English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We present a vision for fully autonomous marine seismic acquisition systems where all activity takes places on the seabed, with no reliance on costly surface vessels. This system would be most suitable to enable low-cost, ondemand reservoir monitoring in deepwater fields, for example, those in the Brazilian pre-salt province. The sensor components of the system are developed or under development (i.e., permanent or semi-permanent sensors on the seabed or in wells). For the source components we propose mobile impulsive or vibratory sources which are towed by an AUV close to the seabed or carried by a RAUV on the seabed. Such a system would be naturally distant from environmental concerns and interference with surface operations, would support simultaneous source operations to increase survey efficiency, and would be resilient against water velocity variations which limit 4D seismic resolution. We consider reflection geometries and longoffset target-oriented refraction geometries, to reduce the load on the source system (much fewer source points). We also discuss the requirements for AUV endurance. Introduction Seismic acquisition requirements in deepwater and the technologies to fulfill them depend critically on the desired business objectives. For exploration, one requires coarse images over large areas at relatively low cost, such as provided by towed streamer narrow azimuth seismic. Where feasible, this technology has evolved into Wide Azimuth (WAZ) seismic, which uses multiple vessels. For field development planning the areas are smaller, but the quality and resolution need to be better. In complex geological settings the use of Ocean Bottom Nodes (OBN) is becoming widespread, especially in areas such as the pre-salt in the Brazilian Santos Basin where multiple simultaneous vessels have not been permitted (precluding the use of WAZ). For reservoir monitoring the requirements become yet stricter with the need for very good data repeatability and frequent or on-demand surveys. Permanent or semi-permanent ocean bottom or in-well sensor systems become preferable in that case. In all cases seismic acquisition faces challenges in terms of cost, ability to execute on demand, interference with other operations, and environmental restrictions. In this paper we focus on solving these problems in the area where they are most acute, namely seismic monitoring, by considering fully autonomous marine acquisition. To this end we discuss existing autonomous seismic sensors and sources and propose new ones. A realistic solution also requires new, fit-for-purpose acquisition geometries. Ultimately, we envision a situation where all activity takes place autonomously on or near the seabed. Such fully autonomous marine acquisition systems would be a very welcome technology solution to monitor the Brazilian pre-salt fields (Lopez, Cox, Hatchell, & Wang, 2017). Indeed, the prolific carbonate reservoirs are being exploited using a combination of water, gas, and wateralternating-gas injection schemes. Due to limitations on the capacity of the FPSO systems to separate gas from the produced liquids, gas cycling (i.e., injected gas that finds its way to the producers sooner than models predict) needs to be avoided, otherwise liquids production will need to be curtailed and oil production will drop. Similar issues occur with injected water. Seismic monitoring could provide information about the distribution of gas and water in the reservoir and prompt actions to avoid them reaching the producers when not wanted. Seismic monitoring would also add value in its traditional use to identify by-passed oil and enable better placement of infill wells. Due to the recent increase in licensing rounds and in the diversity of operators, the opportunity for deployment of automated acquisition systems to monitor the pre-salt is growing. Seabed autonomous acquisition systems would enable cost reductions by removing dependence on costly surface vessels, facilitate on-demand data acquisition, and be distant from environmental concerns and interference with surface operations. In the Brazilian pre-salt it would be 2km away from surface activities and 2km closer to the reservoir. Such location should allow operation of multiple seabed sources and render their likely small power more effective. By-passing the entire water column would make the system resilient to water velocity variations between or during 4D seismic surveys, which ultimately limit the 4D seismic resolution (Theriot, Yin, & Lopez, 2017), especially important where the 4D signals are expected to be small. Autonomous Marine Acquisition – Sensors We consider permanent or semi-permanent sensors as possible elements for an autonomous acquisition system. These may be deployed on the seabed or in wells. (Retrievable sensors that “fly” to the seabed and position and retrieve themselves autonomously could also be considered, if they would not rely on surface vessels.) Permanent Reservoir Monitoring (PRM) systems comprise ocean bottom cables permanently installed on the seabed containing sensors, typically at 100m spacing along the cable, with the cables themselves arranged to cover the area of interest, and the whole system connected to a surface vessel, such as a FPSO. PRM systems installed in deepwater in the Brazilian Campos Basin at O-North (Ebaid, Wang, Seixas, Kumar, & et al., 2017) and Jubarte (Thedy & et al., 2015) have provided very high quality 4D seismic data. Here we consider fiber-optic systems, which are expected to be more reliable in the long term. FULLY AUTONOMOUS MARINE SEISMIC ACQUISITION SYSTEMS FOR RESERVOIR MONITORING  Sixteenth International Congress of the Brazilian Geophysical Society 2 Fiber-optic cable systems can also be installed in wells, using Distributed Acoustic Sensing (DAS) to record VSP data along the borehole where the cable is installed, with the DAS interrogator units located on the topsides or at the seafloor. This application has been matured for reservoir monitoring (Mateeva, et al., 2017) and is available for wells with direct vertical access from the platform. Deployment in the more numerous subsea wells (e.g., all the ones in pre-salt Brazil) is expected in the next few years. Finally, semi-permanent nodes, which can stay on the seabed for years and record seismic data on demand, have been developing using ROVs (Magseis Fairfield, 2015) or AUVs (Lopez, Cox, Hatchell, & Wang, 2017) for node communication and data harvesting. The nodes that use a subsea resident AUV (aka On-Demand OBN) would qualify as autonomous, as they do not depend on surface vessels; these are under development with a specification of 500 days of data recording in 5 years before refurbishing (Lopez, Chalenski, & Grandi, 2018). The options for sensors discussed above and the options for sources to be discussed below are listed in Figure 1 and compared across a number of dimensions: cost (CAPEX and OPEX), system limitations, impact on quality of 4D seismic acquired by the system, reliability in the field, ease of mobilization, status of the market, and level of maturity. The colors are meant to guide the eye for ease of comparison of advantages and disadvantages. Autonomous Marine Acquisition – Sources Autonomous seismic sources that operate similarly as conventional sources have been introduced and tested, for example in the embodiment called Rapid Autonomous Marine 4D (RAM4D) (Chalenski, Hatchell, Lopez, & Ross, 2017) (Patent No. US 2018/0164456). RAM4D uses an Unmanned Surface Vessel (USV) to tow a small airgun array and conduct seismic surveys ‘over the horizon’, although with some degree of supervision (Anderson, 2018). The system is restricted to small sources (e.g., 500 ci) due to the limited space available for the airgun compressor in the typically small USV (vessels up to 12m long have been tested) (Chalenski, Lopez, Hatchell, & Grandi, 2018). Nonetheless, airgun sources of that volume have been shown to be useful in many instances, even to great depths and in reservoir monitoring applications (Chalenski, Wang, Lopez, & Hatchell, 2016). This system uses surface vessels and does not address the issue of interference with surface operations and only partially the issue of environmental restrictions (by using small airgun sources, although marine vibrators could also be towed). Moving the source to or near the seabed has been considered and tested by several authors, when towed by surface or submarine vessels or when activated at fixed locations. Recently Watts (Watts, 2019) has proposed using fixed seabed sources for reservoir monitoring. The main drawback of that concept is the sparsity of source locations, which coupled with a typically sparse set of seabed receivers, makes conventional 3D imaging unfeasible. Reservoir monitoring may still be possible by observing time-shifts on raw data. Here we propose to use mobile seabed seismic sources that are transported in vehicles that navigate near the seabed or roam on the seabed. The “flying” sources could be carried by a special-purpose AUV, while the “roving” Figure 1: Comparison of options for sensors and sources to enable a fully autonomous marine seismic acquisition system for reservoir monitoring. OBN and Conventional sources are shown for reference to traditional acquisition systems. Colors indicate: desirable or manageable (green), limitation or risk (yellow), undesirable (red), and immature (blue).

Fully Autonomous Marine Seismic Acquisition Systems for Reservoir Monitoring