Extensive numerical modeling by SLB using its GeothermEx™ geothermal consulting services showed that reducing turbine inlet pressures would increase production and project longevity. Further analysis showed, however, that increased injection would reduce decline rates significantly and extend the life of one of the world’s largest known geothermal fields.

Located an hour’s drive north of San Francisco, The Geysers geothermal field has 18 power plants fed by more than 300 wells, which supply up to 20% of California’s renewable energy capacity. In its heyday, The Geysers delivered 2 gigawatts (GW) of clean geothermal power. Over the years, however, with multiple operators tapping this large, hydraulically connected field, productivity decline rates increased rapidly. A solution was needed to extend the productive life of the field.

Guided by a government-led technical advisory committee and input from field operators, the first approach was to lower operating pressures (to increase steam flow rates) and improve the steam-to-power conversion efficiencies (to generate more power from a given flow rate). A GeothermEx full-field numerical model simulation showed that these methods would increase steam production by 55 net megawatts (MW) for 10 years. However, the model also showed that field production would subsequently decline, reducing output from 1,250 to a net 475 MW.

To lessen decline rates, GeothermEx services were used to evaluate increasing injection beyond the typical 25% rate. Because dry steam resources have very little excess water that can be used for injection, operators at The Geysers were using fluid from creeks and captured rainfall. However, the modeling showed that much more water would be needed to significantly reduce the decline rate.

The solution was to stop discharging treated municipal wastewater into the creeks instead using it as a new supply of injection water (a method dubbed “flush to flash”). The results were positive: Decline rates were significantly reduced in many areas when injection water was properly targeted, maintaining the commercial potential of power generation at The Geysers, which continues today.