Effects of borehole stability on well log data

Abstract

In this thesis we analyze the effects of borehole irregularities on well logs and develop methods to obtain reliable formation properties from such logs. Data from a well in eastern Venezuela are analysed. Borehole irregularities in this well consist mainly of borehole enlargements perpendicular to the borehole axis due to rock failure under high horizontal stresses. They are observed in three forms: as small scale irregularities of the borehole wall, as-near elliptical shapes oriented along the minimum in situ stress direction, and as an increase of borehole diameter at all azimuths due to large scale failure. Breakout data suggest that maximum stress is oriented NNW-SSE. The suite of logs analyzed include lithology, density, dipmeter and full waveform acoustic logs. The standard log processing and interpretation methods fail because of the effects of irregularities. The analysis of the density log indicates that standard corrections are not enough when diameter is severly elongated. Similarly, acoustic logs processed commercially give unreasonable formation velocities. Guided by theoretical models we evaluate the effects of irregularities on different wave types recorded by full waveforms acoustic logs, i.e refracted P and S, pseudo Rayleigh, Stoneley, flexural etc. We obtain P and S wave velocities from the least affected waves. The density profile is derived from P wave data (Vp-[rho] correlation), with the aid of lithology logs. From the above parameters we obtain the dynamic elastic moduli and then, through known empirical relationships, the static quantities. These values will be used in the next phase of this study as input to finite element models to determine the magnitude of the in situ stress that caused the borehole failures.