Oil Deposits in Diatomites: A New Challenge for Subterranean Mechanics

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the authors(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illus- trations may not be copied. The abstract should contain conspicuous acknowledgement of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-214-952-9435. Abstract Because of their size and difficulties with oil recovery, the oil-bearing diatomite formations attract now special at- tention 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 al- most 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 scan- ning electron microscopy images of the diatomite rock re- veal a disordered microstructure with little grain interlock- ing and cementation. Therefore, fluid flow through the diatomite starts only after changes of the rock microstruc- ture. 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 nonequi- librium 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.