ABSTRACT: A Geomechanical Model of the Arcabuz-Culebra Field Reveals Stress Magnitude and Orientation Changes Due to Production

In many reservoirs, pre-production stress magnitudes are often controlled by the strength of pre-existing faults. During production, horizontal stress magnitudes may change due to poroelastic effects. This can cause a change in stress state which can then lead to stress reorientations.

These effects were observed during a geomechanical study of Arcabuz-Culebra, an overpressured gas field located in the Burgos basin of northeast Mexico. We determined stress orientations based largely using analyses of wellbore breakouts detected by four-arm caliper (dipmeter) logs. Leak-off tests (LOT's) supplemented by minifrac and microfrac tests were used to determine the least principal stress magnitude. The results reveal production-related differences between older and younger areas of the field. In regions of the field less affected by production, the maximum horizontal stress is consistently aligned NNE-SSW, parallel to the regional stress field, and a strike-slip stress state is evident. Where production has been under way for some time, however, stress orientations follow the orientations of seismically imaged faults. And, the principal stress magnitudes are lower within depleted sands. We propose that these differences are due to poroelastic effects which cause a decrease in horizontal stress magnitudes with depletion, and that in this field the stress changes induced a reorientation of the stress field due to reactivation of old faults.

These results are significant because hydraulically induced fractures required to enhance productivity in this tight gas reservoir generally follow the orientation of the greatest horizontal stress.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia