One of the challenges in real-time geomechanics using logging-while-drilling data has been the lack of shear measurements in all formation/mud types. Traditional monopole acquisition only allowed the acquisition of shear in fast formations, so the shear in slower formations, such as in overburden sections, was often missing due to the measurement physics. The introduction of real-time quadrupole shear acquisition certainly has been the game changer in this area, and has the potential to make a significant contribution to the reduction of nonproductive time in the drilling phase through appropriate wellbore stability management.
Deriving shear slowness in quadrupole acoustic logging requires an inversion method that takes into account the dispersion of the quadrupole mode and includes the effects of tool presence. The inversion can be run in both real-time and during memory processing but the required inputs may be different in either case due to the limited data availability in real time. However, the case study considered here shows the differences between real-time and recorded-mode inversions are minimal, and the real-time shear quality allowed accurate geomechanics analysis.
Two complementary models were used in a geomechanics analysis to compensate for the sensor offset difference in the bottomhole assembly (BHA) using the real-time quadrupole shear. The first model is based on geopressure gradients of vertical stress, pore pressure and fracture gradient. The second one incorporates the gradients of shear failure, depth of damage and breakdown. All the workflows used to obtain wellbore stability computation in real time include the determination of lithological facies, the elastic properties of each formation and rock strength parameters. The tectonic strain's correlations from the post-drilling analysis of two key offset wells enabled the modeling of the minimum and maximum horizontal stresses acting on the wall of the borehole from the poroelastic theory. Finally, by applying the Kirsch's equation, we are able to obtain a shear failure gradient of the borehole and also the plastic zone model, inferring the depth of damage in absolute terms from 5% to 10% of the diameter of the borehole in gauge or bit size.
This technique is applicable to any quad-combo acquisition that includes gamma ray, resistivity, density, neutron and sonic (compressional and shear) measurements. It is a proven methodology in fast formation with monopole acquisition, and it is starting to expand its application by adding the real-time quadrupole shear in slow formations. Thus, it enables us to evaluate wellbore stability in the slow formation, an answer finally available in real time thanks to the superior quadrupole shear measurement, workflow and expertise.