25 things you may or may not know about the SLB Platform Express

It is approaching 30 years since the release of the Platform Express (PEX) tool from SLB (Link). The PEX tool, along with the Maxis acquistion software introduced in the same year (1995), marked a significant advancement in wireline logging technology at the time. The tool has proved reliable and effective over the years.

Commonly referred to as a triple combo string, the PEX tool combination aimed to deliver the following key measurements in a single compact toolstring:

• Gamma Ray Logging: Widely used for lithology identification and correlation between wells. Gamma ray logs measure the natural radioactivity of the formation, helping to identify shale and sandstone layers, which is crucial for understanding the stratigraphy and depositional environment.
• Density and Neutron Logging: Density logging measures the electron density of the formation, which can be directly related to formation density and porosity. Neutron logging measures hydrogen concentration, which is indicative of fluid-filled porosity. Together, they provide a comprehensive picture of the formation’s porosity and fluid content.
• Resistivity Logging: This service is critical for identifying the presence of hydrocarbons and determining the fluid content in the pore spaces of rock formations. It measures the resistivity at different depths of investigation away from the wellbore to represent both invaded zone resistivity and “true” formation resistivity, whilst helping to differentiate between water-filled and hydrocarbon-filled reservoirs.

The PEX delivered these standard measurements by combining additional sensors which had not been seen before at time of realease.

Hardware

Platform Express is the marketing name for what is also referred to in software as the HILT (Highly Integrated Logging Tool). So named because it was designed to combine multiple logging measurements into a single compact design, with the aim of reducing the time (and hopefully) the cost associated with logging runs, whilst also reducing the amount of rat hole (additional hole section drilled to provide space for logging sensors). The entire package of the Pex is is 38ft long in comparison to the previous generation triple combo which was 90ft long (link).

The components:

The PEX contains the following main components (from top to bottom):

a.       HGNS – HILT Gamma-Ray Neutron Sonde, not to be confused with HNGS (Hostile Natural Gamma Ray Sonde).

b.       HDRS – HILT Density and Rxo Sonde

c.       AIT-H – Array Induction Tool (H or M) or HALS – Azimuthal Laterolog Sonde

HGNS – HILT Gamma Ray Neutron Sonde – The main sensors inside the HGNS contains a scintillation gamma-ray detector (SGT -which can detect naturally occurring gamma rays), a thermal neutron detector for porosity (CNT) and a single-axis accelerometer whose principal function is to facilitate speed correction of logs (the correct the effects of stick and pull). You may notice that headers on Platform Express logs often have the services names in the title, a CNL (Compensated Neutron log) and GR (Gamma-ray log), this reflects the sensor measurements and outputs.

HDRS – HILT Density and Rxo Sonde section of the tool includes a 3 main components. The HRCC (High Resolution Common Cartridge) containing electronics for control and communications, the HRGD (High Resolution Resistivity Gamma Ray Detector), a skid (plate) with 3 density detectors (known as a TLD detetcor- three lithodensity) and a micro resistivity sensor (known as an MCFL pad). Finally, the HRMS (High Resolution Mechanical Sonde) to apply force to the skid to achieve good contact with the borehole, essential for density measurements due to the sensitivity to standoff (or fluid between sensor and formation). A single-arm caliper is present for measuring the diameter of the borehole and providing good contact of the skid with the borehole wall.

AIT-H – Array Induction Tool – designed to measure the resistivity of the surrounding formation (radially). The AIT-H tool employs an array of induction coils to generate electromagnetic fields that penetrate the formation. One of the key advantages of the AIT tool is its ability to provide detailed resistivity profiles in both conductive and non-conductive borehole fluids (such as saline water-filled formations) and resistive (such as hydrocarbon-filled). Where the salinity is too high (a chart or slb toolplanner is available for this determination) then a laterolog type tool is required such as the HALS (see below). The AIT-H also contains a bottom nose which can measure mud resistivity and SP in conductive mud environments.

HALS – The HALS – tool operates on the laterolog principle, which involves injecting a focused electrical current into the formation through an electrode and measuring the potential difference to determine resistivity. Today, the HALS has been somewhat superseded by the HRLA High-Resolution Lateral Log Array (HRLA) in conductive borehole environments. The HRLA has an array of electrodes outputting resistivity at 5 depths of investigation (as opposed to 2 for the HALS, shallow and deep), in addition the HRLA can be run without the cumbersome bridle (a >60ft special piece of cable run above the wireline head to isolate the tool from the cable). The HRLA will be looked at in more detail in a future post.

25 Things you may or may not know about PEX

General

1. At the time of release of the original PEX (HILT-B) tool the operating temperature rating was -25 degC to 125 degC. This was uprated to 150 degC with the release of the HILT-H (or more commonly known as the Pex 150). Although rated lower than the previous generation of resistivity tool such as the AIT-B (175 degC), the tool was positioned to cover the “Golden Zone” temperature distribution. This zone refers to research which indicates that 97% of conventional oil, and 90% of conventional gas occurs in reservoirs at >60degC and <120degC.  (ref: Source ). However, for opertaions above 150degC careful consideration of alternative options for resitivity needs to be considered.
2. On older logs, PEX toolstrings may include a HTBC (HILT Tool Bus Converter) in the string. As the PEX uses its own internal telemetry (CAN telemetry), it is necessary to use a special cartridge to be able to run older generation tools which use (DTB – Dowhole Tool Bus) telemetry. The HTBC is run above the PEX a to terminate this telemetry and communicate with the tools below. Tools with the modern FTB telemetry can be combined above of below the PEX without a HTBC.
3. When running other wireline tools above the HGNS, consideration should be given to the strength of those assets for the purposes of fishing. Older sonic tools were more likely to fail when subject to higher tension/compressional forces and posed a specific risk of leaving radioative sources in the well.
4. The PEX was the first toolset to be tested under similar loading conditions to LWD tools, specifcally to withstand shock of 250 g repeated 2000 times, with prior generations of logging tools that were designed to withstand 100 g repeated 10 times. The PEX is generally known as a reliable piece of equipment within the industry – to date.AIT-H (Array Induction Tool)
5. The depths of investigation for the AIT-H are 10, 20, 30 60 and 90 inches. The tool can acquire data at a resolution of 1, 2 or 4ft via a parameter AHBLM (Basic log mode). Higher resolutions will reduce the logging speed possible from the documented 3600ft/hr maximum.
6. The AIT-H bottom nose contains a mud resistivity sensor within the bottom nose (Rm measurement) which is used as an input for borehole correction. Any inaccuracies in this measurement can result in borehole correction error when this is used as an input (conductive mud environments). There is a mud resistivity master calibration which must be performed if the sensor is to be used as an input.
7. The AIT-H can be run with an alternative hole finder (nose guide) bottom nose (alternative name not used here). This can be a useful alternative to the standard bull nose configuration. Alternatively, third party providers such as Petromac (ref) are available which can be fitted directly to the bottom of the AIT tool.
8. The AIT-H must be run with a caliper for effective borehole correction.
9. The AIT-H was historically calibrated every 3 months on a large wooden (non-conductive) calibration structure. (image). This frequency has recently been increased to up to 3 years between calibrations based on maintenance indicators.
10. The AIT-H needs to be run with standoffs of known size, the size of these standoffs should be measured (and maximised) for the size of the borehole. Options range from 0.5” (for less than 6.5” hole size) in 0.5” increments to 2.5” (for greater than 11.5” hole size).
11. Both the AIT-H and HALS sections of the tool are “bottom only” tools. This means that you cannot run anything below them. If the PEX needs to be combined with other tools (such as sonic, resistivity imagers etc) then these will have to be run above the AIT/HALS.
12. The AIT-H can be run in hole sizes from 5.5” to 20”. However in larger hole sizes a plastic bowspring was produced to assist in eccentering the tool in the borehole to assist with locating the tool and therefore borehole correction. This is rarely used and availability limited.
13. A new version of the AIT-H was introduced as part of the upgrade to PEX-150 tools. The AIT-M is the current generation AIT-H with a 150degC temperature rating and DTS telemetry system.HDRS – HILT Density and Rxo Sonde
14. The density and Pef (Photoelectric effect) measurements vertical resolution options of between 2, 8 and 18 inches (900, 1800 and 3600ft/hr).
15. The weakest part of the PEX toolstring (excluding HALS) is the HRMS (Caliper) section. The standard HRMS and HRCC are rated to just 4,400 lbs in compression as opposed to 50,000 lbs in tension. However, a “stiffener kit” is available which nearly doubles the compressional rating to 8000lbs. This kit is best fitted to the tool in town before shipping offshore.
16. The HRMS caliper opens up to 24 inches and no large hole kit is available/required.
17. The HRGD (a.k.a TLD – three litho-density detectors) requires a calibration which measures the background signal from stabilisation sources which are built into the tool. Subsequent measurements are then made in magnesium and aluminium blocks (known density) and also with a stainless steel sleeve. It is critical that these blocks and the skid are in good condition for a reliable calibration.
18. For the HRGD, the same radioactive source (Cesium-137) used for calibration must be used for logging otherwise the tool must be recalibrated after the log is completed and the log recomputed.
19. The HRGD in the HDRS historically required calibration every month irrespective of the number of jobs performed. This has been changed in recent years (2021) to the requirement of a calibration check each month (not a full calibration) and if no jobs have been conducted then no master calibration is required for up to 1 year (assuming that the tool passes non-source required checks).HGNS
20. The standard gamma-ray and porosity measurements from the HGNS have a vertical resolution of 2ft and a high-resolution measurement of 1ft (3600ft/hr vs 1800ft/hr)
21. The standard HGNS configuration can be run in hole sizes from 6” to 14”. A large hole kit is available (HGNE) which consists of a modified bowspring which opens up to 22” (availability may be limited in which case alternative eccentraliation could be considered). In holes less than 14″ a standard bowspring is required to force the tool against a borehole, in hole sizes less than 6″ the bowspring needs to be removed.
22. The HGNS neutron tool historically required calibration every 3 months (the longer time period between calibrations a function of the longer half life of the AMBe radioactive source in comparison to the Cesium137 source used for the density measurement). As with the TLD this frequcny has been increased.
23. The HGNS tool includes two detectors and a source. The calibration for the tool requires it to be placed through a hole in the centre of a water filled tank at known temperature using a crane. As the HGNS tool body (metal) wears down, the air gap between the source and detectors changes which directly affects the calibration measurement. Hence, the tool housing diameter must be measured accurately prior to each calibration and entered into the software as a parameter.
24. As the primary interaction with the thermal neutrons emitted by the HGNS radioactive source is with hydrogen atoms, any standoff between the tool and borehole wall will significantly affect the measurement. A wear ring is commonly run on the HGNS to protect the housing from wear. In this instance a parameter needs to be set in the software to compensate for this standoff (parameter SOCO) with the correct size of standoff.
25. As the HGNS tool is originally calibrated in an 8” hole any deviation from this size hole requires correction, hence the parameter HSCO (Hole Size Correction option) needs to be set to yes.

Closing Remarks

Since its launch in 1995, the Platform Express has consistently demonstrated its reliability, providing end-users with high-quality gamma ray, density, neutron, and resistivity data. The PEX encompasses a range of parameters, calibration procedures, and setup requirements that may not be universally known. This article has explored a substantial portion of the technical specifications of the tool. A forthcoming article in our Tool Tuesday series will examine in greater detail the data acquisition and processing cnsiderations (follow us to make sure you are alerted to this article). Future discussions will also include a comparison with alternatives offered by competitors.

Reflecting on its outstanding 30 years of service, the PEX tool has significantly contributed to the quality of petrophysical data. Looking forward, we would like to see enhancements in subsequent generations, notably the elimination of the need for chemical radioactive sources and the achievement of higher temperature tolerances to eliminate the necessity for the use of slim hole versions of the platform.