Applications Of Acoustic Image Logs

Abstract Acoustic image logs have been acquired in the Barua/Motatan and Mara fields as a part of the information acquisition program implemented by Maraven, S.A. to improve the efficiency of reservoir exploitation. Two basic types of image logs exist: those based on fundamentals of resistivity/microresistivity and those based on ultrasonic fundamentals. This work demonstrates experiences in the interpretation and application of the ultrasonic (acoustic) type image log. The acoustic image logs have shown great utility in the characterization of reservoirs by providing indispensable geological data along with associated petrophysical and geomechanical reservoir information. The image observed is from the borehole wall which permits description of the reservoir by identification of stratigraphic features, naturally fractured zones, the orientation of fractures, changes in the acoustic reflectivity of rocks (often related to rock density), thin beds, structural analysis, and orientation of local in-situ stresses. Additionally, the logs have been used to assist in the control of borehole stability, definition of sedimentary facies and changes in the porosity of certain facies allowing definition of flow units. Limitations to acoustic image log measurements and applications are discussed. Environmental and drilling effects which may obscure formation features or generate ambiguous information are included. Introduction Acoustic image logs provide information from the borehole wall and permit description of various reservoir properties by identification of stratigraphic features, fracture intervals, fracture orientation, changes in rock porosity (e.g., rock density), lithofacies, thin stratification, structural analysis, and orientation of local in-situ stresses. The acoustic image log permits analysis in fractured reservoirs including fracture attributes such as orientation, density, aperture, and distribution. These attributes can have an important effect on reservoir productivity and field development. The stress regime can be established through detection of the maximum in-situ horizontal stress thereby determining the major stresses which control hole stability. Cross stratification is a geological element that effects productivity, but is difficult to define using conventional open hole logs. Acoustic image logs have high vertical resolution and with their orientation abilities permit the evaluation of these strata. In sandstone and carbonate formations, study of acoustic image logs permits definition of sedimentary facies and changes in texture are reflected as color tone changes, allowing definition of flow units. Lithofacies have been described using statistically-derived acoustic image properties in carbonate formations as well as information derived from simultaneous acquired acoustic and resistivity images (STAR sm). These techniques have been used with considerable success in the Mara and Barua/Motat fields. Figure 1 shows the geographic location of these fields and their proximity to Lake Maracaibo. Physical Principles of Measurement and Graphical Presentation The acoustic image logs in open holes consist of an image spanning 360 of the borehole wall circumferentially. The tool is magnetically oriented and the principle of measurement is based on an acoustic wave reflected from the borehole wall.