Detecting Opal-CT Formation Resulting From Thermal Recovery Methods in Diatomites

Abstract

Evaluation of the effects of thermal recovery methods upon diatomaceous reservoirs with their inherent high porosity and low permeability is problematic in that diatoms, a main component of their namesake rock, are composed of amorphous, hydrous biogenic silica (Opal-A) and can alter when heated. The opal-A to opal-CT transformation, is readily apparent using imaging methods, X-ray diffraction (XRD), and petrophysical measurements when the rock has been fully converted. In laboratory experiments with partial transformation, these changes, if any, are subtle and easily missed due to the minute amount of alteration products and the substantial amount of natural variability within the rocks. For example, XRD measurements may show an increase of 1 wt % in opal-CT after an experiment. It is not apparent whether additional opal-CT either formed as a result of the experiment or is a relative enrichment caused by the dissolution of more susceptible minerals such as opal-A and pyrite. A new method based on nitrogen sorption was developed to detect silica-phase alteration in diatomaceous samples. We observed that nanometer-scale pore-size distributions as measured via nitrogen sorption and processed using the classic BJH method differ for opal-A and opal-CT reservoir samples. Opal-A samples have less nanometer-scale pore volume (~0.1 cc/g), smaller nanoscale pore sizes (~3.8 nm), and distinct pore-size distributions compared to samples containing opal-CT (e.g., 0.3 cc/g and 6.6 nm). This method detects subtle amounts of opal-CT in that samples containing only 3 wt % (XRD) exhibit a distinct opal-CT peak at 7.8 nm in one example. These nanometer-scale pore-size changes occur whether micrometer-scale pores either increase in size (dissolution) or decrease in size (alteration). This method was applied to reservoir and quarry diatomites before and after laboratory experiments conducted at ambient to 230 °C temperatures, pH values of 6 to 10, durations of 10 hours to two years, different fluids, various pressures, and a gamut of flow conditions including spontaneous imbibition, forced imbibition, and static. Supporting data such as water chemistry and XRD data were also measured. Comparison of before and after BJH pore-size distributions reveals a reduction in peak size when dissolution occurs and a shift to larger nanometer-scale pore sizes when alteration (converting to opal-CT) occurs. Many samples exhibit both characteristics. The inlet side of the cores exhibit more dissolution and alteration than the outlet side of the same core. Other factors could also contribute to these changes in the nanometer-scale pore structure such as fines mobilization and compaction.