Geothermics, Vol. 14, No. 2/3, pp. 449-457, 1985. Printed in Great Britain. 0375- 6505/85 $3.00 + 0.00 Pergamon Press Ltd. © 1985 CNR. RESERVOIR ENGINEERING ASPECTS OF REINJECTION R. N. HORNE Department of Petroleum Engineering, Stanford University, California, U.S.A. (Transmitted by the Government of the United States of America) R.55 Abstract--This paper discusses aspects of reinjection that affect geothermal reservoir performance. It is based on field observations throughout the world. Three specific areas are examined. First, the problem of maintaining reliable and consistent injectivity by avoiding precipitation of dissolved solids. Injectivity loss has been prevented in some fields, but not in all. The second problem is that of determining where the reinjected water goes, in order to analyse the likely behaviour of the system before productivity difficulties begin. In this regard tracers are the most important reservoir engineering tool, and the collective worldwide experience in their use is summarized in tables. The third problem is that of loss of production performance due to invasion of reinjected water, as has occurred in several highly fractured fields. Early recognition of this difficulty is the major purpose of tracer testing; however, the complete interpretation of these tests remains one of the most significant outstanding problems in reservoir engineering. These three aspects of the reinjection problem are discussed in detail and the current status of reservoir engineering approach to these problems is summarized. Examples are given from operating geothermal fields in Japan, New Zealand, the Philippines and El Salvador. INTRODUCTION Reinjection of field and power station waste hot water is the most important problem facing the geothermal reservoir engineer. Reinjection is necessary in all but a few geothermal developments as a means of waste water disposal. Geothermal wells in liquid-dominated geothermal fields produce a mixture of steam and water under turbine inlet conditions, and the water must be separated and disposed of since only steam is useful in the turbine. Vapor- dominated reservoirs produce no water at the wellhead, and therefore have only power-station condensate to dispose of. Geothermal water can rarely be discharged into surface water conduits, since it is at high temperature and also contains dissolved materials (principally silica but frequently trace amounts of dangerous heavy metals, such as arsenic and mercury). The thermal and chemical pollution resulting from surface discharge would be unacceptable in most cases. Power-station condensate is practically free of chemical pollutants but is still above ambient temperature. At the present time reinjection is the most readily available alternative to surface disposal. In Japan, all of the liquid-dominated geothermal fields under production (Otake, Onuma, Onikobe, Hatchobaru and Kakkonda) all reinject almost 100°70 of their waste water, as will the new station at Nigorikawa. The vapor-dominated Matsukawa geothermal field produces only a small amount of chemical-free condensate which is safely discharged to a surface stream with no environmental impact. In other parts of the world reinjection has been less commonly used. In New Zealand, the 180 MW Wairakei geothermal power station exhausts 6500 tonnes/hour of waste of hot water into the Waikato River. An annual total of 156 tonnes of arsenic and 0.006 tonnes of mercury are carried into the river with this water (Axtmann, 1975). These figures should be viewed in context; the mean river flow is 457,000 tonnes/hour, and before development of the power station the natural discharge features in the Wairakei region discharged the same chemicals at 489