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Implications of Production Scale Hydraulic Fracturing by Coal Bed Methane Drainage Operations on the Subsequent Safe and Efficient Mineability of Coal Seams

Underground » Ventilation, Gas Drainage and Monitoring

Published: May 07Project Number: C14011

Get ReportAuthor: Ken Mills, Rob Jeffrey, Winton Gale | Hydraulic Fracturing Technology, SCT Operations Pty Ltd, CSIRO Petroleum

Production scale hydraulic fracturing is routinely used by petroleum operators to stimulate water and gas production in coal bed methane projects.  Where petroleum leases overlap with current or future coal mining leases, production scale hydraulic fracturing has been identified by the coal industry as having potential to adversely impact on the future safe and efficient mineability coal.  This report presents the outcomes of ACARP Project C14011 aimed at assessing the potential for adverse impacts based on experience to date in Australia and the USA.

The outcomes of the project indicate that production scale hydraulic fracture stimulations do not adversely impact on the future safe and efficient coal mining and, in most cases, offer benefits to coal production including the potential for significant reduction in gas related delays.

During discussions on the development of legislation to manage coal bed methane resources in Queensland (Petroleum and Gas (Production and Safety) Bill 2004), the coal industry supported ACARP Project C14011 as a means to better understand the potential impacts of production scale hydraulic fracturing on the subsequent safe and efficient mineability of coal seams. This project aims to consider experience in the USA and Australia of longwall mining in coal seams that have been stimulated using hydraulic fracturing and to assess the relevance of experience in the USA to Australia, given possible variations in strata conditions between the two countries.

Literature Review and Study Tour

A literature review of impacts on coal mining of production scale hydraulic fracturing in the USA and Australia was undertaken.  A study tour was then made to the USA to collect specific detail on the current practices in relations to coal bed methane hydraulic fracturing and its subsequent impact on coal mining operations in the USA.  Meetings were held with representatives from Halliburton in Charleston, West Virginia, and from NIOSH (formerly US Bureau of Mines) and Schlumberger in Pittsburgh.

The experience related to the project team by personnel from coal bed methane service companies, coal mine operators, and research organisations indicated that many hundreds of hydraulic fractures had been mined through in the Northern and Central Appalachian Coalfields and in the Black Warrior Coal Basin of Alabama with no adverse effect on mining reported in any of the various studies that were undertaken.

In these coalfields, coal operators now routinely undertake gas drainage with hydraulic fracturing and by other means to improve conditions ahead of mining and to produce gas for commercial sale.  At least two coal companies in the USA have their own coal bed methane companies that have proved very profitable for them during the period when coal prices were low.  These companies continue to develop hydraulically fractured vertical wells to drain gas ahead of mining with several thousand wells having been fractured and some one thousand of these having been mined through without apparent adverse impacts.

Risk Assessment

The full range of potential impacts of production scale hydraulic fracturing was identified in a risk review process.
This review indicated that mining into foreign objects, particularly steel casing, is considered to be the greatest risk for future safe and efficient mineability of coal. Keeping of records and controls on well abandonment protocols are recommended as methods to substantially mitigate this risk.

Experience indicates that adverse impacts of hydraulic fracture stimulation on strata behaviour are generally imperceptible and, for practical purposes, insignificant.

Petroleum activities may impact on the distribution of gas within the coal seam but this impact is not considered to have a direct adverse effect on the safe and efficient mining of coal, per se.  Given the extended timeframes likely between petroleum operations and subsequent mining operations, the potential for an uneven distribution of gas is significantly reduced.  A net benefit to safe and efficient coal mining from generally lower gas contents would be expected.  Although the loss of revenue to the coal mining operators from high methane areas to support draining of more difficult drain areas is recognised, this is more of a resource allocation issue rather than one that impacts on safe and efficient mining of coal.

The impacts of hydraulic fracture stimulation on coal quality and ventilation control are considered to be insignificant.
The impacts of chemicals introduced with historical hydraulic fracturing procedures on safe and efficient coal mining are found to be of minor significance consistent with experience of routinely mining through fractures in the USA.  A risk of contamination of the groundwater system generally is recognised, although such contamination is an issue that needs to be addressed by petroleum operations in consultation with regulatory authorities irrespective of whether the coal seams are subsequently mined.

Confirmation of Comparison Basis

There have been lingering concerns in some quarters of the coal industry over how transferable experience in the USA might be on the basis of potential differences in stress conditions and material properties that may exist between the two countries.

Comparison between rock properties in the USA and rock properties in Australia indicates that there is a broad range of strength and stiffness properties for coal measure strata in both countries.  The observed range of these properties is essentially similar.  There does not appear to be any fundamental reason, on the basis of material strength and stiffness properties, why hydraulic fracturing experience should not be transferable from the USA to Australia and vice versa.

A broad range of in situ stress conditions is observed in both countries. While the general experience in both countries is that the horizontal stresses are larger than the vertical stress in non-coal strata with a ratio between horizontal stresses of about 2:1, the Black Warrior Coal Basin would appear to be an exception with the horizontal stresses measured in the roof strata between coal seams being somewhat lower than the vertical.

There are also some locations in Australia where the horizontal stresses are lower than vertical in non-coal strata, but such sites are not common and tend to be limited to thick coal seams.

The experience of hydraulic fracturing in the Black Warrior Coal Basin may not be directly transferable to high horizontal stress conditions such as those found in the Southern Coalfield but would be likely to be consistent with thick coal seams and other low stress environments.

Numerical Modelling

Numerical modelling is used to assess the potential impacts at the level of individual development roadways for two scenarios, a deep coal seam such as the Bulli Seam in the Southern Coalfield and a coal seam such as the Mid Goonyella Seam in the Bowen Basin.

The results of this modelling indicate that hydraulic fractures are most likely to be vertical in the coal seam and horizontal in the roof and floor strata giving a likely T shaped geometry in most situations.  The presence of horizontal fractures in the roof coal is expected to reduce the stability of the immediate roof strata.  However, provided gas drainage is effective, there is a second, much more significant effect associated with gas being drained from the coal seam.

Drainage of methane from the coal is found to cause a reduction in the horizontal stresses within the coal seam due to volume changes associated with desorption.  The reduced horizontal stresses in the roof coal are found to significantly increase the roof stability during roadway development.

Vertical hydraulic fractures within the coal would have the potential to cause increased rib instability in areas where roadways are mined in a direction parallel to the major horizontal stress.  However, in practice, it is uncommon for mining to occur in this direction because seam dip, alignment to cleat and other constraints tend to dominate mine layout considerations.  Normal rib support installed to control background rib deterioration would be expected to be effective for controlling any rib instability associated with hydraulic fracturing.  In addition, the hydraulic fractures tend to be localised to a small area and would therefore only affect stability if the roadway rib were within a narrow window of distance from a particular hydraulic fracture.

Recommendations

As a result of this study, we recommend development of a robust record keeping system for wellbore completion details that is maintained for the long term by a government department or similar independent of either petroleum or coal operators, as the best method to mitigate against the most significant potential impacts of production scale gas drainage.

We recommend wellbore abandonment protocols that:

  • Allow re-entry into wellbores by coal operators in the future to mill out or otherwise reduce the impact of steel casing on subsequent coal mining activities.
  • Provide a record of final water level, final gas pressure and total gas captured.
  • Provide a record of hole completion detail including type of casing, perforation horizons, hydraulic fracture treatment details and quantity and type of chemicals used.
  • Determine and record location of the collar and the location of coal seam intersections to an accuracy of 1m in 100m using a down-hole survey tool.

We recommend the operational experience of cutting through casing on the longwall face is reviewed and, if necessary, a trial is conducted to determine the significance of casing for longwall production.

We recommend the potential to encourage corrosion of steel casing as part of the abandonment protocol through control of pH or by other means is investigated.

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