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.