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.
Geological evolution of the Chalk Group in the northern Dutch North Sea: inversion, sedimentation and redeposition