Gas Detection Technique and Equipment to Continuously Monitor Gas in Drill Fluid

A fluid logging system for underground drilling can be used as a complementary method for the recognition of gas and gas-rock outburst prone conditions ahead of the face and along an inseam drill hole.   Mud logging technology has been used in surface boreholes in the oil industry, for a considerable time with great success. An underground version of the system promises to deliver valuable data, however equipment alteration is necessary due to difficult underground conditions and safety requirements.  Project C3077 was aimed at the development and evaluation of a system suitable for Australian underground coal mining conditions. Suitable equipment has been identified and selected, however a gas trap unit still needs to be designed and tested specifically for use underground.

Definition of the Problem and Description of the Proposed Technology

Recognition of gas and gas-rock outburst prone conditions which may exist ahead of the development face is essential for safe and efficient development drivage. Methods used to date are not fully satisfactory.

To complement existing methods a means of logging underground drilling fluids could be useful. Appropriately designed equipment, approved for gassy underground mining conditions, could be useful for recognition of gas and gas-rock outburst prone conditions ahead of the face.

Monitoring the quantity and composition of gases in the drilling fluid should provide continuous real time information about both the gas content and the permeability of the coal seam along an inseam drill hole.

The proposed system is based on surface mud logging design and technology which has been used in the oil industry with a great deal of success. However, transfer of this technology underground requires specific research and the selection of appropriate equipment that will conform to Australian conditions and standards.

The aim of the project was to identify select and where necessary develop equipment and technology for an Underground Drilling Fluid Logging System suitable for Australian underground coal mining conditions.  The specific objectives were to:

• Identify and review existing state-of-the-art equipment for measuring the amount, composition and character of gas influx at particular distances in horizontal, inseam drill holes. 

• Identify and select equipment for an underground drilling fluid logging system suitable for Australian underground coal mining conditions. 
• Prepare cost estimates and guidelines for the final design, purchase and implementation of the system.

Summary of Work Accomplished

Material on the following subjects has been researched:

• surface mud logging technology

• gas detection and measurement
• underground outburst conditions

Conclusions

• All necessary equipment for a system design which conforms to Australian underground coal mining conditions has been identified. 
• Details of equipment and suppliers are specified in the section titled 'Equipment Selection and Cost' and in Appendix 'A'. 
• To introduce the system into underground coal mines, certain changes to underground drilling practices are necessary:

1. the use of a standpipe and stuffing box to direct both drilling fluid and cuttings to the proposed gas trap 
2. installation of a gas trap to separate the gas from the drilling fluid and allow the drilling fluid and cuttings to be discharged continuously to the heading floor or to a special container. 

• The gas separator (gas trap) should be designed specifically for the underground system initially in the form of a cylinder with interior baffles to hinder the flow, and followed by laboratory and underground tests. The gas trap which is proposed is a simple device which does not require a power supply and is without any internal moving parts. 
• The initial design of the system should simply aim at obtaining a gas signature of the strata as a relative gas volume against drilled depth. Once this has been achieved a quantitative approach could be taken. 
• The system prototype is based on in-seam hole conditions and existing underground gas flow parameters at Cordeaux Colliery using a modified drill rig and a newly introduced data logger. 
• The system can also be transferred and used in conditions particular to other coal mines. 
• Equipment has been selected on the basis of technical discussions with manufacturers, suppliers and various specialised companies including: MSA, Control Systems Technology, Mine Instrument Services, Zellweger, Systrix, BHP Research Newcastle Laboratories and BHP Steel Collieries Division. 
• The data logger currently selected (Underground Drilling Monitor) has only one free 'input' which is not sufficient for the system. This needs to be redesigned to accept the following inputs, providing the suggested equipment is used:

1. 40-200 mV (x2) - flow
2. 0-2 V - CH4%
3. drilled depth (already utilised by BHP) 

• For the initial design, air ejector suction can be regulated by a manual valve, however, for the final product, suction should be adjusted by an automatic valve controlled by an Underground Drilling Monitor. Therefore, one output channel would be required in the monitor providing a 4-20mA control signal to the valve.
• For the measurement of gas flow Rosemount Instrument's Differential Pressure Transmitter has been chosen, however, an alternative option does exist: carrying out the flow measurement using passive pressure and differential pressure transducers offered by Transmetrics Inc. They are small devices, very well suited for compact, portable design, however, utilisation of these devices would require some electronic design, signal conditioning and consequent approval for underground use.
• Current off-the-shelf units are preferable for the prototype system where as the final system design should consider the use of passive transducers instead.
• On the basis of the selected equipment and taking into account the above conclusions and recommendations, the prototype system should be designed, constructed and trialed underground to confirm the value of the data delivered by the system. If the trials are successful the final product could be designed with emphasis on automatic operation and portability, which could be achieved using:

1. passive pressure and differential pressure transducers for flow measurement
2. a remote control valve for air ejector suction control 

• The proposed technology offers the only known system to identify and recognise, on a continuous basis, gas content and permeability of a coal seam ahead of a driven heading face.