Print this pageNatural Fracturing Impacts Siliceous Shale Reservoir Development: Cymric Field, California, USA
JOHNSON, SCOTT J.
Geophysical data show pronounced systematic variations in reservoir
anisotropy
orientation in the shallow (< 500 m) Monterey-equivalent reservoir in
the Cymric Field of California. Variations occur over limited areal (< 125 m)
and vertical (<5 m) distances. Two main orientations are evident:
north-northeast and west-northwest, perpendicular and parallel to anticlinal
trends, respectively. Borehole image data indicate that north-northeast
anisotropy is regional maximum horizontal in-situ stress and west-northwest
anisotropy is an oriented natural fracture system and/or a local stress related
to natural fracturing. Andersonian fault theory accounts for local stress
reorientation which is possible due to the small difference in maximum and
minimum horizontal in-situ stress magnitudes.
Variations in reservoir anisotropy
orientation and type result in significant
areal and vertical variations in depletion geometry patterns. In relatively
undeformed portions of the reservoir where fracture permeability is negligible
and regional stress pervades, cyclic steam injection induces a fracture parallel
to regional maximum horizontal in-situ stress. Where natural fracturing is
well-developed, cyclic steam either induces a fracture of reduced width and
height growth or is injected into the oriented natural fracture system. Oriented
fracture systems in thin, widespread, stratum-bound zones focus injection and
facilitate intrawell communication. Understanding depletion geometry patterns as
they relate to natural fracturing is a key step in optimizing field development.