MIT Natural Gas Report Glosses Over Environmental Issues
July 1, 2010 § 1 Comment
MIT’s most recent report on energy is on the Future of Natural Gas, following similar reports on coal and nuclear energy. It is co-edited by Ernest Moniz and Tony Meggs. The latter recently left BP as CTO. As reported in Forbes recently, the report emphasizes the role of shale gas in enabling natural gas substitution of coal. The authors see this as a transitional strategy for a low carbon future. We agree with that and have expressed similar ideas in the Directors Blog.
However, the report is surprisingly shy about discussing the environmental issues seen as facing shale gas exploitation. While we believe these are indeed tractable, they merit much more discussion than they were given. Accordingly we repair some of that omission here.
The most significant issues center on three matters: fresh water withdrawals, flow back water and collateral issues, and produced water handling and disposal.
Fresh Water Withdrawals and Flow Back Water: Typical wells use between 3 and 5 million gallons per well. Industry practice has been to use fresh water as the base for fracturing fluid. The water that returns to the surface after the fracturing step is known as flow back water. Shale operations are unique in that only about a quarter to a third of the water returns, the rest staying in the formation. Also, the flow back water is usually more saline than the injected water. So, in principle it cannot be re-used.
Handling salinity is the first step to water conservation. The key is ability of the fracture water to tolerate some level of chlorides. Recent research has shown that not only is this possible, but that it can be beneficial. The chlorides actually stabilize the clay constituents of the shale and improve production, although companion chemicals such as friction reducers need to be modified. This has two possible implications to water withdrawals. One is that after some measure of treatment, the flow back water should be usable. But because all of it does not return, withdrawals for make-up water will be necessary. This is where the second implication comes in. Moderately saline water from another source could be used since salinity is tolerable. The most important implication of the foregoing is that flow back water could over time be completely re-used and this then ceases to be an issue with respect to discharge.
So, now let us discuss numbers. In current practice the tolerance for chlorides is likely about 40,000 ppm. Flow back water with higher salinity will need to be desalinated to some degree, or diluted by fresh water. In some parts of the country this may be viable. Another option could well be to use sea water, if that were to be the water of convenience. Sea water tends to contain around 30,000 ppm chlorides. That is already in the range of acceptability with the possible removal of some minor constituents. Finally saline aquifers are a potential source. These are in great abundance, with variable salinities. Saline water wells drilled as companion to the gas wells are very likely in areas where fresh water withdrawals compete with agriculture or other endeavors. In general, if the shale gas industry can utilize water unsuited to agriculture and human consumption, then it will be seen in a completely different light.
Water associated with the gas is produced at some stage of the recovery, usually towards the end of hydrocarbon production. In some cases early production occurs due to infiltration of the fractures into the underlying saline water body often present. Whether from connate water or the water layers below, produced water will be very saline, in part because of the age of the rock. Disposal of this water is a major issue, especially in New York and Pennsylvania and can cost upwards of $10 per barrel, when even possible. Concern regarding illegal discharge is high among the residents.
The treatment of produced water represents a significant business opportunity. Several outfits are developing forward and reverse osmosis schemes for desalination. Others are working on bacteria eradication, heavy metal removal and the like, using methods such as membrane filtration and ion exchange. Some of these are already in service on a limited basis.
Produced water offers the promise of being usable for make-up water after some modest treatment. The salinity may be directly tolerable but the bacteria would need to be removed prior to re-use. This is because many of these cause the production of hydrogen sulfide downhole, which makes the gas less valuable and causes corrosion in the equipment.
There have been anecdotal reports of well water contamination by gas, most recently sensationalized by a documentary. The popular literature ascribes two hypotheses to this phenomenon. One is the migration of fracturing operation cracks from the reservoir up to the water body. The other is gas leakage from the well.
Hydraulic fracture cracks will not propagate the significant distances to the aquifers. Were they inclined to do so, they would heal due to the earth closure stresses. In terms of distance, the closest fresh water aquifers are about 5000 ft. and 3000 ft. away, respectively, for the Barnett and the Marcellus. So this really is not likely.
Gas leakage from the well is preventable if the well is drilled and completed correctly. A fundamental feature of regulation has always been to design for isolation of fresh water in all petroleum exploitation, not just in the shale. Between the produced fluids and the aquifer lie two layers of steel encased in cement. The cementing operation is designed for preventing fluid migration. Tests are run to ensure competence of the cement job and remedies are available for shortcomings. At these shallow depths the operation is extremely straightforward and amenable to regulatory oversight.
See Also: New York Times’ response to the study