Factors Controlling Reservoir Permeability in the Leyte Geothermal Field, Philippines

Abstract

Permeability in the Leyte geothermal production field reservoir is highly influenced by the left-lateral movement of the Philippine Fault System that transects the entire geothermal field. Deep wells are designed primarily to target near vertical fault structures of proven or inferred permeability, and well prognoses are based on the 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 the Tongonan field may be attributed to this movement, aside from the different physical and chemical processes affecting the reservoir permeability due to the effects of 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 movements.