Optimising Sand-Propped Hydraulic Fracture Stimulation of In-Seam Gas Drainage Holes

Dartbrook coal mine sponsored a trial of sand-propped hydraulic fracturing as a way of stimulating gas drainage rates from in-seam horizontal gas drainage boreholes. The trial placed 20 propped fractures in a borehole drilled from 10 cut through and 10 fractures in a second borehole drilled from 7 cut through. Both holes were drilled into Longwall 108 and gas rates were measured before and after the hydraulic fracturing work. The borehole at 10 cut through was stimulated by a factor of 180 while the borehole at 7 cut through was stimulated by a factor of 22 by the sand-propped fractures. Because of the very significant effect measured in the trial, sand propped fracturing was carried out in drainage boreholes drilled into Longwall 109  at Dartbrook  in order to accelerate gas drainage before mining. An overall stimulation effect of about 5 was achieved in LW109.

The stimulated gas rates achieved at the trial site at Dartbrook were history matched using a hydraulic fracture model (SIMFRAC) to estimate fracture size and conductivity followed by a coal seam methane reservoir simulator (SIMED II) to match the pre- and post-stimulation gas rates. The fractures are estimated to have extended 30 to 40m from the borehole with 15m of this extent propped with sand. Fracture conductivities of about 10 md-m were consistent with the fracture modelling. However, the reservoir model history matching obtained the best fit to the data using a fracture conductivity of only 1.5md-m.  A smaller conductivity would suggest the sand proppant was distributed over a larger fracture extent but at lower concentration. The model parameters obtained from the history match were then used to study the relationship between borehole spacing and fracture spacing along the boreholes. The predictions of gas drainage found that optimal strategies exist in relation to the design of borehole and fracture treatments. In particular, the effect of increasing fracture density with respect to gas drainage does not have a linear relationship. In fact increases in fracture density beyond a certain value give a diminishing return. Therefore investment in fracturing and drilling needs to take account of this particular optimal value. However the actual strategy used will depend on the relative costs of drilling compared to fracture treatments and the operational objectives. Mine drainage often has to be performed within a short time to fit in with the overall mine operational plan and this constraint must be taken into account in determining the overall optimum design.

This ACARP project was designed to add to the technical capacity to carry out such treatments and to investigate several issues that arose during the work at Dartbrook and later during a fracturing trial at Tahmoor.  The stress and coal conditions at Dartbrook allowed open-hole inflatable packers to be used as a way of isolating short sections of the borehole so that a hydraulic fracture could be placed at a known location. However, higher stresses at Tahmoor resulted in borehole conditions that did not allow successful use of open hole packers.  The packers failed after only 1 or 2 fracture treatments at Tahmoor, which made placing fractures by this means impractical.

Testing of packers by inflating and deflating them in 96mm ID pipe was undertaken in this project. Four series of tests were performed using  air and water to inflate the packers. No signs of packer failure were noted for any of the test series which stopped after 200 inflation-deflation cycles. Table S1 summarises the testing results.

Table S1: Summary of Packer Testing Results

The packer testing indicates that the borehole condition is the most likely cause for early packer failure. Inflatable open-hole packers are damaged by inflating them in oversize or broken-out boreholes.

A caliper tool was designed, built and tested to enable measurement of borehole condition before running inflatable packers. The tool uses strain gauged arms that sense the borehole diameter as it is moved along the hole. The instrument has been used in shallow surface boreholes and in in-seam holes at West Cliff with good result.

Casing the open holes before hydraulic fracturing provides a method of stimulating in-seam holes that are otherwise too damaged. Methods of casing and cementing horizontal holes have been investigated and use of either PVC or fibreglass casing seems possible. Such a cased hole must be notched or perforated to allow fracturing operations to occur. Mechanical and abrasive jet notching methods were investigated and trials showed that both methods work, but the abrasive jet notching can be done more rapidly. Mechanical notching required 10 to 30 minutes to cut through the casing and into the cement while abrasive jet notching requires 1 to 2 minutes to achieve a better notch result.

The stimulation effect achievable with sand propped hydraulic fractures in horizontal in-seam holes was demonstrated at Dartbrook in 2002. Provided reliable methods of placing the fractures into the seam can be developed, the technology will make possible more complete and rapid drainage of gas for hard-to-drain, cross-measure, or entire longwall blocks of coal. This project has advanced the understanding of packer life in open-hole and cased-hole environments and has developed methods to notch cased boreholes for placement of sand-propped fractures. Modeling has been used to demonstrate the relationship between fracture size/ conductivity and borehole/ fracture spacing on gas drainage rates and drainage effectiveness.