Supporting the Mining and Geothermal Industries Since 1994

Sims Professional Services

Sims Professional Services

Geothermal - Wellsite Services

Mud loggers provide needed services that include monitoring of carbon dioxide, documenting drill breaks, calculating mud losses, and collecting cuttings samples.  They log the cuttings but do not have the time for detailed geological analysis.  This is the duty of the dedicated geologist who knows the site-specific geology and the geologic targets. 

Dr. Sims was at the drill rig daily while drilling eight deep geothermal wells in the Salton Sea geothermal field.  Production wells cost about $8 million each to drill and another $8 million to line the casing with nickel alloy.  There are critical real-time decisions that must be made on a regular basis.  One of the most important decisions is the casing depth.  If it is too shallow, then cold fluid may enter the well and lower the enthalpy.  If a major permeability zone is intercepted when the well is hot and the final casing depth has not yet called, it is problematic.  A solid plan is necessary to get to the required temperature without intercepting a major permeability zone, case the well at the correct temperature, and then have a path that follows a favorable direction with regards to the rock fabric on its way to the target.  The geologist must make all of the real-time decisions at the rig, based upon expectations, drill cuttings and drilling parameters.

Another important task for the wellsite geologist is to keep the drilling direction on track.  There should be a well-reasoned target and expectations of what should be intercepted along the way.  If there are faults that will be intercepted, the drillers must know, and respond accordingly, so that severe doglegs can be avoided.  If something unexpected is encountered the wellsite geologist should know this immediately and revisit the model while drilling.  If the geology is instead analyzed only after drilling, as is often the case, it is too late to make adjustments.

At the drill site, Dr. Sims uses methodology developed by Dr. Denis Norton.  Drill cuttings are logged for important information that mud loggers do not record, and each 10-foot sample is analyzed using an x-ray diffractometer that is located in the geology trailer at the well pad.  Detailed mineralogy allows for estimation of reservoir temperature while drilling, and documentation of textures allows identification of potential flow zone permeability.  

We drilled excellent wells at Hudson Ranch, including the 40+ MW well HR 13-2, which is probably the world's most powerful geothermal well.  Our overall success rate was 80% and we learned as much from the wells that underperformed as we did from the stellar wells. 

Minerals that form from geothermal fluids provide constraints for temperature and fluid composition and the mineralogy is best determined by XRD analysis.  The textures of minerals also provide important information and that can be gained only by careful chip logging.  For example, quartz will sometimes be etched and sometimes it will have fresh faces and that gives information about changing fluid conditions.  Also, low temperature minerals may be formed above high temperature minerals, or visa-versa.  The photograph above is euhedral epidote formed over euhedral albite, that formed on breccia fragments.  Much chemistry information can be learned from this single cutting.  The crystal habits and the size of the crystals also provide valuable information regarding fracture aperture and porosity (photograph by Lee Hess, from Sims, D.B., 2013, Geology at Hudson Ranch: The Salton Sea Geothermal Field, San Marcos College, October 10, 2013).

At the Salton Sea, we initially had three senior scientists on the rig logging cuttings, performing XRD analysis, evaluating the data, and developing the proper strategies.   Someone asked the President how he could justify having three Ph.D.'s on the rig daily as wellsite geologists .  His response was to question why you would spend $8 million drilling a well and not have three Ph.D.'s looking after it. 

Each geothermal field is different from others and a wellsite program must be tailored to fit the conditions.  Dr. Sims can help to establish your wellsite geology program and provide training.  He can evaluate the project requirements and bring in essential colleagues who specialize in mineralogy, geochemistry, geophysics and other specialty fields if needed.  

Seismic reflection is one of many potential tools that can be used to target wells.  Mineralogy from XRD analysis, cuttings logs by dedicated geologists, and pressure temperature surveys are entered into the model and these data sets are used to evaluate the usefulness of different geophysical and petrophysical tools.  A detailed drill plan includes path and target expectations and the cuttings and drill data are monitored daily by the senior geologist on site (from Sims, D.B., 2013, Geology at Hudson Ranch: The Salton Sea Geothermal Field, San Marcos College, October 10, 2013)

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