Reaction kinetics determined from core flooding and steady state principles for Stevns Klint and Kansas chalk injected with MgCl2 brine at reservoir temperature

A methodology is presented for determining reaction kinetics from core flooding: A core is flooded with reactive brine at different compositions with injection rates varied systematically. Each combination is performed until steady state, when effluent concentrations no longer change significantly with time. Lower injection rate gives the brine more time to react. We also propose shut-in tests where brine reacts statically with the core a defined period and then is flushed out. The residence time and produced brine composition is compared with the flooding experiments. This design allows characterization of the reaction kinetics from a single core. Efficient modeling and matching of the experiments can be performed as the steady state data are directly comparable to equilibrating the injected brine gradually with time and does not require spatial and temporal modeling of the entire dynamic experiments. Each steady state data point represents different information that helps constrain parameter selection. The reaction kinetics can predict equilibrium states and time needed to reach equilibrium. Accounting for dispersion increases the complexity by needing to find a spatial distribution of coupled solutions and is recommended as a second step when a first estimate of the kinetics has been obtained. It is still much more efficient than simulating the full dynamic experiment. Experiments were performed injecting 0.0445 and 0.219 mol/L MgCl2 into Stevns Klint chalk from Denmark, and Kansas chalk from USA. The reaction kinetics of chalk are important as oil-bearing chalk reservoirs are chemically sensitive to injected seawater. The reactions can alter wettability and weaken rock strength which has implications for reservoir compaction, oil recovery and reservoir management. The temperature was 100 and 130°C (North Sea reservoir temperature). The rates during flooding were varied from 0.25 to 16 PV/d while shut-in tests provided equivalent rates down to 1/28 PV/d. The results showed that Ca2+ ions were produced and Mg2+ ions retained (associated with calcite dissolution and magnesite precipitation, respectively). This occurred in a substitution-like manner, where the gain of Ca was similar to the loss of Mg2+. A simple reaction kinetic model based on this substitution with three independent tuning parameters (rate coefficient, reaction order and equilibrium constant) was implemented together with advection to analytically calculate steady state effluent concentrations when injected composition, injection rate and reaction kinetic parameters were stated. By tuning reaction kinetic parameters, the experimental steady state data could be fitted efficiently. From data trends, the parameters were determined relatively accurate for each core. The roles of reaction parameters, pore velocity and dispersion were illustrated with sensitivity analyses. The steady state method allows computationally efficient matching even with complex reaction kinetics. Using a comprehensive geochemical description in the software PHREEQC, the kinetics of calcite and magnesite mineral reactions were determined by matching the steady state concentration changes as function of (residence) time. The simulator predicted close to identical production of Ca as loss of Mg. The geochemical software predicted much higher calcite solubility in MgCl2 than observed at 100 and 130°C for Stevns Klint and Kansas.acceptedVersio

Reactive flow in chalk at reservoir temperature – Experiment and simulation

This thesis seeks to study how 0.219 M MgCl2 brine flooded through chalk cores at reservoir temperatures will chemically react when flooded through at various flowrates. We used two different chalk cores. The cores were from Stevns Klint in Aalborg, Denmark and Kansas, USA and are referred to as SKA3 and KR30 respectively. A triaxial cell was used for each of the cores for this experiment, with SKA3 being run at 100ºC and KR30 at 130ºC. Pore pressure and confining pressure was maintained at a constant level throughout the experiment, only the flowrate was varied. For SKA3 a pore pressure 0.7 MPa and a confining pressure of 2.0 MPa was used, while KR30 was run at equal pore pressure but lower confining pressure (1.2 MPa). The confining pressures were deliberately low in order to minimize compaction of the cores. The effluents were sampled regularly and then analyzed with an ion chromatography machine to see how varying the flowrate affects the concentrations of ions present in the brine after flooding. From plotting these results, we could see a general trend that lowering the flowrate increased the concentration of calcium present in the effluents. This shows that the brine had a greater chemical effect on the chalk when the flowrate was lower. At higher flowrates, the opposite behavior was shown. The data gathered from core SKA3 showed a greater tendency to deviate from this behavior, often failing to reach steady state. Core KR30 showed no deviation from the general trend, with lower flowrates always increasing calcium concentration and vice versa. Comparing these results with similar tests performed on Stevns Klint cores from the same block as SKA3 (Olsen, A. T, 2020) suggest that temperature was the deciding factor in the trouble to reach steady state, not the chalk type. After completing the floodings the gathered data was then compared to a model developed to predict the concentrations of ions in the flooding of a chalk sample by MgCl2-based brines. A set of mathematical equations was solved to predict ion concentrations at different rates. The tuned model was plotted alongside the experimental measurements for comparison. We were able to fit the model to a satisfactory degree, where it was able to reasonably predict the concentrations of the ions at a given flowrate.This thesis seeks to study how 0.219 M MgCl2 brine flooded through chalk cores at reservoir temperatures will chemically react when flooded through at various flowrates. We used two different chalk cores. The cores were from Stevns Klint in Aalborg, Denmark and Kansas, USA and are referred to as SKA3 and KR30 respectively. A triaxial cell was used for each of the cores for this experiment, with SKA3 being run at 100ºC and KR30 at 130ºC. Pore pressure and confining pressure was maintained at a constant level throughout the experiment, only the flowrate was varied. For SKA3 a pore pressure 0.7 MPa and a confining pressure of 2.0 MPa was used, while KR30 was run at equal pore pressure but lower confining pressure (1.2 MPa). The confining pressures were deliberately low in order to minimize compaction of the cores. The effluents were sampled regularly and then analyzed with an ion chromatography machine to see how varying the flowrate affects the concentrations of ions present in the brine after flooding. From plotting these results, we could see a general trend that lowering the flowrate increased the concentration of calcium present in the effluents. This shows that the brine had a greater chemical effect on the chalk when the flowrate was lower. At higher flowrates, the opposite behavior was shown. The data gathered from core SKA3 showed a greater tendency to deviate from this behavior, often failing to reach steady state. Core KR30 showed no deviation from the general trend, with lower flowrates always increasing calcium concentration and vice versa. Comparing these results with similar tests performed on Stevns Klint cores from the same block as SKA3 (Olsen, A. T, 2020) suggest that temperature was the deciding factor in the trouble to reach steady state, not the chalk type. After completing the floodings the gathered data was then compared to a model developed to predict the concentrations of ions in the flooding of a chalk sample by MgCl2-based brines. A set of mathematical equations was solved to predict ion concentrations at different rates. The tuned model was plotted alongside the experimental measurements for comparison. We were able to fit the model to a satisfactory degree, where it was able to reasonably predict the concentrations of the ions at a given flowrate