Seismic Attributes Analyses In Pre-Production Geothermal Reservoir Simulation

Abstract

Geothermal reservoir models are mainly used to predict how long a system can be economically exploited, what schedule of new wells are required to maintain production, how a system will respond to different rates of production, how to best re-inject fluid into a system and how the natural geothermal features will react to the production. It is necessary to perform modeling in production phase when history matching is possible, but it is useful in pre-production phase as it is more accurate than stored heat calculation. Geothermal reservoir simulation in the pre-production phase refers to predicting the behavior of reservoir and in spite of restricted amount of data it is helpful in field design and initial economic calculations. Croatian case study was preproduction simulation of the geothermal carbonate reservoir drilled by wells in several HC exploration campaign. In the case study main unknowns were connected to fracture distribution in reservoir and fault characteristics. Mitigating influence of the limitations were performed through additional analyses of vintage seismic data set. Seismic attribute analyses were used as indicator of fracture distribution and porosity development in the reservoir interval. Basic attribute analyses included: 1) Hilbert transformation of seismic trace extraction (reflection strength, instantaneous frequency and instantaneous phase) were of great help for reservoir geometry determination 2) Multiattribute calculation enable setting boundary of non-permeable zone were set in simulation model 3) In-house developed Z2Z algorithm appliance – unify different vintages of seismic data and enable easier interpretation and further analyses in combination with energy extraction 4) The Z2Z analyses lead to conclusion that the fractures approximately 30’ to x axis discontinuity grid is denser than on the profiles with angle closer to y axis . Basic visible spacing at x axis is set to 25 m, and 40 m on y axis. The proportional value reducing for both axes is possible having on mind that the level of seismic recognition is 25 m. The attribute that combine energy and Z2Z stack for lower frequencies highlight fracture spacing and their intersection on z axis. The intersection of the fractures in z direction is sparser then in x and y direction. Reservoir model was constructed in Petrasim, four rock types were determined, three of them as variations of dolomites that contribute to water flow: fractured dolomites, semipermeable dolomites, permeable faults, impermeable dolomites. For each rock type primary and secondary porosity parameters were determined. Due to restricted set of data in part of the reservoir well W-1 that part of reservoir couldn’t be determined firmly, so model was set for solution that the reservoir is fractured dolomite, semipermeable and impermeable dolomite. Through seismic analyses proportion of fracture density were determined for each direction. Well test that was run in 1996. was used as calibration of the model. After initialization of the model the testing of the several production / injection schemas and quantities were run in order to test temperature drop on production wells in the geothermal field.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018