Sidewall Core
A sidewall core is a small-diameter cylindrical rock sample recovered from the borehole wall after the hole has been drilled, acquired via wireline-conveyed percussion or rotary coring tools to provide lithology, porosity, permeability, fluid saturation, and biostratigraphic data that complement openhole log measurements.
Key Takeaways
- Percussion sidewall coring (PST) fires hardened steel bullets into the formation to extract plugs typically 1 inch in diameter by 1.75 inches long, while rotary sidewall coring (RST) uses a diamond-studded hollow drill bit to cut undisturbed plugs up to 1.5 inches by 2 inches.
- Rotary cores preserve original fabric and pore structure with minimal compaction, making them preferred for SCAL (Special Core Analysis) measurements including capillary pressure, relative permeability, and electrical properties.
- RCAL (Routine Core Analysis) on sidewall plugs includes helium-injection porosity, Klinkenberg-corrected gas permeability, and grain density, providing key petrophysical calibration points for log interpretation.
- Percussion cores are often severely damaged by the impact, displaying grain crushing, microfracturing, and compaction that can reduce measured porosity by 20 to 40 percent relative to rotary or conventional core values.
- Integration with gamma ray, neutron-density, and resistivity logs allows depth-matched calibration of net pay cutoffs and saturation transforms across the logged interval.
Fast Facts
A single sidewall coring run can recover up to 90 plugs in a single trip, each fired or cut at specified depths identified during log interpretation. Rotary sidewall cores typically cost 3 to 5 times more per sample than percussion equivalents, but yield substantially better quality for SCAL programs. Core recovery rates above 85 percent are considered acceptable for percussion tools in competent formations, though soft or friable sands may yield recoveries below 60 percent.
Tip: When planning a sidewall coring program, prioritize rotary cores at key reservoir intervals where SCAL data will drive relative permeability or capillary pressure models, and reserve percussion cores for stratigraphic and biostratigraphic purposes where fabric preservation is less critical.
What Is a Sidewall Core
A sidewall core is a formation sample extracted from the wall of the borehole after the well has been drilled to total depth or to a casing point, without the need to pull out of hole and run a conventional core barrel assembly. The tool is run on wireline, positioned at target depths identified from real-time or post-drill log analysis, and then activated to recover small cylindrical plugs of rock. This approach is far less expensive and time-consuming than conventional coring and allows geologists and petrophysicists to obtain physical samples from multiple distinct zones in a single run.
The two fundamental methods are percussion (also called bullet cores) and rotary coring. In percussion tools, a spring-loaded or explosive charge fires a hollow steel cylinder into the formation at high velocity. The cylinder, carrying the plug, is then retracted by a retaining cable and stored in the tool's sample chamber. In rotary tools, a small diamond-bit drill head is extended laterally and cuts a clean plug from the formation, withdrawing it intact into the tool magazine. Both methods are limited in penetration depth, typically entering only a few inches behind the borehole wall, but this is sufficient for reservoir characterization in most applications.
How Sidewall Coring Works
Prior to the coring run, a composite log display is prepared by the wellsite geologist and petrophysicist, identifying target depths based on lithology, porosity, and hydrocarbon indicators. These depth picks are loaded into the tool memory or relayed by surface command during the run. The wireline tool is lowered to each target depth, the coring mechanism is actuated, and the sample is stored in a numbered slot in the magazine. Modern tools carry up to 90 shot positions.
On surface, samples are extruded from the magazine, cleaned of mud contamination, and labeled with shot number and measured depth. Fluid cut and odor are noted immediately. Samples are then preserved or frozen if fluid typing and GC analysis are planned. For RCAL programs, plugs are cleaned by Soxhlet extraction or Dean-Stark extraction to remove hydrocarbons and formation water before porosity and permeability measurement. For SCAL, multiple plugs from the same interval may be combined into a composite to build reservoir-representative samples for capillary pressure or wettability tests.
Biostratigraphic samples require minimal preparation: the plug is disaggregated and washed over a sieve to liberate foraminifera, pollen, or other microfossils that constrain depositional age and paleoenvironment. This is particularly valuable in frontier basins or when correlating between distant wells.
Sidewall Coring Across International Jurisdictions
In Canada and the Western Canada Sedimentary Basin, the Alberta Energy Regulator requires that core and sidewall core data be submitted to the Core Research Centre in Calgary, where samples are archived and accessible to subsequent operators. The AER's Directive 059 governs wellbore data submissions, including core analysis reports. Sidewall coring is routine in WCSB deep gas and heavy oil delineation wells, where the Montney, Cardium, and Duvernay formations are primary targets.
In the United States, sidewall coring data is governed by state conservation commission rules and, for offshore wells, by BSEE regulations under 30 CFR Part 250. The USGS and state geological surveys archive donated sidewall core samples from certain basins. In unconventional plays such as the Permian Basin Wolfcamp or Eagle Ford, rotary sidewall cores are widely used to calibrate total organic carbon and maturity estimates derived from spectroscopy logs.
In Norway, the Norwegian Offshore Directorate (now Sodir) mandates that core and sample data from wells on the Norwegian Continental Shelf be submitted to the national petroleum data repository. Sodir's DISKOS database holds sidewall core analysis reports alongside log data and seismic surveys. Rotary sidewall coring is common in North Sea chalk and sandstone reservoirs, where reservoir quality variation across short depth intervals demands high-resolution sampling.
In the Middle East and Saudi Arabia, Saudi Aramco operates some of the most comprehensive coring programs in the world across the massive Ghawar, Safaniya, and Khurais fields. Sidewall cores in the Arab-D and other carbonate reservoirs are used to calibrate NMR log porosity and to identify fracture aperture and secondary porosity. Given the critical importance of these supergiant reservoirs, Aramco typically combines conventional core with dense sidewall coring grids to build three-dimensional geological models with exceptional sample support.
Synonyms and Related Terminology
Sidewall cores are also referred to as sidewall plugs, SWC samples, or bullet cores when obtained by percussion tools. The process is sometimes called sidewall sampling. Related concepts include routine core analysis (RCAL), special core analysis (SCAL), conventional core, porosity, and permeability. The wireline conveyance method distinguishes these from formation tester sampling, which collects fluid rather than rock.
Frequently Asked Questions
Q: Can sidewall cores replace conventional core for reservoir characterization?
A: For most petrophysical calibration purposes, rotary sidewall cores provide adequate data, but conventional core remains superior for sedimentological analysis, fracture characterization, and building full-diameter plug data for flow simulation. Sidewall cores are a cost-effective complement rather than a complete replacement.
Q: How is sidewall core depth accuracy ensured?
A: The wireline tool depth is correlated to the openhole log depth by matching gamma ray readings from the tool's integrated GR sensor against the openhole GR log. Stretch correction and cable weight adjustments are applied. Typical depth accuracy is plus or minus 1 to 2 feet.
Why Sidewall Coring Matters
Physical rock samples are the only means of ground-truthing the interpretations generated from wireline logs, seismic data, and geological models. Sidewall cores deliver this ground-truth at a fraction of the cost and time of conventional coring, making them a near-universal step in well evaluation programs. The petrophysical parameters derived from sidewall core analysis, particularly Archie exponents m and n, formation factor, and capillary pressure curves, directly control volumetric reserve estimates and simulation model inputs. Errors in these parameters propagate through to field development decisions, well count, and recovery factor projections.
Biostratigraphic data from sidewall cores anchors stratigraphic correlation frameworks across basins and provides the chronological control needed to correlate well logs to regional sequence stratigraphic frameworks. In data-sparse frontier basins, a single well's sidewall core program may represent the only physical calibration available for an entire play fairway. The relatively low incremental cost of adding 20 to 40 additional sidewall core shots to a standard program is consistently justified by the value of the calibration data delivered, particularly when those shots target transitional intervals near petrophysical cutoffs where net pay classification is uncertain. Geomechanical applications of sidewall core including unconfined compressive strength (UCS) testing, Brazilian tensile strength measurement, and triaxial stress testing on rotary-cut plugs are increasingly standard deliverables that feed directly into wellbore stability models and completion design workflows.