Wave propagation through soils is a non-destructive method used to characterize soil stiffness properties and is the basis for geophysical interpretations. Due to the difficulty of measuring shear waves through soft cohesive soils with ultrasonic transducers, bender elements are typically used, which couple to the soft clay. Unfortunately, the stress limitation of bender elements precludes it from testing above 1.5 MPa. A novel setup using ultrasonic transducers, with electronic conditioning, is developed in order to overcome the difficulties of transmitting a shear wave through a soft material. The successful fabrication of triaxial cell endcaps fitted with ultrasonic piezoceramic elements has enabled the measurement of compressional and shear wave velocity of clays over a wide stress range (0.5 10 MPa). This research compares the shear modulus measurements using three different technologies that encompass a stress range of 0.1 to 70 MPa. A variety of resedimented materials and plasticities are tested in order to characterize the Vp and Vs as a function of stress during K0-consolidation. Resedimentation creates controlled, uniform specimens whose results can develop a backbone for velocity behavior independent of field heterogeneities and disturbance. The Vp and Vs results show different trends as a function of stress for the different plasticity materials. However, the Vp/Vs ratio, which is a common indicator of unloading and lithology, has a strong dependence on the liquid limit (wL). A model is developed to predict the Vp/Vs as a function of stress for a given wL. In addition to K0 loading, the effects of unloading are investigated. Unloading introduces secondary compression, which significantly alters the stiffness results. A method is developed to integrate the secondary compression data with normally consolidated data based on the concept of apparent preconsolidation pressure. Finally, the dynamically-obtained stiffness parameters are compared to staticallyobtained constrained modulus values from static compression. By using Gassmann’s fluid substitution and a corrective X factor, a new technique is developed that can predict the dynamic bulk modulus based on the static CRS measurements. Thesis Supervisor: John T. Germaine Title: Research Professor, Tufts University

Stiffness characterization of mechanically-compressed cohesive soils using wave propagation

Pore pressure in sediments beneath salt in the Gulf of Mexico varies widely creating a potentially dangerous and difficult drilling challenge. Estimating elastic parameters of sediments beneath salt is key to the prediction of pore pressure and reducing the drilling risk in exiting the base of the salt. In this paper we investigate the ability of 2D waveform inversion to recover the elastic parameters in the sedimentary layer beneath the salt from a walk-away VSP (vertical seismic profile) carried out with the receivers in the salt, with the objective of estimating pore pressure at the base of the salt (to be estimated using traditional methods). We propose an effective method for performing the inversion and apply this method to a blind test of a large and realistic synthetic dataset. To facilitate the design of a VSP survey suitable to this type of inversion we also present an analysis into the effects of the receiver array receiver spacing. It is shown that the resulting velocity estimates are sufficiently accurate to predict the pore pressure within the limits required for drilling.

Investigation into the use of 2D elastic waveform inversion from look-ahead walk-away VSP surveys

Drilling on Target With VSP Lookahead Technique in Deepwater India Exploratory Environment

The well was drilling salt when high volumes of gas were encountered. Mud weight was raised and gas volumes decreased. Drilling continued for a short distance before gas volumes again increased. At 19,596' the well took a 5 bbl influx while still in salt. The well was shut well in, mud weight raised to 15.9 ppge, and the well was static. Drilling was resumed and an additional 14’ were drilled to 19,610‘ MD where the well lost circulation. Bottoms up from the well suggested that the bit might be at base of salt. This would be approximately 1,300’ shallow to prognosis and it was suspected this might be an inclusion in the salt. Predrill modeling for the VSP suggested that reasonable data could be acquired via a rig source zero offset survey. The decision was made to pull out of the hole and run the VSP to determine if the well was close to or at the base of salt and to determine preferred sidetrack options. 3D Pre-survey design and acquisition

Successful imaging in a structurally complex environment, using a Look‐Ahead VSP

Drilling on Target With VSP Look-Ahead Technique in a Deepwater India Exploratory Environment

Summary VSP techniques have long held out the promise of ahead-of-the-bit pressure predictions, but a variety of data acquisition and interpretation issues have limited this potentially important application. In particular, P-wave velocities ahead of the bit, if determined from inversion of upcoming reflected amplitudes, contain the assumption that the up-hole velocity trend can be simply extrapolated downward. This assumption is inappropriate in a case where a deeper pressure seal causes a reversal of the trend. However, a simple interpretation technique (Thomsen, 1992), requiring only the picking and registration of upward traveling (i.e., reflected) P-waves and mode-converted S-waves allows straightforward determination of Vp/Vs ratios ahead of the bit using only zero-offset VSP data. The Vp/Vs ratios are derived without explicit knowledge of either the interval P-wave velocities or the interval S-wave velocities. Vp/Vs ratios are lower in the presence of higher effective stress (lower pore pressure) (Mueller et al., 1991). Vp/Vs ratios can be interpreted for pore pressure in several ways, including comparison against local Vp/Vs compaction trends (as measured by dipole sonics). Vp/Vs ratios appear to be more sensitive than P-wave interval velocities to effective stress variations, especially at low effective stress (hard over-pressure) Eberhart-Phillips et al. (1989).

/pdf/vsp-derived-vp-vs-ratios-for-pressure-prediction-ahead-of-5b0dmf2to1.pdf

VSP-Derived Vp/Vs Ratios for Pressure Prediction ahead of the Bit

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VSP-Derived Vp/Vs Ratios for Pressure Prediction ahead of the Bit