GRRM/How To/Execution/Reservoir

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Applying the Methodology to Geothermal Projects




Execution Indices for Estimated Reservoir Characteristics
Flow Tests | Lithologic Cores | Temperature logs | VSP| Passive Seismic | Acoustic Reflectivity | Resistivity | FMI | Packer Test

The following tables provide the details to select the appropriate execution index values for the GRRM reservoir attribute. For each technique used to estimate reservoir characteristics, select the 1 - 5 index value of the category that most closely reflects the understanding of how the analytic method and technique were followed that collected the project's data.

Note: if more than 1 technique is used, there will be as many execution index values as techniques, at minimum. If multiple sources of data are used (such as literature searches and independent sampling), there will be an execution index value for each source.

Flow meter tests Lithologic cores Temperature logs
5
  • Impeller flowmeters used for velocities of > 5 ft/min, and heat-pulse or electromagnetic flowmeters used for velocities of less than 0.1 ft/min.
  • Tests run at three or more pumping rates and at zero.
  • Results correlated with temperature and pressure logs at all wells.
  • Geologist on-site during entire coring.
  • Follows consistent labeling, record-keeping, and description methods.
  • Hydrothermal alteration minerals examined in thin section.
  • Frequency of faulting and fracture orientation measured.
  • Stratigraphic sequences followed in plane view across multiple cores.
  • Log is differentiated by depth (i.e. possible to monitor geothermal gradient).
  • Known lithologic descriptions available to verify variability from corresponding cores.
4
  • Impeller flowmeters used for velocities of > 5 ft/min, and heat-pulse or electromagnetic flowmeters used for velocities of less than 0.1 ft/min.
  • Tests run at two pumping rates and at zero.
  • Results correlated with temperature and pressure logs at some, but not all, test wells.
  • Geologist on-site during entire coring.
  • Follows consistent labeling, record-keeping, and description methods.
  • Hydrothermal alteration minerals examined in thin section.
  • Frequency of faulting and fracture orientation measured.
  • Stratigraphic sequences followed in plane view across multiple cores.
  • Log is differentiated by depth (i.e. possible to monitor geothermal gradient).
  • Known lithologic descriptions available, but created from drill cuttings, not cores.
3
  • Impeller flowmeters used for velocities less than 5 ft/min.
  • Test run at one pumping rate and at zero.
  • Results not correlated with temperature and pressure logs.
  • Core received from previous studies
  • Evidence of inconsistent labeling, record-keeping, and description methods
  • Spot examinations of hydrothermal alteration minerals in thin section
  • Frequency of faulting and fracture direction noted
  • No cohesive map correlating stratigraphic sequences from regional cores
  • Log is not differentiated by depth.
  • Lithologic descriptions are not available.
2
  • Results taken from previous third-party studies of the area (either literature or contractors) with little or limited information on survey methods, replication, or error.
1
  • Assumed from studies of analogous geothermal settings, or extrapolated from studies of nearby areas.
Vertical Seismic Profiling (VSP) and reflection seismic Passive seismic Acoustic Reflectivity
5
  • Results correspond (or can be tied to ) fracture systems and stratigraphy represented in well logs.
  • Interpreted sections identify major seismic reflectors and faults on top of the actual data.
  • Cross sections are provided both with and without interpretation.
  • 3-D reflection results
  • Vertical Seismic Profiling (VSP) available along with multicomponent surface data
  • Appropriate inversions and corrections to explain amplitude and frequency variation
  • Pumping tests monitoring seismicity with pressure changes have been completed.
  • Well-documented historical records of natural seismic activity covering the entire geothermal field OR significant time series and resolution to identify areas of active faults and fissures.
  • Interpretations of permeability and active faults completed by an analyst with >5 years experience.
  • Acoustic caliper, transit times, and amplitude of televiewer logs are collected at high quality to identify fine fracture permeability.
  • Location, strike and dip of fractures, and lithologic contacts can be identified in all logs.
  • Entire signal is digitized as waveform.
  • Cycle skipping can be constrained to identify fractures (i.e. not due to improper signal, detection level, or gas in the fluid).
  • Spacing (1-ft receiver) allows identification of lithologic contacts as sharp deflections.
4
  • Results mostly correspond with fracture systems and well log stratigraphy.
  • Interpreted sections are provided both with and without interpretation.
  • 3-D reflection results
  • Vertical Seismic Profiling (VSP) available along with multicomponent surface data
  • Most appropriate corrections to explain amplitude and frequency variation (i.e. frequency dependent (AVO-AVA) as a function of azimuth)
  • Pumping tests have been completed, and seismicity partially monitored (selected areas, or only after evidence of earthquakes).
  • Documented historical records of natural seismic activity covering the majority of geothermal field OR time series and resolution to identify areas of active faults and fissures.
  • Interpretations of permeability and active faults completed by an analyst with >2 years experience.
  • Acoustic caliper, transit times, and amplitude of televiewer logs are collected at sufficiently high quality to identify fracture-driven permeability.
  • Location, strike and dip of fractures, and lithologic contacts can be identified in vast majority of logs.
  • Entire signal is digitized as waveform.
  • Cycle skipping can mostly be constrained to identify fractures (i.e. minor issues due to improper signal, detection level, or gas).
  • Spacing (1-ft receiver) allows identification of lithologic contacts as sharp deflections
3
  • Results can only be tentatively related to fracture systems and well log stratigraphy.
  • Cross sections are provided with interpretation only.
  • 2-D reflection results
  • Vertical Seismic Profiling (VSP) not available with multicomponent data
  • No (or limited ) corrections to explain amplitude and frequency variation
  • Pumping tests have been completed, and seismicity partially monitored (selected areas, or only after evidence of earthquakes).
  • Historical records of natural seismic activity covering the selected areas of the geothermal field OR seismicity assumed from analog areas.
  • Interpretations of permeability and active faults completed by an analyst with <2 years experience.
  • Acoustic caliper, transit times, and amplitude of televiewer logs are collected at moderate quality, enough to identify fracture-driven permeability.
  • Location, strike and dip of fractures, and lithologic contacts can be identified in most logs.
  • Entire signal is digitized as waveform.
  • Cycle skipping prevents fracture identification (i.e. identifiable issues due to improper signal, detection level, or gas in the fluid).
  • Spacing does not allow identification of lithologic contacts as sharp deflections.
2
  • Results taken from previous third-party studies of the area (either literature or contractors) with little or limited information on survey methods, replication, or error.
1
  • Assumed from studies of analogous geothermal settings, or extrapolated from studies of nearby areas.
Resistivity Formation Microimaging (FMI) Packer Test
5
  • Appropriate quantitative corrections (fluid composition and well size) are made for well effects to obtain true formation resistivity.
  • Data is plotted in parallel with other geophysical wireline logs to define contact depths.
  • Thorough geologic descriptions are available to compare and verify resistivity changes.
  • Ability to calculate porosity values across formation unit (Archie’s equation)
  • Combination of single-point resistance and electromagnetic-induction readings
  • Combined with other geophysical wireline measurements (e.g. azimuthal resistivity imager, or induction imager)
  • Available for significant thickness/depth of reservoir (i.e. able to identify heterogeneity)
  • Evaluated by an experienced analyst for fractures, faults, and lithology
  • Corresponds to other lithology cores and subsurface geology
  • Completed in-situ, and replicated in lab with core section.
  • Replicated in multiple wells.
4
  • Appropriate quantitative corrections (fluid composition and well size) are made for well effects to obtain true formation resistivity.
  • Data is plotted in parallel with other geophysical wireline logs to define contact depths.
  • Some geologic descriptions are available, allowing verification of all major resistivity changes.
  • Ability to calculate porosity values in most of formation unit (Archie’s equation)
  • Electromagnetic-induction readings for all boreholes; some single-point resistance logs
  • Combined with minimal geophysical wireline measurements (e.g. temperature, pressure)
  • Available for significant proportion of reservoir
  • Evaluated by an experienced analyst for fractures, faults, and lithology
  • Minimally corresponds to other lithology cores and subsurface geology (e.g. cannot fully trace faults).
  • Completed in-situ, and not replicated in lab with core section.
3
  • Quantitative corrections (fluid composition and well size) are not made for well effects- apparent formation resistivity.
  • Other geophysical wireline logs do not help to define contact depths.
  • Minimal or conflicting geologic descriptions are available, rendering some resistivity changes inconclusive.
  • Not able to calculate porosity values across formation
  • Either single-point resistance and electromagnetic-induction readings
  • Combined with no other geophysical wireline measurements (e.g. temperature, pressure)
  • Available for small, limited piece of reservoir.
  • Evaluated by an analyst for fractures, faults, and lithology
  • Does not correspond to other lithology cores and subsurface geology (e.g. cannot fully trace faults).
  • Completed in lab with core section heated to reservoir temperature.
2
  • Results taken from previous third-party studies of the area (either literature or contractors) with little or limited information on survey methods, replication, or error.
1
  • Assumed from studies of analogous geothermal settings, or extrapolated from studies of nearby areas.


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