Stress Induced Changes in Elastic Wave Attributes in the Wattenberg Field, Colorado, US

I have investigated the influence of stress on elastic wave behavior. Elastic waves are rich in information content. Not only do elastic waves carry information about geologic structure, they carry geomechanical information as well. The objective of this work was to examine elastic wave behavior with rock physics and geomechanical modeling, to extract actionable information for use in engineering designs and field development decisions.;I used Techlog, a software package provided by Schlumberger to analyze all of the well-centric data. I used Computer Modeling Group's (CMG) reservoir simulator and its geomechanics module (GEM) to compute the flow coupled geomechanical stresses.;The Fort Hays, Codell and Muddy Formations are mostly isotropic. Thin intervals within the J Sand and Carlile Formations show stress induced HTI anisotropy. The Niobrara Formation and bounding shales show strong VTI anisotropy with Thomsen's epsilon and gamma values ranging from 0.1 to 0.5. The Niobrara Formation is overpressured. Dipole sonic data shows the top of pressure to be 1300 feet above the top of the Niobrara Formation. The sonic data also shows the Pierre Formation above the top of pressure to have a P-wave velocity change of 580 ft/s with a 1000 psi change in effective stress. However, below the top of pressure, the P-wave velocity changes by 2150 ft/s for the same 1000 psi change in effective stress. Linear slip theory (LST) was used to fit the ultrasonic velocity measurements conducted on Niobrara Formation core. These velocity measurements showed even the stress sensitivity was anisotropic; the vertical velocities showed 5 times the stress sensitivity as the horizontal velocities. The linear elastic stress models did not yield an accurate vertical stress profile; the bounding shales, the Sharon Springs Formation above, and the Carlile and Graneros Formation below, exhibit ductile strain that increased the minimum horizontal stress gradient by 0.1 psi/ft over the linear elastic stress estimates. During production, depending on the permeability anisotropy of the stimulation and the initial horizontal stress imbalance, the horizontal stresses increase at different rates causing the stress field to rotate 90 degrees.;The entire stratigraphic column is anisotropic. In addition, the stratigraphic column exhibits significant stress-induced velocity anisotropy. This complicates time-lapse seismic measurements because both the stress sensitivity and effective stresses vary with injection and production. Numerical stress modeling is required to estimate the initial vertical stress profile and time dependent changes in effective stress due to injection and production. Engineering opportunities for improved recovery are possible based on the geomechanical insights and workflows developed in this dissertation.