Fracture permeability in the Matalibong-25 corehole, Tiwi geothermal field, Philippines Page: 4 of 11
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PROCEEDINGS, Twenty-First Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 22-24, 1996
SGP-TR-151
FRACTURE PERMEABILITY IN THE MATALIBONG-25 COREHOLE,
TIWI GEOTHERMAL FIELD, PHILIPPINES
Dennis L. Nielsonl,Wilson C. Clemente2, Joseph N. Moorel and Thomas S. Powell3
1Earth Sciences and Resources Institute
Department of Civil and Environmental Engineering
University of Utah
Salt Lake City, Utah
2Philippine Geothermal, Inc.
Metro Manila, Philippines
3Unocal Energy Resources Division
Santa Rosa, CaliforniaABSTRACT
The Tiwi geothermal field is located in southern
Luzon on the northeast flank of Mt. Malinao, an
andesitic volcano that was active 0.5 to 0.06 Ma.
Matalibong-25 (Mat-25) was drilled through the Tiwi
reservoir to investigate lithologic and fracture con-
trols on reservoir permeability and to monitor reser-
voir pressure. Continuous core was collected from
2586.5 to 8000 feet (789 to 2439 meters) with greater
than 95% recovery. The reservoir rocks observed in
Mat-25 consist mainly of andesitic and basaltic lavas
and volcaniclastic rocks above 6600 feet depth (2012
meters) and andesitic sediments below, with a transi-
tion from subaerial to subaqueous (marine) deposition
at 5250 feet (1601 meters). The rocks in the reservoir
interval are strongly altered and veined. Common
secondary minerals include chlorite, illite, quartz, cal-
cite, pyrite, epidote, anhydrite, adularia and wairakite.
An 39Ar/4UAr age obtained on adularia from a quartz-
adularia-cemented breccia at a depth of 6066 feet
(2012 meters) indicates that the hydrothermal system
has been active for at least 320,000 years. Fractures
observed in the core were classified as either veins
(sealed) or open fractures, with the latter assumed to
represent fluid entries in the geothermal system.
Since the core was not oriented, only fracture fre-
quency and dip angle with respect to the core axis
could be determined. The veins and open fractures
are predominantly steeply dipping and have a mea-
sured density of up to 0.79 per foot in the verticalwell. Below 6500 feet (1982 meters) there is a
decrease in fracture intensity and in fluid inclusion
temperatures.
INTRODUCTION
The Tiwi field is located on the northeast flank of Mt.
Malinao, an eroded and extinct Quaternary stratovol-
cano located along the Bicol arc of southern Luzoin,
Philippines (see inset of Figure 1). Discovered ini-
tially in the 1960's by the Philippine Commission on
Volcanology, and later developed by Philippine
Geothermal, Inc. as a contractor to the Philippine
National Power Corporation, the field now has an
installed capacity of 330 megawatts, and averages a
78% capacity factor. Case histories of the field and
data on field performance are presented in Alcaraz, et.
al. (1989), and Gambill and Beraquit (1993).
Well Matalibong-25 (Mat-25) was completed on
March 15, 1992, as a vertical slim-hole through the
reservoir in the western part of the Tiwi field (Figure
1). The interval above the reservoir was drilled by
conventional rotary methods and completed with 7
inch (17.8 cm) casing cemented from the surface to
2563 feet (781 meters). The well was then continu-
ously cored, with CHD-134 core collected from
2586.5 to 5000 feet (789 to 1524 meters) followed by
CHD-101 core to the targeted depth of 8000 feet
(2439 meters). Core recovery was greater than 95%209
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Nielson, Dennis L.; Clemente, Wilson C.; Moore, Joseph N. & Powell, Thomas S. Fracture permeability in the Matalibong-25 corehole, Tiwi geothermal field, Philippines, article, January 24, 1996; United States. (https://digital.library.unt.edu/ark:/67531/metadc884723/m1/4/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.