Correlating Reservoir Pressure Changes with Production and Injection in the Tiwi Geothermal Field, Republic of the Philippines

Abstract

In November 2005, capillary tubing was installed in well Matalibong- 25 (Mat-25) in the western Matalibong production sector of the Tiwi Geothermal Field, Philippines, to begin monitoring pressure changes in the deep liquid reservoir on a continuous (5 min interval) basis. This was followed by a similar installation in Mat-29 and later Mat-30 to extend coverage of the pressure monitoring to the more northern area of the Matalibong production sector. In addition, transducers were installed to monitor wellhead pressures at both Mat-25 and Mat-30 in July 2008 on the basis that the changes in wellhead pressure reflected the pressure changes in the overlying steam production zone. With the precision provided by the continuous pressure monitoring, it has been possible to clearly show that there is stronger interaction than expected between the Matalibong production area and the injection wells (Mat-21 and Mat-33) located to the north, both from the noted responses to short term production/injection changes and the overall correlation between the pressure changes and net “voidage”. This would not have been possible using the previous method of analyzing periodic downhole surveys. It has also shown that it is important to maintain an open mind in reviewing data as it was not expected that there would be such a strong interaction, considering the distances involved, even though there was other evidence (changes in discharge characteristics, tracer tests, micro-gravity, etc) that indicated some interaction was occurring. The ability to monitor pressures on a continuous basis is now an important aspect of resource management in the Matalibong Sector of the Tiwi field as increasing deep pressures are causing the liquid-steam interface to rise and negatively affect the productivity of steam wells. The data gathered has improved our knowledge of this process and will help in designing a long term reservoir management strategy to mitigate this issue.