Introduction
This article offers an overview of different types of borehole seismic survey, then compares a relatively new fibre-optic technology against the more traditional geophone tools in use. It delves into crucial considerations for our customers when planning to utilise fibre-optic cables for borehole surveys as part of standard wireline data acquisition, drawing on one&zero’s experience and insights acquired through the delivery of recent operational assurance and processing services.
Read on if you would like to hear more detailed discussion on the following considerations:
- Optical fibre VSP surveys can be completed in 1-2 hours of rig time as part of a multi-tool run offering potentially significant time and therefore cost savings. However, there may be additional time and cost considerations to factor into the operation.
- Data quality depends on cable contact with the borehole wall, affected by well geometry and condition, with operational risks of cable slack needing careful planning and monitoring.
- Fibre-optic cables are less available globally (currently), requiring advanced planning and specialist knowledge for equipment setup and remote data delivery.
- Fibre-optic cables are more expensive and cannot be spliced if damaged, necessitating careful handling and review of replacement costs at pre-ops planning phases and contract awards.
- Fibre-optic cables are suitable only for recording signals along the cable axis, not for surveys requiring three-component seismic imaging or detailed reservoir analysis.
- Processing challenges can include picking first breaks and deconvolving data, with noise management by removing bad traces, potentially affecting formation top images.
Types of borehole seismic survey
Borehole seismic, as the name suggests, relates specifically to when seismic waveforms, generated by a seismic source (typically at surface), are collected downhole. Borehole surveys typically provide higher resolution images compared to surface seismic techniques, as it overcomes many of the limitations associated with wave attenuation and scattering. These types of surveys enable direct measurements of seismic velocities in the well, offering precise time-depth correlations essential for subsurface characterization. Borehole seismic data is typically used to complement surface seismic data, providing detailed information at the wellbore location that can be integrated with the broader view obtained from surface seismic, creating a more complete picture of the subsurface.
Two common types of survey which are run for wireline operations are Checkshot Surveys and Vertical Seismic Profiles (VSP). These are often confused by those unfamiliar with these types of surveys and so a brief description of the differences are provided below for clarity:
Checkshot Survey
Purpose: The primary goal of a checkshot survey is to measure the time it takes for seismic waves to travel from the surface (measured via a hydrophone) to various depths in the borehole.
Data Collection: It has traditionally involved deploying geophones at specific depth intervals in the well and recording the arrival time of a seismic signal from a surface source.
Resolution and Detail: Less shots are required as we are only interested in the first-time break (the time it takes the sound wave to travel from source to receiver and not the rest of the waveform).
Applications: Checkshot data is used for time-depth correlation, enabling the adjustment of seismic reflection data to the actual depths encountered in the wellbore. It is mainly used to calibrate the seismic time with the depth in the well.
Simplicity and Cost: It is a simpler, quicker (typically three good “shots” per level required) and typically less expensive operation than conducting a full VSP survey.
Vertical Seismic Profile (VSP):
Purpose: VSP is a more detailed seismic method that records the full seismic wavefield as a function of depth in the well, both for downgoing and upgoing waves.
Data Collection: Uses geophones (or fibre-optic cable) placed inside the wellbore to record seismic signals from a source on the surface (or sometimes from a source in a nearby well or downhole – see below).
Resolution and Detail: Provides high-resolution seismic images of the subsurface formations and can be used to identify geological features around the wellbore.
Applications: VSP data is used for detailed seismic imaging, providing information on the geological structure, stratigraphy, and rock properties around the well. It is also used to help interpret surface seismic data.
Complexity and Cost: A VSP survey is typically more time-consuming (typically 5 repeatable shots per level), and expensive than a checkshot due to the higher level of detail (repeatability required) and the amount of data collected.
Types of borehole VSP survey
To complicate things, there are also several different types of VSP survey which are performed based on a function of data requirements and wellbore geometry – summarised below:
ZERO-OFFSET VSP (A, C)
- Seismic sensors are placed in the borehole vertically below the seismic source.
- The survey provides a direct measurement of upgoing and downgoing waves from a source close (how close has to be modelled) to the wellbore, leading to a clear and accurate acoustic image of the rock layers immediately surrounding the well.
OFFSET VSP (B, D, E)
- The seismic source is located at a certain horizontal offset from the wellbore.
- Enables imaging of subsurface features away from the vertical well path.
DRILL-BIT LISTENING (F)
- The drill bit itself is used as the seismic source.
- Receivers placed within the well record the seismic waves generated by the drill bit, providing real-time data while drilling.
WALK-AWAY VSP (G, H)
- The seismic source moves away from the borehole during the survey.
- Captures a broader range of subsurface information at different angles.
WALK-ABOVE VSP (I)
- Seismic sources are moved along the surface directly above the vertical array of geophones in the well.
- Provides high-resolution data from directly beneath the survey line.
CROSS-WELL SURVEY (J)
- Seismic energy is transmitted from one well and recorded in another well.
- Allows for imaging the properties between wells.
INTRA-WELL SURVEY (K)
- Both the seismic source and receivers are located within the same wellbore.
- Can provide detailed information about the wellbore environment and immediate surrounding formation.
The Seismic Source
An essential element of the survey involves producing seismic waves. Various seismic sources exist, available in multiple sizes, which are chosen based on factors including well depth (greater depths necessitate more power to reach the sensor), the desired frequency range, energy loss due to different formation types, environmental considerations (such as wildlife conservation), and the levels of background noise.
For borehole seismic, sources used include:
Air Guns (Offshore/Land): These are pneumatic devices that release high-pressure air into the water, creating a bubble that expands and contracts rapidly. This action generates acoustic energy that propagates through the water and into the subsurface, where it reflects off geological structures. Air guns are mainly used in marine seismic surveys.
Vibrators (Land): Seismic vibrators generate waves through the vibration of a plate against the ground surface. They provide a controlled, continuous source of seismic energy and are typically used in land seismic surveys. The frequency and amplitude of the vibrations can be adjusted to optimize the data quality for different geological settings.
Dynamite (Land): is a traditional seismic source where an explosive charge is detonated in a small borehole, providing a high-energy seismic pulse that is very effective at penetrating deep geological formations. However, due to its destructive nature and safety concerns, its use has declined in favour of the above non-explosive sources.
Downhole Seismic Receivers/Sensors – Geophone shuttles and fibre-optic cable
Geophone shuttle style tools are the most commonly used seismic receivers for borehole seismic survey today. Examples include the Versatile Seismic Imager (VSI) from SLB, Sercel’s Geowave (GeoWave® II | Sercel) used by Baker Hughes and Avalaon’s Geochain (Downhole Equipment and Ancillaries – Avalon Sciences Ltd) used by Halliburton. They implement a high quality motorised clamping tool (or shuttle) holding a cartridge of 3 orthogonally arranged GAC sensors (Geophone Accelerometers). Each shuttle is separated from the next using a length of wireline cable (typically 15m spacing but can be adjusted) and can be strung together to create a tool string of between 1 up to 40 shuttles (for VSI) and more (see data sheets from links above). Of course, increased rig-up time and potential for individual shuttle failures must be weighed up against efficiency of acquisition, and detailed planning against operation experience is necessary to optimise the arrangement for specific types of VSP surveys. But in general, from experience, a 4 or 8 level VSI array can acquire 150-200 VSP levels in a rig time of between 10 and 15 hours.
A relatively new technology to the market are DAS fibre cables (type hDVS) such as SLB’s NOVA cable (optiq-seismic-ps.ashx (slb.com). The fibre optic is built into the core of a multiconductor e-line cable which means it can be used to run standard wireline tools, prior to the seismic survey which can take as little as 1-2 hours. With this approach it is possible to acquire seismic survey data across the entire length of the wellbore at the same time. This method of acquisition uses a surface interrogator to supply a light pulse to illuminate the fibre and to record the back-scattered signal which varies when deformed by an acoustic signal. The returning signal is formed from the signal difference between two points (pulse width), and the distance between the points is the gauge length, something set up by knowing the expected bandwidth and some trials before acquisition. Ultimately, with some mathematical transforms, both VSI and DAS results can be made comparable to geophone or hydrophone data from surface seismic.
Technical Advisory
(Reduce Risk, Reduce Cost, Maximise Quality)
Below are some considerations to consider when planning a fibre optic type survey over a traditional geophone array:
➡️The major operational benefit of using a fibre optic cable for a VSP survey, is that it can be acquired in as little as 1-2 hours of rig-time (when used as as part of a multi-tool logging run) and costs for the service reflect this value added. Operator’s should be aware that a cable change on the wireline unit will be required prior to operations. Additionally, operator’s should be aware (preferably at the time of tender) that different cables may incur different per-run costs, hence a cable change may be required in between runs to prevent additional costs dependent on the contract. This will impact any time saving and needs to be factored in to any cost-benefit analysis.
➡️ Fibre optic cables are typically much more expensive to manufacture than standard (non-fibre optic) cables and cannot be spliced in the event of being cut or damaged. Therefore, damaging the cable, or cutting in preparation for fishing, will result in a replacement charge. Hence operators should review the cable replacement costs prior to operations alongside any tension/risk modelling performed.
➡️ As with any borehole seismic sensor, data quality will be a function of contact (of the sensor) with the borehole wall. As this is a cable that is in contact along the length of the well bore, contact will be primarily a function of well geometry (deviation) and borehole condition (rugosity, washout, breakout, fractures etc). Cable contact will be improved, particularly at lower angles (~<20 degrees), by slacking off the wireline cable. However, this will introduce additional operational risk which needs to be planned for effectively and monitored closely at the wellsite. Historically one&zero domains have noticed particularly noisy areas of data at zones of borehole rugosity, variable tension in the cable, or at the point of a strong formation break. In our experience during processing, DAS cables are prone to severe noise in wells with less than 5-7° deviation.
➡️ Fibre optic cables are much less readily available globally than standard geophone shuttle type tools. Operations need to be planned well in advance. Specialists are required that have knowledge of how to configure the surface equipment (collectors) and data delivery is done remotely (not at the wellsite) as is the case with standard geophone based VSPs. All of these factors introduce potentially risks to the operation which need to be managed effectively.
➡️ Fibre optic cables only records the signal component which is along the cable axis, so is not suitable for vertical incidence, walkaway, walkaround, or any borehole survey needing to evaluate detailed three component seismic imaging, AVO, or other parameters related to reservoir characteristics. However, it will record both P reflections and shear conversions (down and up).
➡️In processing the challenges have been to:
o Pick first breaks – for checkshot data and good synthetics
o Deconvolve data – for a good corridor stack and well-seismic match.
Noise has been observed to be coherent and did not stack out, so the most reliable method was to simply remove the bad traces. The downside of this was potential removal of crucial formation top images at the wellbore. With formations that are fairly flat, picking a corridor away from the TWT (Two-Way Traveltime) curve did not compromise the accuracy too much. However, in different scenarios this should be considered at the data acquisition planning phase. The lack of 3 components limited the accuracy of any true seismic imaging to compare with the surface seismic.
➡️Seismic While Drilling (SWD) can offer an alternate means of acquiring Checkshot/VSP data without the need for a wireline run at all and has been used to great effect by some operators. Details will be covered in a future post.
Closing Remarks
In the evolving landscape of borehole seismic operations, the comparison between traditional geophone tools and the innovative fibre optic technologies reveals a dynamic shift towards potentially imporved efficiencies. Fibre optic-based Vertical Seismic Profile (VSP) surveys, with their capability to be completed in a significantly reduced timeframe offer a promising avenue for operators to save both time and costs. However, this efficiency comes with its own set of challenges, including the necessity for meticulous planning due to the unique requirements.
one&zero stands at the forefront of assissting in these operational challenges, offering expert services that encompass the entire spectrum of borehole seismic operations. From the initial planning stages, where strategic decisions are made to optimise the use of fibre optic technology along with environmental considerations, through to the oversight of the preparation of equipment and wellsite execution and data QC. Moreover, the post-job phase benefits from one&zero’s experience in data processing and analysis, where challenges such as noise management and data deconvolution can be navigated to deliver clear and actionable insights.
one&zero’s expertise not only mitigates the inherent challenges associated with this advanced technology but also leverages its benefits to the fullest, ensuring that our clients achieve not just operational efficiency but also a deeper understanding of their reservoirs.
Please remember to follow our LinkedIn page for future articles.
#BoreholeSeismic #OpticalFibre #Processing #MMO #PAM #SeismicWitness
