Summary
Diatoms and radiolarians are microorganisms that precipitate Opal-A to form
siliceous tests that accumulate on the seafloor to form siliceous oozes.
Progressive diagenesis of these deposits during burial results in thick, highly
compressible reservoirs of exceptionally high porosity and low permeability,
not unlike the chalk reservoirs of the North Sea. During burial and
over time, the amorphous silica phase (Opal-A) becomes unstable and gradually
changes in its structure to more stable, ordered Opal-A' and crystalline forms
or phases of silica, namely Opal-CT and quartz. The Opal-A → Opal-A' →
Opal-CT → quartz transformation results in a naturally occurring densification
and compaction process that is accelerated by an application of heat.
Reservoir compaction and surface subsidence can usually be controlled by
injecting fluid to control the effective stress. However, in
heavy-oil diatomite reservoirs undergoing steam injection, the injected fluid
causes competing effects: it controls effective stress to some degree, yet at
the same time it accelerates compaction and subsidence.
This paper describes selected results of a diatomite laboratory testing
program and features of a unique thermal reservoir simulator formulated to
handle the effects on compaction caused by stress, temperature, and
time-dependent strain (creep). Elevated temperature in amorphous
Opal-A diatomite is shown to be capable of causing a sample compression of 25%
or more and a severe reduction in permeability. The effects of thermally
induced compaction are expected to accelerate surface subsidence as diatomite
steam projects mature.
Introduction
There is a class of problems involving reservoir compaction of cohesive
rocks (e.g. chalk, shale, and diatomite) in which the effects of stress are of
a second-order importance compared to those of temperature. The
injection of cold seawater in North Sea chalk reservoirs under conditions of
invariant effective stress has led to continued compaction and subsidence (Cook
et al. 2001; Sylte et al. 1999). The North Sea chalks are nearly pure calcium
carbonate, and it is well known that the solubility of calcium carbonate
increases as the water temperature decreases. Thus, even under
conditions of unchanging effective stress, one would expect gradually
increasing dissolution of calcium carbonate and compaction as the reservoir
temperature of the chalk (~ 270°F) is gradually lowered by cold seawater
injection (Dietrich 2001). In the giant Wilmington field of California,
the shaly siltstones that are interbedded with the unconsolidated sands have
recently been shown to be much more susceptible to thermally induced compaction
than to stress-induced compaction (Dietrich and Norman 2003). And
finally, diatomite is known to undergo a silica-phase transformation as
temperature is raised, whereby amorphous Opal-A is converted to a more dense,
crystalline Opal-CT. The injection of steam into California diatomite
reservoirs is expected to accelerate this naturally occurring process and lead
to rapid densification and compaction. In each case, for chalk, shaly
rocks, and diatomite, there is both a laboratory and field basis that
demonstrates the dominant role played by temperature.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
8 April 2005
- Revised manuscript received:
19 April 2006
- Manuscript approved:
8 August 2006
- Version of record:
20 March 2007
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