5 Experimental procedures
5.3 Routine core analysis
5.3.1 Porosity measurement
The saturation method was used to measure porosity. This method involves measuring the mass of the core sample before and after the sample is saturated with a fluid. The mass difference is measured corresponding to the total the volume of fluid that has saturated the sample.
This volume corresponds to the total pore volume of the core assuming that it is 100% saturated with the fluid. The density of the fluid used is known and the bulk volume, pore volume and porosity can be calculated using material balance (Graue, 2006).
The chalk core is cut from an outcrop before the core surface is cleaned by hand with Ekofisk brine and then dried at 80 °C for 24 hours in a heating cabinet. The length and width of the core sample are measured with a caliper before it is saturated with a liquid in a vacuum apparatus.
Saturating the core
Two methods of saturating the core samples have been applied in this thesis. Both methods use the principle of the saturation method, as explained above, but with a slightly different procedure.
Figure 5.3 shows the first method of saturating the core with the use of a vacuum apparatus. The Ekofisk brine was vacuumed to a pressure below 10 mbar and the core was vacuumed to below 1 mbar before saturation. It is important to get below these pressures to avoid underestimating the porosity and avoid getting air inside the core, leading to a three phase system.
Figure 5.3: Experimental procedure of measuring porosity in a vacuum apparatus. Modified from (Christophersen, 2012).
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The second method is to saturate the core in the core holder itself. This is the best suited method when using a CT/PET scanner to avoid movement of the core holder between scans. It is also less time consuming when using fractured core samples.
Figure 5.4 shows the experimental setup when saturating an air filled core sample directly in the core holder. Both end pieces have a valve to close the desirable connections whereas one was connected to a vacuum pump and the other was connected with a pipe to a cylinder filled with oil. The core sample is vacuumed below 1 mbar before it is spontaneously saturated with oil. By measuring how much oil the core is saturated with minus the dead volume, pore volume and porosity can be calculated by material balance.
Figure 5.4: Illustration of saturating the core directly in the core holder.
5.3.2 Permeability measurement
Permeability measurements were conducted using the method described in section 1.2.1, by the use of Darcy’s law. Assuming that the cores were 100% saturated with the Ekofisk brine, the cores were flooded with several constant injection rates until the differential pressure stabilized. Figure 5.5 shows the experimental for permeability measurements.
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Figure 5.5: Experimental setup for measuring absolute permeability. The same setup is used when draining the cores.
5.3.3 Drainage
The brine saturated core samples were drained with n-decane until the irreducible water saturation was obtained. The cores were drained with up to 4 PV of Ekofisk brine in both directions of the core with a differential pressure equivalent to 2 bar/cm of their length until the brine production stagnated. The experimental setup was identical for permeability measurements, displayed in Figure 5.5. The injection rate during permeability measurements was slowly increased to the desired rate to control the buildup of the differential pressure versus the confinement pressure. This procedure was followed to avoid rapid changes in pressure and possibly damage the core plugs.
5.3.4 Fracturing the core samples
In order to study recovery mechanisms by CO2 in fractured reservoirs, most of the cores were cut along the center of the cylinder axis with a band saw to represent a fracture. The cores diameter become 1 mm less from the width of the saw blade and fracture surface becomes smooth with no roughness.
Two methods of calculating the new pore volume after fracturing was applied. The first method was to subtract the volume removed by the saw blade and the second method was to measure the weight difference before and after cutting the core. Both methods included the assumptions that the porosity distribution and fluid saturations did not change upon cutting the core.
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Assuming that the volume removed has the shape of a rectangular box the new pore volume can be calculated by subtracting the fracture volume, V, from the initial pore volume, PV:
( )
frac fracture
PV =PV − ⋅
φ
V , Vfracture = ⋅ ⋅L D Dcut (5.1) where PVfrac is the pore volume after fracturing, is the measured porosity, L is the length of the core, D is the initial diameter and Dcut is the diameter of the part that are cut away perpendicular to the fracture plane. The new porosity can also be estimated using weight measurements:frac frac
initial
PV PV m
= ⋅m (5.2)
where mfrac is the weight of the core after cutting and minitial is the initial weight of the core. Some of the cores where cut dry and saturated directly in the core holder as explained in section 5.3. In these cases the pore volume was measured based on the volume of oil that imbibed into the core and the dead volume of the core holder.
To allow reproducibility in experiments with the same fracture permeability and fracture-to-matrix transmissibility, a special designed spacer was placed in the fracture (see Figure 5.6). The spacer was 1 mm thick and made of polyoxymethylene (Pomeroy, 1933). It contained three separate partments connected with high conductivity flow channels. Each of these partments represents an open fracture with 1 mm width.
Figure 5.6: POM spacer with three separate apartments to simulate open fractures.
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