Abstract

With world-wide application of LWD acoustic technology, it becomes important to assess the data accuracy and quantify the errors in the LWD acoustic velocity measurements. We use a linear-regression analysis to estimate the velocity/slowness error for both wireline and LWD array acoustic tools. The analysis indicates that the measurement errors from earlier LWD tools are approximately two to three times larger than the errors from wireline measurements, due to the presence of drilling noise and the tool wave interference in the LWD conditions. The analysis is also performed for a 9.5-in. LWD tool that has recently been developed for holes ranging from 12 in. to 20 in. The error analyses on the data from the 9.5-in. tool show that the estimated slowness errors are about the same order as those from wireline measurements, demonstrating that "wireline quality" of acoustic measurements can be achieved with LWD technology.

Introduction

The past decade witnessed a rapid development of LWD acoustic technology to address exploration and formation evaluation needs for various formations and hole sizes. A 9.5-in. tool has recently been developed for holes ranging from 12 in. to 20 in., suited for compressional and shear velocity measurements in unconsolidated sediments of deepwater reservoirs. The application in these hole sizes is predominantly for seismic calibration and tie-back. The velocity data are also used for pore pressure prediction and formation evaluation. Because of the important applications, the accuracy of the LWD velocity data has always been a major concern since the inception of the technology.

Acoustic measurements in the "while-drilling" environment have two apparent complexities when compared with the wireline measurements. The first is various drilling-related acoustic noises that contaminate the measured acoustic data. The second is the design of an acoustic isolator to effectively isolate acoustic tool waves while maintaining sufficient tool strength for the drilling. Because of the complexities, LWD acoustic measurements are commonly believed to be less accurate compared to their wireline counterparts. Oftentimes following LWD measurements, wireline acoustic data are acquired as a benchmark to check the quality and accuracy of the LWD data.

To assess the accuracy and quality of the acoustic measurements, we analyze errors in the acoustic slowness/velocity log curves for both wireline and LWD acoustic tools. The results are compared between LWD and wireline measurements to show how the presence of tool waves and drilling noise degrades the data accuracy. Data and analysis results will also be compared for the early LWD tool and the newly developed 9.5-in. tool. The comparison demonstrates that the improvement made on the new tool significantly improves the data quality and reduces the error of the velocity measurement. The estimated error is about the same order as that from wireline measurements, demonstrating that "wireline quality" of acoustic measurements can be achieved with LWD technology.

Acoustic Tool System Overview

The new 9.5-in. acoustic LWD logging system architecture is shown in Figure 1. There are three main sections to the system, a) multipole-acoustic transmitter, b) acoustic isolator, and c) multipoleacoustic receiver array.

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