Previous Talks

Wednesday, February 13th, 2013

Stress determination in active thrust belts: An alternative leak-off pressure interpretation

Brent Couzens-Schultz and Alvin Chan, Shell


In thrust belts, fluid flow through critically stressed fractures will occur at pressures less than the overburden stress, which is the minimum stress. We propose that low leak-off pressures obtained in active thrust belts may result from this mechanism, leading workers to infer that apparent minimum stresses are 30 to 60% less than the overburden stress in some compressional settings. Traditionally, leakoff pressure data have been used to constrain the magnitude of minimum stress, assuming that the rock is dilating against the minimum stress during a leak-off test. In our new interpretation, we constrain the stress state by assuming that the leak-off test causes shear failure along pre-existing weaknesses rather than tensile opening. While this mechanism has been discussed in a small number of borehole stability and hydraulic fracture papers, it has not been directly applied to leak-off tests. We considered this interpretation because we observed that some leak-off tests imply an apparent contradiction between the stress states from the standard interpretation of leak-off tests versus the stress state inferred from geologic and geophysical evidence in tectonically active thrust belts. We present two examples with one in an onshore fold-thrust belt and one in a deepwater fold-thrust belt. Our new interpretation of stresses based on shear failure resolves the contradiction and also provides a constraint on the maximum horizontal stress in the fold-thrust belts.

Speaker Biography

Brent Couzens-Schultz joined Shell in 1997 after completing his PhD in geology at Texas A&M University.  He is a structural geologist who has worked on thrust belt mechanics and fault seal research.  More recently, he has worked on rock properties and pore pressure prediction for many of Shell’s exploration projects.  He is currently the focal point for the Shell’s global pore pressure prediction team.

Alvin Chan joined Shell in 2005 after receiving his PhD degree in Geophysics at Stanford University. His early research works at Shell were mainly on the integration of structural geology, rock physics and geomechanics in exploration settings with complex geology. Since 2010, Alvin has been working as a geomechanics specialist in New Orleans focusing on operations, basin-scaled modeling and wellbore stability studies for Shell’s Gulf of Mexico Deepwater Assets.