Transient pressure analysis of geothermal wells fractured during well testing

Fracturing during injectivity testing can take place in geothermal wells when the reservoir has low permeability or when the well has significant skin damage. The transient behavior (pressure falloff) of these wells cannot be matched using existing well test analysis methods. At the same time, modelling fracturing in geothermal re-servoirs using rock mechanics and commercial finite element software is complicated due to several field un-certainties (e.g. formation height, reservoir permeability and porosity). In addition, rock mechanics data (rock stress, strain and Young’s modulus) are normally unknown in geothermal fields. This makes it difficult to develop an appropriate fracture model that matches the field test data. This study attempts to develop a fracture model without integrating rock mechanics. The model is setup with a simple grid using the TOUGH2 geothermal reservoir simulator and validated using the advanced pressure derivative transient analysis. Multiple subsets of fracture geometries were developed to represent the different stages of fracture closure during pressure falloff. The PyTOUGH code was used to simplify the running of the different fracture stages. The results are very promising and provide a clear justification and explanation for the commonly en-countered fractured well behavior. This model should be of use in matching data from geothermal wells with similar pressure response

Reinjection in geothermal fields: A review of worldwide experience

The worldwide experience of reinjection in geothermal fields is reviewed. Information from 91 electric-power producing geothermal fields shows that: a reinjection plan should be developed as early as possible in field development and it should be flexible as it is likely to change with time. The optimum reinjection strategy depends on the type of geothermal system. For vapour-dominated systems which can run out of water reinjection should be infield. While for hot water and liquid-dominated two-phase systems (low-enthalpy and medium-enthalpy) reinjection is likely to involve a mix of infield and outfield injection. In general infield reinjection provides pressure support and thus reduces drawdown and the potential for subsidence, whereas outfield reinjection reduces the risk of cold water returning to the production area. Deep reinjection reduces the risk of groundwater contamination and ground surface inflation. The proportion of infield to outfield reinjection and the location (deep or shallow) is case specific and typically the infield reinjection rate will vary with time as part of the steam field management strategy.Reinjection Worldwide review Geothermal fields Power production Reinjection returns Recharge