First EAGE Workshop on Fibre Optic Sensing

Distributed Acoustic Sensing (DAS) is being applied to an increasingly broad number of use cases across the whole life cycle of a hydrocarbon asset. There is a pressing need for the industry to adopt a standard data format to transfer, exchange and store this data, to address the huge data volumes and the inter-relational aspects of different versions of the data with respect to a given original recording set and the description of the data acquisition system . By formulating extensions to an existing packaging convention, making use of file formats optimized for large data arrays and specifying an indexing process, petabytes of data become manageable with rapid and precise access to specific recordings or processed data. A standardized method to store, access and extract data greatly facilitates exploring new uses and applications for DAS by researchers in operator or oilfield companies as well as academia. Implementations to date are discussed in the paper.

We analyze perforation shots recorded by a downhole DAS array. These sources induce guided waves, which propagate in a low-velocity shale reservoir. They are dispersive and contain a broad frequency content, reaching up to 700 Hz. They are especially valuable in a horizontal cross-well configuration, in which case they propagate through previously stimulated areas. Guided S-waves are very sensitive to the presence of fluid-filled fractures. P-waves undergo a slowdown but are overall less affected. Using simple geometrical considerations, we can estimate the horizontal extent of the fracture system. It shows considerable spatial variability, indicating that stimulation effects are not constant. This study is the first large-scale use of guided waves recorded by DAS. They hold tremendous potential for high-resolution imaging and inversion of subsurface properties before and after stimulation. As such, they open new possibilities for the use of seismology in optimizing unconventional hydrocarbon recovery.

The strong and regular acoustic signal provided by working pumps spreads along a fractured well depending upon the propagating fracture geometrical and physical attributes, therefore, one can devise fracture properties by interpreting downhole DAS data. We describe a quantitative DAS data inversion workflow and give a synthetic example of hydraulic fracture monitoring. Specifically, profiling the wellbore in real time in terms of Normalized Injectivity Indices (NII) calculated out of DAS pump noise data is described, and inversion for fracture parameters by matching NII to a forward model of pulse propagation in a fractured wellbore is outlined. The attractiveness of the proposed method is that the pumping noise is present at each frac job and that every pump generates a plurality of harmonic frequencies, thus there’s always plenty of data for inversion. We also stress that interference of the pump signal with the flow noise should be accounted for in DAS interpretation.

Europe is working hard to meet its first climate goals. With High Temperature Aquifer Thermal Energy Storage (HT-ATES) large quantities of green energy can be stored in the subsurface in the form of heat. It is important that a site is characterised well as leakage from the reservoir would degrade the efficiency of the system, and could also endanger potable water supplies. Seismic imaging is currently the method with highest resolution and the largest de-risking capability for geothermal projects. This study pursues the use of fibre-optic distributed acoustic sensing (FO-DAS) technology in seismic acquisition to improve the de-risking ability of seismic data in urban HT-ATES settings. In 2019 TNO and GSB combined a dense surface based high resolution seismic survey with a FO-DAS VSP survey on a potential ATES site in the city of Brussels, Belgium. Besides the technology demonstration, targets were aquifer + seal continuity and depth away from the well as well as acoustic velocity of the sediment overburden for H/V Spectral Ratio calibration. The intermediate result indicates that with FO-DAS it is possible to image the subsurface relatively easy, fast, at low cost and with low environmental impact, even in busy seismically noisy urban areas.

Distributed Acoustic Sensing (DAS) is a recently established technology that has considerable potential for monitoring subsurface microseismic activity. Increasing numbers of wells have fibre optic cable installations and with a DAS interrogator these cables can be converted into high-density high-coverage seismic arrays. However, DAS systems can have higher noise floors than that of geophone arrays which make these dense datasets technically difficult to handle and a challenge to accurately pick P- and S-phase arrivals from. Traditional manual approaches are not viable on such sizeable data volumes, and hence automated solutions must be developed for efficient analysis.

Here we test how conventional filtering and picking methods can be combined and applied to a DAS microseismic dataset. These approaches are tested on a single hydraulic fracture stage recorded by Silixa Ltd. iDAS system. 302 events were automatically detected and windowed. We manually pick the events to approximate a velocity model and then locate the events. We then test filtering (median, Wiener, bandpass, F-K) and picking methodologies including standard STA/LTA and a guided STA/LTA. The combined approaches that we have developed have produced equivalent results to manual picking but are significantly faster while analysing much higher density of traces.

For real-time seismic data quality control and delivery, distributed acoustic sensing (DAS) systems need to synchronize with the seismic controller. We describe a DAS system that has the ability to encode all auxiliary seismic signals, such as GPS timing, time break signal, vibe sweep, ground force, and hydrophone pressure, directly onto the optical data stream using a series of piezo-electric fiber stretchers in-line with the sensing fiber. Similar to conventional, wireline or permanent geophone VSP data acquisition systems, the time break signal is utilized to trigger data acquisition, which ensures precise time synchronization at sub-millisecond accuracy between the DAS system and the seismic source. The DAS system can operate in either master or slave mode relative to the seismic source, with synchronization via radio or 4G telemetry between the seismic controller and the DAS unit. The system enables effective data management and the real-time generation of 1 ms sampled seismic and navigation data that conforms to the industry-standard SEGY data format; thus eliminating latency between acquisition and field data deliverables.

The Bida and the Sokoto Basins are two of Nigeria’s inland basins that constitute another set of a series of Cretaceous and later rift basins in Central and West Africa whose origin is related to the opening of the South Atlantic. Geophysical aeromagnetic interpretation has assisted in the interpretation of the geology of the basins. Organic geochemical studies show that the Kudu Shale in the Northern Bida Basin equivalent to the Ahoko Shale in the Southern Bida Basin and the Dukamaje Formation (dark shales and limestones) in the Sokoto Basin constitute the source rocks in the potential petroleum system. With averages for source rock thickness of 40m, area of basin of 45,000km2, TOC of 9.0wt%, and HI of 220mgHC/gTOC, charge modeling indicates 623 million barrels of oil equivalent extractable hydrocarbons in the Basin at the appropriate kitchens.

Fibre-optic Distributed Acoustic Sensing (DAS) offers the ability to record seismic waves with high density measurements in time and space, thus enabling a detailed analysis of subsurface structure and the seismic waveform. The aim of this study is to compare the suitability of high-quality geophones and fibre-optic cables for site characterisation of potential nuclear waste repositories, using the Forsmark site in Sweden as a case study. Initial results of a walkaway vertical seismic profile (VSP) survey are presented here using data recorded on geophones and DAS data recorded on linear, helical and helical engineered cable using two different interrogators. The deployment of one linear and two helical cables in the same borehole enables a comparison of their sensitivity to seismic waves in field data. Of particular interest is the difference between the SNR of data recorded on geophones and the engineered helical cable as part of the new generation of DAS systems.

Helically wound cables (HWCs) have been introduced to address the problem of broadside insensitivity of DAS to P-waves. They are well suited for applications that mainly involve P-waves like surface seismic, but downhole applications like microseismic monitoring involve a wider range of propagation directions and the need to also record S-waves, requiring a broader sensitivity analysis to determine their suitability. Here we model the response of HWCs by projecting the strain tensors of P- and S-waves onto helical fibre geometries. We show that although HWCs boost broadside sensitivity of P-waves they have a destructive effect on S sensitivity, due to the deviatoric nature of the S-wave strain tensor. Illustrating this with a synthetic microseismic waveform, we find that S amplitudes are highest for straight fibres, and decrease as the wrapping angle in lowered, eventually vanishing at ∼35°. For smaller wrapping angles S amplitudes increase again, but with opposite polarity. We conclude that HWCs are ill-suited for microseismic applications, but may provide valuable supplementary information. We suggest a potential application for a cable with multiple helixes with differing wrapping angles, which could exploit the differences in response of P- and S-waves to provide an automated method of classifying arrivals by phase.

The interest in utilizing fiber-optic cables in surface seismic and borehole acquisition designs has led us to establish a test facility in Houston to explore DAS latest developments and technologies. We show the latest results of acquiring zero-offset and offset vertical seismic profiling datasets in the shallow land well in Houston. The datasets are recorded to assess multiple interrogation boxes provided by different vendors. The datasets are also collected using different cable configurations such as cemented DAS cable behind casing, pumped cable inside control line, and a fiber on a spool. We also demonstrate the advantages and disadvantages of utilizing either multi-mode or single-mode fiber-optic cables for recording VSP seismic data. Our objective is to provide the best mode procedure to acquire quality DAS data for different purposes such as checkshot retrieval, velocity analysis, and subsurface imaging.