| Abstract |
The stable isotope data during the commercial operations of Tongonan-I production field fi-om1990 to 1995 are compared with their pre-exploitation levels and with the trends of well chemistry and discharge enthalpy. The changes on the stable isotope composition of the fluids from production wells and reinjection lines provided evidences on two reservoir processes, namely, mixing of reinjected waters with the reservoir fluids, and steam addition induced by localized drawdown of the deep liquid reservoir. The entry of reinjected fluids in the production sector of TIPF evinced on the enrichment of heavy isotopes and CI, in the liquid-dominated wells of Mahiao and Sambaloran sectors. This process is also marked by the decline in T(Si0) of the waters, C02td in vapour and well discharge enthalpy. The wells lying within 1.50 km radius from the TIPF reinjection sink, like wells 101, 213 and 214, are highly affected by the reinjected fluid breakthrough. However, because of only about 1.00 to 1.50% 8'0 difference between the reservoir fluids and the reinjected waters, initial influx of reinjected waters into the production wells did not clearly evince on the changes of the stable isotopes. Steam addition is evident on the production wells proximate to the Upper Mahiao sector, such as wells 102, 106, 108 and 110. This process is manifested on the incoherent trends of d80 and 8H. The T(Si0) of the fluids remain unchanged while the discharge enthalpy of the wells exceeds 1800 kJ/kg. Based on the patterns of 8'0 and @H stable isotopes will be useful as natural tracers of reinjection water breakthrough into the liquid-dominated reservoir of Malitbog and South Sambaloran sectors, as well as on the entry of cooler fluids from the north into the Upper Mahiao sector. However, stable isotopes may not be effective tracers of reinjected fluids in the wells of Upper Mahiao sector because of their high steam contents and the nearness of their isotopic composition with the reinjected waters. |