Fluid pressure sensing with fiber-optic distributed acoustic sensing

Abstract

Measurement of interwell hydraulic interference is a fundamental method of characterizing the permeability structure of geothermal, carbon sequestration, and petroleum reservoirs. A new system of pressure measurement is demonstrated that utilizes fiber-optic cable. In the laboratory, fiber-optic distributed acoustic sensing (DAS) was used to measure oscillating pressure signals employed at mHz frequencies. DAS measures oscillatory strain rate along the fiber-optic cable caused by oscillatory pressure changes. Pressure was measured in a water-filled reservoir subjected to an oscillating water level. Because the native measurement of the DAS system is strain rate, the quality of the measurement degrades with longer oscillation periods and smaller pressure changes. Tests showed a linear relationship between DAS strain and hydrostatic pressure for short oscillation periods (<10 s), but a poor relationship for longer periods (>100 s). The approach exhibits poor sensitivity to fluid pressure compared with piezoelectric transducers. However, because fiber-optic cable can withstand harsh environments and measurements are distributed (every 0.25 m along the fiber-optic cable), further refinement may make DAS useful for geofluid monitoring. This is especially true given that geomechanical, seismic, and temperature monitoring may be conducted along the same cable.

Distributed Acoustic Sensing Using Dark Fiber for Near-Surface Characterization and Broadband Seismic Event Detection

“…While long period sensitivity is a major limitation of many inertial seismic sensors (e.g., accelerometers, short-period geophones, smartphone sensors), the long period response of DAS is currently a topic of active research with only limited available data 24 ; teleseismic earth motion (strains near 1 × 10 −8 ), for example, may be dominated by thermal expansion of the fiber-optic cable (strains on the order of 1 × 10 −6 ) depending on the frequency studied as well as the depth, composition, and condition of the fiber-optic cable and conduit. Recent studies 24,56 have used shallow hydrogeologic pump tests in a well with a fiber-optic cable to show that DAS has sensitivity to 9.4 × 10 −3 Hz (period = 1080 seconds) oscillations in strain induced by the variable confining pressure, presumably due to Poisson effects. This subject is complicated by the known directionality of DAS cables 57,58 , which for the horizontal geometry of telecommunications dark fiber cables is theoretically insensitive to vertically-incident compressional motion (P-waves).…”

Distributed Acoustic Sensing of Strain at Earth Tide Frequencies

“…DAS is typically used to measure strains in frequencies of Hz or kHz. In previous work we have demonstrated in the laboratory [7] and field [8,9,10] that frequencies in the range of mHz can be extracted from DAS signals. The current study focuses on laboratory studies in which we show that periodic strains can be extracted in the microHertz (μHz) range, i.e., a frequency of 23.1 (μHz) or a period of 0.5 day.…”

“…However, our success in the mHz frequency suggested earth-tide measurements might be possible. We have measured, for example, periodic fracture displacements in a rock borehole of less than 0.6 nm at a frequency of ~1 mHz [10] and displacement of fiber in the laboratory of less than 1 pm at a frequency of ~10 mHz [7].…”

Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock

“…Pressure changes in a water column will cause the fiber to compress radially, which may result in a measurable displacement (lengthening) of the fiber according the Poisson's ratio of the fiber glass. Our laboratory tests indicate that heads of about 10 cm or more are necessary to make a measurable displacement in the fiber (Becker, Coleman, et al, 2017), so it is not a very sensitive direct measurement of fluid pressure. In any case, variations in fluid pressure in an open wellbore are generally uninteresting in shallow wells.…”

“…Because of the native measurement displacement rate, signal‐to‐noise degrades at lower frequencies. In the experiments reported here, fiber displacement of less than 1 nm was measured at millihertz frequencies (Becker, Coleman, et al, 2017).…”