Shallow Gas Confirmation by Slow Shear Wave Using New LWD Sonic Quadrupole Technology
Authors: Truong Son Nguyen , Pradana Almagfiro , Hashim M Faris , Aznor M Zafril , Ahmed Aqil , Zolhaili Azzril , Maeso Carlos , Fui Kent Wong , Yamamoto Hiroaki , S Branta 
Published: March 2015

A re-development drilling campaign was planned for a brownfield in South East Asia. In previous campaigns, conducted between 10 and 20 years ago, limited data had been acquired in the shallow to intermediate sections. At the time extensive wellbore stability issues led to stuck pipe events and lost wireline strings. The absence of data from the shallow sections resulted in difficulties for seismic ties and identifying potential shallow hazards.

The planned development phase involved long 12 ¼ in hole sections from a very shallow depth, with extensive borehole builds from vertical to 68°. In order to acquire shallow hole information, log data requirements led to a long bottom hole drilling assembly including multipole sonic measurements.

The sonic measurements were acquired using a new multipole sonic tool in an 8 inch collar. Real time quality control using transmitted coherence peaks and pumps off stations gave confidence in the real time compressional data. Post processing of the full recorded mode waveforms confirmed the real time values. For shallower intervals Leaky-P dispersive processing allowed determination of formation compressional signals (differentiating formation and mud where they are close in value). Formation shear values were always slower than the mud and so were not available from the Monopole signal. The Quadrupole mode contained slow shear through the majority of the section. Shear data was seen in the range of 275 - 920 usec/ft. The compressional and shear data is the shallowest borehole sonic data acquired in the field to date. Presence of shallow permeable gas was confirmed by good quality shear sonic data in a highly unconsolidated formation. The sonic data was also used for seismic inversion. Historically acquisition of shallow interval sonic data has been problematic in South East Asia due to soft formations and wellbore stability issues. This paper demonstrates the use of LWD mulitpole sonic to address this challenge to reduce drilling risk and geological uncertainty.

Introduction

Sonic measurements have evolved significantly from initial rather simple application for correlating surface seismic sections in the 1950s (Aron et al.,2008). A sonic while drilling tool (LWD) was first introduced in 1995 with Monopole measurements only. Theoretical and laboratory work conducted in the 1980s described the use of quadrupole methods for borehole sonic shear measurements (Winbow, 1985, Schmitt, 1988 and Chen, 1989). More recently, quadrupole sonic LWD tools have been introduced in the industry (Tang et al., 2002, Market et al., 2007, and Kinoshita et al., 2008) to address the need for a continuous shear measurement in applications such as geomechanics, amplitude versus offset (AVO) modeling and other seismic interpretation, and Vp/Vs to detect the presence of gas.

In offshore South East Asia, the shallow section formations are commonly poorly consolidated and exhibit very slow acoustic properties. Formation shear slowness (DTS) is always slower than the mud slowness in these conditions and is not available from the Monopole signal. The older generation LWD Monopole tools therefore could not detect the shear values in this environment. With borehole conditions deteriorating with time in these poorly consolidated formations, wireline sonic data has been very difficult to obtain. This resulted in limited sonic data acquired over the shallow section in the past for seismic tie in and shallow gas hazard identification. Therefore there has been a high demand to have an LWD tool which can give good quality continuous compressional and shear slowness in this region. The measurements are applicable to most of the brownfields in the South East Asia region.