Borehole Geophysical Methods

Borehole geophysics involves the analysis of geophysical data that are collected within boreholes, wells, or test holes. Boreholes provide access to the subsurface, and borehole methods can produce continuous, in situ records of the properties of intersected formations, formation fluids, borehole fluids, and well construction. Data collected using borehole methods are diverse and may provide more physical and chemical information about the subsurface than available from drilling, sampling, or surface geophysics alone.

In the pursuit of oil and gas exploration and production, numerous borehole geophysical tools have been developed to investigate key reservoir characteristics. Thus, borehole methods have been used to estimate the porosity, permeability, fluid content, and saturation of the borehole-intersected formations. Because such properties relate to groundwater systems, many borehole methods have been adapted and applied to groundwater supply, recharge, monitoring, and remediation studies and modeling (Mussett and Khan, 2000).

Borehole geophysics is also used to determine well construction and water quality as well as the lithology, bed thickness, and fracture content of intersected formations. The addition of borehole geophysics to groundwater sampling allows for a more comprehensive approach to the construction and identification of key well components (e.g., screens and casing). Stratigraphic interpretation and correlation are also aided by the use of borehole geophysics, which can allow the lateral extrapolation of quantitative point data identified in core samples.

The variation of a physical property over the length of borehole is recorded against depth and graphically displayed as a geophysical “well log”. A log is collected by a cable-suspended tool (or “sonde”) that is moved up and/or down the borehole using a winch fitted with a depth counter. Logging systems typically consist of a power supply, recording unit, and processing modules, which digitize data and allow logs to be corrected, collated, and analyzed using various software.

Most borehole geophysical principles are comparable to those of surface methods, and the two survey types are often considered together when setting up comprehensive investigations. Under ideal circumstances, borehole tools can investigate a volume of the formation beyond the borehole disturbed zone that is many times larger than that of the core. Because of the nature of the borehole measurement, the main advantages of borehole geophysics are increased measurement resolution and depth references for the data set.

Because formation properties are not simply related to the measured geophysical quantities, multiple logs are often collected concurrently to take advantage of their synergistic nature. Much more can be learned by the analysis of a log suite, and this approach produces separate lines of evidence that will ideally converge to support an interpretation. Thus, each imaging technique is utilized based on its detection capability, and the results are used in tandem to validate or repudiate predictions.

Borehole geophysics can provide a wealth of information that is critical in gaining a better understanding of subsurface conditions needed for groundwater and environmental studies. Borehole methods can be subdivided into seven broad categories, and those listed below include the most commonly used method subtypes.

Electrical Borehole Geophysics

Electromagnetic Borehole Geophysics

Nuclear Borehole Geophysics

Seismic Borehole Geophysics

Physical and Imaging Borehole Methods

Fluid Column and Flow Borehole Methods

Reference

Mussett, A.E. and Khan, M.A., 2000, Well Logging and Other Subsurface Geophysics, in Looking into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, p 285-305.

Borehole Geophysical Methods

Reference

Environmental Geophysics