Laboratory Quartz Growth Kinetics for Predicting Reservoir Quality

Hubert E. King, Amy B. Herhold, Robert T. Stibrany, and Stephen D. Cameron
ExxonMobil Research and Engineering Co, Annandale, NJ

Many deep siliciclastic reservoirs have low porosity due to thermally-activated quartz cement growth. However, some deep reservoirs have “anomalous” preserved porosity because quartz cement growth has been inhibited. The ability to quantifiably predict such porosity is a significant factor in reservoir quality forward modeling. Understanding quartz cement growth kinetics is one of the keys to predicting the amount of preserved porosity. Extracting quartz growth rates from field data can be problematic due to the convolution of multiple effects on the kinetics. By studying quartz growth in the laboratory, we can impose a well-defined diagenetic environment, and therefore separate out the impact of different control parameters. Our work is aimed at quantifying quartz precipitation kinetics in the laboratory under a variety of simulated near-reservoir conditions. We are measuring growth rates in a closed system at buffered circumneutral pH using well-cleaned and well-characterized natural quartz sand. Surface preparation of the quartz is important to these experiments. For example, minor amounts of feldspar grains and amorphous silica commonly present on the quartz surface strongly interfere with precipitation measurements. We are able to measure very small amounts of quartz precipitation by accurately measuring the silica concentration in solution. We vary the starting silica supersaturation, pH, and salt content to mimic the effects of different reservoir pore-fluid chemistry. Initial results show that increases in pH and salt content both increase the rate of quartz growth. These fundamental studies will provide the basis for further research to quantify different cement-inhibition mechanisms for use in reservoir-quality models.