RhoVe Method II Empirical Density-Temperature-Effective Stress Transform

Abstract

The rhob-velocity-effective stress (RhoVe) method represents an empirical approach to pore pressure analysis and calibration utilizing a series of model-driven, genetically-linked “virtual” rock property relationships. The method is fundamentally a two-parameter approach (a-term and alpha) that are used to construct a velocity-vertical effective stress (VES) and a density-VES family of curves that can be applied to a well of interest where convergence of the two transformed properties offers a robust solution. When the a-term is set as a function of alpha, RhoVe method is reduced to a single parameter (alpha’) that includes the effects of compositional changes related to clay diagenesis. A sub-regional study of subsalt wells located in the south-central Deepwater Gulf of Mexico (DWGoM) confirmed the presence of a plateau, or upper limit in a-term relationships (sonic versus density cross plot space), in a narrow band that approximately coincides with Bowers published Gulf of Mexico “slow” trend for shales, and is consistent with the effects of ongoing chemical compaction. Once calibrated, the construct represents a fully-populated” petrophysical (shale-only) model volume that can be queried and interrogated to perform advanced calculations. A new method of calculating pore pressure from temperature is presented that both frames the structural-stratigraphic history of fine-grained clastics in a sub-regional setting and allows for an interpretation of local diagenetic effects. Post-drill pore pressure analysis results for the initial subsalt study area were expanded to include other (non-subsalt) areas of DWGoM, and the RhoVe method was extended to produce a family of density-VES relationships applied as a function of temperature (RhoVe-T), that can account for the effects of cementation and other diagenetic factors related to fluid expulsion and ongoing chemical compaction. The method utilizes a single master power law reference relationship between temperature (in degrees Fahrenheit) and alpha’ that is applied as an instantaneous series that spans the entire DWGoM. The study documents the effects of apparent load transfer due to late-stage S/I clay layer reorganization that is initiated at mudstone densities on the order of 2.4 gm/cc (+/-0.05gm/cc) and ~200o F (95o C). Apparent load transfer continues beyond the threshold temperature until an equilibrium state is achieved within the mudstone fabric that allows clay densities to once again increase, marked by a concomitant increase in effective stress. Recognizing the delay in the temperature window beyond the initial threshold temperature (200-250o Fahrenheit) allows for a model-driven, deterministic prediction of increasing compaction trend relationships for the deeper section. Density-derived alpha’-temperature power law solutions are directly extended to include sonic velocity and acoustic impedance relationships tied to VES. A single temperature-alpha’ function transforms both sonic and density data for the entire stratigraphic section, including Plio-Pleistocene/Miocene/Oligocene and older Wilcox-equivalent Paleogene shales and mudstones. Accounting for the effects of ongoing chemical compaction and diagenesis using alternate associations extends the predictability of high-velocity, high-density, low-effective stress rock types such as those found in the Deepwater Gulf of Mexico Miocene and Wilcox-equivalent Paleogene mudstones and older onshore unconventional shale-play reservoir sections.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018