Case History - Cased Hole Dipole Sonic Applications in Mexico
Authors: Barajas Jose Sanchez , Patino Antonio Hernandez , Garcia Eduardo Rodriguez , Mesa Harold , Torne Juan , Kessler Calvin , Varsamis Georgios , Blanch Joakim 
Published: January 2008

Abstract
Crossed dipole sonic logging data has traditionally been used in the openhole environment to:identify natural fracture systems and their orientation,evaluate far field stress data for 3D stimulation modeling, andseismic correlations and synthetic modeling.
Materials presented inthis paper will show open hole and cased hole comparisons of flexural/shearwave slowness data and the anisotropy analysis in both fast and slowformations. The paper will show through-casing fracture identification examplesfrom the Cretaceous-Jurassic formations in Southern Mexico that have beenconfirmed with inflow analysis. The paper will also show applications ofthrough-casing anisotropy analysis combined with oriented perforating in theBurgos Basin in North Eastern Mexico. These techniques have resulted in lowerbreakdown pressures on stimulation treatments due to minimizing near wellboretortuosity. 2D and 3D analytical and finite-difference wave propagationsimulators have been developed to verify the open-hole and cased-holescenarios. The finite-difference simulations are used as a predictive tool toidentify cased-hole conditions that are either favorable, or unfavorable forcased-hole dipole acquisition.

Introduction
Dipole logging requires the propagation of a low frequency out-of-phaseflexural wave that is detected by the acoustic receivers on opposite sides ofthe logging tool. To accomplish this symmetry is required. The wireline dipolesonic tool must be centralized in a near round borehole. For through casingdipole logging, the casing should also be centralized, and the material in thecasing-formation annulus must be radially uniform. The effects of boreholeovality, tool centralization, or casing centralization on waveform propagationare illustrated in Fig. 1 for a 1.5 kHz center frequency flexural wave. Lowerfrequency flexural/dipole transmitted pulses have longer wave lengths, thanhigher frequency flexural wave propagation, thereby slightly reducing the phaseshift due to borehole geometry effects. An elliptical borehole is common indeviated wells due to eccentralization of the drill string and bottom holeassembly. Acquiring crossed dipole sonic data at low frequency and in two axes(X and Y) often minimize these environmentally induced effects.
The crossed dipole sonic logging tool is frequently run open hole in what isreferred to as a "quad-combo" (natural gamma ray, eccentered dual spacedneutron, density, dual induction, or dual lateralog with appropriate standoffs, centralized dipole sonic, and navigation tool). One such logging toolstring is configured such that the X-dipole measurement is aligned with thedensity pad, which due to gravity effects will seek the low side of theborehole in deviated well conditions. Therefore, the X dipole measurement maybe affected by tool positioning effects in the angle build sections of deviatedwellbore. The Y-dipole measurement is orthogonal to the density pad and in poorborehole conditions often provides quality shear wave slowness data. Aspreviously discussed, centralization is required, but in certain bore holeconditions this may not be operationally possible. Acquiring crossed dipoledata through casing is a viable option, provided the borehole conditions arefavorable. Unfavorable conditions for through casing dipole logging areexcessive washouts, poor radial placement of cement, lack of casingcentralization, and poorly consolidated formations. Advanced waveformprocessing, such as slowness anisotropy analysis, which is used to determinethe fast and slow shear wave travel times and their corresponding orientationrequires that a navigation device be run simultaneously with the dipole sonictool.1 In open hole logging conditions magnetometers andaccelerometers are utilized. In deviated cased hole logging conditionsmulti-axes accelerometers can be used. In near vertical cased hole loggingconditions a gyroscope is required. Anisotropy analysis can be performedwithout tool navigation data, to determine the magnitude of the anisotropy, butthe orientation of the fast and slow shear waves is referenced to the toolface, which can rotate while logging. This is frequently done in geologicconditions where the anisotropy is know to be a result of natural fracturing, and fracture orientation is not desired.