Permeability of Stylolite-Bearing Chalk
- Ida Lind (Maersk Olie og Gas AS) | Olav Nykjaer (Maersk Olie og Gas AS) | Soren Priisholm (Maersk Olie og Gas AS) | Niels Springer (Geological Survey of Denmark)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 986 - 993
- 1994. Society of Petroleum Engineers
- 1.6 Drilling Operations, 5.5.2 Core Analysis, 4.6 Natural Gas, 1.6.9 Coring, Fishing, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.1.5 Geologic Modeling, 5.8.7 Carbonate Reservoir, 1.14 Casing and Cementing, 5.6.2 Core Analysis
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Permeabilities were measured on core plugs from stylolite-bearing chalk of the Gorm field in the Danish North Sea. Air and liquid permeabilities were measured in directions parallel to and perpendicular to the stylolite surface. Permeability was measured with sleeve pressure equal to in-situ reservoir stress. Permeabilities of plugs with stylolites but without stylolite-associated fractures were equal in the two directions. The permeability is equal to the matrix permeability of non-stylolite-bearing chalk. In contrast, when fractures were associated with the stylolites, permeability was enhanced. The enhancement was most significant in the horizontal direction parallel to the stylolites.
Stylolites are common in North Sea Chalk reservoirs (Albuskjell, Tyra, Ekofisk, West Ekofisk, Dan, and Gorm, Fig. 1). Typical stylolites of the Chalk have amplitudes of <1-cm size and clay-bearing drapes of <1-mm thickness. Stylolite-associated leaching, which can be prominent in limestones, has not been reported in the Chalk. In some reservoir intervals, stylolites are associated with fractures. These stylolite fractures commonly terminate against stylolite seams, perpendicular to or at least at high angles to the stylolite surface (Fig. 2). These fractures are 0.5 to 5 cm long and 0.01 to 0.1 mm wide.
In the Gorm field, stylolites are common with a vertical density of two to four stylolites per foot. Densities up to 20 stylolites per foot have been noted. A decreasing stylolite density is observed as porosity increases (Fig. 3). A similar trend was noted in the Upper Cretaceous chalk of England.
The influence of stylolites on effective permeability has been discussed by several authors. The consensus, based on petrography, is that stylolites in North Sea chalks act as barriers to flow in the vertical direction owing to their planar horizontal fabric and concentration of clay minerals. Flow in the horizontal direction may be enhanced if open fractures are associated with the stylolites. The present study does not support the assumption that stylolites form vertical barriers.
Actual measurements of stylolite permeability are few. One reason is that plugs tend to break along stylolite surfaces during drilling and routine core analysis, rendering them unsuitable for permeability measurement.
The objectives of this work were (1) to determine whether reliable measurements of permeability across and along stylolites are possible and, if so, under what test conditions; (2) to determine the influence of stylolites on the permeability of the chalk; and (3) to examine the relationship among stylolite-affected permeability, petrography, and mineralogy.
|File Size||3 MB||Number of Pages||8|