Identifying Fractures using Fluid Inclusion Stratigraphy: Insights from Fluid Inclusion Thermometry

Abstract

Identification of open, permeable fractures in geothermal wells for fluid extraction and possible stimulation (EGS) is attempted using fluid inclusion stratigraphy (FIS) techniques. FIS analyzes fluid inclusion gaseous species in well cuttings to characterize fluid types at depth. We further investigate the application of this technique to locating open fractures by correlating unique fluid inclusion gas signatures to thermometric data of major fracture swarms.

Multiple fracture systems from three cored wells were sampled for calcite and quartz mineralization in addition to transparent mineral phases from the adjacent host formation. Their fluid inclusion trapping temperatures and salinities were analyzed using a Linkam PR600 thermometric stage. In each well, production zones identified by temperature and geophysical logs have a fracture density of >5 fractures/10m with apertures >20mm each. Production fracture fluid inclusion populations exhibit uniform salinities and trapping temperatures for both vein and wallrock minerals that correlate well with production fluids. FIS data, sampled at 5-10m intervals over production zones, typically exhibit anomalously high intensities, or peaks, for most gaseous inorganic species; N2, Ar, CO2 and He are commonly associated with large-aperture (>50mm) fractures and fracture swarms. Non-producing zones have similar fracture densities as productive fractures systems, but differ by having multiple fluid inclusion populations typically recording trapping temperatures 20-50