An increasing awareness of the role played by natural fractures and of theimportance of hydraulic stimulation of petroleum reservoirs has lead to newdevelopments in analytical testing and equipment:
The Electromagnetic Goniometer (EMG200)
In-situ stress orientations from velocity anisotropy measurements andepifluorescence analysis
The X-ray Computerized Tomography (CT) Scanner.
The Electromagnetic Goniometer provides a rapid fracture measurementsystem that is unique and more accurate than any other core fracture measuringsystem to date.
Acoustic Velocity Anisotropy and Epifluorescence Analysis can be used todetermine in-situ horizontal stress orientations. Variations in compressionalacoustic wave velocity are caused by micro fractures that form when core isremoved from the ambient stress regime. Epi-fluorescence analysis of orientedthin sections permits measurement of micro fracture orientations and screensthe core for other heterogeneties which may cause acoustic velocity anisotropy.Ambient horizontal stress orientations can be used to predict hydraulicfracture propagation direction.
The CT scanner provides a state of the art method for evaluatingfractured rocks. Core can now be viewed to evaluate the three dimensionalrelationship of matrix and fractures.
The evaluation of fractured reservoirs throughout the world has becomeincreasingly important in the last 10 years. Recent texts by Nelson1, Aguilera2 and Van Golf-Racht3illustrate the importance of fractures in hydrocarbon reservoir geology anddiscuss the different approaches to understanding these complicated reservoirs.A recent advance in fracture measurement technology is the Electronic Goniometer, a fast and accurate instrument for measuring fracture orientationand recording descriptive fracture data. Another new technology, acousticvelocity anisotropy performed in conjunction with epi-fluorescence thin-sectionanalysis of microfracture orientations is an effective and economic means ofdetermining in-situ stress orientations. X-ray Computerized Tomography (CT)Scanning technology provides images of slices of core permitting evaluation ofthe three dimensional aspects of matrix and fracture interaction.
The Electromagnetic Goniometer, EMG-200, uses a three dimensional digitizer, interfaced with a PC work-station. The PC performs data storage and datamanipulation functions (Garrett et a1 4) The EMG- 200 consists principally ofthe 3SPACE, three-dimensional digitizer developed by Polhemus Navigation Systems (Raab5). The digitizer consists of an electromagneticsource, a sensor, and an electronics unit. The source and sensor each containthree orthogonally oriented coils. The source is mounted a few inches below thetable top where the core sample is placed for study. The sensor is housed in ahand held stylus. Currents flowing through the source coils induce magneticfields that in turn induce small, but detectable currents in the sensor. Theelectronic unit interprets this information to generate the position andorientation the stylus. Stylus location is determined to an accuracy of 0.032inches root mean square (RMS) with a resolution of 0.016 inches RMS. Thedigitizer is capable of collecting data continuously, or with a footswitch, transmitting discrete points to the host computer.
Just above the source rests the core holder which accommodates cores rangingfrom 5 to 15 cms in diameter and up to 75 cms in length.
