The term open hole service refers to the geophysical investigation of the borehole prior to the completion of a well. A slimhole tool is lowered into the borehole after drilling is completed, and the hole has been conditioned with drilling fluids, by a winch with a stainless steel wire containing electrical conductors that relay data from the tool connected at the end back up to a laptop computer where the data is digitized and presented in a readable format for engineers to evaluate. This data record is generally presented in a vertical format of 1 inch is equal to 20 feet of vertical borehole data (vertical scales can be manipulated to meet specific needs of client).
Tools commonly applied for this analysis are generally Natural Gamma Ray, Spontaneous Potential, Single Point Resistance, 4 Focused Resistivity, and Current and can all be executed on a single trip into the borehole and presented on same data log of the borehole studied. The information provided will assist in the determination of geological changes within the vertical column of the borehole such as where sands are confined between clay layers; in addition determinations can be made concerning the potential water quality within a sand through the Spontaneous Potential curve. Not as often a caliper log can be performed on a borehole to determine the rugosity of the annulus itself in order to determine if a washout is present and aids in the calculations for the amount of cement grout that will be required to seal the casing to the borehole wall itself. Additional open hole borehole tools that can be applied at request are fluid temperature and resistivity that can assess physical properties of groundwater and are good tools for geothermal well applications; fluid flowmeters and samplers are also available.