Chemical Logging

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Exploration Technique: Chemical Logging

Exploration Technique Information
Exploration Group: Downhole Techniques
Exploration Sub Group: Well Log Techniques
Parent Exploration Technique: Well Log Techniques
Information Provided by Technique
Lithology:
Stratigraphic/Structural:
Hydrological: Presence and geochemical composition of fluid producing zones
Thermal: Calcium-alkalinity ratios versus depth assist in defining warm and hot water aquifers
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Chemical Logging:
Chemical logging produces a chemical profile of the formation fluid within a well based on the measurement of changes in the chemical composition of the drilling fluid during drilling operations.
Other definitions:Wikipedia Reegle


 
Introduction
Chemical logging utilizes returned drilling fluid as a transport mechanism for sampling water-producing zones in the subsurface. Geochemical analyses can be performed on the returned drilling fluid in order to determine the location (depth) and chemical composition of the water. A fundamental assumption of this technique is negligible mixing of the drilling fluid as it is brought to the surface mud pit. [1]

The composition of the returned drilling fluid depends on its level of dilution (based on the flow rate of the water-producing zone), chemical composition of the formation water, and initial chemical composition of the drilling mud. Interpretation of the data is complex due to these factors.[1]

Geochemical analyses consist of pH, conductivity, and hardness measurements as well as determining chloride, bicarbonate, fluoride and silica concentrations.[1]
 
Use in Geothermal Exploration
During a chemical logging study at the Raft River Geothermal Test Site, returned drilling fluid samples were collected every 60-120m in order to perform geochemical analyses. Positive results were obtained through this sampling and the chemical log was capable of resolving aquifer systems within the well. Through the correlation of the chemical log with a lithological log from the same well, water type could be determined. Additionally, the chemical log indicated the presence of hot water aquifers ~15m prior to drill penetration.[1]

A major advantage of this technique is that drilling operations are not disturbed in order to obtain samples.[1]














 
References
  1. 1.0 1.1 1.2 1.3 1.4 (McAtee and Allen, n.d.) "Chemical Logging, A Geothermal Technique"




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