Because of their size and difficulties with oil recovery, the oil-bearing diatomite formations attract now special attention worldwide. For example, the giant diatomaceous oil fields in California, Lost Hills and Belridge, contain some 10 billion barrels of oil in place. Diatomaceous strata have peculiar geological structure: as a result of the cyclic deposition, the diatomite rocks are layered across width scales ranging from tens of meters to sub-millimeter. The diatomite rock is very fragile and its fracture toughness is low: the inter-layer boundaries are weakly connected and ready to part when the fluid pressure changes. When intact, the diatomite has porosity of 50-70% and is almost impermeable (0.1-1 md). Oil production from the diatomites was always difficult and started only 30 years ago after the introduction of hydrofracturing. The scanning electron microscopy images of the diatomite rock reveal a disordered microstructure with little grain interlocking and cementation. Therefore, fluid flow through the diatomite starts only after changes of the rock microstructure. The hydrofractures are not single vertical cracks, but are complex, multiply connected regions of damaged rock.
The current models of fluid-rock systems, e.g., Refs.,1,3,19 cannot capture the dramatic rearrangements of the diatomite microstructure caused by fluid withdrawal and injection, and have little predictive capability. In particular, these models cannot capture the intense rock damage during hydrofracturing, followed by the nonequilibrium countercurrent imbibition with the ensuing rock damage and hysteretic effects. To understand and predict reservoir behavior in the diatomite and limit well failures, a new micromechanical approach has been developed.
