Carbonate Cement Dissolution during a Cyclic CO2 Enhanced Oil Recovery Treatment

  1. Sadoon Morad
  1. L. K. Smith

Published Online: 17 APR 2009

DOI: 10.1002/9781444304893.ch21

Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution

Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution

How to Cite

Smith, L. K. (1998) Carbonate Cement Dissolution during a Cyclic CO2 Enhanced Oil Recovery Treatment, in Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution (ed S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304893.ch21

Author Information

  1. Institute for Energy Research, PO Box 4068, Laramie, WY 82071, USA

Publication History

  1. Published Online: 17 APR 2009
  2. Published Print: 29 MAY 1998

ISBN Information

Print ISBN: 9780632047772

Online ISBN: 9781444304893

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Keywords:

  • carbonate cement dissolution during cyclic CO2 enhanced oil recovery treatment;
  • water sample collection and analysis;
  • cyclic enhanced oil recovery treatments;
  • cyclic CO2 treated fields;
  • mineral dissolution amount and rates;
  • modelling calcite dissolution

Summary

Diagenetic and reservoir studies usually require original, uncontaminated formation water. However, abundant information can be obtained using water produced from enhanced oil recovery projects. In this study of cyclic CO2 well treatments in six oilfields in Wyoming, USA, produced water chemistry was monitored for several weeks after the CO2 treatment. In all cases, increased concentrations of calcium, magnesium, bicarbonate, silica and aluminium indicated mineral dissolution. The ratio of calcium to magnesium provided information about the relative dissolution rates between calcite and dolomite, assuming congruent dissolution. At very high PCO2 the dolomite dissolution rate may approach that of calcite. Comparing post-treatment mineral saturation indices for the produced water indicated that the ratio of calcite to water in the reservoir is more important to dissolution rates than PCO2, because reservoirs with higher carbonate mineral/water ratios were more oversaturated with respect to calcite than reservoirs with higher PCO2. Also, from the mineral saturation indices it was found that injected CO2 can come to equilibrium with carbonate minerals in the reservoir in approximately 6 months (at P = 18 MPa). This type of information is useful in scaling up laboratory dissolution experiments to field conditions. For the aluminosilicate minerals dissolution and/or alteration is more difficult to discern from the water chemistry. However, aluminium concentrations peaked later than silica concentrations in the two wells where such analyses were performed. From this, it was surmised that a more silica-rich mineral (such as feldspar) dissolved/altered first, followed later by the dissolution/alteration of a more aluminium-rich mineral (such as clay). The shape of the concentration–time profiles also provided information about the reservoir. In a good treatment, where oil production increased after the CO2 injection, the concentration for all the ions, except aluminium, peaked immediately after production recommenced and then followed a steady decline back to pretreatment levels. In unsuccessful treatments ionic concentrations peaked much later, or peaked and did not decline. This information from produced water was used to assess reservoir heterogeneity, which led to guidelines for choosing the wells most likely to be good candidates for cyclic CO2 treatments.