GEOCHEMISTRY IN AN UNCONVENTIONAL OIL RESERVOIR: EXPERIMENTAL INSIGHTS INTO EFFECTS OF FLUID-ROCK INTERACTIONS ON PORE-LINING MINERALS

Interactions between injected fluids and minerals in unconventional reservoirs impact subsurface mineral distributions that affect porosity and fluid flow. Hydrothermal experiments comprising rock, formation fluid, and fracturing fluid at simulated reservoir conditions evaluate the effect of hydraulic fracturing fluid on fluid-rock interactions. Synthetic formation fluid and two Parkman Formation core samples from the Powder River Basin are reacted at 90°C and 200 bars in a rocking autoclave (fluid:rock ratio ≈ 10:1) for at least 28 days. Injecting synthetic hydraulic fracturing fluid into the ongoing reaction changes pH, ionic strength, relative abundance of ions, and redox potential. Reaction of injection fluid, formation fluid and rock continues for a minimum of 28 days.

The sandstone consists primarily of quartz, K-feldspar, plagioclase and carbonate cements; smaller amounts of authigenic and diagenic clay minerals including chlorite, illite, mixed-layer illite-smectite, and kaolinite are present. Each experiment uses 0.5-1 cm cubes of sandstone to preserve pore structures and promote petrologic evaluation of changes to pore-lining clays. The formation fluid (ionic strength ≈ 0.2 molal) contains 0.19 molal NaCl and millimolal to molal quantities of SiO₂, Ca, Mg, K, SO₄, Fe, and HCO₃. Synthetic formation fluid composition is based on formation fluid chemistries reported for wells targeting the Parkman Formation and calculated to be saturated with respect to minerals present at the start of each experiment. During each experiment, in-situ fluid samples are collected up to ten times prior to injection and at least ten times after injection of fracturing fluid.

Fluid samples are analyzed for total dissolved carbon by coulometric titration, anions by ion chromatography, and major, minor, and trace cations by ICP-OES. Bench pH values are paired with aqueous analyses to calculate in-situ pH. Iron species are analyzed by spectrophotometry. Distribution and chemistry of pore-lining minerals are evaluated by SEM-EDS before and after each experiment. Analytical data and geochemical calculations are integrated to evaluate fluid-rock interactions for initial conditions including organic-rich and organic-lean rock samples.