Goaf Drainage drilling

Australian Drilling Industry Association – Emerald Sundowner, Sept 2018

Picture origin: http://peonyden.blogspot.com/2006/07/nature-of-longwall-mining_115275389231661027.html

Goaf - definition:

That part of a mine from which the coal has been removed.

The Issue:

The goaf can liberate vast quantities of seam gas rapidly as intact seams around a mining operation are fractured and their trapped & pressurised gas is suddenly free to escape.

Coal origins: Late Permian Age Bowen Basin swamp, approx 260 Million years ago.(Australia was closer to the Sth Pole)

Background:

BOWEN BASINGEOMETRY

Collinsville to Theodore500km long

TYPICAL BOWEN BASIN STRATIGRAPHY

Most underground mining is carried out in:• the Rangals

(Leichhardt seam),• the Moranbah Coal

Measures (Goonyella Middle seam), and

• the German Creek Formation (German Creek seam)

Sample Longwall Goaf - section view.

Overlying ‘roof seam’- intact

Overlying ‘roof seam’- intact

Longwall Goaf section view showing Goaf Wells capturing seam gas and transporting it to the surface.

Sample Longwall layout with Goaf holes on the Tailgate side of the Face

Longwall mining direction

Goa

f

Goa

f

Goaf Drainage holes

TGate MGate

German Creek Seam sample strata cross-section

GERMAN CK

CORVUSTIERI 2TIERI 1

AQUILA

PLEIADES

FAIRHILL

CANIS / LEPUS

HERCULES

Indicative zone ofcontinuous fracturing

Production casing set 10m below the Fairhillseam:Task: seal off the overlying aquifers!

Open hole section to terminate 5m below the CORVUS floor.

SurfaceTertiaries Goaf Drainage hole

Example cross-section along the Longwall block, German Creek formation seams

Pleiades

Aquila

Tieri 2

Corvus

German Ck

Tieri 1

10

0m

50 - 200m

Collapsed or

fractured strata

Goaf Drainage holes

Goonyella Middle (mining horizon)

Sample stratigraphy of the Moranbah Coal Measures.

Goaf gas seams present both above and below the mined seam. Presents a more complex goaf capture challenge.

Goonyella Lower

Overlying P seam & Goonyella Upper

Production casing seated:• Below base of weathering, or• Below old tertiary gravels / sands, or• 10m below an overlying seam aquifer

Ground levelConductor casing

Indicative layout of goaf wells

Final hole depth to be approx. 5-10m below the lowest overlying seam.

Contingency slotted steel liner(if required)

Top of liner casing to sit a minimum of 15m above the production casing seat

Mining target seam

Open hole diameter ranges from 8” to 17”.Diameter driven by gas flow requirements.

Production casing flange

Final hole depth to be >10m above target seam.

Lowest overlying seam

1. Drill2. Case3. Drill out cement4. Drill to TD5. Run liner

1. Drill2. Case3. Drill out cement & to TD4. Run liner

1. Drill2. Case & Line3. Drill out cement & to TD

Drill / Case / Drill Possible option?Drill / Case & Line

Drill to TD / Case

Pros:Larger diameter portion below casing increases flow capacity

1. Drill to Prod Casing depth

Varying goaf drilling / casing / lining practices

2. Cement Prod’ncasing

3. Drill on to TD

Pros: Easy to case off

Cons:Smaller diameter portion below casing restricts flow capacity

1. Drill to TD

2. Cement Prod’ncasing

Cons: More challenging to cement casing effectively

4. Insert slotted liner (if req’d)

3. Insert slotted liner (if req’d)

Pros:Install casing & liner in one pass.Lge dia hole with lining

1. Drill to TD

2. Cement Prod’ncasing & install liner (packer)

Cons: Sourcing gear ($) to cement effectively

3. Drill out shoe

Key design points, Cementing & VerticalityKey design points:• Case off overlying aquifers.• Do not drill into the mining seam (typically 10m stand-off).• Production casing must contain seam gas pressure & cope with vacuum.• Maintain an open gas path to the surface.

Cementing:• Seal off surface and near surface aquifers. Water bore rules apply.• ≥ 20mm cement thickness surrounding the Conductor casing.• ≥ 15mm cement thickness between Production and Conductor.• Casing centralisers at 12m spacing.

Verticality:• 2% or less. Offline holes may miss the gas source, or shear off.

Thanks for your time!

Any questions

Australian Drilling Industry Association – Emerald Sundowner, Sept 2018

John Weissman: Weisstech Pty Ltd, Sept 2018