A Comparison of Hydraulic Fracture Field Experiments, Including Mineback Geometry Data, with Numerical Fracture Model Simulations
Authors: Jeffrey R.G. , Settari A. 
Published: December 2007

Hydraulic fracture stimulations, including the fracture geometry data from mineback mapping, for two crosslinked HPG gel treatments and one water treatment performed in vertical wells sited over underground coal mines in Queensland and New South Wales, are analyzed and compared to numerical fracture model predictions.

The treating pressure gradients for all three treatments were higher than 1.1 psi per foot although the fractures produced were predominantly vertical. The two crosslinked gel treatments produced fractures that were propped to a greater extent in the seam down-dip direction. The treatment using water formed two propped sub-parallel fractures and a small horizontal fracture at the top of the seam. Stress conditions in the rock and coal seams were significantly different at the sites.

Laboratory parallel-plate scale-model studies are presented that demonstrate the nature of gravity current movement of dense fluid (both clear and slurry) down-dip in an inclined fracture model. Measured velocities are compared to theoretical calculations and to field measurements of asymmetrically propped fractures exposed by mining.

Introduction

Starting in 1990, a number of hydraulic fracturing experiments have been conducted over underground coal mines in Queensland and New South Wales, Australia. These experiments were designed to collect data on fracture propagation in coal at full scale. The sites were characterized by conducting micro-fracture stress measurements in the rock above and below the coals seams, by conducting well, step-rate, and minifrac tests in the coal, and by logging and conducting mechanical property tests on the core recovered from the rock and coal. Additional geotechnical, site geology and structure data was obtained from existing mine databases and from underground geology mapping. The treating equipment was extensively instrumented and 14 channels of data were recorded during injection and falloff In all cases, the treatment data included injection rate, fluid specific gravity, and surface and bottom-hole pressures. Eventually, when the mine advanced to the area of the wells, the fractures created were mapped and proppant samples were taken.

At the German Creek mine, a total of four fractures, one resulting from a full-scale treatment and three from smaller experimental treatments, were mined and mapped. All four the these treatments produced fractures that were propped to a greater extent in the seam in the down dip direction from the well. As a result of this minethrough data, a laboratory-based study was carried out to investigate transport of proppant by gravity current processes.

The field and laboratory experiments are summarized in this paper and compared to numerical fracturing model predictions.

The Field Experiments

Of the three field experiments analyzed here, the two that used crosslinked gel fracturing fluid, were performed at Central Colliery, German Creek Mine and the one that used water fracturing fluid, was performed at Munmorah Colliery. The German Creek Mine is located in central Queensland and mines the German Creek coal seam using long wall mining methods. Munmorah Colliery is located in New South Wales, just North of Sydney, and mines the Great Northern coal seam using bord-and-pillar mining methods. The treatments are summarized in Table 1 and site conditions are summarized in Table 2.

At the time of the treatments, mining had approached to within about 400 meters of the two holes at Central and to within about 150 meters of PHKM-2 at Munmorah.

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