Process for recovering methane and/or sequestering fluids

Details Process for recovering methane and/or sequestering fluids Process for recovering methane and/or sequestering fluids

2000-12-18

2002-07-02

Suchfield, George (Department: 3672)

Wells

Processes

Distinct, separate injection and producing wells

C166S263000, C166S280100, C405S129400

Reexamination Certificate

active

06412559

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of coalbed methane and, in particular, to a process for sequestering fluids, such as carbon dioxide and hydrogen sulfide, in a coal bed, and/or recovering methane from a coal bed.
BACKGROUND OF THE INVENTION
Coalbed methane (CBM) has become a significant component of U.S. natural gas supplies. CBM production increased to 2.9 Bscf/day of gas supply in 1997, accounting for about 6% of total U.S. natural gas production (Stevens et al., “Enhanced Coalbed Methane Recovery using CO
2
Injection: Worldwide Resource and CO
2
Sequestration Potential” SPE 48881; 1998).
Most CBM reservoirs are produced under primary recovery methods, i.e., without secondary recovery methods involving injection of recovery-enhancing fluids. The proportion of original gas-in-place that can be recovered is dependent on reservoir properties, in particular, the absolute permeability of the coal bed. In high permeability reservoirs (>20 millidarcy (md)), recovery can theoretically be up to 80% of original gas-in-place. CBM recovery in moderate permeability reservoirs (5 to 20 md) can range from 50 to 70%, while recovery in low permeability reservoirs (≦5 md) can range from 10 to 50%. CBM recovery is also dependent on production economics. Presently, low permeability reservoirs are unlikely to produce CBM at commercial rates without some form of enhanced recovery. Moreover, the volume of CBM remaining after primary production, especially in moderate and low permeability reservoirs, is significant. For example, it is estimated that primary production in developed areas of the San Juan Basin alone, which are generally high permeability reservoirs, may leave behind as much as 10 Tscf of natural gas in areas with depleted coal beds (Stevens et al., ibid).
New technologies have been proposed for enhanced coalbed methane recovery (ECBM) to recover a larger fraction of CBM in place. The two principal variants of ECBM are (1) inert gas stripping by injecting nitrogen (N
2
), which is a weaker adsorbing gas (WAG) than CH
4
, and (2) displacement desorption by injecting carbon dioxide (CO
2
), a stronger adsorbing gas (SAG) than CH
4
.
Generally, as an injected WAG enters a coal bed through a wellbore, the partial pressure observed for CBM in the vicinity of the wellbore is substantially reduced. Most significantly, it is believed that the CBM partial pressure in the wellbore vicinity can be reduced to particularly low levels as a WAG is injected. Consequently, it is believed that as the CBM partial pressure is reduced, the CBM desorption rate from coal increases dramatically and the CBM is swept substantially through the coal bed in a mixture with the WAG to a production well. The production rate of the WAG and CBM is controlled by the total pressure in the formation, which is maintained as high as possible by injection during this process. Some WAG is sorbed into the coal, but there is a net reduction in the total gas (i.e., CBM and WAG) content of the coal.
By contrast, generally, as a gas that is more strongly adsorbing than CH
4
is injected into the coal bed, it is believed to be preferentially adsorbed into the coal. Since the SAGs are generally not produced, this process works well for both enhanced CBM recovery and sequestration of SAGs, such as CO
2
or hydrogen sulfide (H
2
S). And there is a net increase in the total gas (i.e., SAG and CBM) content of the coal. Also, the SAG is typically trapped in situ and is not produced unless the injected SAG front reaches the production well (i.e., breakthrough). At breakthrough, this type of SAG injection and CBM displacement process would be terminated.
Thus, a secondary benefit associated with a SAG injection/CBM displacement process, such as the CO
2
-ECBM process, is that it can sequester large volumes of CO
2
. There is an increasing concern that some gaseous effluent streams from industrial processes may cause environmental problems, and, as a result, these streams should not be released into the atmosphere. CO
2
is a constituent of many gaseous effluent streams released from industrial processes and whose release into the atmosphere is causing increasing concern. Should global restrictions on CO
2
emissions be promulgated, CO
2
-ECBM could be one of the few profitable technologies for sequestering CO
2
. For instance, tradable credits for CO
2
sequestration could dramatically improve CO
2
-ECBM economics over current performance levels.
Some global warming proponents relate excess nitrous oxide (N
2
O), as well as CO
2
, emissions to climatological change. Also, nitrogen oxide (NO
x
)) emissions, such as nitric oxide (NO) or nitrogen dioxide (NO
2
), in sufficient concentration, can be toxic to health and the environment. Additionally, sulfur oxide (SO
x
) emissions, in sufficient concentration, can contribute to the production of “acid rain,” which can have a detrimental effect on various plant and aquatic life.
Thus, it is possible that many or all of these gases could become more stringently regulated, at least in certain market-developed countries or regions, such as the United States, Canada, Japan and Europe. Consequently, this prospect of increasing regulatory stringency for some or all gaseous emissions can hamper many industries because the combustion of virtually any hydrocarbon fuel with air produces an effluent containing CO
2
, N
2
, and gaseous combustion products.
For instance, various countries, including, among others, France, Germany, the United Kingdom, Australia, the United States, Canada and Japan have agreed to seek internal approval and adoption, within their respective jurisdictions, of the Kyoto Protocol. The Kyoto Protocol ensued from the United Nations Framework Convention on Climate Change, held in December, 1997 at Kyoto, Japan. Under the Kyoto Protocol, each participant agreed in principle to “implement and/or further elaborate policies and measures in accordance with its national circumstances” to, among other things, enhance energy efficiency and protect reservoirs of certain atmospheric gases not controlled by the Montreal Protocol (e.g., CO
2
).
Generally, under the Kyoto Protocol, the participating countries agreed to limit emissions of greenhouse gases specified under the Protocol, including CO
2
, CH
4
, N
2
O, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF
6
), as well as work towards reducing the overall emissions of these gases by at least 5% below 1990 levels in the target period of 2008 to 2012. To date, no legislative amendments to the U.S. Clean Air Act Amendments of 1990 (CAAA) have been passed that would require facilities operating in the U.S. to comply with the Kyoto Protocol greenhouse gas emissions target. Nonetheless, the present U.S. administration has made a policy decision to promote voluntary compliance with the Kyoto Protocol. Accordingly, companies operating in the U.S. that have significant CO
2
emissions have been encouraged to voluntarily work towards the Kyoto Protocol's target level for the greenhouse gases specified. Also, if good progress towards the Protocol's goals is not shown, it is possible that some further amendments to the CAAA could flow from the Kyoto Protocol. CAAA amendments conforming with the Kyoto Protocol could also be motivated if models are developed to more definitively measure and predict the extent of global climate changes based on current and projected gaseous emissions. Thus, limiting the gaseous emissions, particularly from coal-fueled power generation plants, while maintaining an energy efficient power generation process, is becoming a more important commercial objective.
In addition to being a hydrocarbon combustion product, CO
2
can be produced by natural processes and released to the environment during a non-combustion process. For example, CO
2
is produced by thermal and biogenic processes, which are believed to form hydrocarbons such as oil, natural gas, or coal. CO
2
often is recovered with these hydrocarbons and released to the atmosphere by vari

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