Petrophysical Characterization of the Utsira Sand – CCUS

Petrophysical properties are important to understand the nature of the CO₂ storage reservoir and to understand the mechanisms at play. Petrophysical properties like porosity and permeability are key parameters for a safe long-term storage of CO₂ but also for the injection operation itself. The vertical and horizontal distribution of these properties helps in understanding the the storage capacity and migration of CO₂ in the reservoir.

The CO₂ is stored in the Utsira formation which acts as the reservoir rock. The highly porous and extremely permeable Utsira sands are organized into approximately 30m thick packages. These packages are separated by thin (predominantly 1 m thick), low permeability shale layers (Norland Shale caprock),

The properties used in the dynamic simulation model are summarized on the CO₂ DataShare website. The grid properties are reproduced here from Singh et al, as those used in the 2011 benchmark.

Parameters	Symbol	Unit	Reference value	Range
Caprock porosity	φcap	%	35	34-36
Utsira porosity	φfmn	%	36	27-40
Shale porosity	φsh	%	34	31-38
Caprock permeability	kcap	mD	0.001	0.00075 – 0.0015
Utsira permeability	kxy	mD	2000	1100 – 5000
Shale permeability	ksh	mD	0.001	0.00075 – 0.0015

Sleipner Utsira sand reservoir properties indicate the presence of high permeability for flow of CO₂ through the reservoir along with high well injection capacity.

There have been studies describing the changes in the petrophysical properties of rock

Porosity

The porosity is determined from the thin sections, core plugs and wireline logs.

The thin sections for the Utsira sand core (Well 15/9-A23) are presented in the figure above. The
grains consist primarily of quartz with some feldspar and shell fragments and are predominantly angular to sub-angular. The porosity from the thin sections is estimated to be around 36.0%-40.1% with an average of 38.0%.
The technique of liquid invasion using water and/or helium on core plugs yielded effective porosities ranging from 35%-42.5%.

Wire-line log density data is based on the analysis of six wells with suitable logs yielded porosities ranging from 35%-39% . The porosities determined by three different methods range from 35%-42.5%. (Reference: Zweigel et al)

Permeability

The permeability of Utsira Sand was measured on four 1.5 inch cores of varying lengths. The average permeability of the core analysis is approximately 2 Darcy. No significant difference in horizontal and vertical permeability can be demonstrated from the results. Variability in results represents changes composition and physical properties in the between samples rather than measurement uncertainty.

The Sleipner region’s Utsira sands have no well-test data. In contrast to the extremely high porosity, the figures for permeability derived from the core samples appear low. The Utsira sand’s particle size distribution might provide an explanation. The pore spaces between the large particles may remain open due to an insufficient supply of intermediate-sized particles, leading to high porosity, while the fines may obstruct the pore necks, which tends to lower permeability.

Net-Gross ratio

According to data from five wells in block 15/9 with adequate logs, the N/G for the majority of the Utsira Sand beneath the base of the 6.5 m thick shale layer in the Sleipner area ranges from 0.90 to 0.97. (Reference: Zweigel et al)

Discussion

In the vicinity of Sleipner, the Utsira Sand is a highly porous (30-40%), very permeable (1-3 Darcy), weakly consolidated sandstone, lying at depths between about 800 m and 1100 m, with a thickness of about 250 m around the injection site.

The porosity and absolute permeability are assumed to be dependent of saturation and pore pressure (ref: Peter at al). Since Utsira is a sandstone formation, the effect of CO₂ dissolution is expected to be minimal on the rock properties.