| Abstract |
Eight deep wells have been drilled into intra-calderic lavas and pyroclastics to the southwest of Miravalles volcano; Guanacaste, Costa Rica. These intersect a liquid dominate^ geothermal aquifer below 800m with a maximum temperature of 257 C. The aquifer is apparently structurally not stratigraphically bounded and the permeability is dominantly secondary, due to fracturing of the host rocks. The field is due to go into production in the early 1990's. Under static conditions (i.e. when the wells are closed) no significant microgravity differences (33(iGal at the 68% confidence level) were observed between surveys in March 1987 and January 1988. Five microgravity surveys from January to March 1988 also revealed no significant changes. During a five day well testing period in late March 1988, when 126,000 tonnes of water were produced from 4 wells, gravity increases were observed following well closure. These were studied in detail using 4 LaCoste and Romberg gravity meters (G.105, G.513, G.840 and D.61). Measurements revealed a sharp increase of 30-40 |J.Gal within 10 minutes of well closure and a further rise of 30-60 (iGal over the next week, levelling off to pre-testing values. These changes have been modelled in terms of pressure-related expansions of the steam zone base within the aquifer and of depressions in the shallower water table associated with production. Neither model is entirely satisfactory because rapid movement of very large masses of water (10"-10' tonnes to cause the gravity changes observed) is probably unrealistic. These experiences however, will allow a more useful experiment to be designed for the next and longer 5 month well testing period beginning in late 1988. |