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Published: Tuesday, 10 June 2014 21:29

Written by Administrator

Amendments to formation water in coal seams

Gregory DiCenzo

Historically, coal solubility has been modified through pH changes: either to extreme acid or alkaline environment. Also contributing to desired state changes are temperature and pressure.

Though the seam can receive acid or base via injection, it is not practical to modify temp or pressure. The recent “Plasma Pulse” technology from Novas Energy would have no effect on coal solubility, although it may be a tool for resolving flow in a formation. Acid is frequently used in wells drilled into coal to clean the coal face, removing cement and obstructing coal fines from fractures.

Solvents may be used in a coal seam to raise solubility of some classes of molecules or otherwise create highly local states that might bridge polar and non-polar character, thus making, for example, coal functional groups slightly more available to agents such as bacteria acting in their vicinity.

Ethyl acetate is slightly soluble in water (08.3g/100 mL) and might raise the solubility of alkane or phenolic compounds from a coal seam. In this scenario, too, permeability may be increased via the action of precipitants being removed. It is known to be safe (at a few percent) to microbia and to increase larger coal compound availability.

Pyridine is used in analysis of coal, producing “coal extracts” – though the result may not so much be a solution as a fine dispersion. In either case, pyridine is known to swell coal and lignite ex situ. Pyridine is polar, with a moderate dialectric constant – but is aprotic and can not contribute to hydrogen bonding. Pyridine injected into a seam under pressure is unknown. Solvents such as pyridine, diesel and related compounds injected into oil wells to reduce paraffin are mixing with crude oil and do not present immiscibility problems with consequent pressure buildup. Tetrahydrofuran is similar to pyridine and has less toxic homologues.

Traditional preparations of coal extracts might use p-toluenesulphonic acid in pyridine to depolymerize coal – since coal is largely the byproduct of dehydration reactions enroute to lignite, but tending more to decarboxylation reactions during transformation to sub-bituminous coal. Indeed, de-polymerization is a common step in ex situ process; carbon monoxide is also used towards this end under pressure. However, this chemical process may not be possible in situ, in what is a largely aqueous milieu.

Effects of dialectric property change of fluid in situ is unknown; gas adsorption and desorption and the effect on permeability of sub-bituminous coal is known to affect permeability with both positive and negative correlations. Thus, in situ, addition of a miscible solvent (pyridine) may have unpredictable effects – possibly deleterious through flow resistance effects. “Safe” solvents to be injected into coal formation water would be partially water soluble solvents such as ethanol or ethyl acetate, neither of which would raise viscosity but would likely increase wettability of the carbon substrate and, at low levels, would be bio-safe. At this time, real benefits of ethyl acetate and ethanol addition to formation water are hypothetical.

Surfactants

Surfactants are diverse: ionic, zwitterionic, nonionic. Used in purely aqueous solutions, they primarily reduce surface tension or help mobilize lipid compounds in a micelle scheme. Surfactants are also used in the oil field in “polymer flooding” methods, usually to combat paraffin buildup or to recover oil in a water-bearing formation. In the latter case, interfacial tension is decreased, allowing oil globules to flow in the water channel. Alkaline polymer surfactants are used (which react with acidic components in the oil) and simple polymers are used at low rates.

Surfactants are also used in coal processes and aid in dust reduction; in situ, it is known that surfactants can mobilize coal fines after fracking – such that they can reduce permeability by clogging fractures. Thus, it is preferable that coal fines not be moved to the mobile phase, and should probably remain “oil-wet” – except that bacteria would also be excluded.

Thus, surfactants may have permeability- decreasing properties if used in situ by mobilizing particles. While helpful in oil reservoirs, they could be undesirable in coal seams.

Yet, borrowing from oil field amendments, coal seam injectate may benefit from foam and viscosifying agents which help improve penetration and sweep, thereby increasing the concentration of bio-chemo contact. Fracking, with traditional propant characteristics, will also increase penetration of bacteria in previously excluded space, thus increasing reactions sites on an area basis.

Microbially enhanced oil recovery (MEOR):

MEOR is a sister technology to coal solvent injections. Generally, indigenous and adapted microbes are stimulated in an oil bearing stratum, thus increasing their numbers and activity. The activity of MEOR typically is to achieve the same effect that polymer flooding achieves: decreasing the interfacial tension between oil and water phases, thus stimulating movement of oil through a formation. In some cases, it is considered that the biological activity also improves the quality of the in place oil.

In practice, limiting nutrients are provided to the microbes which are otherwise carbon heterotrophs. In some cases, microbes may be injected into a formation. MEOR can occur in wither aerobic or anaerobic settings, the latter requiring sugar amendment, typically molasses. In this case, the microbes are in place producers of otherwise desirable cellular products.

Effects of MEOR actors:

- decrease viscosity through alteration of the larger crude compounds

- surfactant production, causing movement of oil into and through the formation
- gas production: increases pressure which drives oil into moving

- acid and/or solvent production and excretion

The mere increase of biomass may cause plugging, which is usually reduced through other reservoir management methods. Otherwise, objectives for MEOR researchers are to increase bio-surfactant production and enhance the viscosity increasing effects of indigenous microbes. The latter is desirable where injecting only reaches less permeable portions of a formation, greatly reducing the expected volume of treated formation. From this perspective, rational use of MOER methods may promote solubilizing results.

In summary:

Whether stimulating indigenous bacteria to increase bio-surfactant or otherwise increase viscosity, or adding surfactants or otherwise increasing viscosity may be preferred targets for management of solvent sweep, physical parameters of the formation water would probably benefit from modifications.

Chemical parameters of the solvent profile may also benefit from modification: immiscible solvents such as pyridine or THF may be of use but at low rates to avoid over-pressuring of the formation through increased viscosity but also via effects on swelling, porosity and permeability.

More water-soluble solvents such as ethyl acetate or ethanol can be somewhat effective but are also bio-labile and short-lived. It is known that these are not harmful to the indigenous microflora, but return on investment is unknown.

pH Adjustments are temporary, and can be lost upon restoration to typical formation pH’s. Permanent pH changes are probably impractical – and may also affect the biota.

Other factors should be considered to enhancing in situ treatment of coal seams:

- increase the sweep of the injectate

- fracturing to increase contact of microbia with coal surface

- use shut-in methods rather than recirculation methods to allow for biofilm buildup and cost savings

Miscible, mutual solvents may perform acceptably in coal seams; numerous formulations are available from oilfield service providers (e.g. Weatherford) but their biostatic activity may not be known in some cases as the formulas are protected trade secrets. The intention of their use would be to create zwitterionic character in highly localized areas near coal surfaces. It is not known if this effect could occur in situ in a biogenic gas coal seam.

“Best guess” suggestion for treatment of ongoing coal processes: a mild solubility enhancer like polymer surfactant addition. A surfactant could diffuse over time; merely miscible solvents like pyridine would not move via diffusion. Surfactants coating coal fines could create bio-amenable zones for bacterial contact. And, they may create “wetted” surface of coal for both enzyme and cell contact.

Rational use of MOER techniques to change chemico-physical parameters might be too speculative at this time and also require an unknown bio-response time.

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