The Cyclically Restored in Situ Petrophysics CRISP Method for Analysis of Petrophysical Properties of Unconsolidated Oil Sands Reservoirs: Overview and Testing Update

Abstract

Routine core analyses of unconsolidated oil sands often yield unreliable and inconsistent porosity and permeability values due to the destruction of in situ textures or fabrics during core retrieval and sampling processes. To overcome the drawbacks of routine core analysis we developed a new method, namely "Cyclically Restored In Situ Petrophysics (CRISP)", for analysis of petrophysical properties of unconsolidated oil sand reservoirs. The new approach begins with a replication of in situ texture via cyclic compaction of unconsolidated oil sands in a uniaxial piston cell with incremental higher axial loadings that mimic historic overburden pressure cycling induced by glacial cycles through the Pleistocene. After the texture restoration, the sample is flooded in situ with various liquids and/or solvents and gases to obtain multiple porosity and permeability data points. Forward and backward flow can be applied to test permeability in both directions. After analysis the sample is dried, weighed, and the grains can be further analyzed for particle size distribution, mineralogy, or other parameters. The preliminary test program investigated the accuracy and precision of the new method (CRISP) and compared CRISP to the commonly-used sleeved-plug net overburden analysis (NOB) method. Results indicate that CRISP permeability measurements to simulated formation brine are highly repeatable, with variance of 0.71% (mDarcy) for a study of 531 samples from McMurray Formation, and of 0.15% (mDarcy) in a 150 sample Lloydminster Fm. study. For both sets of samples, the brine permeabilities range from 1 to 5000 mD. The preliminary results also show that CRISP outperforms the sleeved-plug net overburden method (NOB) in precision, with vastly better conformance between repeated samples, and also yields lower porosities that agree more closely with presumed in situ porosities given geological constraints and geophysical log data than the NOB method. Further, CRISP requires equivalent time for analysis as the NOB approach, and uses the same format of samples. CRISP therefore represents a significant improvement for petrophysical properties analysis in unconsolidated oil sand reservoirs for better and more realistic reservoir evaluation and subsequent engineering development.