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Borehole seismic imaging is a technology that has long been proven in the oil and gas industry, both for exploration purposes and enhancing production and field development projects. But there is still room for improvement.
Gaining higher resolution reservoir imaging data in real time downhole provides critical information to help operators optimize development drilling processes. With borehole seismic tools having evolved from single-component sensors in the late 1970s to the modern seismic array tools of today, these tools play a key role in reducing drilling decision risks.
But the upstream industry still faces an array of challenges in the area of borehole seismic sources, according to one research and technology development coordinator.
Member companies of the Industry Technology Facilitator (ITF) sat down recently to identify and discuss the current shared challenges faced by the E&P sector in this area. There are, according to the ITF, many technical limitations with the available borehole seismic source technologies.
But that means there is great potential for further advances in this area to create economically attractive technical solutions.
Finding new ways to accurately chart wells is always a key R&D priority for the oil and gas industry, and the ITF (made up of 29 operator and service companies) is on the hunt for novel technologies for potential funding. These technologies can address a wide range of applications, including seismic while drilling (SWD) for exploration and development as well as downhole sources for cross-well seismic tomography, enhanced reservoir imaging, and permanent reservoir monitoring. These applications likely will require different borehole source types, ITF said, and can be for onshore, offshore, normal, or HP/HT wells.
In particular, a major motivation in the organization’s call for proposals is the need for new technological solutions to enable real-time SWD for two specific applications:
Pore pressure prediction in potential HP/HT wells; and
Geosteering to locate small, deep reservoirs targets.
Those seismic sources selected will be part of a testing program to be prepared as part of a joint industry project and carried out at the Michigan Technological University test facility. The testing program also will involve the acquisition, processing, and interpretation of seismic data from downhole and surface arrays, including low-frequency seismometers in shallow 183-m (600-ft) boreholes. Vertical boreholes do exist for the deployment of sources; however, holes of any configuration can be drilled.
In areas of poor seismic data quality, MWD often is used for well steering and abnormal pore pressure prediction. SWD is part of the MWD suite of tools. Two different techniques can be used:
The direct SWD technique, where a downhole
receiver records surface seismic sources; or
The reverse SWD technique, where a borehole seismic
source is recorded on surface receivers.
Downhole seismic receivers have enjoyed major technical advances over the past 30 years, said the ITF, but downhole seismic sources have not. Many borehole sources have been developed, tested, and used, including piezoelectric vibrators, airguns, sparkers, and noise generated by the roller cone drill bit. However, these SWD sources have so far failed to become commercial for the depths, temperatures, and pressures required.
Existing limitations of direct SWD techniques include downhole receivers that cannot sustain high temperatures encountered in HP/HT wells. In mountainous terrains, vibrator access might not be possible outside of the rig area. Drilling shotholes for explosive sources also can be costly.
The available alternative reverse SWD techniques also have technical limitations. For example, in HP/HT wells or foothill areas the present borehole seismic sources are not well adapted to the deep reservoirs usually found in these domains, airguns cannot be deployed at great depth due to mud pressure, and piezoelectric vibrators or sparkers do not emit enough low frequencies and are not powerful enough. Roller cone drill bits also often have to be replaced by PDC tools due to the hard rock formations, and these tools are not a good generator of seismic noise. When roller cone drill bits can be used as seismic sources, they mainly generate shear waves, with their compressional (P) wave energy often too weak to be recorded at surface, the ITF said.
Listed in its “Call for Proposals” are the principle applications that the ITF said its members are looking to have addressed by any borehole seismic source technology:
Domains where these are likely to be used include wells where real-time SWD data are needed and where conventional downhole tools cannot be easily deployed or deployed at a reasonable cost; deeper, complex, and smaller reservoirs; extended-reach and horizontal wells on land and offshore; and onshore unconventional hydraulically fractured reservoirs. It highlighted two domains in particular where there is clear potential commercial value: