Origins of Tensleep Formation Water Chemistry and the Prediction of Scale Formation During CO₂ Treatments, Bighorn Basin, Wyoming

Ryoji Shiraki, Thomas L. Dunn

Ninety-four water analyses for Tensleep Formation water, Bighorn Basin, Wyoming were collected. The low total dissolved solids (TDS) and the lack of a typical NaCl-type solution suggest that the original formation waters were completely flushed out of the Tensleep Formation due to penetration of meteoric water and that subsequent reaction between the rock and meteoric water produced new water chemistries.

Two types of water chemistry have been identified in the Tensleep Formation water. Type A is rich in Ca²⁺, Mg²⁺, SO₄^2-, and HCO₃^-, and has very low Cl^- contents and a good linearity between [Ca²⁺ + Mg²⁺] and [SO₄^2- + HCO₃^-], suggesting that this chemistry is controlled by the dissolution and precipitation of anhydrite, gypsum, calcite, and dolomite. Type B has higher Cl^- content and the relation between [Ca²⁺ + Mg²⁺] and [SO₄^2- + HCO₃^-] is less clear than in Type A. Speciation by SOLMINEQ.88 indicates that the saturation index of anhydrite and gypsum is a function of T S. Type A is saturated with respect to gypsum at about 3,500 mg/l TDS, but Type B is slightly undersaturated even for higher TDS. Both Types A and B are highly supersaturated with respect to calcite and dolomite.

Geochemical modeling of the water chemistries indicates that Type A has greater potential for sulfate scale as a result of CO₂ treatment. Further experimental studies and modeling are required to determine the true carbonate scale potential of Tensleep waters during CO₂ treatment.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995