Mini-Frac Analysis in Oilsands and their Associated Cap Rocks Using PTA Based Techniques

Abstract

Abstract The Alberta Energy Regulator (AER) requires mini-frac tests to be performed on thermal or polymer injection projects as part of the licensing process. Mini-fracture closure stress in the cap rocks is the key item used in determining the maximum operating pressure (MOP) for injection wells. The MOP has a significant impact on the economics of the project. This paper will establish a physics based interpretation method that covers all injection/fall-off mini-frac tests. In the field, mini-frac tests are typically performed with multiple injection/fall-off cycles (usually 5 to 7 cycles) on each zone. Multi cycle testing is unique to the oilsands and their associated cap rocks. Multiple zones are also tested in a well, in both pay intervals (typically McMurray sands) and cap rocks (Clearwater shales etc.). Each cycle is analyzed for closure events and hopefully a consistent stress is found for each zone. Industry currently faces a conundrum as there is no agreement among analysts as to how to interpret these tests. As a result, reports submitted to the AER follow different methodologies. This may partially explain the wide range of reported closure stresses within individual basins. This paper will show using field mini-frac data that traditional interpretation techniques can lead to ambiguities/incorrect interpretations. These issues can be overcome by using a pressure transient analysis (PTA) based interpretation approach. PTA has a long history of analyzing injection/fall-off and production/build-up tests where the rock fabric does not change during the test. The PTA interpretation methodology has remained remarkably consistent over the past 25 years. It can be summarized as follows: first identify flow regimes with a special kind of derivative plot, and second use flow regime specific specialized plots to calculate formation properties. This paper will show that PTA can now also handle dynamic fracturing (i.e. mini-fracturing) and all of its associated special cases; which include pressure dependent leak-off, height recession/transverse storage and tip extension. Mini-frac analysis is now just a sub-set of the larger PTA methodology. It has the added advantage in that a rules based procedure can be established for the entire interpretation workflow.