Summary The first exploration-and-production downhole field trial of distributed acoustic sensing (DAS) fiber-optic technology was conducted during the completion of a tight gas well in February 2009. DAS is a novel technology that allows the detection, discrimination, and location of acoustic events on a standard telecom single-mode fiber several kilometers long. Using a combination of the measurement of backscattered light and advanced signal processing, the DAS interrogator system segregates the fiber into an array of individual microphones. To date, the technology has been applied mainly in the defense and security industries. One of the most exciting applications for downhole application of DAS is in the area of hydraulic fracturing of tight-sand and shale-gas reservoirs. Balancing the cost of hydraulic-fracture stimulation with the production benefit is crucial in tight-sand and shale-gas developments because, after drilling costs, the completion is the largest single cost component of the well. Recordings can be made while tools are run in hole, bridge plugs are set and perforations are shot and during the fracture-stimulation treatment. The technology is sufficiently reliable and sensitive to detect and monitor these in-well activities. The fidelity of the recordings made during hydraulic-fracturing and flowback operations provides a step-change improvement in the ability to perform real-time and post-job diagnostics and analyses of the stimulation. The different case studies presented in this paper will illustrate how, even in its earliest form, DAS has the potential to enhance the capability of monitoring and understanding in-wellbore activities. The technology enables the optimization of hydraulic-fracturing design and execution, which can drive down completion costs and lead to increased well productivity and ultimate recovery.

Time-lapse (4D) seismic monitoring of pressure-induced changes in depleting gas fields reveals that detectable differences in seismic arrival times are observed above the reservoir interval. Geomechanical models of depleting reservoirs predict that as a result of reservoir compaction due to pressure depletion, changes in the long-wavelength stress and strain fields occur in the rocks bounding the reservoir. Models incorporating the geomechanical stress and strain field changes predict changes in the two-way arrival times that are compared with actual time-shift observations at a depleting gas field in the North Sea. The geomechanical-based predictions are in good agreement with the observations. Detecting geomechanical changes in the over-and underburden rocks opens up new ways of using 4D data, especially in places where the signal from the reservoir rocks is small.

In order to make commercial and development decisions effectively and more rapidly, new appraisal and testing technologies are needed to maximize early data collection and subsequent subsurface understanding as quickly as possible. For Unconventional Gas and Light Tight Oil (UGLTO) projects, some of this critical data can be derived from hydraulic fracture stimulation and inflow profiling activities. For UGLTO projects, achieving an optimum hydraulic fracture stimulation is a continuous endeavor beginning as early as possible; and balancing the cost of completion vs. production performance is critical as the completion/stimulation is a large cost component of the well and heavily influences production rate/ultimate recovery. The fast paced development and introduction of new completion technologies requires diagnostic technology that can help us understand stimulation effectiveness, assess new completion technologies, and evaluate which zones are the most productive. One emerging technology, fibre optic distributed sensing has the potential of providing key insights during both the hydraulic fracturing and initial flowback. The passive nature of fibre optic sensors allows intervention-free surveillance, which makes fibre-optic technology an effective platform for permanent sensing in producing wells. Until recently, the oil & gas industry fibre optic sensing technology has focused mainly on temperature (DTS) profiling along the wellbore. In 2009, it was first demonstrated how fibre optic distributed acoustic sensing (DAS) can also be used for downhole applications. Where hydraulic fracture diagnostics based on DTS alone in the past sometimes yielded ambiguous results, the combination of both acoustic and temperature sensing provides a step-change improvement in the ability to perform real-time and post-job diagnostics & analyses of the stimulation. The different horizontal well case studies presented in this paper will illustrate how the combination of DTS and DAS has the potential to enhance the monitoring, assessment, and optimization of openhole and limited entry designed hydraulic fracture stimulation treatments.

Deciding on the optimum spacing between fractures and selecting the optimum fracture treatment parameters is a key challenge in designing the hydraulic fracture stimulations of Unconventional Gas and Liquid Rich Shale (UGLRS) wells. To make those decisions more effectively and more rapidly, (downhole) hydraulic fracture diagnostic tools can be used which provide a better understanding of how and where fractures initiate and what the distribution of fluid and proppant volume is downhole. One emerging technology, fiber optic distributed acoustic sensing (DAS) has the potential of providing such key diagnostic insights during hydraulic fracturing operations in real-time. This paper describes some of the background technology and presents the results of several hydraulic fracture stimulation (HFS) diagnostic case studies. The results illustrate how DAS has been used to perform real-time monitoring for both open-hole multi-stage fracturing and "Cemented Plug & Perf Completions". DAS has provided valuable insight as to the stimulation effectiveness. The technique has also provided insights into effective zonal isolation when using mechanical isolation during the hydraulic-fracturing process that would otherwise not have been possible. It also complements other HFS diagnostic technologies (e.g. tracers, micro-seismic, distributed temperature sensing (DTS), production logs (PLT)). DAS monitoring of hydraulic fracture stimulation can help accelerate the learning curve and drive performance improvements. Installation of fiber optic cables early in a field's life or when entering a new geological/geo-mechanical situation can allow for accelerated optimization of future wells.

Distributed Acoustic Sensing (DAS) deployed in a wellbore can be used to detect P-waves and S-waves generated in the subsurface. In a well which has geophones and a DAS cable deployed, micro-seismic events have been detected on both instruments at the same time, establishing the concept of using DAS as a micro-seismic detector. While DAS is still less sensitive than geophones, it has the advantage of being non intrusive and a permanent installation, so both recording in treatment wells and 4D recording concepts are realistic options that can be implemented once the fibered cable is installed.