Diagnostic fracture injection testing (DFIT) has become an established method of well testing in low permeability unconventional reservoirs – particularly in shales and coal seam gas (CSG). As well as providing in-situ measurements of permeability and reservoir pressure, the test allows estimation of the closure pressure (and fluid loss characteristics) for the formation under investigation, which is assumed to be a measure of minimum horizontal stress (σh).

Arrow Energy is currently conducting a wide-scale exploration and well testing programme in the Bowen Basin region of Queensland, Australia, in support of its planned CSG to liquefied natural gas (LNG) project. This programme is mainly focussed on evaluation of formations within the Moranbah and Rangal Coal Measures; these are typically present as thick, low permeability zones.

Traditional Drill Stem Test (DST) and Injection Fall-Off Test (IFOT) results have proven unreliable within such low permeability and under-pressured reservoirs. The limited drawdown generated by DSTs in such formations has historically been an issue for these, and other, reservoirs. Similarly, difficulties have been encountered with regard to the appropriate management of IFOT injection rates in the Bowen Basin coals, which can fracture at very low rates; the resultant diagnostic typecurves needed for analysis of such a "failed" test have tended to introduce an unnecessary level of uncertainty in the results.

At present, the preferred exploitation method for these formations is extended-reach, open-hole, horizontal in-seam wells. As such, very few instances of cased-hole completions are available. Whilst the DFIT technique has previously been widely applied in cased-hole completions, largely due to its prevalence as part of hydraulic fracture stimulation treatment in North America, its application has been extended successfully to open-hole testing as part of this programme. Arrow has to date completed over 140 open-hole DFITs in this region, with an approximately 80% success rate in interpreted parameters with reasonable confidence.

The results of this programme have been incorporated into a basin-wide interpretation of regional stresses and subsurface properties. These results have been used, in combination with Mechanical Earth Models (MEM), to evaluate optimal lateral well orientation, hydraulic fracture treatment design, as well as the mapping of variable stress gradients in an effort to better understand the distribution of permeability trends across different geo-domains.

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