Underground » Strata Control and Windblasts
This project was conceived following the first successful application of hydraulic fracturing to control caving at Moonee Colliery. The routine use of hydraulic fracturing at Moonee has provided a unique opportunity for development and refinement of hydraulic fracturing technique in a mining application. This final report summarises the experience at Moonee and describes the technical developments that have been made possible through the funding provided for this project by ACARP.
The initial investigation of hydraulic fracturing at Moonee, the introduction and integration into routine operations and the ongoing refinement of the technique over five longwall panels are described as an example of how this technology can be successfully applied. The path was not always smooth, good luck was certainly a factor at times, but it is also a story of the application of careful observation and measurement, scientific analysis combined with engineering judgement and a high level of ongoing commitment by everyone involved.
Hydraulic fracturing is widely used in the petroleum industry to stimulate oil and gas production from underground reservoirs and is a well-developed technology. The technique is also used on a small scale to measure in situ stresses in exploration boreholes. It is essentially a process whereby fluid under pressure is used to create a fracture in rock that grows outward from a single injection point in a direction governed by the stresses acting in the rock strata. Armed with a sound understanding of the stress field, the rock properties and the geomechanical processes active in a particular mining environment, it is possible to use hydraulic fractures to bring about, in a cheap and efficient manner, specific outcomes that are favourable to mining.
In the case of Moonee, the desired outcome is to induce the goaf to fall while men are not present on the longwall face. In other situations, hydraulic fracturing might be used to precondition a take-off road, or reduce the severity of periodic weighting, or simply to induce caving to provide a stable working environment. There is a wide range of applications. The technique requires upfront understanding of the environment, but once this understanding has been developed, hydraulic fracturing can be easily integrated into routine operations.
The process of hydraulic fracture growth in the conglomerate strata at Moonee Colliery is similar to that described in the extensive petroleum literature, but differs in several important ways. The fractures at Moonee grow in an environment where the minimum principal stress acting across the fracture plane is low or even tensile. The free surface created by mining has a significant effect on the fracture opening compliance and this effect must be taken into account when calculating fracture width and volume if models are to reliably calculate the hydraulic fracture growth rate and size. In this project, modifications have been made to existing numerical models. These modifications are described and the matching of model data with field data is presented.
Understanding the geomechanical processes is an integral part of being able to usefully apply hydraulic fracturing technology. Models of strata behaviour can range from simple conceptual models to complex numerical codes. A range of such models are described and examined in the context of controlling caving at Moonee and also in the context of controlling periodic weighting in a sandstone channel environment.