Abstract Discrete fracture and matrix (DFM) homogenization, simultaneously capturing reservoir layers and contained fractures, is an alternative to discrete fracture network (DFN) upscaling. Here, the DFM approach was applied to a fractured carbonate reservoir. Honouring geostatistical data from well logs, near-well multilayer reservoir models were constructed and analysed. Fracture aperture variations were modelled with a new semi-analytical model including a special treatment of layer-restricted fractures. Important results concern both pre-processing of stochastically generated DFMs for finite-element meshing, and the ensemble permeability values obtained by numerical homogenization of single v. multilayer models, respectively. Upscaling by volume averaging of vertically stacked single-layer DFMs results only in a fraction of the equivalent horizontal permeability that is obtained by homogenization of the multilayer models. Inspection of the flow patterns shows that this discrepancy arises because many fractures contact each other at layer boundaries fostering cross-flow. This effect is further enhanced where fractures intersect multiple layers. Compared to earlier DFN models for this reservoir, the DFM-derived fracture and matrix ensemble permeabilities are up to four times higher, highlighting how important it is to include the rock matrix into equivalent permeability calculations.

Abstract The Upper Cretaceous (Senonian) Chalk in Kent, SE England, is considered with the aim of establishing the tectonic history of the basin in which it was deposited, based on the chronology of fractures and an understanding of the role of these fractures in controlling fluid movement in high-porosity-low-permeability sediments. The earliest brittle structures in the study area are NE-SW-striking, flint-filled shear fractures, with dips of c. 60°, which were formed when the maximum compression (σ 1 ) was vertical and were utilized as channels for fluid movement during flint filling. Flint also occurs along bedding planes, suggesting a diagenetic source. This phase was followed by the development of NW-SE-striking fracture swarms containing fractures ranging between vertical joints and steeply dipping hybrid fractures with acute dihedral angles of c. 40°. The absence of flint along these fractures indicates that they formed after diagenesis of the Chalk. NW-SE-striking, subvertical, regularly spaced, through-going joints then formed as a result of a NW-SE regional compression linked to the Alpine collision. The final stage in the basin history relates to the formation of bed-parallel and vertical (i.e. bed-normal), bed-restricted, systematic and unsystematic fractures associated with uplift and unloading. To model fluid flow through the fracture network present in the Chalk, a finite-element-finite-volume modelling was carried out. The fracture geometries mapped in the field were discretized using unstructured hybrid element meshes with discrete fracture representations. The permeability of fractures was calculated from the cubic law and the petrophysical properties of the rock matrix were taken from Chalk reservoirs in the North Sea. In the models, a constant pressure was applied at the top of the oil-saturated, fractured Chalk while water was injected at the base. In spite of greater density, the water preferentially displaced the oil from the fractures and migrated faster through the fracture swarms and joints than through bed-restricted fractures and the rock matrix. Almost 830f the total flow within the model occurred through the fractures. The results of the field study, combined with those of the numerical modelling, suggest that fracture swarms have a strong impact on the movement of fluids in fractured and faulted reservoirs.