Flow rig

Core-floods were conducted in a high pressure, high temperature, purpose-built closed flow rig (Figure 3.1). The flow rig was constructed specifically for the experiments described in Chapters 4 and 5, following a similar design to that used by Krevor et al. (2012) [12], and optimised to perform steady-state relative permeability measurements with the corrosive CO2-

brine fluid system at pressures of 8-25 MPa, temperatures up to 90◦C, all while maintaining CO2

and brine in chemical equilibrium under conditions of mutual saturation. Dual high pressure syringe pumps for CO2 and brine (Teledyne Isco, model 500D) were used to co-inject fluids.

and brine could continuously circulate with one pump in each pair maintaining flow and one pump refilling. Fluids were circulated through a core holder (Phoenix Instruments, custom) in a horizontal orientation, to a two-phase separator (Vinci Technologies, custom). From the separator, fluids were returned to the refilling pumps via a brine return line connected to the base of the separator or a CO2 return line connected to the top of the separator. The fluid level

in the separator could be viewed through a sapphire window. This ensured that the correct fluid would be returned to the correct pump throughout each experiment. A back pressure syringe pump (Teledyne Isco, model 500D) on the outlet side of the core was used to maintain the system pressure and a confining pressure syringe pump (Teledyne Isco, model 100DX) applied an overburden of 3-5 MPa over the experimental pressure to the core. All flow lines and pumps were constructed from a corrosion resistant nickel-based steel alloy (HC276 Hastelloy). Pressure was measured at the inlet and outlet face of the core using high accuracy pressure transducers (Digiquartz Intelligent transmitter, Model 410K-HT-101, oil-filled). A bypass line was used to circulate CO2 and brine outside the core, to ensure the fluids were mutually saturated with

respect to one another before injection. This was confirmed by monitoring the back pressure pump volume and the fluid level in the separator. When the interface in the separator stopped rising and the volume in the back pressure pump stopped decreasing, this indicated that no more CO2 was dissolving into the brine and the fluids were saturated with respect to one another.

This fluid equilibration step was performed at the pressure, temperature and salinity conditions particular to each experiment, prior to beginning each displacement, to ensure all experiments were performed between immiscible fluid pairs (CO2-saturated brine and brine-saturated CO2)

and any change in saturation in the core was due to displacement alone. Fluid saturations were measured using a medical X-ray CT scanner (Universal Systems HD-350) using a voltage of 120 kV, current of 225 mA and exposure time of 1 s. The core holder sat on an insulated perspex trough inside the CT scanner, attached to an integrated sliding table (Figure 3.2). The lines immediately at the inlet and outlet of the core holder were coiled and semi-flexible so that the core could be moved during scanning. Scans of 1 mm thickness were taken at 3-5 mm intervals along the length of the core, with an x − y resolution of 512 pixels or 234.4 µm.

The flow lines were heated to experimental temperature using a system of resistance heating wires and thermocouples attached to four PID controllers and surrounded by two layers of insulating foam. The CO2, brine and back pressure pumps were heated by circulating silicon

oil from heating baths through insulated heating jackets surrounding each pump and the two- phase separator was housed inside an oven. The core and confining fluid was heated using a polyimide Kapton insulated heating mat (Omega Engineering) wrapped around the outside of the core holder and controlled via a thermocouple inserted into the confining fluid and a PID controller. Temperature was continuously monitored at 27 positions around the flow rig to ensure the experimental temperature was maintained throughout the experiment. Temperature could be maintained at ±5◦C around the flow rig as a whole, and at ±2◦C or better at the

Figure 3.1: High pressure, high temperature flow rig used for steady-state relative permeability experiments.

locations of large fluid volumes (pumps, core and separator) and at the inlet line of the core. Pump pressures and volumes, and the pressure drop across the core were also continuously recorded using a data acquisition system.

The core holder was constructed from aluminium, which is transparent to X-rays, and could accept cores with a diameter of 3.8 cm and a maximum length of 25.5 cm. The end pieces were constructed of the same corrosion resistant nickel-steel alloy as the tubing (HC276 Hastelloy) and were indented with concentric grooves to spread the fluids evenly over the inlet face of the core.

The core was wrapped in a layer of heat-shrinkable teflon, nickel foil and a second teflon layer, then inserted into a rubber and fluoropolymer elastomer Viton sleeve and placed into the core holder (Figure 3.3). The first teflon layer filled the pore space on the surface of the core, preventing any bypass of fluids between the core and the sleeve, while the second teflon layer kept the nickel foil in place when inserting into the rubber sleeve. CO2 is soluble in teflon and

Viton, so the nickel foil acted as a barrier to prevent CO2 diffusing out the sides of the core

into the confining fluid. The foil layer extended over the end pieces of the core holder to ensure the CO2 flowed into the core, not around the outside

De-ionised water was used as a confining fluid and was injected between the interior of the core holder and the Viton sleeve. Pressure transducers were connected via a pair of lines which had no fluid flow to minimise the fluctuation in the readings.

Figure 3.3: Schematic of X-ray transparent core holder and core wrapping for 1.5 inch diameter cores.