We use a coupled poro-elastic model to calculate the change in stresses at the crest of dipping structures, due to a regional over-pressured field. We show that localized flow field at the crest of the structure may increase horizontal and vertical stresses above the lithostatic value, and therefore very high leakoff values, even greater than the overburden, are possible at the crest of these dipping structures. We also show that principal stresses may rotate, and the direction of the least principal stress can become vertical. Furthermore, the elevated pore pressures can lead to very low effective stresses, and hence localized fractures. We also explore how the stress field is affected by permeability anisotropy: higher horizontal permeabilities lead to more significant changes in the horizontal stress. Our analyses illustrate that traditional basin-modeling assumptions - such as a vertical stress equaling the overburden and a horizontal stress equaling a fraction of the overburden - are not valid at the tip of dipping structures and may misrepresent borehole stability and trap integrity.
This paper studies the stresses around the crest of dipping structures. We use the term dipping structures to refer to inclined permeable bodies (e.g. sand reservoirs) encased in low permeability sediments that are usually overpressured, for example due to rapid sedimentation. Due to the higher hydraulic conductivity of these structures, a local flow is established along them, from areas of higher pore pressures (high overburden) to lower pressures (low overburden) ([1-3]). High horizontal stress values are often reported near the crest of dipping structures, and horizontal stresses higher than the overburden have been measured, indicating rotation of the least principal stresses (e.g. Auger Basin, [3]). In addition, pore pressures at the tip often converge to the least principal stress, leading to fracture openings.
