Hydraulic fracturing is used at Moonee Colliery to induce caving as part of the routine operation of this longwall mine. Measurements undertaken to successfully introduce hydraulic fracturing to Moonee and pressure records routinely obtained from each treatment provide a unique opportunity to develop and test a new model of hydraulic fracture growth near a free surface. This paper presents the results of the comparison for several fracture treatments, demonstrating that the model is able to match the treatment data.
INTRODUCTION
Moonee Colliery is owned and operated by Coal Operations Australia Limited and the mine is located just south of Newcastle, NSW at Catherine Hill Bay. Mining extracts 3.2 m of the Great Northern seam, leaving, on average, 1.8 m of roof coal and claystone above the seam. This weak roof coal sequence typically caves immediately behind the supports leaving the 30 to 35 m thick conglomerate section bridging the 100 m wide longwall panel.
Hydraulic fracturing is used at Moonee Colliery to grow fractures in the roof rock behind the face. The fractures formed are horizontal and parallel to the base of the conglomerate. Their growth produces caving of the massive conglomerate roof strata at an interval designed to avoid natural caving events. A new model that accounts for the strong interaction of the hydraulic fracture with the base of the roof conglomerate (a free surface) has been developed. The development and application of the model are the subject of this paper.
down both toward the rib line and the face. Further back from the face, the arch maintains an essentially constant shape with a maximum height or 15 m above the base of the conglomerate at the centerline of the panel (see the profile in Fig. 1 labeled "old panel"). Figure 1 contains calculated contours of the vertical stress magnitude on vertical sections across and along the longwall panel.
