Dipole Shear Imaging behind casing: Extending the borehole Acoustic Imaging envelope to Brown-fields
Authors: Srivastava Ashok , Yamamoto Hiroaki , Ahmed Shabbir , Roberts Jonathon , Cantin Fabrice , Moreau Olivier-Daniel , Bounoua Dr. Noureddine 
Published: May 2016

The potential for imaging bed boundaries and fractures using the borehole acoustic reflection survey (BARS) technique is well documented. Traditionally, this type of imaging has been conducted with a monopole source and imaging the reflected P and the mode-converted transmission waves (Pto S and S to P). Recently the BARS methodology has been applied to shear data from a dipole source. Much of the published work is in an open-hole environment. In our case study a BARS image was acquired in a layered carbonate reservoir in a horizontal well behind casing. The objective was to acquire a base image prior to a planned hydraulic fracturing job for comparison with the post-fracturing image. Both, monopole and dipole data were used to do the imaging. Data acquired shows that the BARS technique overcame uncertainties associated with; arrival of the casing modes, cement conditions, attenuation of the reflected signal at the casing/cement/formation interfaces and provided reliable results behind casing up to a distance of 100 ft away from the well.

Introduction

BARS (Borehole Acoustic Reflection Survey) or SWI (Single Well Imaging) for detecting near borehole events at sub seismic scale was first demonstrated by Hornby in 1989. The methodology adopted involves imaging P waves from a monopole source. Later Yamamoto (1999) and other demonstrated that borehole crossing features could be imaged using S to P conversion downdip and P to S conversions updip of the event. More recently Tang and others (2009) demonstrated this methodology using dipole acoustic data. Published data does not show much experience of acoustic imaging in a cased well through a porous reservoir. A porous reservoir presents a highly attenuating environment making imaging "near borehole features", especially in a cased well very challenging. Acoustic imaging can be significantly beneficial if it can be successfully done in these environments.

Successful imaging requires the reflected signal to have sufficient amplitude to differentiate it from noise. There are losses as a consequence of the formation Q and geometrical spreading which are well understood however a significant amount of energy is lost at energy transfers from tool to borehole, borehole to formation, formation to borehole and borehole to tool. These transfers are frequency dependent and difficult to predict by models. Numerical work has been done in past to predict radiation and reception pattern in a borehole but the work considers simple geometries as compared to the real conditions especially of a cased well scenario.

In a cased borehole our biggest challenge is to transfer energy from the borehole to the formation. We propose that one way to overcome the uncertainty of the borehole transfers is to have a wideband transmitter with a range of frequencies of excitation. This would ensure that the preferred frequency for a particular geometry would most likely be excited and we would get good energy transfer at the interfaces.