Abstract

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.

Introduction

We face nowadays a new period in the development of subterranean mechanics: the science of flow, deformation and fracture in natural rock-fluid systems. Important practical problems with oil and gas recovery, water supply, and more recently with the disposal of nuclear and chemical wastes, are forcing researchers to reconsider and modify the established flow theories that, with relatively minor changes, have dominated earth sciences since the late thirties. These modifications lead to substantially new physical and mathematical formulations, and are required to tackle practical problems with, e.g., oil production from the North Sea chalks and the diatomites in California, liquid nuclear waste seepage at Hanford, WA, or with nuclear waste isolation under the Yucca Mountain project, NV.

In classical approach, mass, momentum and energy conservation laws and their invariance properties alone are insufficient to obtain a closed consistent system of equations of fluid flow in real rocks. To obtain such a system, it is necessary to endow continua with physical properties. This means that a model of each continuum is employed, which is adequate for the classes of fluid and rock motions of interest. Such models, called constitutive equations, provide the necessary relationships between the properties of the motion, or the states of the continua, and acting internal forces.

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