ABSTRACT: Characterizing Fractured Reservoirs in the Timor Sea Area Using High-Resolution Borehole Images and the Stress Tensor

King, Malcolm¹, Alison Mabillard¹, and David Castillo²
(1) Shell Development Australia, Perth, Western Australia, Australia 
(2) GeoMechancis International, Inc, Magill, Australia

Several Timor Sea area wells have encountered hydrocarbons where reservoir permeability appears to be dominated by natural fractures. Although observations of natural fractures were pervasive in the ultrasonic and microresistivity image logs, identification of the unique fracture population responsible for supporting fluid flow depends in part on the state of stress resolved on these features. Using detailed knowledge of the ambient stress field and natural fractures, we have been able to calculate the shear and normal stresses for each individual fracture and determine the shear failure potential for enhancing and maintaining fracture permeability along the critically-stressed fractures. Critically-stressed fractures are those where the ratio between the applied shear stress and the effective normal stress exceeds the coefficient of sliding friction characteristic of the rock mass. Determining the ambient stress state in these wells involved mapping from wellbore image logs drilling-induced wellbore failure (wellbore breakouts and tensile wall fractures). Pressure and drilling information was also used to calculate the stress tensor which was consistent with the observed wellbore failures.

The results indicate that the Timor Sea area is in a strike-slip stress regime (SHmax>Sv>Shmin) where the SHmax orientation (maximum horizontal stress) is NE-SW and sub-parallel to the convergence direction between Australia and Indonesia. Examples presented include results from the Kelp Deep-1 well drilled in the Zone of Cooperation Area A (ZOCA 95-18).

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