massive cold-water injection program. This oper- ation takes advantage of the extreme tempera- ture differences between the injectate and the formation—more than 149°C [268°F]—and the formation’s relatively high coefficients of thermal contraction to create fractures. Three injection stimulations were conducted
on one low-permeability well in the Cianten Caldera that lies within the boundaries of the Salak concession. These stimulations included injection of about 9.8 million bbl [1.6 million m3] of water. To evaluate the impact of these treat- ments on injection performance, the operator used a modified Hall plot and analysis that indi- cated fracture development within the formation (below right). Injectivity improvements were also quantified through periodic pressure-falloff tests and the creation of a geomechanical reservoir simulation model calibrated against field his- tory.15
The final analysis concluded that injectiv-
ity had been increased significantly. Two additional wells drilled in the area will undergo the same type of stimulation to allow injection of water produced from the high-temperature core of the reservoir.
The Great Heat Exchange Hot dry rock—HDR—reservoirs represent par- ticularly high-potential geothermal systems. The total amount of heat that may be unlocked from these reservoirs worldwide through injection or fracturing has been estimated at 10 billion quads—about 800 times more than that esti- mated for all hydrothermal sources and 300 times that available from hydrocarbon reserves.16
14. Kovac KM, Lutz SJ, Drakos PS, Byersdorfer J and Robertson-Tait A: “Borehole Image Analysis and Geological Interpretation of Selected Features in Well DP 27-15 at Desert Peak Nevada: Pre-Stimulation Evaluation of an Enhanced Geothermal System,” Proceedings of the Stanford University 34th Workshop on Geothermal Reservoir Engineering, Stanford, California (February 9–11, 2009).
15. Yoshioka K, Pasikki R, Suryata I and Riedel K: “Hydraulic Stimulation Techniques Applied to Injection Wells at the Salak Geothermal Field, Indonesia,” paper SPE 121184, presented at the SPE Western Regional Meeting, San Jose, California, USA, March 24–26, 2009.
16. Duchane D and Brown D: “Hot Dry Rock (HDR) Geothermal Energy Research and Development at Fenton Hill, New Mexico,” GHC Bulletin (December 2002),
http://geoheat.oit.edu/bulletin/bull23-4/art4.pdf (accessed August 11, 2009).
“Quad” is a short term for quadrillion and is a unit of energy equal to 1015 BTU [1.055 × 1018 J]. It is the equivalent of about 180 million bbl of oil [28.6 million m3]. For reference, the total 2001 US energy consumption was about 90 quads. The total HDR resource numbers published by Duchane and Brown were calculated by summing the thermal energy content of rock beneath the Earth’s land masses at temperatures above 25°C [77°F] from the surface to 10,000 m [33,000 ft]. While these numbers seem astronomical and do include resources that are impractical to recover because they are low tempera ture or are unreachable, they still represent an enormous amount of energy.
A S I A
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I N D O N E S I A Darajat
I N D O N E S I A > Salak field, Indonesia.
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> Evaluating injection performance. A modified Hall plot provides a qualitative indicator of injection performance. The Hall integral (orange) is a straight line if the well skin factor does not change over time. A steeper slope indicates some type of flow resistance, such as plugging or scaling, while a shallower slope indicates formation stimulation. In subtle cases, such as this one in Salak field, plotting the Hall derivative (blue) on the same scale improves the diagnosis. A derivative curve above the integral curve indicates increased resistance and below the integral curve—as shown here—ongoing stimulation. This analysis confirmed fracture development during cold-water injection in the field. (Adapted from Yoshioka et al, reference 15.)
AUT09–RVF–08
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