Geothermal processes – a numerical
approach for reservoir evaluation

General approach to reservoir evaluation

• Data collection
• Building a conceptual model
• Natural state numerical modeling
• Production modelling

Bodvarsson and Witherspoon, 1989

Components of a conceptual model

- Reservoir geometry
- Distribution of hydraulic and thermal properties
- Directions of fluid movement
- Location of inflow/outflow zones, and hydraulic boundaries
- Location of heat inputs and outputs
- Temperatures
- Two-phase zones

DATA SOURCES

• Geological data
      – Data from surface outcrops (structure, lithology)
      – Borehole data (stratigraphy, hydrothermal alterations)
• Geochemical data
      – Chemical composition
      – Geothermometers
• Heat and mass flow measurements
      – Temperature and discharge of springs, geysers, and fumaroles
      – Temperature gradient measurements in shallow boreholes
• Geophysical data
      – Surface geophysical data: Resistivity, gravity, seismic, self-potential
      – Down-hole data: well logs, T and P profiles, flow rates, flowing enthalpies

The purpose of numerical modelling

• To understand the physical functioning of a system
• To estimate generating capacity
• To design/optimise geothermal utilization system
• To predict the behavior of the system (temperature, pressure) during extraction
• To estimate chemical changes during production
• To predict environmental impacts
• To help to evaluate different conceptual models of the field
• Etc.

Geothermal processes in numerical models: groundwater flow equation

Diffusion equation:

Geothermal processes in numerical models: heat transport equation

Darcy equation:

Fully coupled due to T dependence of density and viscosity

Numerical models to solve heat and flow transport equations

Data needed :

– discretization in 3 dimensions (∂x, ∂y, ∂z)
– hydraulic and thermal properties
– initial conditions
– boundary conditions

Discretisation methods

Boundary conditions

• Model base: upflow rates + enthalpies, heat flow
• Top boundary: constant temperature and atmospheric pressure, recharge
• Lateral boundaries:

– No-flow
– Background temperature + pressure
– In/outflow rates + enthalpies

Calibration

• Flow rates and enthalpies of wells
• Temperature and pressure profiles
• Production enthalpies and pressure changes
• Pressure (head) time series