| Abstract |
Wayang Windu Geothermal Field includes an active tectonic region and volcanic events which provide complex deformation zones in the reservoir; many fractures are formed and connected to result in a reservoir pathway for geothermal fluids. From the appearance, fractures have certain pattern directions which are likely influenced by the development of major fault and major stress orientations. However, only several areas are indicated as fluid pathways which have correlation with the fault deformation. Natural fractures are formed by deformation from tectonic movement or volcanic events and are distributed through main structures (faults, collapse structures, doming structures). Based on observation the fracture pattern at Wayang Windu is interpreted to mainly result from tectonic events which is seen from the uniform orientation and correlation with major stress orientation. The general structural trends have main orientation around 30-40 deg. but due to active tectonic movement and topography there are some perturbations following the local stresses. Three wells are considered to demonstrate the general fracture characterization in the northern and southern parts of the reservoir. For the fracture orientations, MBB-2 and MBE-4 wells show the orientations are dominated with NE-SW (30-60 deg) trends although there are some orientations to N-S or W-E. These show the possibility that trending major faults which contribute to the reservoir are parallel with the major stress. Different characterization is seen in the southern part, in the WWT-2 well the fractures are more dominated with N-S trends (350 to 30 deg) that may be influenced by major deformation from N-S trends although the local stress measured inside WWT-2 has E-W direction. Subsequently the fracture trends are compared with the permeable zones measured from spinner, temperature, and drilling data. The results show that while there are excessive fractures developed in the Wayang Windu reservoir only small number are included in the permeable zone. Several possibilities may contribute as barriers, starting from the intensive clay gouge in fault core that may close pathways, limited connectivity between fractures, or the stress which is already peturbated making the fractures tend to close with certain pressure depletion. In summary, the stress orientation changes is one of the important thing to see the potential open fractures trends which resulting more connectivity and conductivity for the geothermal fluids. This area will be a prospect target for upcoming drilling program. |