Injectivity Restoration of a Hot-Brine Geothermal Injection Well
Authors: Messer P.H. , Pye D.S. , Gallus J.P. 
Published: December 2007

Production testing of wells in an Imperial Valley geothermal reservoir Production testing of wells in an Imperial Valley geothermal reservoir required reinjection of heat-depleted brine water. Reinjection of this water supersaturated with silica resulted in severe injectivity impairment. An acid stimulation technique successfully restored the original injectivity.

Introduction

Electric power generation from geothermal liquid-dominated reservoirs involves tremendous water disposal requirements. For example, generating enough electrical power for a city with a population of 400,000 from such a power for a city with a population of 400,000 from such a geothermal system would involve a water injection rate greater than the total injection rate for all current California waterflood projects. Reinjection of geothermal fluids from hot-brine sources in the Imperial Valley, CA, will be necessary because long-term surface disposal probably will not be permitted, land subsidence tendencies probably will not be permitted, land subsidence tendencies will be minimized, and the reservoir will be recharged. An individual injection well will be required to dispose of heat-depleted brine at rates up to 100,000 B/D. Maintaining injectivity for the brine disposal requirements associated with geothermal power generation will be essential for long-term, economic operation.

The problem of injectivity loss associated with concentrated brine reinjection into hot formations has not been addressed often in the literature. Cuellar and Einarsson et al. discussed waste-water reinjection tests conducted for almost 1 year at the Ahuachapan geothermal plant in the Republic of El Salvador. By maintaining the injected brine above the silica saturation conditions, no wellbore scaling or injectivity loss was observed. Similarly, Chasteen reviewed the condensate disposal history at the Big Geysers dry-steam field in northern California and brine injection from tests conducted in New Mexico and in the Imperial Valley. Based on limited injection volumes, no injectivity impairment was observed. However, Kubota et al. noted an injectivity decrease and silica-scale buildup in injection wells at the Otake Geothermal Field in Japan while injecting brine under supersaturated silica conditions.

Most available literature discusses reinjection in naturally fractured formations; little information is available concerning the injection of concentrated brines in porous formations similar to the sedimentary deposits in the Imperial Valley. Testing in the North Brawley Field in the Imperial Valley revealed that severe injectivity damage results if these hypersaline brines are not handled properly before injection in the sedimentary formations. properly before injection in the sedimentary formations. Even with the best brine handling conditions, long-term injectivity loss still may occur. An acid stimulation procedure was developed to restore the injectivity loss procedure was developed to restore the injectivity loss resulting from heat-depleted brine injection for one North Brawley Field well.

Regional Geology and Field History

The Imperial Valley, also known as the Salton Trough, is a graben structure filled with Miocene to recent sediments. The structure is located along the junction of a large continental crustal plate to the east and an oceanic plate to the west and south. This junction has exposed the plate to the west and south. This junction has exposed the sedimentary basin to high heat flow from the hot mantle, which has led to the development of several thermal anomalies. The North Brawley Field is one heat anomaly located about 3 miles north of Brawley, CA. Six wells have been completed in the last 2 years by Union Oil Co. of California.

JPT

P. 1225