Detection Within The Wellbore Of Seismic Signals Created By Hydraulic Fracturing

Abstract

Abstract Early in 1978, Sandia Labs participated in massive hydraulic fracture mapping experiments with Amoco in the Wattenburg area. On two of these massive hydraulic fractures in the Sussex formation, a downhole, wall clamped, three-axis geophone was tested. On the first experiment, the system was clamped in the open hole section during the breakdown phase. On the second experiment, the system was located in the lubricator during the main fracture and was lowered into place after shut-in. Breakdown pump of the first experiment was conducted in four phases. The formation was first broken down and shut-in for a quiet period and then three 5000 gallon stages of fluid without proppant were pumped with a quiet period after each one. Following the last quiet period, flow back was started and half way through a shut-in was scheduled for the fifth and last quiet period. During even the smallest flow rates, the noise induced into the geophones was extremely large and masked any other seismic activity. During the quiet periods, several seismic events were observed. These apparently are from two sources:motion associated with permanent movements of the fracture face permanent movements of the fracture face andhigh frequency impulsive sources possibly associated with thermal possibly associated with thermal fracturing. Following the 124,000 gallon fracture on the second experiment, the seismic system was lowered into place and clamped into the casing 50 feet above the open hole section that had been fractured. Seismic signals were recorded for approximately six hours after shut-in when the test was terminated. Both types of signals seen on the early experiment also appear to be present after the fracture treatment. Introduction With the increased use of massive hydraulic fracturing, the knowledge of fracturing dimensions and orientation has increased in importance. The efficient and economic placement of wells for the optimum development of a field will require that fracture orientations be known. Fracture detection and orientation techniques received a considerable effort by El Paso Natural Gas in their Pinedale Field in 1974 and 75. The importance of determining fracture orientations was demonstrated by their research program in fracture mapping and the economic implications were described in reference 2. Seismic detection of fracture signals has been ongoing for several years. In an early attempt to detect fractures at Oak Ridge, surface recording of seismic signals was utilized. The fact that seismic signals are created by hydraulic fracturing and that fracture faces may be mapped by determining the originating point of the signals has been well established. The frequency content of the seismic signals and the attenuation of the earth makes it imperative that seismic recordings be made close to the fracturing if the locations are to be determined. Extensive seismic recordings that were made at the surface during a massive hydraulic fracture in the Wattenburg area by Sandia proved to be incapable of determining fracture orientation. However, seismic signals can be received in the wellbore and these received signals used to map the orientation and plan view of the fracture in the vicinity plan view of the fracture in the vicinity of the wellbore. Following the Wattenburg experiments in 1976, where surface seismic signals were not detected, Sandia initiated their program to develop a borehole seismic program to develop a borehole seismic recording system.