Borehole Science

Most geophysical methods use measurements acquired on the Earth’s surface or from airborne/satellite systems to image and characterize the subsurface structure and properties. In all of these cases borehole measurements provide crucial hard data points to calibrate such remote sensing methods. Boreholes provide our only way of sampling the actual subsurface rocks and fluids. They also give us an access point for placing measurement sources and receivers closer to reservoir targets for improving measurement accuracy and resolution. When ERL was founded in 1982, borehole science research formed the backbone of the Lab’s focus particularly in borehole acoustic logging and Vertical Seismic Profiling (VSP) applications. These continue to be important areas of borehole research for the Lab, along with EM and passive downhole microseismic monitoring applications.

Work in Borehole Science is focused on developing methods to estimate in-situ mechanical properties, permeability, and stress proximal to the borehole wall and also extending outward in the interwell regions of any reservoir – for oil and gas, geothermal, or CO2 sequestration. Foundational to all such estimation methods is our continuing research on developing a fundamental understanding of rock physics and rock-fluid interactions. We continue to work on the critical problem of sensing ahead of the bit in the drilling process to find ways of predicting the presence of overpressured zones that are serious safety and environmental hazards, especially as offshore oil and gas drilling extends into ultra deep water settings. A related application of this work is improved geosteering methods for long-reach directional drilling. Early work in developing the fundamental understanding of acoustic logging methods has been extended into the area of logging-while-drilling applications. The Lab’s strengths in seismic imaging have also been leveraged into methods of imaging around boreholes using VSP, crosswell, and single well geometries. Permeability estimation methods using tube wave analysis from acoustic logs and VSP data are augmented by studies focusing on electroseismic and seismoelectric conversion. In-situ subsurface stress estimation is another important area of focus using borehole logging data as well as microseismic events generated by re-activated faults in producing fields or induced hydraulic fractures in tight reservoirs. Often these small earthquakes are measured in downhole seismic arrays. In all of these applications we combine theory, numerical modeling, laboratory physical modeling, and field data analysis to develop, validate, and calibrate our methods.

Like all of our efforts at ERL, Borehole Science research is part of an integrated approach to being able to estimate and predict the variations in rocks and fluids in the subsurface. Much of the work is focused in the area of seismic imaging and characterization, but many projects, such as electroseismics, include a multiphysics perspective. The connection between mechanical and fluid flow properties is also a focus as we relate seismic phases (e.g., Stoneley waves) to fluid flow. Stress estimation from guided borehole waves are compared with estimates obtained from microseismic data and geomechanical models derived from GPS and InSAR measurements.