| Title | Characteristics of the Matalibong Steam Zone, Tiwi Geothermal Field, Philippines |
|---|---|
| Authors | Menzies, Anthony; Villasenor, Larry; Sunio, Eugene; Lim, Wilhelmino |
| Year | 2009 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Tiwi; Matalibong; Production; Superheated Steam |
| Abstract | The Tiwi geothermal resource was initially liquid-dominated but pressure drawdown in the early years of production resulted in the formation or expansion of a shallow steam zone in Naglagbong and later in the Matalibong, Kapipihan and Bariis sectors to the west. The Naglagbong steam zone was quenched by the influx of groundwater in the 1980’s but the steam zones to the west have persisted and continue to provide a significant percentage of the overall steam production at Tiwi. Production from the steam zones was initially thought to be saturated steam but in the mid-1990’s corrosion in the surface facilities was found to be associated with volatile chloride production, suggesting that superheated steam was being produced. Further investigation indicated that 15 of the Matalibong Steam Zone wells were producing steam with up to 60°F of superheat, and this had probably started to occur in the late 1980’s. Since the mid-1990’s, the conditions in some of the Matalibong Steam Zone wells have changed due to a rising steam-water interface which has effectively reduced the volume of the steam zone by flooding some of the deeper production zones. This in turn has resulted in a reduction in enthalpy and steam production from four wells while two others have stopped producing due to enthalpy decline. However, 10 wells with shallow permeable zones continue to produce either saturated or superheated steam, with Mat-22 and -23 still producing >200kph steam (>11MW) each after 15 to 20 years of continuous superheated steam production. The relatively low production declines seen in both the saturated and superheated steam wells in the Matalibong Steam Zone and the low pressure decline indicates that recharge must be occurring to the steam zone. This is considered unusual as superheated steam production is usually a sign of a declining resource where fluid reserves are being depleted. The recharge fluid appears to be a combination of boiled reservoir water, which originates close to the Bariis upflow, and influx of groundwater from the sides and possibly top of the reservoir, as indicated by the presence of Tritium in the produced steam. As this water and steam move through the hot rock above the steam-water interface, it boils and is superheated before being produced. This appears to be the most reasonable explanation for the sustainability of the superheated steam zone considering that the liquid level is now <400ft below the production zones in the remaining superheated steam production wells. The focus of reservoir management is now to stabilize the steam water interface so that the existing wells will continue to produce steam and possibly to induce the steam-water interface to move deeper, which could allow wells with deeper production zones to again produce saturated or superheated steam. |