Rock mechanical properties such as Poisson's Ratio and Young's Modulus are essential to understanding well completions and predicting stimulation responses. Utilization of these in-situ measurements to design well completions has proven effective by allowing entry points to be placed within "like rock" groupings, yielding higher perforation efficiency during stimulation. However, the economic and operational burden to collect this data often prevents it from being collected regularly.
Current industry practice is to calculate these rock mechanical properties from dipole sonic logs. Unfortunately, the present level of technology for sonic tools yields measurement of, at most, three stiffness coefficients (C33, C44, C66) in an ideal well. If the target formation is isotropic this is sufficient because only two coefficients are required to generate a fully realized solution. In a vertically transverse isotropic (VTI) formation, however, we must estimate the remaining unknowns including the critical C13 tensor (Quirein & Cheng, 2014 Modified ANNIE). Therefore, in order to fully describe the rock mechanics matrix, additional measurements or technology is required.
Continuous, high-resolution measurements of near-bit drilling induced vibrations provide an alternative technique to obtain these stiffness coefficients. Vibrations are recorded by mounting a set of tri-axial accelerometers on a 12" bit sub positioned directly behind the bit. In this recording configuration, the near-bit accelerations provide a measurement of the forces acting at the bit and the motions of the bit-rock interaction.
Typical values of near-bit accelerations used to represent the forces acting on the formation can be on the order of several gravities. Typical values of the displacement of the bit as determined from processing the near-bit accelerations are on the order of several micrometers. Because these values represent accelerations and displacements as opposed to stress and strain, geometric or other scalar corrections are applied to determine relative values of the mechanical rock properties. Thus, using these innovative new stress-strain relationships with respect to the material symmetry of formation and well orientation, it is possible to obtain the stiffness coefficients and thereby the rock mechanical properties.
There are, of course, potential issues with measuring rock mechanics while drilling; one is the question of whether the Young's Modulus and Poisson's Ratio are static or dynamic. The DDL data is more closely associated with the dynamic data since it is calibrated to the dipole sonic tool response. Therefore, static to dynamic corrections are still required. Also, changes in downhole assemblies and drilling techniques must be accounted for in the processing.
A comparison from a two-mile lateral in the isotropic Bakken reservoir near Williston, ND, is presented contrasting open-hole horizontal logs including triple combo, dipole sonic, image logs and this new downhole rock mechanics measurement. Post-processed data shows rock mechanical properties can be positively identified using this new technology.