed parallel to the 
sandwich to maintain a space between the layers for the high pressure feed 
bearing. The mesh and the membrane sandwich are then rolled up, and are 
then wrapped with a protective plastic sheet 64 which also keeps the 
assembly tightly wound, as illustrated in FIG. 5. Individual modules or 
assemblies 50 can be mounted in series in a pipe housing to furnish a 
complete membrane system. 
All of the above support mechanisms are generally known in the art, and 
thus form no part of the present invention. Various other support 
membranes can as well be used, and various parallel and series connections 
in membrane modules, with and without recycle, are also possible. In 
operation, it is also possible to reduce the pressure on the permeate side 
of the membrane to increase the pressure differential across the membrane 
so as to increase the rate of separation. Further still, other operating 
modes can be, for example, pulsed or steady-state. 
The resistance of suitable methane-permeable membranes to hot brines must, 
of course, exist, at least to the temperatures encountered during use. The 
temperatures encountered in geothermal brines can be as high as 
150.degree. C. and higher, hence the membranes must be able to withstand 
at least a temperature of 150.degree. C. 
If organic membranes are used, the chemical structures should be selected 
to withstand the in situ temperatures. Some membranes having suitable 
chemical structures include polytetrafluroethylene, polyolefins, and 
polypeptides. If resistance to higher temperatures is necessary, it can be 
incorporated in the usual manner by use of organo-metallic membranes. 
In the table below are listed the resistance to water and temperature of 
several polymeric membrane materials, and any one of these materials may 
be used for the membranes in accordance with the method of the invention. 
TABLE I 
______________________________________ 
Polymer Resistance to Water and Heat 
Temperature, .degree.F. 
Water, 
Polymer Type ASTM Test D759 
ASTM Test E96 
______________________________________ 
Acrylonitrile-butadiene- 
styrene (ABS) 190-220 G 
Cellulose Acetate 
150-200 G 
Cellulose Triacetate 
300-420 G 
FEP Fluoroplastic 
440-525 G 
Teflon 500 G 
Nylon 6 200-400 G 
High Density PE 
250 G 
Polyimide 750 G 
Polysulfone 350 G 
Polyethorsulfone 
400-450 G 
______________________________________ 
Acetal Copolymer, 10% NaCl solution, 180.degree. F. 180 days; +0.5% cut 
change, slight discoloration 
Nylon Zytel 101, 10% NaCl solution, 73.degree. F. 365 days; no change 
observed. 
As indicated above, the method of the present invention permits methane and 
other similar fuel components to be removed from underground brine such as 
found in many geopressurized geothermal zones. The method furthermore 
eliminates the environmental problems associated with land subsidence on 
brine withdrawal and disposal. Broadly, the method includes the steps of 
incorporating a gas permeable membrane to a gas collection system, 
continuously or intermittently subjecting part or all of the membrane 
permeator to the liquid brine containing dissolved and other fuel 
components, permeating the fuels through the membrane, and collecting the 
permeated methane and other fuel components while rejecting the brine. The 
method may be used to recover the methane from the brine in situ, or at 
ground level. Also, while the described embodiment of the invention is 
specific to the removal of methane from the brine found in geopressurized 
geothermal zones, the same method can be used to remove the methane 
dissolved in the water found in hydro-pressurized zones. In the latter 
case, the temperatures encountered usually are substantially lower. The 
pressures encountered likewise may be substantially lower.