Acid Fluid-Rock Interactions with Shales Comprising Unconventional Hydrocarbon Reservoirs and with Shale Capping Carbon Storage Reservoirs: Experimental Insights

Abstract:

Injection of HCl is often a first stage in the hydraulic fracturing process. These acidic fluids react with marls or shales in unconventional reservoirs, reactions generally comparable to reaction between shale caprocks and acidic, carbonated formation waters in a carbon storage reservoir. Hydrothermal experiments examine acid fluid-rock interaction with 1) an unconventional shale reservoir and 2) a model shale capping a carbon storage reservoir. In the former, unconventional reservoir rock and hydraulic fracturing fluid possessing a range of ionic strengths (I = 0.01, 0.15) and initial pH values (2.5 and 7.3) reacted at 115°C and 35 MPa for 28 days. In the latter, a model carbon storage reservoir (Fe-rich dolomite), shale caprock (illite), and shale-reservoir mixture each reacted with formation water (I = 0.1 and pH 6.3) at 160°C and 25 MPa for ~15 days. These three experiments were subsequently injected with sufficient CO₂ to maintain CO₂ saturation in the water and allowed to react for ~40 additional days.

Acidic frac fluid was rapidly buffered (from pH 2.5 to 6.2 after 38 hrs) by reaction with reservoir rock whereas the pH of near-neutral frac fluid decreased (from 7.3 to 6.9) after 47 hrs. Carbonate dissolution released Ca and Sr into solution and feldspar dissolution released SiO₂ and Li; the extent of reaction was greater in the experiment containing acidic frac fluid. All three carbon storage experiments displayed a similar pH decrease of 1.5 units after the addition of CO₂. The pH remained low for the duration of the experiments because the immiscible supercritical CO₂ phase provided an infinite reservoir of carbonic acid that could not be consumed by reaction with the rock. In all three experiments, Ca, Fe, Mg, Mn and SO₄ increase with injection, but slowly decline through termination of the experiments. This trend suggests initial dissolution followed by re-precipitation of carbonates, which can be seen in modeling and SEM results. New clay minerals form in experiments emulating both unconventional and carbon reservoirs. Smectite precipitates in the latter, the identity of clays in the former is still being determined. Both systems are initially dominated by acid reactions; subsequent differences in geochemical behavior derive from the amount and source of acidity and buffering reactions of the rock.