Microcylindrical Log

The microcylindrical log (MCFL, or Micro-Cylindrically Focused Log) is a pad-mounted microresistivity wireline logging tool that measures the electrical resistivity of the very shallow formation zone immediately adjacent to the borehole wall — typically investigating 1 to 5 centimeters into the formation — using a cylindrically focused electrode array pressed against the wellbore wall to minimize the borehole fluid effect on the measurement, providing the shallow resistivity reading that, combined with deeper-reading resistivity tools, defines the invasion profile used to characterize the flushed zone, determine water saturation, and identify moveable hydrocarbons in the near-wellbore region invaded by drilling fluid filtrate during the period the well was being drilled.

What Is a Microcylindrical Log?

The electrical resistivity profile around a drilled wellbore is not uniform. During drilling, the mud hydrostatic pressure exceeds formation pore pressure, forcing mud filtrate into the permeable formation — the filtrate displaces native formation fluids (oil, gas, or brine) from the region nearest the wellbore, creating a series of concentric zones: the flushed zone (where filtrate has displaced most of the original pore fluid), the transition zone (partially invaded), and the uninvaded zone (containing native formation fluids). Each zone has a different resistivity depending on its fluid content and the resistivity difference between the filtrate and the native formation water.

Measuring this resistivity profile at different depths of investigation reveals both the formation's native properties (from the deep-reading tool that samples the uninvaded zone) and the near-wellbore invasion characteristics (from shallow tools that sample the flushed and transition zones). The microcylindrical log samples the shallowest zone — the flushed zone immediately at the wellbore wall — providing the boundary condition in the invasion profile from which deeper-reading tools derive their formation evaluation information.

The practical value of the MCFL in reservoir evaluation is the moveable hydrocarbon indicator it provides when combined with deeper resistivity data. If the flushed zone shows high resistivity (low water saturation — oil remaining even after filtrate flushing) compared to the deep formation resistivity (which shows the undisturbed oil saturation), the difference quantifies the oil that the filtrate displaced — and by extension, the oil that production pressure can displace. This comparison is among the most powerful tools available from standard wireline logging for assessing which reservoirs will actually produce oil when perforated, without requiring a production test.

MCFL Applications in Formation Evaluation

Invasion diagnosis using the MCFL and resistivity suite requires plotting the three resistivity readings (Rxo, Ri, Rt) on a linear scale against depth to identify the invasion profile pattern — a "step" profile (Rxo much higher than Rt) indicates deep invasion into an oil-bearing formation by fresher filtrate that displaced the more saline formation water, with the flushed zone retaining some oil (moderate Rxo) and the deep formation still containing oil (high Rt, deep resistivity); a "annulus" profile (Rxo low, Ri high, Rt moderate) indicates that the formation water salinity gradient between filtrate and native brine has created an intermediate resistivity ring around the wellbore where the mixed fluids produce a resistivity peak; an "anti-" profile (Rxo lower than Rt) indicates formation water is more saline than mud filtrate, consistent with non-hydrocarbon-bearing formations where invasion reduces resistivity near the wellbore.

Mud cake thickness interpretation from the MCFL pad resistance provides secondary diagnostic information about the formation face — tight formations with low permeability build thick, low-permeability mud cakes that separate the MCFL pad from the formation and cause the tool to measure mud cake resistivity rather than formation flushed zone resistivity; the mud cake effect is visible as an anomalously low MCFL reading immediately adjacent to a tight (high deep resistivity) formation, and is distinguished from a genuine low-resistivity flushed zone by the presence of a distinct, thick mud cake on the caliper record and by the inconsistency between the very low MCFL and the high porosity implied by the density and neutron logs at the same depth.

Microcylindrical Log Across International Jurisdictions

Canada (AER / WCSB): WCSB conventional oil and gas reservoir evaluation in the Devonian carbonates and Cretaceous sandstones uses the standard triple-combo logging suite including microresistivity for invasion characterization — AER requires that conventional well log data be submitted to the Well Data Management System (WDMS) as part of the well completion documentation, including the resistivity suite from which MCFL data can be retrieved for regional reservoir characterization; WCSB tight reservoirs (Montney, Cardium, Viking) often show limited filtrate invasion due to their low permeability, making the MCFL-derived Rxo close to the true formation resistivity Rt, reducing the usefulness of the invasion contrast for moveable hydrocarbon detection in these low-permeability formations where production testing is the more reliable productivity indicator.

United States (API / BSEE): GoM deepwater formation evaluation uses advanced array resistivity tools (HRLA, RT Scanner) that provide multiple depths of investigation including a shallow measurement analogous to MCFL as part of a comprehensive resistivity inversion that characterizes the invasion profile in deepwater sands — BSEE requires that deepwater well logs be submitted to the BOEM data submission portal, and the resistivity data including shallow microresistivity provides the invasion characterization used in the formation evaluation reports submitted with the well completion documentation; USGS and BOEM use MCFL and shallow resistivity data from deepwater well logs in regional resource assessment databases.

Norway (Sodir / NORSOK): Sodir's Fact Pages database includes wireline log data submission requirements for all NCS wells, with resistivity logs including shallow microresistivity forming part of the standard NCS log data package; NCS formation evaluation for North Sea sandstone and chalk reservoirs uses standard invasion analysis combining MCFL or equivalent shallow resistivity with medium and deep readings to distinguish hydrocarbon-bearing from water-bearing zones and to estimate moveable hydrocarbon content in the reservoir interval — Sodir uses this formation evaluation data in resource classifications for Norwegian petroleum reserve reporting.

Middle East (Saudi Aramco): Saudi Aramco's Arab Formation well evaluation uses the full resistivity suite including MCFL for invasion characterization in Arab D carbonate reservoirs — the invasion profile in Arab Formation carbonates provides critical information about the dual-porosity nature of the carbonate (matrix versus fracture) pore system, since fractures and vugs may show different invasion depth and rate than the matrix porosity, and the MCFL shallow investigation sees the combined flushed zone properties of both pore types; Aramco's petrophysical interpretation for Arab Formation wells includes MCFL data in the invasion model used to estimate true formation water saturation independent of the invasion correction uncertainty that would apply if only deep resistivity data were available.