Faults as fluid flow barriers and their role in trapping hydrocarbons Grp 2

Faults often present barriers to migration of hydrocarbons, or trap hydrocarbons by either offsetting the pay zone against non- reservoir or through deformation processes that reduce pore throat radii within the fault zone

Development of a better understanding of both the structure and dynamics of fault/fracture networks is important in many areas of applied science and basic research. In many materials failure occurs as a result of the growth and coalescence of cracks. In particular, information concerning the distribution of faults and fracture that are too small to be detected in seismic surveys but are large enough to have an important impact on fluid flow is important. The transport properties of a heavily fractured reservoir can be completely dominated by transport through the fracture network or the fractures can act as barriers to flow.

Whether fractures act as barriers/seals or conduits will depend mostly on their surface properties (e.g., roughness, mineralogy, strength, infilling), their spatial distribution (including parameters such as length, width, continuity, spacing, orientation and dip) and the state of stress. Fracture sealing can be due to several mechanisms, and one of the most common is the formation of gouge zones due to shearing. Laboratory experiments have been performed on fractures in different rock types. In these tests conductivity has been monitored over a range of normal and shear stresses, the latter resulting in shear displacement along the fracture plane. Tests have been performed on fractures in hard rocks such as gneiss, granite, syenite and schist, and sedimentary rocks including chalk, shale and different sandstones. Based on the results from these laboratory experiments, the major factors controlling the permeability of single fractures and fracture cross-flow are the stress dependent fracture aperture (e), the fracture internal flow paths (tortuosity), the fracture surface properties like roughness (JRC), fracture wall strength (JCS) and the intact rock uniaxial compressive strength (sc), the fracture normal stress (sn) and shear displacement (ds) related dilation (dn) and gouge formation. Whether fracture conductivity and fracture cross-flow increase or decrease with ongoing shear, depends mainly on these parameters and the stresses acting in the fracture plane. Fracture asperity damage (crushing, gouge formation and pore size reduction) depends strongly on the ratio of the fracture wall compression strength (or intact rock uniaxial compressive strength if the fracture surface is not altered) to the normal stress level (JCS/ sn), the ratio of JCS to shear displacement (JCS/ ds) and the fracture surface morphology, expressed by the joint roughness coefficient (JRC).

In the North Sea and Haltenbanken faults may, depending upon the degree of juxtaposition or clay smearing, be sealing relative to flow across the fault s/AbsKongsberg96/