Use of Radioisotopes as Tracers in Geothermal Wells

Abstract

Long term development and successful utilization of geothermal fields for electric power generation is primarily dependent on the response of the reservoir to extensive exploitation. For geothermal fields which are in the early phases of development, only a limited discharge history is available. The site under investigation, the Tongonan geothermal field which is the most promising of four (4) sites in the Philippines under advanced stage of development, can be classified as such . The drilling program was initiated in 1976 and more than 30 drillings have since resulted in producing wells with an average well output of 10.5 MW. The minimum anticipated available capacity is 720 MW and the maximum, 1,000 MW. To determine large scale response of this reservoir to a sustained withdrawal of geothermal fluids over 25 years, the nominal plant life of the generating equipment, a field drawdown test has been planned. This involved discharging a number of wells simultaneously for a period of one to two years. Radioisotope techniques were incorporated into the drawdown tests. Two specific applications were conducted: 1. Assessing if horizontal permeability connections exist between two (2) well bores. This is an important factor in determining the minimum practical well spacing that can be achieved in field development without any interwell pressure reactions occurring which could lead to rapidly declining mass outputs; 2. Tracing the subsurface movements of reinjected spent fluid. The fate of this fluid is of some concern since "cooling" could result should substantial quantities return to the producing wells. Reinjection if found feasible can however provide an economical solution to the environmental problem posed by toxic metals present in geothermal fluid.