Time-Lapse Ultrasonic Measurements of Laboratory Hydraulic-Fracture Growth: Tip Behavior and Width Profile
- J. Groenenboom (Delft U. of Technology) | D.B. van Dam (Delft U. of Technology) | C.J. de Pater (Delft U. of Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2001
- Document Type
- Journal Paper
- 14 - 24
- 2001. Society of Petroleum Engineers
- 5.6.5 Tracers, 5.1.1 Exploration, Development, Structural Geology, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.2 Pipelines, Flowlines and Risers, 5.1.6 Near-Well and Vertical Seismic Profiles, 1.14 Casing and Cementing, 3 Production and Well Operations, 4.3.4 Scale, 1.2.3 Rock properties, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.5.2 Core Analysis
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We performed acoustic measurements in a time-lapse sequence in scaled laboratory tests. The advantage of time-lapse measurements is that the fracture response can be separated from the background signal. As a consequence, not only can the hydraulic fracture be detected, but its shape and geometry can be measured during its growth. This application requires the combined information of both compressional- and shear-wave measurements. We apply this technique to propagation, flowback tests, and reopening of hydraulic fractures.
Acoustic waves excite diffractions at the fracture tip. These diffractions are used to locate and to characterize the fracture tip. The acoustic measurements indicate that we can distinguish between a dry tip and the fluid front of the fracture.
Shadowing of shear-wave transmissions allows estimation of the moment of fracture initiation. The width profile of the fracture is determined with compressional-transmission measurements. This application is based on the fact that the attenuation and time delay of compressional transmissions are proportional to the fracture width. Analysis of a flowback test shows that the fracture closed at the wellbore but remained open farther away from the wellbore.
Design of hydraulic fractures is based on assumptions about the fracture geometry and on a description of the target formation. Fracture behavior in subsurface formations is not only a complicated process, but there is often a lack of sufficient formation data. Therefore, we studied the fundamental fracture process in laboratory tests in conjunction with novel measurements of the fracture geometry using active acoustics.
Medlin and Massé1 pioneered acoustic monitoring of hydraulic-fracture growth in laboratory tests. They showed the possibility of measuring fracture length by detecting transmission losses of compressional waves. On the basis of their results, we decided to build a state-of-the-art acoustic-monitoring system in our triaxial load frame.2,3 These experiments resulted in the first detection of strong compressional diffractions scattered from the perimeter of the fracture.2 We used arrival time of the diffraction to estimate fracture length.2,3 We recently extended the method to determine fracture width3,4 and improved the setup to detect shear waves that contain useful information on fracture closure and reopening.3,5
The measurements proved to be extremely valuable in the laboratory, and they are also promising for determining the dimensions of fractures in the field. As yet, only a small number of field treatments have been combined with active acoustic measurements, and these have been only partly successful because of the complex acquisition geometry and conditions at depth. Both differential vertical-seismic-profiling (VSP) experiments6-8 and crosswell experiments9-11 have shown the possibility of monitoring fracture propagation by use of the phenomenon of shear-wave shadowing. When propagation of the shear wave is disturbed by creation of a hydraulic fracture, transmission of the shear wave is lost or shadowed because shear waves cannot propagate through fluid. Hence, shear-wave shadowing indicates that the fracture intervenes the ray path from source to receiver. Field tests were used in this way to estimate the global fracture geometry. Active measurements can also be used to determine small-scale details of fractures near the borehole. Recently, advances in sonic tools with dipole sources operating at low frequency have shown that it is possible to monitor fractures from the same borehole.12
The advantage of acoustic monitoring in the laboratory instead of in the field is the flexibility to try various acquisition geometries for relatively low cost. Laboratory experiments provide a unique opportunity to gain information about the acoustic data that can be acquired and their application.
The first section of this paper describes the experimental setup. The second section discusses the observations of acoustic diffractions and their relation to the fracture tip. We use these diffractions to locate the position of the fracture tip. Estimation of fracture size by use of the direct diffractions allows prediction of the arrival time of more complicated events, including those related to surface waves along the fracture. With this arrival-time prediction, we can interpret the various events that have been measured in the laboratory. In addition, we show that the acoustic diffraction measurements can distinguish between migration of the fluid front and the dry tip.
The third section of the paper discusses observations and applications of transmission measurements. Shadowing of shear-wave transmissions allows us to determine the moment of fracture initiation. Compressional waves are partially transmitted. We propose a waveform fitting procedure and show that accurate and reliable estimates of fracture width can be obtained. By combining several transmission measurements of different source/receiver combinations, we reconstruct the full fracture width profile during propagation and closure.
Weijers13 gives a detailed description of the experimental setup for the fracture experiments, and Savic2 and Groenenboom3 describe the acoustic part. We limit ourselves to those parts of the experimental setup that are necessary to understand the data we show in this paper.
A 0.3-m cubic block is stressed in a true-triaxial-compression machine. The load frame consists of three perpendicular compression systems, each of which can independently deliver a maximum force of 3500 kN (Fig. 1). For this block size, the maximum pressure that can be imposed on each side is 40 MPa, which corresponds to the effective in-situ stress at a depth of approximately 3000 m. Because the sample is not supported at the ribs, it is impossible to apply pore pressure to the rock.
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