INTRODUCTION
This paper describes the microseismic mapping of repeated injections of drill cuttings into two separate formations at a test site near Mounds, OK. Injections were performed in sandstone and shale formations at depths of 830 and 595 m, respectively. Typical injection disposal was simulated using multiple small-volume injections over a three-day period, with long shut-in periods between the injections. Microseismic monitoring was achieved using a 5-level array of wall-locked, wireline-ran, triaxial- accelerometer receivers in a monitor well 76 m from the disposal well. Results of the mapped microseismic locations showed that the disposal domain was generally aligned with the maximum horizontal stress, with some variations in azimuth, and that wide variations in height and length growth occurred with continued injections. These experiments show that the cuttings injection process can be adequately monitored from a downhole, wireline-ran receiver array, thus providing process control and environmental assurance.
Injection of slurries of ground drill cuttings into a deep formation is an environmentally attractive, cost-effective method for disposing of hydrocarbon- contaminated cuttings (Malachosky et al. 1993, Louviere & Reddoch 1993, Hainey et al. 1997, Bruno et al. 1995). However, the exact processes of fracturing, disaggregation, fracture branching, volumetric storage, and changes induced in the reservoir have not yet been fully identified (Abou- Sayed et al. 1989, Moschovidis et al. 1994, Wilson et al. 1993, Weng et al. 1997).
The Drill Cuttings Injection Experiment at a site near Mounds, OK, has the objectives of identifying the important mechanisms through a suite of monitored injections and the subsequent coting of the created fracture process zone. Monitoring techniques in this experiment include microseismic imaging, surface and downhole tiltmeter mapping, treatment pressure monitoring and the application of various tracers. This paper describes only the microseismic results conducted during July 1998. The Mounds site layout is shown? in Figure 1 and consists of a three-well pattern drilled to greater than 900-m depth. The injection well is in the center with the microseismic monitor well 76 m south of the injection well and the tiltmeter monitor well 43 m north of the injection well. The anticipated hydraulic fracture azimuth (Smith et al. 1985) is approximately east-west, so this layout was designed to give clear views of both wings of any
induced fracture system from either monitor well.
As shown in Figure 2, injection experiments were conducted in two zones including an open-hole section of the Wilcox sandstone at a depth of approximately 830 m and at perforations from 591- 5?/m in the Atoka shale. In both zones, 5-level receiver arrays (Warpinski et al. 1998) were damped across from the injection zone. These were then oriented using perforations and primer-cord shots in the treatment well and were left in place to monitor all of the injection tests. The spacing between individual receivers was 15.2 m.
