Sidewall

Key Takeaways

• Sidewall coring provides direct rock samples from specific formation intervals for laboratory analyses that cannot be performed on cuttings (which are small, mixed-depth chips that have lost their original orientation and spatial context) or on wireline logs (which measure bulk formation properties without providing physical material for examination): sidewall cores are used for mineralogical analysis (X-ray diffraction to identify clay types and cement phases that influence petrophysical log response), petrographic analysis (thin section and scanning electron microscopy to characterize pore geometry, cementation, and grain contacts that control permeability and irreducible water saturation), reservoir geomechanics (unconfined compressive strength testing and elastic modulus measurements on undamaged plugs), source rock geochemistry (Rock-Eval pyrolysis on sidewall cores from potential source intervals), and reservoir fluid typing (gas chromatography of fluorescence from cores cut in oil zones to distinguish oil-stained from residual oil zones); conventional rotary sidewall coring (using a hollow diamond-tipped coring bit attached to a wireline motor and pressed into the formation by an anchor pad on the logging tool) produces a 2.5 to 4.0 cm diameter by 4 to 8 cm long cylindrical plug with minimal thermal or mechanical disturbance, compared to percussion sidewall coring (in which a hollow steel bullet is fired into the formation by a propellant charge and retrieved with the rock sample inside) which provides a smaller (2.5 cm by 2.5 cm) sample with some mechanical disturbance from the bullet impact; rotary sidewall cores are preferred for geomechanical and petrographic work where sample integrity is critical, while percussion cores are faster and less expensive when many samples at close depth intervals are needed for stratigraphic or geochemical screening.
• Sidewall formation testing using a probe formation tester (MDT, RCI, FTWD) applies a hydraulic packer or a porous probe pad against the borehole wall to extract a small volume of formation fluid while recording the pressure response, determining the formation pore pressure (from the pressure drawdown and buildup during the test) and the formation permeability (from the mobility k/mu derived from the inverse slope of the pressure versus square root of time during the initial drawdown); the probe geometry (the small area of the probe orifice relative to the formation face) makes this a point measurement that samples the formation within a few centimeters of the borehole wall, with the mobility estimate therefore reflecting the near-wellbore permeability of the invaded zone rather than the true formation permeability at reservoir conditions; the pressure measurements from multiple sidewall tests at different depths in the same well (pressure versus depth profile) are used to identify fluid contacts (where the pressure gradient changes from hydrocarbon gradient to water gradient), calculate the formation water density (from the water gradient), and detect pressure compartmentalization (where different reservoir intervals are at different pressures, indicating flow barriers or depleted zones from offset production); the wireline formation test has largely replaced the drill stem test (DST) for fluid contact identification and pressure profile determination in exploration and appraisal wells because it can be performed at many depths in a single wireline run in 8 to 24 hours, compared to the 3 to 7 days required for a cased-hole DST with production testing.
• Sidewall neutron tools (including Schlumberger's CNL and Halliburton's DSN-B) press a neutron source and detector against the borehole wall to reduce the volume of borehole fluid between the source and the detector, minimizing the borehole fluid contribution to the neutron measurement (which would bias the apparent porosity toward the hydrogen content of the borehole fluid rather than the formation); in liquid-filled wells (oil-based mud or water-based mud), the sidewall contact reduces the borehole diameter correction required for the neutron measurement by eliminating the standoff between the tool and the formation wall; in gas-filled wells or wells drilled with air or foam (where the borehole fluid has very low hydrogen content), the sidewall contact is essential for a usable neutron measurement because without contact the measurement would be dominated by the air or foam in the borehole rather than the formation hydrogen; modern azimuthal neutron tools (Schlumberger's AIT, Halliburton's HRLA) use multiple receiver spacings at different azimuths to simultaneously measure the borehole size (from the near-spacing readings) and the formation porosity (from the far-spacing readings), partly correcting for the standoff effect without requiring direct sidewall contact for the tool body.
• Sidewall electrical measurements (micro-resistivity pads, image logs) use direct contact with the borehole wall to measure the resistivity of the very near-borehole formation (within 1 to 2 cm of the wall), which has been flushed by mud filtrate invasion and reflects the resistivity of the flushed zone (Rxo) rather than the true formation resistivity (Rt) measured by the deep-reading induction or laterolog tools; the micro-spherically focused log (MSFL) and the shallow focused log provide Rxo measurements that are used in combination with the deep resistivity to calculate invasion depth (from the contrast between Rxo and Rt) and to estimate Sxo (the flushed zone water saturation), which combined with Sw from the deep resistivity gives the movable hydrocarbon saturation (Sxo - Sw = movable hydrocarbon fraction); the Formation MicroImager (FMI) uses a 192-button pad array to produce a high-resolution (5 mm) resistivity image of the borehole wall that reveals sedimentary structures (bedding, lamination, cross-bedding), fractures (open fractures appear as sinusoidal conductive features, healed fractures as sinusoidal resistive features), borehole breakouts (stress-induced enlargements in the direction of minimum horizontal stress), and drilling-induced fractures (DIF, induced by the thermal and mechanical stress of drilling), providing essential structural, geomechanical, and petrophysical information at a resolution far exceeding any other logging measurement.
• Sidewall sampling in contaminated formations (where the near-borehole formation is invaded by mud filtrate and the original formation fluid has been partially or fully displaced) requires careful sample conditioning before the fluid type at reservoir conditions can be determined from the sample: fluid sample pumping using an MDT or RCI formation tester extracts formation fluid through the probe for a period sufficient to displace the mud filtrate from the near-wellbore region and collect a sample that is representative of the reservoir fluid; contamination monitoring using the optical fluid analyzer (OFA) module in the formation tester measures the color, methane content, and GOR of the sampled fluid continuously during pumping, allowing the engineer to determine when the mud filtrate contamination level has dropped to an acceptable level (typically less than 5 percent contamination for a high-quality sample) and to trigger the sample bottle closing mechanism when the sample quality criterion is met; fluid samples collected with less than 5 percent contamination provide accurate PVT data for reservoir simulation, saturation pressure determination, and flow assurance analysis, while highly contaminated samples (greater than 20 percent contamination) cannot be used for quantitative PVT work and should be characterized as "trend samples" useful only for qualitative fluid typing.