Prediction of Geochemical Formation Damage in the Vicksburg Formation of South Texas

ABSTRACT Formation damage is broadly considered to be the occlusion of pore throats by debris. This occlusion is generally thought to result from the introduction of fine-grained particles to the formation, or the migration of particles already present in the formation. We contend, however, that particles and cements may be precipitated in situ by disruption of geochemical equilibrium in the reservoir. The Olivarez #2 well was completed in the Oligocene Vicksburg Formation (Z-sand), using a KCI-based completion fluid and was traced using a fluid of similar composition. The well began to unload and flow immediately and was killed using 14ppg CaBr2. Some damage was suspected to have occurred at this point because the initial production tests did not see flow rates comparable to other wells in the field. A second frac of the formation was then undertaken using CaCl2/CaBr2-based fluids, but no significant improvement in flow rates was realized. Our analysis of this problem focused on rigorous geochemical modeling of the reservoir and the changes that would be induced if fluids were added to the reservoir fluid-rock system. Calculations accounted for activity coefficients via the extended Debye-Huckel relationship and for elevated temperatures by the van't Hoff or Kirchoff equations. Geochemical analysis of reservoir environments utilizing these equations yields a more accurate picture than can be reached using Stiff-Davis or similar approaches. Consideration of sulfates, carbonates, and silicates concurrently is also only possible with this type of analysis. The results of our modeling indicate that the loss of KCI-based completion fluids to the formation may have resulted in the precipitation of both carbonate and silicate phases within the formation. Subsequent attempts to improve deliverability by tracing produced no improvement because the loss of CaCl2- and CaBr2-bearing fluids to the formation during kill and tracing procedures apparently caused additional silicate-phase precipitation. We recommend geochemical reservoir characterization as a critical part of proper reservoir management. Such work, when done in a timely fashion, can help predict the tendency of a reservoir system to produce irreparable precipitate damage. This type of work may also help curtail the use of costly clean-up or stimulation procedures where the geochemistry of the system will render them ineffective.