In-seam Drilling for Gas Drainage — Rotary Drilling

• No directional control
• Excessive drilling metres
• Ineffective coverage
• Fatal Outburst can happen due to inaccurate borehole coverage

Gas and Outburst Prevention

• Improvements in directional drilling technology
• Drilling rates comparable to that of rotary drilling
• Rotary drilling is still to be utilized where historically adequate
• All mines required to develop an Outburst Management Plan
• Directional drilling the only form of drilling to provide planned and verified coverage

Introduction of Directional Drilling

• Oil field technology modified for underground
• Boreholes drilled with a down-hole motor
• Boreholes progressively surveyed for steering and to determine location
• Locate boreholes in desired horizon and location

Key Components for Directional Drilling

• Suitable stable environment
• Experienced project supervision
• Experienced drillers
• Proven and competent procedures and training
• Drill rig with high pressure water pump
• Downhole motor
• PCD drill bits
• Survey system
• Suitable and safe site management

Directional Drilling Environment

• Requires environment suitable to support directional drilling
  • Target strata (coal or stone) has to be stable (drillable)
    • High pressure ejects broken strata into borehole
    • Fracture strata collapses into borehole
    • Clay strata swells into hole with water contact

 

Electro-hydraulic Drill Rig for Longhole Directional Drilling

Directional Drilling – Downhole Motor

• High pressure water pumped down drill rods is forced through the power section and rotates the drill bit
• Various torque characteristics and rotational speeds available

 

Downhole Motor Deflection

• Standard 1.25 deg bend
• 96mm OD PCD bit
• 18.7mm deflection at bottom of the bit
• Provides the ability to be steered

DHM Steering Control — Vertical Response

• Change in vertical angle over 6m – limited curve rate

Drill Site

• Drill site provided to include all equipment
• Avoid interaction with mine transport movements
• High safety standards and operational procedures
• Ventilation requirements to manage gas, electrical requirements, water supply and pumping

Gas/Water Management — Standpiping

• Fully grouted standpipe
• Pressure rated fittings
• Fittings seal borehole and control returns
• Pressure tested for known high pressure intersections

Flat-faced PCD Bits

Borehole Surveying – DDMS (Directional Drill Monitoring System)

Borehole Drilling Patterns

From right to left:

• Rotary drilling initially
• Conversion to directional drilling
• Improved drilling and steering skills

Progressive Seam Profile Definition

• Drill up to intersect roof
• Extrapolate seam roof line
• Pull back to suitable location and branch
• Advance 50-100m to next roof intersection

Thick Seam Profile Definition

• Progressive intersections with the seam floor

Borehole Design

• Borehole is designed in horizontal plane to suit positioning on the mine contour plan
• Seam contours are based on surface borehole interpretation and mine RLs

Design Profile

• Borehole Profile based on intersections of design borehole with seam contours
• Seam profile is provided to be used as a guide by the drillers
• it is only an estimation of what the seam profile is
• Drillers have to progressively define the seam profile as they advance

Profile Plot

• Exaggerated vertical scale – angles exaggerated
• Actual seam profile differs from the design profile
• Seam roof is extrapolated ahead of each roof intersection
• Each intersection revises the seam profile

Lateral Deviation Plot

• Lateral Deviation is plotted progressively
• Aim is to follow the design line to position the borehole as required

Drilling Records – allows interpretation of drilling results

Driller to keep records of:

• Depth
• DHM orientations
• Feed pressure
• Water pressure and flow
• Comments of strata drilled
• Survey results

Longhole In-seam Drainage / In-seam Exploration

• Long in-seam holes drilled for gas drainage provide exploration information

Exploration with Long Gas Drainage Borehole

• Seam profile defined by standard roof intersections
• Seam discontinuities identified

Fault Definition

• Consecutive stone intersections at similar depth indicate presence of a structure
• Prefer to have some indication of likely direction of displacement from geologist
• First intersection can be extended upwards to find displaced seam
• Second intersection can be extended down to find displaced seam
• Infinite possibilities

In-seam Exploration – Fault Identification/Definition

• Multiple intersections defined location of fault
• Sacrificial branch upwards defined the displacement of the fault

In-seam Exploration – Fault Identification/Definition

• Multiple intersections defined location of fault
• Sacrificial branch upwards defined the displacement of the fault
• Additional drilling confirmed no further faulting beyond

Undulating Seam

• Seam profile changes
• Unexpected roof or floor intersections at unrecoverable angles
• Additional metres drilled

Thin Undulating Seam

• Sample profile has seam dips varying between 0 and 4 degrees
• Seam thickness 1.5m
• Additional metres drilled in stone trying to find seam
• Borehole curve rate limited to 2.0 degrees/6m

Intersecting Stable Target Seam

• 11 degree relative angle of intersection
• Overshoots and intersects roof

 

• 7.0 degree relative angle of intersection
• Near roof but stays in seam

Seam Definition – bogging coal conditions

• Used when target seam is broken and unstable
• Penetrate coal, assess for stability, progress until boggy, pull back
• Require stable stone strata above or below seam
• Require stable interface at seam intersection
• Prefer reasonable penetration rate in stone strata
• Prefer easy branching in stone strata
• Used muds to improve stability and flushing

Seam Definition – bogging coal conditions

• Drill in stable strata above or below the seam
• Curve up or down sharply to intersect seam
• Pull back before bogging
• Extrapolate seam profile
• Branch and continue

CONCLUSIONS

• Directional drilling is an established form of in-seam exploration
• All drilling requires a stable environment to maintain an open borehole and avoid blockage and/or bogging
• Additive muds can improve stability but are limited in high pressure, broken environments
• Alternative access is available by drilling in stable stone strata above or below the target seam with intersections at regular intervals
• The drilling stability can be assessed with limited exposure to bogging and loss of equipment