Silica Dissolution From Oil Sands

The major mineral component of the Alberta Oil Sands deposits (excluding those in carbonate rocks) is quartz that occurs either in coarse or in microcrystalline form (chert). Most in situ processes are being proposed for the recovery of bitumen involve the injection of large volumes of water and steam into the formation. The dissolution of SiO₂ in the high temperature regions of the production zone and its subsequent precipitation in cooler parts of the formation could lead to a significant decrease permeability. This reaction is suspected to have been the cause of large pressure drops in the Shell and Petro Fina field experiments (REDFORD and COTSWORTH, 1974).

Cementation and formation of overgrowths as a result of the precipitation of silica from the interstitial solutions are common causes for porosity and permeability reductions during diagenesis (PETTIJOHN, POTTER and SIEVER, 1972). This has also been demonstrated experimentally (CECIL and HEALD, 1971; ERNST and BLATT, 1964; HEALD and RENTON, 1966; PARAGUASSA, 1972; SPRUNT and NUR, 1976). However, the geological literature on diagenesis is of little help in predicting the behaviour of silica during steam injection because the time scales involved are altogether different. Most of the experimental work on cementation reported in the literature was carried out at pressures of 100 Mpa or higher, and temperatures up to 400°C, whereas during the injection of steam into the oil sands the maximum temperatures that will be reached are in the range around 300°C at the corresponding saturated water vapour pressure (8.58 Mpa). During in situ combustion the temperature can rise as high as 600°C, but the pressure will remain below 100 Mpa.

Dissolved silica also takes part in reactions such as: H₄SiO₄ + kaolinite + calcite ⇄ montmorillonite + H₂CO₃ (BAYLISS and LEVINSON, 1971). The degree to which formation porosity and permeability are affected by the dissolution and precipitation of silica depends to a large extent on the kinetics of these reactions. Also, the dissolution of quartz can be the rate-determining step in mineral formation reactions, as was shown by COURNOYER et al. (1975) for the formation of zeolite from an aluminate solution that was in contact with solid quartz.

To complement other studies in this laboratory an experimental study of the dissolution and precipitation rates of quartz in the oil sand-water system was carried out. The present report describes and interprets the results of dissolution rate experiments conducted at pH levels between 5.6 and 8.7 and at temperatures ranging from 150°C to 250°C, both in the presence and in the absence of bitumen. Attention is paid to the way in which the experimental set-up has influenced the results, in particular with respect to the fraction of the sand surface that actually takes part in the dissolution process. Product minerals that formed during some of the dissolution experiments are described and identified with the aid of scanning electron micrographs and x-ray diffraction. Also, the results of some preliminary precipitation rate experiments are discussed. In a final section of the report the steady state SiO₂ concentration and dissolution rate in the ARC-AOSTRA 150 cm physical simulator are calculated as a function of distance from the injector, using the rate equations derived in the earlier sections. Steady state concentrations and SiO₂ precipitation rates are calculated for a hypothetical field case.