Revolutionising subsurface monitoring: a cost-effective 4D gravity workflow

Is there a minimum gravity difference needed for the inversion to show results?

There is no minimum gravity difference needed for the inversion to create a representative density distribution in the subsurface. The main limiting factor will be the resolution of the gravity meter and effects from environmental noise and changes in elevation from uplift or subsidence, or put more simply “what gravity change can actually be detected in the field?”.

It’s been reported that modern Gravimeters can detect changes on the scale of 0.001 mGal, however, this is likely within the threshold of environmental noise or GPS resolution (remembering the standard Free-Air correction formula is h*.3086).

Eiken et. Al in their paper “Gravimetric monitoring of gas production from the Troll Field” have reported intrasurvey repeatability measured as a standard deviation to 4-5 micro-gal between 2002-2006.

Why could we see Microgravity or Mass Changes in Geothermal become more positive over time? There are several studies in the Kawaerau Field and Olkaria where after some time the Gravity Changes throughout the whole field become positive?

Can Volsung predict that kind of trend

Positive gravity response can either be a result of Mass increase or changes in collection height due to variable ground surface elevations.

There are multiple case studies of positive gravity responses due to the influx of water in an Oil or Gas field during Production or Enhanced Oil Recovery.

In a Geothermal field you may see positive gravity anomalies for several different reasons; zones of reinjection, natural downflow, resaturation of steam zone, and more.

The paper “Using Microgravity Measurements to Track Reinjection in Liquid Dominated Reservoirs” by Trevor M. Hunt provides a great explanation while referencing real world case studies.

Volsung can predict the surface gravity response from an evolving reservoir based on physical and geospatial properties (temperature, depth, density, pressure, etc) for water and steam based on the simulated exploitation of a reservoir.

What is the best data that can be used to constrain a density inversion?

A conceptual model that defines the extent of your reservoir and geologic units to focus the inversion along areas of expected permeability is the best starting point for your inversion.

Leapfrog Energy enables the construction of such a model where VOXI can use the resulting .lfm file directly for building different constraint grids to guide our inversion to a geologically reasonable result.

A reservoir simulation model that exports the expected density changes from the exploitation of your reservoir is also important. Volsung can export the 3D density changes from the planned exploitation of your reservoir at important time stamps. This 4D density distribution can be used as a starting model for your inversion.

What reference density of initial density value for the inversion should be used if you are studying geothermal time lapse with a microgravity survey?

You should use the density differences calculated from your Reservoir simulation software.

In the case of this webinar, we used the density changes from Volsung as a starting model for our inversion. This should help guide the inversion to something that is geologically and physically reasonable based on the planned injection and production intervals within our reservoir.

What accuracy in elevation is required?

The highest accuracy possible for your GPS elevations.

Microgravity surveys will record minute variations in the surface gravity response. Because, in Geothermal specifically, we are recording the gravity changes from moving fluid in the ground it is important to consider the effects of subsidence.

The natural decay of the gravity signal (1/r^2) and the Free-Air correction (0.3086 mGal/m) which compensates for the changes in the gravitational field above sea level will produce different results depending on the collection elevation.

The Free-Air correction will generate a difference of 0.003 mGal with a .01m difference, which is within the range of expected differences in a 4D microgravity survey. This should be considered when choosing the equipment for your 4D microgravity survey.

How do you incorporate altitude change in the inversion?

The collection height can be changed in the VOXI Viewer. This allows you to incrementally change the altitude for each time step in your project.