Inversion of Advanced Full Waveform Sonic Data Provides Magnitudes of Minimum and Maximum Horizontal Stress for Calibrating the Geomechanics Model in a Gas Storage Reservoir

Abstract

AbstractIn 1D Geomechanics projects, calibration of stress is extremely important in the construction of a valid Mechanical Earth Model (MEM). The minimum horizontal stress data is usually available from LOT (Leak-off test), XLOT (Extended Leak-off test), Open hole stress tests, or cased hole Mini Fall off Test (also called Diagnostic Fracture Injection Test – DFIT). These traditional measurements have a few deficiencies.This paper presents an application to a gas storage field, where stresses are derived from a newer unique approach where the radial variation of acoustic velocity from advanced dipole sonic logging tool is inverted to obtain stress. These derived stresses are then utilized to calibrate the 1D MEM for the gas field. This approach addresses some of the deficiencies of traditional approaches of deriving stress magnitudes. The characteristics of this approach are:Both the minimum horizontal stress (Shmin) and maximum horizontal stress (SHmax) are obtained. Traditional approach provides a value for Shmin only.These measurements are obtained in reservoir zones at multiple depth intervals. Traditional approaches provide a measurement at one depth only.This method is applicable to all formations that are acoustically stress sensitive.This method is relatively more cost effective as compared to traditional approaches where rig time is a premium considering that advanced sonic logs are routinely run for other applications as well.The sonic data was processed to derive a dispersion plot (velocity versus frequency). From this plot, a plot of velocity versus radial distance is derived. As Kirsch equation provides a radial variation of the stress with distance for a hole drilled in a uniform stress field, and variation of stress translates to variation with velocity, a parametric inversion is utilised to derive the stress.In this gas field, no LOT or XLOT were available. However, the advanced dipole sonic logging tool was recorded in a well recently drilled and some of the formations were found to be acoustically stress sensitive as observed from the dispersion plot. Therefore, this inversion technique was applied and Shmin and SHmax were obtained that were extremely useful to calibrate the 1D MEM.This technique is unique in the industry and complements existing methods to obtain stress measurements for 1D MEMs where stress measurements are either not available or expensive to obtain. This method is not widely used. The authors hope that this paper will illustrate how this method was used in this gas field and encourages other users to investigate the application of this method in their fields, when traditional data is limited.