| Abstract |
The use of geochemical tools based on gas chemistry has become increasingly important in monitoring the reservoir processes in Tongonan due to the expansion of the two-phase zone resulting from the increase in the rate of pressure drawdown in 7997 when more power plants were installed. The methods involving gas-gas and gas-mineral equilibria (SNUC and FT-HSH methods) have been applied to evaluate the progress and extent of the pressure drawdown, The results of the SNUC method indicate increasing steam fraction and decreasing partial pressure of dissolved COP across the field. These are consistent with increasing temperature of phase separation from about 260░C to 290░C in the central part of the field between 7997 and 7998, which implies - lower depths of boiling and/or vertical expansion of the steam zone. The lateral extent of this process is established by increases in calculated steam fractions, increasing discharge enthalpies and decreasing brine flows from the production wells. The FT-HSH diagrams indicate vapour gain by as much as about 5%. Conventional water and gas chemistry monitoring tools such as trends of Clres, C02td and quartz geothermometer have been meaningful in evaluating the secondary processes. Injection return persists in Upper Mahiao sector in the north and in Malitbog sector in the south, as indicated by continuously increasing Clres and decreasing COztd in the affected wells. Aside from this process, the pressure drawdown has also induced migration of cooler peripheral waters from the northeastern boundary of Upper Mahiao sector. ln Malitbog sector, which largely represents the outflow zone, the fluid movements induced by pressure drawdown has provided local liquid recharge. Slight increases in CIres and decreases in C02td suggest recharge of boiled fluids from the vicinity of some wells. |