| Abstract |
Permeability in the Leyte geothermal production field reseruoir rs highly influenced by the left-lateral movement of the Philippine Fault Sysfem that transects the entire geothermal field. Deep wells are designed primarily to target near vertical fault structureso f proven or inferredp ermeability,a nd well desrgns are based on calculated intercepts of these faults. Sfudies on microtectonics of specific areas indicate that permeable structures coincide with areas under extensional regime while faults with limited permeability are located in areas under compression. Since the Philippine fault branches into three major fraces in the area, fhese faults a/so define the boundan'eso f the exploitabler esource. The West and Central Fault Lines delimit the permeability wesf of the resource, while the extent of the permeability to the easf is yet to be defined. Majority of the compressrbnafla ultst rendingN t4l-SE appear near the trace and NE of the Central fault line, while faults under extensional regime are sub-parallel to the Philippine Fault. Records of tectonic earthquakes srnce the initial production in 1983 and the expansion in 1997 indicate that there are minor tectonic activities in the area, indicating that there is continuing movement along the Philippine Fault. The changing in-situ permeability characferisfics of the faults transected by the wells in Tongonan may be attributed to this movement aside from the different physical and chemical processes affecting the reseruoir due to continuous steam production. Based on old and recently drilled wells, there appears to be an enhancement of permeability in some faults, and fhis may initially be attributed primarily to tectonic processes and secondary to changes in reseruoir fluid phase due to drawdown. The new model suggests that the Central Fault Line is the boundary of the permeable areas on fhe east and the impermeable area on the wesf. Future expansion drilling for both production and injection wells is proposed to target structures east of the Central Fault Line. |