Impact of Shear Velocity Estimation on Wellbore Stability

Abstract

Introduction: The main purpose of this study is to determine a shear velocity prediction scheme for deep offshore Niger delta accurate enough for use in wellbore stability. There are few wells in the area that contain measured shear velocity data and they are located close to each other. The available logs were analyzed with the following equations: (a) Gardner equation and (b) Mudrock equation to determine which setting is more applicable in the deep-offshore setting. The most common method is the Greenberg and Castagna (1982) which utilizes a linear relationship between compressional and shear velocity in each lithology. Proposed Solution The Greenberg - Castagna method was used to predict the Vs in the study area using a control well with known sand and shale coefficients. New lithological (sand and shale) coefficient were derived using the data from the wells in the study area. This new calibration shows that the locally calibrated coefficient is lower than the controlled in offshore Niger delta. The local sand and shale trends are also closer than the controlled sand and shale trend. This therefore suggest smaller AVO response in sand and shale boundaries. This difference led to further examination of the data in the study area using Mudrock equation and Gardner equation in other to explain the variation. Methodology/Approach A petrophysical analysis is carried out in the field and a robust lithologic model is developed. This model captures the volume of key minerals (volume of quartz, shale and calcite where available) in study area which would drive the calibration constants to be used for that field. The Greenberg - Castagna Vp-Vs approach is lithology dependent. A fluid substitution to 100% brine saturated is performed to calibrate the shear velocity model and this is particularly important in the reservoir zones. After this is done, the model is calibrated by adapting the constant/ linear and quadratic factors of the Greenberg - Castagna equation for each of the minerals. This calibrated model is applied to other wells in the field for validation and shear velocity forward modeling/ prediction. Conclusion The locally calibrated coefficient is significantly lower than the published Greenberg - Castagna values. Even though these values are localized, they produce better Vs estimates in a test well. Forward modeling also showed that the local coefficient predicts smaller AVO response at sand- shale boundaries than the controlled coefficient.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018