Think tank pushes boundaries for borehole seismic imaging

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.

New seismic technologies can address a wide range of applications, including enhanced subsalt imaging. (Image courtesy of WesternGeco)

The challenge

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.

Applications

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:

• SWD for velocities to calibrate and predict pore pressure;
• SWD for updating the velocity model around the well;
• SWD for updating the time/depth conversion;
• SWD for high-resolution imaging ahead of the bit;
• High-temperature applications for real-time time-depth relationships;
• SWD for geosteering to locate small, deep reservoir targets and to reach several target zones in a short time;
• Permanent borehole sources for permanent reservoir monitoring;
• Strong P-wave sources that can be placed in both vertical and horizontal wells and have a downward radiation pattern;
• Borehole sources for high-resolution imaging of hydraulic fractures in unconventional reservoirs; and
• Borehole sources for cross-well seismic that have a downward radiating pattern.

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:

• In the Gulf of Mexico, reservoir formations are located beneath salt structures, which cover extensive areas of the basin. A reliable SWD source would provide the ability to image formations ahead of the bit without disrupting the drilling operations; provide accurate time-to-depth conversion of surface seismic data; and provide a look-ahead, real-time pore pressure prediction capability.
• In deeply buried reservoirs such as in the foothills where the size of the target reservoir is small compared with the depth, conventional surface seismic imaging may be difficult to obtain (such as in areas of thrust zone tectonics and where there may be problems with access in foothill areas). A successful SWD source would enable geosteering to optimally place the wellbore. By increasing the ability to meet the target reservoir, this will prevent the need to drill a sidetrack borehole (which currently would cost upwards of $5 million). Other related items and goals on the wish list of ITF members for SWD sources include no interference with standard rig operation and no need for pauses in drilling and mud circulation, continuous real-time survey acquisition and processing, seismic broadband to under 2 Hz for pore pressure prediction, and the ability to switch the source on or off on demand. According to the ITF, there is no restriction on the type of source apart from being nondestructive and not damaging the borehole. The maximum outer diameter (OD) of the source for many areas should be 6? in. and if possible 6 in., while ideally the source should be capable of handling frequencies to 100 Hz. The above list of requirements from the ITF is not exhaustive, but it illustrates that potential advances in borehole seismology could provide economically attractive technical solutions for use in exploration, development, and production.