A basic rock characterisation was performed on all four core samples (Table 5.2). Absolute per- meability was measured with experimental brine at experimental conditions using the method in Section 3.1.4. Mercury intrusion capillary pressure curves and pore throat distributions were measured using a Micromeritics Autopore IV 9500 Porosimeter and converted for CO2 (Fig-
ures 5.2 and 5.3). These distributions were used alongside CT-measured porosity maps and porosity profiles to assess the heterogeneity of each core. Thin section photos are shown in Fig- ure 5.4 and brief rock descriptions are provided in the following sections. All four samples are classified as quartz arenites using the Folk classification scheme [252,253]. Grain size is assessed following Wentworth (1922) [254] and roundness following Pettijohn et al. (2012) [255].
Table 5.2: Rock sample characteristics
Exp Rock φ Kabs L Pentry, |∆Pc| Pore throat diameter
[-] [D] [m] [Pa] [µm]
19-24, 37-8 Bunter 0.260 ± 0.012 2.201 ± 0.113 0.151 1964.4 30.1 25-28, 39-40 Ormskirk 0.271 ± 0.014 12.143 ± 0.787 0.127 1096.6 46.4 29-32, 41 Captain 0.267 ± 0.007 1.145 ± 0.098 0.235 1862.3 28.3 33-36, 42-43 Paaratte 0.280 ± 0.003 2.328 ± 0.575 0.228 2909.1 24.0
Figure 5.3: Pore throat size distributions. Mean pore throat diameter is estimated by fitting a gaussian to the pore throat distribution.
5.3.1
Bunter Sandstone Formation
The Bunter core had a porosity of 0.26 and a measured absolute permeability to brine of 2.2 D. This is a higher permeability than the core plug average from the Gordon, Esmond and Forbes (kabs ∼ 400 mD, φ ∼ 0.2) fields but matches well with the porosity-permeability trend of mea-
sured core plugs [235]. The Bunter Formation is a medium-grained sandstone composed mainly of sub-angular to sub-rounded quartz grains with a minor component of detrital K-feldspar, clay and carbonate clasts (Figure 5.4). Some altered quartz and K-feldspar overgrowths are present, as well as an intergranular cement mainly composed of dolomite [256].
5.3.2
Ormskirk Sandstone Formation
The Ormskirk core had a porosity of 0.27, which is at the upper range of plug values typical of the South and North Morecambe field [223]. The core sample had an extremely high perme- ability of ∼ 12 D, well in excess of the measured range of 0.0001 to > 1 D [223]. The Ormskirk Formation is a medium-grained, mature sandstone predominantly composed of sub-rounded to rounded quartz grains. The presence or absence of illite as a pore lining cement is a major influence on the permeability of this formation [43,222]. However, no illite was observed in thin section (Figure 5.4) and the particular sample used for core-floods contained only dolomite and quartz cements. Absolute permeability was measured with a confining pressure of 5 MPa, which does not reflect the stress state that would be present in the reservoir. It is likely that the measured absolute permeability would be much closer to the field average if performed with a higher confining pressure. As the main goal of these experiments is to assess the impact of varying the viscous-capillary force balance on the interaction between fluid flow and capillary
Figure 5.4: Thin sections of Bunter, Ormskirk, Captain and Paaratte sandstones in plane (right) and cross-polarised (left) light.
heterogeneity, it is only the capillary heterogeneity that needs to be preserved.
5.3.3
Captain Sandstone Formation
The Captain core has a permeability of 1.1 D and porosity of 0.27, similar to samples from the Goldeneye field [38, 257]. Much of the Captain Sandstone Formation is poorly consolidated and in the Captain field permeabilities over 7 D are not uncommon [39, 258]. However, a more consolidated and hence lower permeability core sample was obtained due to the difficulties of drilling core from poorly consolidated sandstone. The core is dominated by fine to medium- grained, angular to sub-angular quartz (> 90%) with a minor feldspar component and some authigenic clay and intergranular cements of quartz, kaolinite and calcite [257].
5.3.4
Paaratte Sandstone Formation
The Paaratte core has a porosity of 0.27 and permeability of 2.3 D similar to published plug measurements for this formation [248, 250]. The core is composed of > 95% fine-grained, sub- angular quartz with intergranular dolomite cement [248].
5.4
Experimental conditions
Drainage and imbibition relative permeability curves between CO2 and brine were measured
under capillary and viscous limited conditions and residual trapping experiments were per- formed to characterise the flow and trapping behaviour for four reservoir sandstones, using the methods described in Sections 3.1.6 and 3.1.9.
Pressure, temperature and salinity conditions were chosen so as to be representative of likely injection sites in each formation, either by using site specific conditions or regional averages for the formation (Table 5.3). Pressure and temperature for the Bunter core were taken from the Hewett field and a regional average salinity for Bunter Formation reservoir brines in the Southern North Sea was used [34, 235, 259]; the conditions of the South and North Morecambe fields were used for the Ormskirk core [221, 225]; pressure and temperature for the Goldeneye field and regional Captain Sandstone brine salinities were used for the Captain core [41,243,257]; and conditions for the Paaratte core were taken from the Otway storage site [246, 250].
Two sets of drainage and imbibition relative permeability core-floods were performed on each core; one at high flow rate, viscous limited conditions and the other at low flow rate, capillary limited conditions. The particular flow rates selected depended on the heterogeneity in the rock core and the absolute permeability. As all the cores had relatively little observable heterogeneity
Table 5.3: Experimental conditions and fluid parameters for CO2-brine core-floods.
Experiment Rock T P Sa IFTb µ
nwc µwd ρnwc ρwe
[◦C] [MPa] [mol kg−1] [mN m−1] [µPa s] [µPa s] [kg/m3] [kg/m3]
19-24, 37-8 Bunter 53 13.1 1.0 34.7 45.9 581.6 604.4 1021.9 25-28, 39-40 Ormskirk 33 12.7 4.3 38.7 70.4 1198.0 797.6 1118.7 29-32, 41 Captain 80 18 1.0 32.9 41.1 403.8 540.1 1003.4 33-36, 42-43 Paaratte 63 12.5 0.1 34.2 32.4 454.4 437.7 989.4
a NaCl brine,b [9, 203], c [6],d [2],e[7].
in porosity (see CT-measured porosity maps in Section 5.5) and high absolute permeability (> 1 D), a flow rate of 20 ml min−1 was used for all the viscous limited experiments. However, in practice, there will still be some control on the fluid distributions exerted by regions of high capillarity. The capillary limited experiments were performed at flow rates of 0.2 - 4 ml min−1 (see Table 2).