Evaluating Hydraulic Fracturing in Cased Holes With Cross-Dipole Acoustic Technology
- X.M. Tang (Baker Atlas) | D. Patterson (Baker Atlas) | M. Hinds (Baker Atlas)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2001
- Document Type
- Journal Paper
- 281 - 288
- 2001. Society of Petroleum Engineers
- 1.14 Casing and Cementing, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.1 Waterflooding, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3 Production and Well Operations, 5.8.7 Carbonate Reservoir, 2.2.2 Perforating, 3.3.2 Borehole Imaging and Wellbore Seismic, 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.6.5 Tracers, 5.6.1 Open hole/cased hole log analysis, 4.1.5 Processing Equipment, 1.6 Drilling Operations
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A cross-dipole technology was used to evaluate a carbonate formation in southeastern New Mexico to determine fracture trends in a waterflooded environment. The measurements were first made in open hole, then in cased hole before and after fracture stimulation. The cross-dipole data were processed to find the amount of shear-wave anisotropy and the associated azimuth. The results demonstrate that stimulated fractures create a substantial anisotropy and a well-defined azimuth behind casing. More important, by evaluating the anisotropy magnitude and azimuth from the cased-hole data, we can determine the fracture extent along the borehole and its azimuth in the formation. The fracture extent is also consistent with that from a radioactive tracer measurement. The results of this study suggest that cross-dipole acoustic logging is an effective technology for cased-hole fracture stimulation evaluation.
Hydraulic fracture stimulation in cased boreholes can be effectively performed by pressurizing perforations through casing. But evaluating the stimulation result presents a formidable task. The presence of casing makes it difficult to evaluate and detect the vertical extent and azimuth of the stimulated fractures. This paper presents a solution to this problem using cross-dipole acoustic logging technology.
Natural or stimulated fractures parallel to and intersecting a borehole create azimuthal shear-wave anisotropy around the borehole. The amount of anisotropy gives an indication of fracture intensity, and the associated fast-shear polarization azimuth gives the strike of open fractures. For open holes, this fracture-induced anisotropy can be effectively measured with cross-dipole acoustic logging.1 Application of this technology to cased holes has been hindered by two factors. The first is the concern of the effect of casing and cement on the cross-dipole measurement, and the second is the lack of an effective device to measure the tool's orientation inside casing.
We performed numerical modeling to show that a cross-dipole tool can measure shear-wave anisotropy through casing and cement provided that they are well bonded with the formation. A gyroscope device allows for measuring azimuth in cased wells. With these foundations, we can apply cross-dipole technology to cased-hole analysis.
A well in southeastern New Mexico was chosen to evaluate the cross-dipole technology for cased-hole applications. This well was drilled into a carbonate formation at a depth of approximately 6,800 ft. Petrophysical analysis was performed on this well to obtain the geological formations inherent in the area. This example is located in the northwest edge of the Central basin platform and is bordered to the west by the Delaware basin, to the east by the Midland basin, and near the northwestern shelf and the Captain Reef trend. These basins are parts of what make up the Permian Basin.
The goal of this test was to determine the fracture trend in this waterflooded field. First, openhole logging in the well was performed to locate the zones of interest and to determine the amount of anisotropy that existed before casing the well. The openhole logging found two intervals of interest near the bottom of the well. These intervals are 6,650 to 6,770 ft and 6,168 to 6,370 ft, respectively. After the borehole was cased, the same logging measurements were repeated for comparison purposes. The intervals of interest were then completed in two separate stages. The first stage included only the interval of 6,650 to 6,770 ft. It was hydraulically fractured and tagged with three radioactive isotopes: scandium (Sc-46), iridium (Ir-92), and antimony (Sb-124). The scandium was pumped with fluid into the perforations during the fracture stimulation. The fracture was subsequently sand-tagged with iridium (I-92) and antimony (Sb-124), both being solid proppant. As shown by the tracer analysis results (see Figs. 6 and 7 later in this paper), the bulk of the fracture was placed from 6,646 to 6,710 ft; the fracture grew down 20 ft below the bottom perforation and up to 6,545 ft, with a gradually reduced migration up to 6,450 ft. The second completion stage was for the interval of 6,168 to 6,370 ft, which was fractured without any radioactive isotopes. The radioactive tracers can be detected with a cased-hole spectral gamma ray device so that the various stages of the hydraulic fracture can be monitored for their vertical migration away from the perforations. Along with the post-stimulation spectral gamma ray measurement, the cross-dipole acoustic logging was repeated. The results of this logging, together with those of the openhole and prestimulation cased-hole logging runs, were analyzed to determine the vertical extent and azimuth of the stimulated fractures.
In the following, we demonstrate the theoretical modeling results for cross-dipole logging through casing, and we present the measurement results for openhole and pre- and post-stimulation cased-hole logging runs. Finally, we interpret the results and provide the conclusions of this study.
Cross-Dipole Measurement Through Casing
A dipole acoustic tool performs a directional measurement by inducing and receiving flexural (or bending) waves along the borehole. A cross-dipole tool consists of two sets of dipole transmitter-receiver systems facing 90° apart (Fig. 1). The cross-dipole tool measures azimuthal shear-wave anisotropy around a borehole. This anisotropy has two orthogonal polarization directions, referred to as the fast-shear and slow-shear wave polarization directions, respectively. The magnitude of the anisotropy is measured by the fast and slow shear-wave velocity (or slowness) difference. During cross-dipole logging in an anisotropic formation, the borehole flexural wave motion induced by a source transmitter splits into fast and slow waves. These two waves are received by in-line and cross-line receiver arrays on the tool. For the in-line receivers, the maximum receiving sensitivity is in the source vibration direction, while for the cross-line receivers, this sensitivity direction is perpendicular to the source direction. The tool acquires a four-component array data set (two in-line and two cross-line). These data are processed with an array waveform inversion method.2 This method computes the fast and slow dipole-shear waves from the four-component data and matches the fast and slow waves across the array to determine the magnitude and azimuth of the anisotropy simultaneously.
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