Underground » Ventilation, Gas Drainage and Monitoring
Coal seam gas pre-drainage through underground-to-inseam (UIS) horizontal boreholes is one of the common practices for the management of personnel safety and environmental concerns in relation to gas concentration in underground coal mines. In the design of UIS boreholes, local experience is invaluable; however, an experienced-based design is not necessarily optimal from both economic and technical perspective. Poor designs may lead to inadequate gas pre-drainage thus posing risks to the health and safety of mine personnel, delaying production and increasing financial burdens (e.g., due to overdesigning).
The general aim of this project was to develop an informed, data-driven methodology for the design and optimisation of UIS gas pre-drainage boreholes by accounting for the gas flow, sorption and stress behaviours of the coal seam. The project resulted in developing a three-dimensional numerical gas flow model that incorporates the processes of matrix gas diffusion, fracture gas flow and stress evolution in a heterogenous domain with complex UIS borehole geometries. Also, as part of this project, a UIS borehole design and optimisation methodology was developed and implemented on a zone of a longwall panel in an underground coal mine located in southeast of Australia. The methodology consists of data gathering, geological modelling, numerical model calibration, defining scenarios of UIS borehole patterns, simulating gas drainage for each scenario and processing of simulation results for determining the optimal case based on given threshold limit value (TLV) of gas concentration and drainage (lead) time.
The project objectives were:
- An extensive literature review and sample/data collection from the nominated coal mine;
- Understand the effect of different system properties including gas content, cleat system and in situ stresses on gas release in a laboratory scale;
- Develop a numerical model capable of explaining the physical processes observed using the fundamental understanding obtained from the laboratory experiments;
- Construct a geological model of the coal seam using available data such as cleat direction, stress orientation and magnitudes, gas content, downhole logging data, seismic data, etc;
- Perform the numerical simulation of gas drainage through UIS boreholes and calibrate the model;
- Perform a sensitivity analysis to assess the effect of different parameters on the gas drainage as well as placement of different wells; and
- Outline the developed methodology for the optimisation of UIS pre-drainage boreholes.
Major deliverables of the project are:
- A design and optimisation methodology for UIS gas pre-drainage boreholes;
- An in-house finite element-based coal seam gas drainage model that incorporates matrix gas diffusion, gas flow in cleats and coal mechanical deformation processes;
- A coal permeability evolution model based on the effect of evolving stresses (due to pressure depletion and matrix shrinkage induced by gas desorption) on coal cleat permeability (represented by a second rank tensor).
Conducting laboratory experiments on different coal specimens with high and low cleat/fracture system shows that existing models overestimate sorption stresses and a thermodynamic inconsistency in measuring the sorption - stress coupling coefficient exists in conventional approaches. We thus propose a new experimental technique to measure the sorption - stress coupling coefficient that satisfies the thermodynamics requirements. Experimental results reveal that as pore gas pressure increases, a smaller fraction of the swelling strain induced by gas adsorption can be recovered under stresses up to 40 MPa. In fact, the behaviour of coal specimens when allowing it to expand and adsorb gas is significantly different to that when they are allowed to compress and expel gas. Our experimental results also show that the hydromechanical response in gas desorption is very different from that of the gas adsorption under external stresses and a significant hysteresis exists.
This study demonstrates the development and implementation of a workflow for the design and optimisation of underground-to-inseam pre-drainage. The workflow consists of geological/property modelling, numerical modelling, calibration, and optimization. Downhole geophysical logs and other commonly available data can be used to construct a geological and property model for numerical ACARP simulation of gas drainage. Previously developed methods to extract coal flow-mechanical properties from downhole logging data were initially used and kriging was then employed to distribute these properties from around the boreholes to a 3D property model. These properties are used in numerical simulations. An in-house numerical coal model, NETCoal, which couples the processes of matrix gas diffusion, fracture gas flow and rock deformation, were used to model gas drainage from underground-to-inseam horizontal boreholes and evaluate the drainage performance of drilling patterns. A shape factor was effectively used to calibrate the numerical model by the actual field cumulative gas drainage. This study demonstrates that the drainage performance of a drilling pattern depends on in-situ stress state, initial permeability, diffusion coefficient and cleat orientation to different extents. Numerical results show that the total length of underground-to-inseam boreholes has a strong non-linear correlation with the drainage time. These correlations together with plots of drainage percentage versus total borehole length can be used to evaluate an optimal underground to-inseam drilling pattern based on constraining lead times for critical areas of longwall panels. The procedure proposed in this study can enhance the efficiency of coal seam gas pre-drainage practice by reducing the cost of underground-to-inseam drilling.