gravel-pack log, Oil & Gas Glossary

What Is a Gravel-Pack Log?

A gravel-pack log is a cased-hole nuclear wireline measurement, typically a gamma ray or neutron log, run before and after gravel packing a sand-control completion to evaluate the placement quality of the gravel pack by detecting voids, incomplete packing, or gravel bridging that would allow formation sand to bypass the pack and enter the wellbore during production.

Key Takeaways

Pre-pack and post-pack gamma ray or neutron log comparison identifies where gravel has and has not been placed.
Neutron-activated tracers added to the gravel carrier fluid enable direct imaging of gravel placement through gamma ray logging.
A properly packed annulus shows uniform log response; voids or bridging create anomalous intervals on the post-pack log.
Gravel-pack log quality directly determines whether the completion will control sand or allow sand production that damages surface equipment.
Temperature logs are also used to identify gravel-pack placement through the heat of hydration when cement is used as a packing fluid.

How Gravel-Pack Logs Work

The gravel-pack evaluation logging process typically involves two logging passes at the same well and tool string configuration: a baseline log run before the gravel placement, and an evaluation log run after gravel has been pumped into the annular space between the screen and the open formation (in open-hole gravel packs) or between the screen and the casing perforations (in cased-hole gravel packs). The baseline log establishes the natural gamma ray or neutron porosity response of the formation and completion hardware in the unperturbed condition. The post-pack log measures the same interval after gravel placement.

In the simplest approach using natural gamma ray logging, the gravel, typically sized 20/40 or 40/60 mesh high-purity silica sand or ceramic, has very low natural gamma ray (near zero) compared to formation sand and shale (10-200 API units). Where the gravel has been properly placed, the post-pack gamma ray log reads lower than the baseline because the formation-adjacent measurement volume is now partially filled with low-activity gravel. Where voids exist, the gamma ray remains at baseline levels. In radioactive tracer approaches, a radioactivated species (typically iridium-192 or sc-46 or a non-radioactive stable chemical tracer) is added to the gravel slurry at known concentration. After placement, a gamma ray log detects the radioactive tracer wherever gravel was placed, directly mapping the gravel distribution in the annulus.

Gravel-Pack Log Applications Across International Jurisdictions

In Canada, gravel-pack logging is used in WCSB heavy oil wells at Cold Lake and Lloydminster where Clearwater and McLaren Formation unconsolidated sands require sand control to prevent wellbore collapse and pump damage during thermal production operations. AER completion records require documentation of sand control completions including gravel pack placement verification; gravel-pack log data is submitted as part of the completion report for wells using slotted liner or screen with gravel pack. Production wells that produce sand after gravel packing have gravel-pack logs reviewed retroactively to identify void locations that enabled sand production.

In the United States, gravel-pack logging is a standard quality-control procedure for Gulf of Mexico deepwater completions in weakly consolidated turbidite sands. BSEE well completion requirements for sand-control completions include documentation of gravel-pack placement quality; failed gravel packs detected on post-placement logs require assessment of whether re-packing is needed before the well is put on production. The economic consequences of a failed gravel pack in a deepwater well — sand production that can destroy ESP pumps costing USD 500,000 or more, and wellbore fill that may require a costly remediation workover — make the investment in gravel-pack logging quality control easily justified. In Norway, Equinor's Oseberg and Brage fields, which produce from unconsolidated Brent Group sandstones, use gravel-pack completion evaluation logs as part of the standard completion quality assurance programme. In the Middle East, Saudi Aramco's Khurais and Manifa fields targeting unconsolidated carbonate and sandstone reservoirs use gravel-pack logs to verify placement in long horizontal completion intervals.

Fast Facts

The gravel used in oilfield gravel packs is not garden gravel but highly engineered, carefully sized silica sand or ceramic media. API gravel pack sand specifications require that at least 96% of particles fall within the designated sieve size range, that roundness and sphericity exceed minimum values (round grains create higher permeability packs than angular ones), and that the material has very low solubility in 12% HCl and 3:1 HCl:HF (mud acid) to ensure it survives acid stimulation operations without dissolving. The permeability of a properly packed annulus of 20/40 mesh gravel at the appropriate stress can exceed 500 darcies, far higher than any natural formation, ensuring that the pack itself offers essentially zero flow resistance to produced fluids.

Gravel-Pack Log Interpretation

Interpreting a gravel-pack log requires comparing the pre- and post-pack runs on the same depth scale after careful depth matching. Key features to identify include: intervals where the post-pack gamma ray is uniformly lower than pre-pack (good pack quality, consistent gravel distribution); intervals where the post-pack gamma ray is identical to pre-pack (void or incomplete pack, formation not contacted by gravel); and intervals with erratic changes in the post-pack relative to pre-pack (bridging or non-uniform gravel distribution). Depth mismatches between the two runs can create apparent pack-quality anomalies that are actually depth registration errors rather than true voids; careful depth-shifting alignment is required before any interval-specific quality assessment.

Tip: When planning a gravel-pack evaluation programme using radioactive tracers, obtain regulatory approval for tracer use before the completion job rather than assuming it is automatically permitted. Radioactive tracers require licensing under nuclear materials regulations in every jurisdiction. In Canada, CNSC licensing is required; in the United States, NRC or Agreement State licensing applies; in Norway, the Norwegian Radiation and Nuclear Safety Authority (DSA) regulates tracer use. Stable chemical tracers (non-radioactive isotope-tagged compounds) may be available as an alternative that eliminates the radioactive material licensing burden while still providing direct gravel placement imaging through downhole spectroscopy tools.

Gravel-pack log is also referenced as:

Pack evaluation log — the functional name used in completion engineering documentation when describing the post-placement quality evaluation programme for any type of sand control completion
Tracer log — used specifically when a radioactive or chemical tracer has been added to the gravel slurry and the log is detecting the tracer distribution rather than the inherent gamma ray contrast of the gravel
Sand control evaluation log — the broader category including gravel-pack, frac-pack, and standalone screen evaluation logs; used when the full suite of sand control completion quality measurements is referenced

Frequently Asked Questions

What happens if the gravel-pack log shows a void in the pack?

A void in the gravel pack represents a section of the completion where the annular space between the screen and the formation has not been filled with gravel. In this void section, the formation sand is in direct contact with the screen mesh, which is sized to stop only the coarser sand fraction while allowing fine sand and fines to pass through into the wellbore. During production drawdown, the unsupported formation sand adjacent to the void can mobilise, migrate through the screen, and accumulate in the tubing or at the pump intake. Remediation options for a void identified on the gravel-pack log include a remedial gravel-pack squeeze into the void zone, a frac-pack extension, or wellbore sand management through controlled drawdown and production rate limits that prevent sand mobilisation velocity from being exceeded at the void location.

Why is the gravel-pack log run both before and after placement?

The pre-pack baseline log is essential because formation gamma ray varies naturally from interval to interval, and the absolute gamma ray value after pack placement depends on both the formation background gamma ray and the overlay gravel signal. Without a pre-pack baseline, it is impossible to determine whether a low gamma ray reading in the post-pack log reflects good gravel placement or just a naturally low-gamma ray formation section. The difference between pre- and post-pack logs (the delta-GR) isolates only the gravel contribution to the gamma ray response, providing a direct and formation-independent measure of gravel placement quality at each depth.

Why Gravel-Pack Logs Matter in Oil and Gas

Sand production is one of the most economically damaging production problems in unconsolidated reservoir operations. A single sand production event can fill thousands of metres of production tubing, destroy an electric submersible pump (ESP) costing half a million dollars or more, and require a costly workover to clean out the wellbore before production can resume. Properly executed gravel packs that are verified by post-placement evaluation logging prevent the vast majority of these events. In the Gulf of Mexico deepwater, where hundreds of wells produce from unconsolidated Miocene and Pliocene turbidite sands, the systematic use of gravel-pack evaluation logs as a completion quality-control tool has dramatically reduced the frequency of sand-related equipment failures and production deferments compared to the early deepwater era when pack quality was assumed rather than verified. The cost of the logging evaluation is orders of magnitude smaller than the cost of a single preventable sand production workover.

Gravel-Pack Log: Definition, Completion Quality Evaluation, and Sand Control Logging