Laboratory measurements of rock strength: Implications for estimating in situ stress from wellbore failures in shallow marine mudstones

摘要

Quantifying in situ horizontal and vertical stress magnitudes is one important step toward understandingrnfault strength and mechanical processes at depth along major faults, yet direct measurements are generallyrnrare. One method for estimating in situ stress magnitude is to use compressional borehole breakout (BO)rnwidths observed in logging data to compute far field stresses. Stresses obtained by this method arernstrongly dependent on rock unconfined compressive strength (UCS) and friction coefficient (μ).rnHowever, because direct measurements of rock strength typically undersample the formation, UCS isrncommonly estimated using P-wave (V_p) velocity as a proxy. Several relations linking UCS to V_p havernbeen developed for shales, but there are few experimental data to define similar relationships forrnunlithified sediments as are commonly encountered in the shallow subsurface (i.e. < ～1-2 km depth). Here,rnwe report on a suite of triaxial experiments designed to measure the UCS of marine mudstones from <1rnkm depth from Ocean Drilling Program (ODP) Site 1174, and to develop a relationship between V_p andrnUCS appropriate for these materials. We find that UCS values defined by our experimental data arernsystematically 1.3-2 MPa lower than those predicted by existing V_p-UCS relationships for shales. Thisrnimplies that in situ horizontal stress magnitudes estimated from wellbore failures may be substantiallyrnlower than previously estimated. We explore the implications of our experimental results by applyingrnthem to estimate far field tectonic stress magnitudes from BO widths at ODP Site 808 and IntegratedrnOcean Drilling Program (IODP) Site C0002.