Simulating Geochemical Fluid-Rock Interactions at the Fracture-Fluid Interface for a Shut-In Well

Abstract

Fluid-rock interactions between hydraulic fracturing fluids (HFFs) and unconventional reservoirs have been shown to cause dissolution and precipitation of minerals. Increased hydrocarbon production can be observed after an extended shut-in period. We contextualize mineral-fluid reactions by simulating the fracture-fluid interface at reservoir conditions over an extended timeframe. We pair a hydrothermal experiment with a geochemical model to investigate reactions occurring at the fracture-fluid interface. The experiment reacts five rock cubes (each ∼1 cm³) from the Wall Creek member of the Frontier Formation in SW Wyoming with a HFF developed in our lab. Experiments simulate a shut-in well at in-situ conditions by reacting fluids and rocks at 115°C (240°F) and 350 Bar (5070 psi) over an 8-month timeframe. The experimental design allows for fluids to be collected over time, and for rock samples to be analyzed before and after the experiment. Fluids collected from experiments were analyzed for major and trace elements by ICP-OES, total dissolved carbon by coulometric titration, and anions by IC. Rock samples are evaluated by SEM-EDS and XRD before and after the experiment. Geochemical models are developed using Geochemist's Workbench to aid in evaluating and interpreting experimental results and to evaluate the reliability of model predictions. In the fluid chemistry, increasing Ca and Sr concentrations correlate with pH increase due to the dissolution of carbonate cement. Dissolution of calcite cement can open permeable pathways for hydrocarbon flow into fractures. pH, Ca, and Sr reach a steady state after ∼110 hrs of fluid-rock reaction. Compared to our other developing datasets, we show that changes in surface area can alter reaction rates at the fracture-fluid interface. Dissolved silica concentration does not reach a steady state; it steadily increases in concentration during the experimental timeframe from 1.3 ppm to 17.2 ppm. This suggests that dissolution reactions involving silicate minerals may contribute to increased well productivity after long-term shut-in, because dissolution of silicate minerals may increase porosity and permeability within the reservoir.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016