Underground recovery of natural gas from geopressured brines

A technique for recovering natural gas from geopressured brines contained in a first aquifer in which the brine is conveyed from the high pressure geopressured aquifer to an underground chamber at lower pressure at a location remote from the geopressured aquifer, the lower pressure within the chamber causing the gas to be released from the brine after entry into the chamber. The released gas is conveyed from the chamber to a suitable gas outlet for use while the brine within the chamber by virtue of its pressure is conveyed to a second upper level discharge aquifer at a location remote from the chamber.

INTRODUCTION 
This invention relates to techniques for recovering natural gas and, more 
particularly, to techniques for recovering natural gas underground from 
geopressured brines. 
BACKGROUND OF THE INVENTION 
In many regions of the world, particularly, for example, in the Texas and 
Louisiana Gulf Coast regions as well as other regions of the western 
United States, extensive reservoirs of deep, geopressured saline waters, 
or brines, that are saturated with natural gas, i.e., methane, are 
present. Such brines exist in large formations of geopressured tertiary 
sandstones which contain huge aquifers at depths typically from a few 
thousand feet to over 25,000 feet. The particular aquifers in the above 
geographical regions are characterized by relatively high pressures which, 
for example, can be as much as twice the normally expected pressures in 
other aquifers at a comparable depth and may be characterized by 
relatively high temperatures of the order of 150.degree. C. or greater. 
The geological conditions of such aquifers tend to favor the formation of 
methane gas from the organic matters present. 
The methane gas content which is dissolved in such brines has been 
estimated to be approximately 1.0 cu. ft./gal. of water, recent tests 
showing that in exemplary regions 40 to 50 cubic feet of methane is 
dissolved in each barrel of water taken from an exploratory well at a 
depth of 16,500 feet. 
United States geological surveys have reported that the geopressured salt 
water in onshore reservoirs of this nature may contain as much as 24,000 
trillion cubic feet of methane and it has been further estimated that even 
more gas may be found offshore. In fact, one estimate has indicated that 
the geopressured gas reserves of the entire Gulf of Mexico region may be 
more than 100,000 trillion cubic feet, an amount which could go far toward 
solving the problem of energy shortage if such reserves can be 
appropriately developed. 
DISCUSSION OF THE PRIOR ART 
Present proposals for recovering the natural gas which is dissolved in 
geopressured brines introduce considerable difficulties. Such proposals 
involve bringing the highly saline water to a position above the earth's 
surface and utilizing the pressure of the saline water at such point to 
perform work by supplying the brine to hydraulic turbines which permit the 
pressure energy to generate electricity, for example. The pressure on the 
saline water at the output of such turbines being reduced, the methane gas 
dissolved therein is released therefrom and then recovered. The pressure 
may be further reduced for more complete recovery of the dissolved gas. 
Bringing the water to a point above the earth's surface for such processing 
requires equipment at the surface which must be capable of handling 
extremely large volumes of saline water, which water, following the 
turbine operation and the gas recovery process, must then somehow be 
disposed of. For example, even a relatively small generating plant, e.g., 
one which generates approximately 25 megawatts of electrical energy by the 
use of such pressure energy, sometimes coupled with the use of the thermal 
energy present in the brine, might require in an exemplary system a water 
throughput of about 17 million gallons per day. The handling and the 
disposal of such quantities of water using presently known techniques 
becomes an extremely difficult problem. 
While it has been suggested that such highly saline water be treated to 
bring it to a quality suitable for surface discharge, such treatment turns 
out to be economically impractical. Even at a conservative brine 
concentration of about 10,000 milligrams per liter, upwards of 700 tons 
per day of salt would have to be removed from the water, an extremely 
costly process using presently known techniques. Moreover, the recovery of 
the salt as soda ash or chlorine seems an unfeasible approach since there 
is no real market for such large quantities at present. Accordingly, 
rather than purify the water for surface use or disposal, such systems 
propose to reinject the brine into the earth. Such reinjection processes 
give rise to both environmental and technical problems. 
Moreover, the use of such highly concentrated brine solutions gives rise to 
corrosion problems for the turbines to which it is supplied as well as to 
other equipment which is used in the process. Such corrosion difficulties 
only further aggravate the handling and processing of the brine and 
further increase the expense of the presently proposed processes. 
It is desirable, therefore, to devise a technique for recovering gas from 
such geopressured brines without giving rise to the above problems 
associated with systems presently proposed. 
BRIEF SUMMARY OF THE INVENTION 
In one preferred embodiment of the technique of the invention, gas can be 
recovered from the geopressured brines without bringing large quantities 
of saline water to the surface, which after use and methane removal must 
then be disposed of. For example, in a preferred embodiment of the 
invention an underground chamber is formed below the earth's surface at a 
selected depth within the hydrostatic pressure zone. A suitable well 
drilled from the earth's surface through the underground chamber extends 
to a lower aquifer below the underground chamber in which the geopressured 
brine resides. The brine, which is saturated with natural gas, is conveyed 
from the lower aquifer to the underground chamber. The brine enters the 
chamber at a relatively high pressure and the lower pressure maintained 
within the chamber causes the natural gas to flash, or be released, from 
the brine, the latter forming a reservoir within the chamber. The portion 
of the well from the earth's surface to the chamber may be used to convey 
the released gas to the surface where it can be collected for use. 
Alternatively, this well may be blocked and a separate gas well drilled 
from the earth's surface to the underground chamber for the purpose of 
conveying the released gas to the surface. A conduit (or a series of 
conduits), which leads from the reservoir of brine within the chamber to a 
second upper, or discharge, aquifer above the lower aquifer is used to 
convey the lower pressure brine from which most of the methane has been 
removed, from the chamber into the discharge aquifer, which is at a 
pressure still lower than that in the chamber. 
Thus, the natural gas is recovered from the brine without the need for an 
extensive handling and processing system and without the need to dispose 
of the very large quantities of highly corrosive brines as in presently 
proposed systems. Moreover, the invention provides environmental, as well 
as process, advantages which are attractive and avoid the environmental 
problems which face the presently proposed systems.

As shown in the FIGURE, an underground chamber 10 is mined out at a 
selected depth below the surface 11 of the earth. For simplicity, the 
underground chamber is shown as generally rectangular in configuration, 
although its exact shape need not be limited thereto. In the embodiment 
shown, the location of the underground chamber 10 lies within what is 
generally designated as the hydrostatic pressure zone below the earth's 
surface, which pressure zone will extend from the surface of the earth to 
the top of what is designated as the geopressure zone, which may begin at 
depths of from 5,000 to 10,000 feet, for example. A liquid well 12 is 
drilled from the earth's surface through the underground chamber 10 to the 
geopressure zone comprising an aquifer 13 containing brines saturated with 
natural gas, e.g., methane gas. The upper portion 12A is suitably blocked 
following completion of the well while the lower portion 12B extends from 
the underground chamber 10 to lower aquifer 13. 
A suitable remote controlled valve and throttle assembly 14, which can be 
of a type well known to those in the engineering art, for example, is 
positioned at the upper end of well 12B above its entry point into the 
chamber 10. A gas well 15 is drilled from the earth's surface 11 to the 
underground chamber 10 for the purpose of collecting and removing the gas 
from the chamber. In some systems the well 12A may be utilized for the 
removal of the gas, thus dispensing with the need to provide a separate 
gas well 15. A conduit 16 leading from chamber 10 to a discharge aquifer 
17 is utilized to remove the lower pressure brine from which most of the 
methane has been released from region 10 as discussed below. 
In accordance with the operation of the system shown in the FIGURE, the 
brine under high pressure which is present in aquifer 13 is automatically 
conveyed from such aquifer to chamber 10 via well 12B, the flow thereof 
being suitably controlled by a remote control valve and throttling 
assembly 14 at the entrance to the chamber. The pressure within the 
underground chamber is controlled at a suitable value by means of a 
pressure control valve 18 at the top of the gas well and by the valve and 
throttling system 14 at the top of well 12B. The pressure in chamber 10 
will generally be controlled at a pressure which is sufficiently low that 
a maximum amount of the methane gas which is dissolved in the brine is 
released therefrom within the chamber. The brine entering the chamber 
forms a reservoir 19 in the lower region thereof. 
Because of the pressure difference between the pressure within chamber 10 
and the pressure at the surface, such released gas is automatically 
conveyed through gas well 15 and pressure control valve 18 to the surface 
where it can be appropriately collected for use in accordance with 
standard techniques well known to the art. 
While the pressure in the chamber will be so controlled as to release most 
of the gas in the brine, such pressure is still generally controlled so as 
to be sufficiently high to drive the brine in reservoir 19 to the upper 
level discharge aquifer 17 due to the pressure difference between the 
pressure within the chamber and that at the discharge aquifer. The level 
of the brine in reservoir 19 may be controlled by appropriate adjustment 
of the pressure in chamber 10, by the rate of gas withdrawal, and by the 
valve and throttling system 14. A level indicator/transducer (not shown) 
coupled to an automatic control system familiar to those skilled in the 
art may be provided. 
Once the system is set into operation, the operation thereof proceeds on a 
continuous basis, the brine being continuously released therefrom and 
conveyed from chamber 10 to the surface, and the brine being continuously 
conveyed from chamber 10 to upper level aquifer 17. 
In the preferred embodiment shown in the FIGURE the chamber 10 below the 
earth's surface effectively acts as a natural container. No pumps are 
needed since the pressure differences discussed above are such that brine 
and gas are appropriately conveyed as desired without them and the need 
for manufactured processing units is effectively minimized. The remote 
control valve and throttling assembly 14 can be fabricated of appropriate 
material available to those in the art which will not be subject to 
corrosion. 
The preferred technique of the invention specifically described above 
avoids the necessity for the handling of brine in processing equipment 
which is subject to corrosion and also avoids the problem of disposing 
from the surface extremely large quantities of such highly corrosive 
brines which arises with the presently proposed systems. In the particular 
embodiment shown, for example, the brine is suitably disposed of directly 
from underground chamber 10 into a natural aquifer 17 via conduit 16. The 
overall gas release and disposal process proceeds substantially 
automatically once the system is placed into operation. 
As one illustrative example of a system which could be utilized in 
accordance with the specific embodiment shown in the drawing, a 
geopressured aquifer, or brine zone, 13 may be located, for example, at 
approximately 15,000 feet below the surface 11 of the earth. It has been 
estimated that the methane gas content thereof will be approximately 1 
cubic foot of gas, at standard temperature and pressure, per gallon of 
water. The salinity of such brine may be up to 160,000 parts per million 
(ppm). In an exemplary system the underground chamber 10 may be located at 
approximately 1000 feet below the surface of the earth, at a position 
reasonably near an upper level discharge aquifer 17 which may be located, 
for example, approximately 500 feet below the surface of the earth. 
In such configuration the hydrostatic pressure at lower aquifer 13 may be 
approximately 6500 psi while the actual pressure (due to the geopressure 
effects) may be about 13,000 psi. At the level of chamber 10 the 
hydrostatic pressure may be about 430 psi while the brine pressure may be 
about 5600 psi. The pressure within chamber 10 may be controlled by means 
of pressure control valve 18, and by the valve and throttling system 14, 
at about 600 psi. The hydrostatic pressure at the upper level aquifer 17 
may be about 215 psi. 
Thus, the pressure difference between aquifer 13 and the upper end of well 
12B at the entrance to chamber 10 is sufficient to force the gas-saturated 
brine upwardly therethrough (as shown by the arrows) into chamber 10 where 
such brine becomes depressurized and releases most of the methane gas 
dissolved therein. The gas, because of the pressure difference between the 
pressure within the chamber and the pressure at the surface of gas well 
15, is conveyed upwardly to the surface as shown. The pressure difference 
between pressure within chamber 10 and the pressure at upper aquifer 17 is 
sufficient to convey the brine from the reservoir 19 thereof in chamber 10 
to upper aquifer 17 where the brine is appropriately distributed 
throughout the aquifer. 
The fabrication of liquid and gas wells 12 and 15, respectively, and the 
formation of chamber 10 are all well within the skill of those in the art. 
For the above illustrative example, the dimensions of the chamber may be 
such that the length is approximately ten feet, the height approximately 
eight feet, and the width approximately five feet. 
A single source well 12 which is 7" in diameter is estimated to be capable 
of bringing approximately 40,000 barrels per day, or 1.68.times.10.sup.6 
gallons per day, of brine from a geopressure aquifer 13 at about 15,000 
feet to the chamber 10 located at approximately 1000 feet. Since the 
geopressure at aquifer 13 is approximately double the hydrostatic pressure 
at such depth, and the pressure drop, as the brine flows upwardly in the 
source well, is about 100 psi for each thousand foot length, the pressure 
at the entrance to chamber 10, accounting also for the loss in hydrostatic 
head pressure, provides a brine pressure of about 5600 psi at such entry 
point. 
If the pressure of the chamber is controlled to be at about 600 psi, the 
methane solubility in the saline water is reduced from 1 cubic feet/gallon 
to about 0.04 cubic feet per gallon. If 90% of the methane which is 
released from the solution can be recovered, the gas production rate for 
such a typical illustrative example will be approximately 0.86 cubic feet 
of methane gas per gallon of brine, or about 1.44.times.10.sup.6 cubic 
feet of methane gas per day. 
To facilitate separation of the gas bubbles from the brine within the 
chamber, the depth of the reservoir of brine in the chamber is controlled 
to remain below about 18 inches for the illustrative example and the 
residence time of the brine within the chamber is of the order of 
magnitude of approximately twenty to thirty seconds. The gas leaving the 
brine enters the gas space above the reservoir for removal via gas well 
15, the gas velocity in the pressure reduction chamber 10 being typically 
less than about 0.01 foot per second to insure adequate disengagement of 
entrained water. A conventional demister device of a type well known to 
those in the art, can be located at the lower end of gas well 15 so as to 
further assist in removal of water droplets from the gas. 
While a single source well 12 for supplying brine to a single chamber which 
ultimately produces gas for conveyance via a single gas well 15 has been 
depicted in the FIGURE, the invention need not be limited thereto. Thus, 
one or more source wells 12B may be utilized to feet the same chamber, or 
a plurality of chambers may be utilized, each one being feet by one or 
more source wells. Further, one or more outlet conduits 16 may be utilized 
at each chamber for discharging to one or more aquifer regions 17. 
Further, one or more gas wells 15 may be utilized to convey the separated 
gas to the surface from each chamber. Appropriate debris filters may also 
be fitted upstream of the gas well, as required, as well as the demister 
device mentioned above. The discharge conduits 16 may also be fitted with 
suitable flow control or check valves as desired. 
In some applications where the chamber pressure is reduced essentially to 
hydrostatic pressure, pumps may be utilized to inject the de-pressurized, 
de-gassed brine from reservoir 19 into an upper level aquifer 17 via one 
or more conduits 16. If desired, an impingement baffle may be located 
above the source well entrance into the chamber so as to deflect the 
incoming brine and distribute it within the chamber. Further, appropriate 
means well known to those in the art for pressure regulation and liquid 
level control within the chamber may also be utilized. 
The location of underground chamber 10 is not limited to the specific 
illustrative example discussed above and the configuration of the overall 
system may be appropriately modified both in the locations of the various 
components thereof and in the dimensions and shapes thereof as desired by 
the particular application for which the system is to be used. Further, 
the underground chamber may be one that is naturally formed. 
Other modifications of the invention may occur to those in the art within 
the spirit and scope thereof and the invention is not to be considered as 
limited to the specific embodiments discussed above, except as defined by 
the appended claims.