Factors Controlling Reservoir Permeability at the Leyte Geothermal Production Field, Philippines

J.A. Caranto and M.P. Jara

PNOC-Energy Development Corporation, Merritt Road,
Fort Bonifacio, Makati City 1201 Philippines

Abstract

Permeability in the Leyte geothermal reservoir is highly influenced by the left-lateral Philippine Fault System that transects the entire geothermal field. Deep wells were designed primarily to target near vertical fault structures of known or perceived permeability, and well designs were based on calculated intercepts of these faults. Studies 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 traces in the area, these faults also define the boundaries of the exploitable resource. The West and Central Fault Lines delimit the permeability west of the resource, while the extent of the permeability to the east is yet to be defined. This is supported by studies on the anatomy of volcanoes in strike-slip fault systems that suggest higher permeability east of the identified sigmoid bounded by the West and Central Fault Lines.

Previous studies of the Philippine Fault reveal an average movement rate of about 1.9-2.5 cm/yr based on kinematic models by Barrier et al., (1991) and 2.0 cm/yr from fault displacement measurements (Aurelio, 1992). Microseismic monitoring also indicates an average of 2 tectonic events per day, and there are even more events along the Philippine Trench located on the east. Records of tectonic earthquakes since the initial production in 1983 and the expansion in 1997 indicate that there are no major tectonic activities in the area, but there is continuing movement along the Philippine Trench and the Philippine Fault. The changing in-situ permeability characteristics 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 reservoir due to continuous steam production. Based on old and recently drilled wells, there appears to be an enhancement of permeability in some faults, and this may initially be attributed primarily to tectonic processes and secondary to change in reservoir phases. The different stress regime in the field is likewise re-evaluated to determine how the regional stress affects the local fault movement.