Monitoring and diagnosing completion during hydraulic fracturing operations provides insights into the fracture geometry, inter-well frac hits and connectivity. Conventional monitoring methods (microseismic, borehole gauges, tracers, etc.) can provide a range of information about the stimulated rock volume but may often be limited in detail or clouded by uncertainty. Utilization of DAS as a fracture monitoring tool is growing, however most of the applications have been limited to acoustic frequency bands of the DAS recorded signal. In this paper, we demonstrate some examples of using the low-frequency band of Distributed Acoustic Sensing (DAS) signal to constrain hydraulic fracture geometry. DAS data were acquired in both offset horizontal and vertical monitor wells. In horizontal wells, DAS data records formation strain perturbation due to fracture propagation. Events like fracture opening and closing, stress shadow creation and relaxation, ball seat and plug isolation can be clearly identified. In vertical wells, DAS response agrees well with co-located pressure and temperature gauges, and illuminates the vertical extent of hydraulic fractures. DAS data in the low-frequency band is a powerful attribute to monitor small strain and temperature perturbation in or near the monitor wells. With different fibered monitor well design, the far-field fracture length, height, width, and density can be accurately measured using cross-well DAS observations.

A seismic source (50) is buried in a multi-layered subsurface formation below a fast layer (30) and above a reflecting interface (10). The seismic source (50) excites a critically refracted (CR) wave that travels laterally along a fast layer bottom interface (35), and emanates downwardly into a slow layer (40) that is below and adjacent to the fast layer (30). One or more receivers (60), positioned below the fast layer (30) and above the reflecting interface (10) are used to detect seismic waves (84, 86). The one or more receivers (60) are positioned within a borehole (65). At least one reflected CR wave is isolated from the received signals, which is a CR wave that has reflected off of the reflecting layer (10) below the one or more receivers (60). A seismic profile of the multi-layered subsurface formation is created, using the at least one reflected CR wave. Time-lapse seismic monitoring of hydrocarbon extraction operations, such as steam injection, is also provided.

Monitoring and diagnosing completion during hydraulic fracturing operations provides insights into the fracture geometry, inter-well frac hits and connectivity. Conventional monitoring methods (microseismic, borehole gauges, tracers, etc.) can provide a range of information about the stimulated rock volume but may often be limited in detail or clouded by uncertainty. Utilization of DAS as a fracture monitoring tool is growing, however most of the applications have been limited to acoustic frequency bands of the DAS recorded signal. Low-frequency Distributed Acoustic Sensing (DAS) signal is used to constrain hydraulic fracture geometry. Events like fracture opening and closing, stress shadow creation and relaxation, ball seat and plug isolation can be clearly identified.

Embodiments presented provide for a fully automated method of high-resolution interval velocity estimation for vertical seismic profile-type data.

Methods are provided for tracking carbon dioxide (CO.sub.2) migration in a hydrocarbon-bearing reservoir located under a cap rock in a formation. In one embodiment, at least one seismic source and a plurality of receivers are located in spaced boreholes in the formation with the sources and receivers located near or at the reservoir so that direct paths from the sources to the receivers extend through the reservoir. CO.sub.2 is injected from the borehole containing the seismic sources into the reservoir, and the sources are activated multiple times over days and seismic signals are detected at the receivers. From the detected signals, time-lapse travel delay of direct arrivals of the signals are found and are used to track CO.sub.2 in the reservoir as a function of time. In another embodiment, the sources and receivers are located above the reservoir, and reflected waves are utilized to track the CO.sub.2.

A survey method includes generating a first amplitude modulation signal by amplitude-modulating a carrier wave repeating the same pattern at a predetermined cycle in each of a plurality of vibrators with a modulation signal whose cycle is 1/m times the predetermined period and is different for each of the vibrators, transmitting the seismic wave based on the first amplitude modulation signal, generating a second amplitude modulation signal in one or more receivers, the second amplitude modulation signal being identical to the first amplitude modulation signal generated by any one of the seismic vibrators, generating a reception signal in each of the one or more receivers by receiving a synthetic seismic wave in which the seismic waves generated by the seismic vibrators are synthesized, calculating a correlation value between the reception signal and the second amplitude modulation signal, and analyzing characteristics of the medium on the basis of the correlation value.

Downhole tools and method for a well. At least some of the example embodiments are methods of imaging a formation around a first borehole, including: focusing first outbound acoustic energy, launched from a tool with the first borehole, on a volume within the formation spaced away from the first borehole, the focusing creates a first virtual point source (VPS) that creates a first return acoustic energy; receiving the first return acoustic energy from the first VPS at a plurality of seismic sensors; and determining a parameter of the formation between the first VPS and a location of the seismic sensors using the first return acoustic energy.

A pressure wave is generated within a first well extending into a subterranean formation. A pressure response associated with the pressure wave is detected from a second well extending into the formation. Information is then determined, based on the pressure response in the second well, wherein the information is associated with at least one of the formation and a fracture connected to at least one of the first well and the second well.

Embodiments presented provide for a fully automated method of high-resolution interval velocity estimation for vertical seismic profile-type data.

A pressure wave is generated within a first well extending into a subterranean formation. A pressure response associated with the pressure wave is detected from a second well extending into the formation. Information is then determined, based on the pressure response in the second well, wherein the information is associated with at least one of the formation and a fracture connected to at least one of the first well and the second well.