The evolution of formation testing hardware from the first-generation tools to the advanced third-generation formation testers demonstrates significant technological advancements in reservoir characterisation and fluid sampling. Each generation has built upon the strengths and addressed the limitations of its predecessors, resulting in tools that are more efficient, accurate, and capable of providing high-quality data under a wide range of formation conditions.
Wireline formation testers are typically run for one or more of the following objectives (ref, ref)
- Measure reservoir pressure and temperature
- Determine reservoir pressure gradients and fluid contacts
- Identify fluid types for each flow unit
- Collect large quantities of formation fluid samples for reservoir studies
- Acquire fluid samples with minimal filtrate contamination and possibly bring them to the surface at reservoir conditions
- Recognize fluid types and obtain various fluid properties at the wellbore, such as bubblepoint pressure and viscosity
- Capture representative samples to quantify mercury levels
- Capture and preserve H2S and other unwanted contaminants in samples
- Determine oil/water-relative permeabilities
- Measure horizontal and vertical permeabilities of the formation
- Assess near-wellbore heterogeneities
- Obtain storativity typically from multiprobe or packer-probe interference tests
- Conduct stress testing to determine formation parting pressure
A key element of any formation testing and sampling toolstring design is the probe. A probe acts as the communications interface between the tool flowline and formation. Sealing is provided using a packer (usually rubber) surrounding the probe to isolate it from the wellbore fluids, this ensures that the measurements reflect formation properties rather than mud or wellbore conditions.
Which probe should you use?
When selecting a probe for formation testing, end users should consider several key variables to ensure optimal performance. The inlet cross-sectional area is crucial; larger areas suit low-permeability or unconsolidated formations by minimizing drawdown and sand production, whilst smaller areas can perform better for formations prone to plugging from mudcake or loose particles. Probes must be capable of generating sufficient drawdown pressure to initiate and maintain fluid flow without causing excessive pressure drop that can lead to phase changes or formation damage. In high viscosity fluids, Probes with larger filter areas and inlet cross-sectional areas are preferable as they reduce the resistance to fluid flow, minimising the risk of clogging and ensuring efficient fluid extraction. Whilst in low viscoisty fluid probes with smaller inlets and filters may be able to handle these fluids without excessive drawdown.
Each operation should be planned and modelled to esnure the optimum probe is selected. Offset well infomation and local experience is critical in this process.
Types of Probes
Standard or Single Probes
- Optimised for medium to high-permeability formations (ref)
- Equipped with a filter piston to keep debris out. This filter is interchangeable.
- Different packer and probe sizes are available and detailed in this article below.
Large-Inlet Probes
- Optimised for formations with low mobility or loose, sandy conditions, although is proven in a large range of mobilities. A larger inlet reduces the pressure needed to draw fluid and decreases the time required for pressure tests and fluid sampling.
- The larger cross-sectional area allows more fluid to be drawn in, reducing the drawdown on the formation.
Long-Nose Probe (0.1521 sq in) (SLB Only)
- High Filter Area Ratio: The probe has a high ratio of filter area to inlet cross-sectional are. This makes it highly effective in areas with thick mudcake or high-permeability, unconsolidated, or drilling-damaged formations.
- Control Over Loose Material Intake: The small inlet area limits the amount of loose material entering the probe, which is beneficial for formations with thick mudcake or loose material from drilling damage.Designed for thick mudcake or high-permeability formations.
- Small inlet area was designed to limit loose material intake.
- Increased drawdown pressure has been observed to lead to higher sand production in unconsolidated formations.
- Effectiveness in unconsolidated formations depends on the degree of unconsolidation and drawdown during testing.
- Long nose probes are not commonly seen today.
Martineau Probe (0.1273 sq in with standard packer) – SLB Only
- Designed for use in soft, plastic, and shaly sands.
- Features a large canister filter instead of a standard filter piston. The Martineau probe has a filter area of 20.69 sq in, which is significantly larger than the filters of the standard probe (16.7 times larger) and the large diameter probe (4.8 times larger).
- This large filter area allows the Martineau probe to handle much more debris before clogging, making it highly effective in challenging formation conditions despite its drawbacks.
- Is also available with large area packer increasing the inlet area to 3.941 sq in.
- These probes are less commonly seen today. Without a filter piston, the probe filter is exposed to wellbore fluid and mudcake, which increases the risk of the filter getting quickly plugged in certain conditions. In addition, The small inlet area results in larger drawdown pressures compared to other probes, which can be a disadvantage in some situations.
Elliptical (Oval) Probe
- Effective in tight laminated and anisotropic reservoirs.
- Larger open-flow area reduces drawdown in tight formations.
Dual Probes
Dual probe design has evolved over the years and they are not directly comparable. The SLB dual probe module (rarely seen in the wild today) was typically used in conjunction with a single probe to simultaneously utilise three sample probes in the formation. This setup helps establish pressure relationships and fluid communication among the probes (horizontal and vertical observation probes). The vertical probe when run above the dual probe was only 2.3ft away (vertical distance) and hence was ideal for vertical interference testing. This close proximity also lends itself to act as a “guard” probe whilst sampling and was used as the precursor to the focussed sampling probes (quicksilver/sentinel – see below) that we see commonly run today.
The Baker Hughes RCX MAGNA (ref) is equipped with a large 66 square inch flow area. This is the largest surface area of all the conventional probes (i.e. excluding inflatable packers). It has two large flow-area probes positioned radially, 180 degrees from each other at the same depth to enhance sampling and testing in ultra-low mobility reservoirs.
Halliburton’s dual probe offering includes the dual probe, and continuity probe. The continuity probe offers a large flow area (2×15.093 sqin) and is designed for both conventional and low mobility testing and sampling. Here is a summary of its key features and functionalities:
- Large Flow Area Probe Pad: Deployed for effective testing and sampling in conventional and low mobility reservoirs.
- Dual Oval Pad: Equipped with dual oval pads, each providing a large flow area of 15.093 square inches.
- Vertical Interference Testing: The two probes means it is capable of performing vertical interference tests to evaluate formation characteristics.
- Anisotropy Testing: Facilitates anisotropy testing to determine vertical and horizontal permeability ratios (Kv/Kh).
- Full Pad Redundancy: Ensures operational reliability through redundant pad systems.
- Borehole Compatibility: Suitable for boreholes ranging from 5 7/8 inches to 17 ½ inches in diameter.
- Low Mobility Sampling: Effective in low mobility formations, enabling accurate sample collection.
- Vuggy and Fracture Formations: Designed to handle vuggy and fractured formations.
Focussed (Guard) Probe
- Minimizes mud contamination by using dual intake ports.
- Outer probe produces mud filtrate, while the inner probe produces filtrate-free formation fluid.
The guard probe is specifically designed to minimize mud contamination in fluid samples. key features include:
- Dual Intake Ports: Inner Port: Similar to a standard single probe, primarily producing filtrate-free formation fluid after initial mud filtrate production. Outer Port: Surrounds the inner port, producing a mix of filtrate and formation fluid. The outer port helps in initially handling the mud filtrate invasion.
- Probe Configuration: Two packers envelop and separate the inner and outer probes, sealing them against the borehole wall. Each probe has separate flowlines, pumpout modules, and pressure gauges, along with downhole fluid analyzers for real-time monitoring.
- Operation: The outer probe first deals with the mud filtrate, allowing the inner probe to produce cleaner formation fluid. This setup ensures that the fluid drawn by the inner probe is mostly free from mud contamination, improving the quality of the fluid samples.
This dual-probe system effectively reduces contamination, ensuring that the fluid samples are cleaner and more representative of the formation fluids. This technique was originally designed to obtain high quality (low contamination) samples in thick zones with high vertical permeability but has been proven in a variety of environments. Care should be taken in think beds and areas of lower mobility, in the latter pump type selection and achievable flow rates are crucial to success.
Dual / Straddle Packer
A straddle/dual packer module is an advanced tool used in formation testing to isolate a section of the borehole and conduct fluid extraction and pressure transient tests.
The module has two inflatable packer elements that isolate a borehole interval ranging from approximately ¼ foot to several feet (certain vendors can provide different sized spacing between packers elements)
Once the packers are inflated, a large section of the sandface is exposed, allowing fluid to be withdrawn from the isolated interval and then from the formation. This production area is several thousand times larger than that of conventional probes.
The module keeps the fluid flow evenly spread around the wellbore. This balanced flow helps improve the quality of pressure data and ensures a cleaner formation by reducing issues caused by uneven flow.
The dual/straddle packer module can achieve significantly higher flow rates than conventional style probes. This provides additional benefits such as better formation clean-up when sampling in tight formations and for transient testing / vertical interference enhancing the strength of the pressure signal recorded at observation probe.
Consideration needs to be given to the difference in operation between dual packers and conventional probes, the number of “set and retract” cycles will be much less than with conventional probes and the time to set the packers and remove the volume of mud filtrate from the space between the packer elements (interval volume) will take time (varying based on hole size). Physically the downhole equipment is much bulkier than standard probes with a larger surface area in contact with the borehole whilst sampling. In addition, using the dual packer in a well under losses needs very careful planning to mitigate risk.
Single Packer Module (SLB Only Currently)
The single packer radial probe design (Saturn) brings the benefits of dual packer design into a package that is simpler and faster. Reduced station time helps to minimize operational risks. It has been designed to cater for a wide range of environments including:
- Wide permeability range down to 0.01 mD
- Heavy oil and near-critical fluids
- Unconsolidated formations
- Thinly laminated formations
- Rugose and unstable boreholes
Flow areas:
- 59.49 in² for 5-in version
- 79.44 in² for 7-in and 9-in versions
- 159.49 in² for the extra large 7-in version
Focussed Radia Probe (ORA – SLB Only)
The ORA platform has a number of potential advantages over historical formation testing and sampling (dependent on the application) including, but not limited to:
- Higher Flow Rates: The platform’s pump capacity of up to 108 barrels per day (bbl/d) allows for efficient fluid extraction and handling, reducing the time required for pressure tests and fluid sampling, improving transient testing measurements.
- Wide Flow Management Range: Capable of managing flow rates from 0.05 to 108 bbl/d, suitable for a wide range of formation conditions.
- Dual-Flowline Architecture: Enhances flow management and improves sampling purity, allowing for efficient fluid extraction even in challenging conditions.
- HPHT: Effective in high-pressure high-temperature (HPHT) environments
In relation to the probe specifically, this is in effect a focussed sampling probe (quicksilver) and radial probe (Saturn) rolled into a single package to take advantage of both methods. This opens up the possibility of high quality samples in environments which have traditionally been challenging including (but not limited to) tight formations, unconsolidated formations, and near-critical fluids. The surface inlet area is one of the highest below dual/straddle packers at 110 sq in (ref).
Technical Advisory
Below is a summary of the different probe inlet areas for those presented in this article.
Selecting the right probe for formation testing and sampling presents several key challenges. While the inlet area is a critical factor, it is by no means the only consideration. Geological conditions, formation characteristics, fluid properties, pressure, and temperature all play significant roles in the decision making process, along with the formation testing and fluid analysis objectives. Ensuring the right balance among these variables is essential for achieving accurate and efficient results.
one&zero has a proven track record in planning and executing hundreds of operations, both in person and remotely, for multiple clients globally. Our experienced consultants possess domain-level expertise and vendor experience, ensuring that we can provide the most reliable and independent technical support for your operations. For more information or to discuss your specific needs, please contact us at [email protected].
