clean

Clean in petroleum engineering and petrophysics is a descriptive term applied to reservoir intervals, formation sections, or produced fluids that are free of, or contain only minimal quantities of, clay minerals, shale laminations, or other contaminating materials that degrade reservoir quality, complicate log interpretation, or reduce product quality; a clean formation or clean sand is a sandstone or carbonate interval with clay volume (Vcl) below approximately 5 to 10 percent, permitting direct application of Archie's equation to resistivity log data for accurate water saturation calculation without clay conductance corrections, and providing the maximum intergranular porosity and permeability achievable at a given burial depth in Western Canada Sedimentary Basin clastic reservoirs. In WCSB petrophysical log analysis, the clean interval designation is operationally defined by the gamma-ray log response: clean sandstone zones in WCSB Cardium and Viking reservoirs read 15 to 30 API units on the GR log (reflecting only the natural radioactivity of quartz and feldspar grains and trace K-feldspar), in contrast to adjacent shale intervals reading 90 to 150 API units; the clean GR baseline (GRclean) established from the cleanest sandstone in the log section is the reference point for all clay volume calculations using the linear gamma-ray equation (Vcl = (GRlog minus GRclean) divided by (GRshale minus GRclean)), making the correct identification of the clean baseline critical to all downstream petrophysical calculations including effective porosity, water saturation, net pay cutoff application, and hydrocarbon pore volume estimation for WCSB reserves reporting under NI 51-101. The clean interval designation in WCSB production engineering also extends to produced fluids: clean oil is crude oil meeting the basic sediment and water (BS&W) specification for pipeline delivery, typically less than 0.5 percent BS&W for WCSB light and medium crude at custody transfer, compared to the unprocessed wellstream that may contain 30 to 70 percent water and dispersed formation sand at initial production; clean gas refers to sales gas meeting H2S (below 4 mg/m3), CO2 (below 2 percent), and water dew point specifications for entry into the Alberta gas transmission network after field processing.

• Clean formation identification and GR baseline establishment in WCSB petrophysical log analysis: Identifying clean formation intervals in WCSB Cardium, Viking, and Montney log sections is the first step in any petrophysical workflow because the clean GR baseline determines the clay volume (Vcl) for every depth level, and Vcl drives effective porosity, Waxman-Smits water saturation, and net pay cutoff calculations throughout the interval. The clean GR value in WCSB Cardium sandstones typically falls between 18 and 28 API units in the cleanest, most quartz-rich intervals; in WCSB Montney siltstones, clean GR ranges from 25 to 45 API units because the fine-grained quartz and dolomite matrix has slightly higher intrinsic radioactivity than coarser Cardium sandstone. Clean baseline selection requires careful log scanning across the full formation interval to find the absolute minimum GR reading that is geologically reasonable (not an anomalously thin bed, washout zone, or tool artifact), cross-checking against neutron-density porosity crossplot position (clean sandstone should plot near the sandstone matrix line) and core mineralogy (XRD confirming clay-free composition) where available. In WCSB wells without core, the clean GR baseline is typically set at the 5th percentile of the GR distribution within the reservoir formation interval, which in practice corresponds to the cleanest 5 percent of beds sampled and provides a stable reference unaffected by occasional anomalously low GR readings from thin potassium-depleted volcanic ash layers or drill-bit induced washouts that can appear as spuriously low GR values.
• Clean sand net pay and net-to-gross ratio in WCSB reservoir characterization and reserves estimation: Net pay in WCSB Cardium and Viking reservoirs is defined as the subset of gross reservoir interval that meets the cutoff criteria for clean sand classification, minimum effective porosity, and maximum water saturation; the clean formation criteria (Vcl below the cutoff, typically 0.25 to 0.40 depending on WCSB pool-specific correlations between clay volume and permeability) is the first filter applied in net pay determination before porosity and saturation cutoffs are assessed. Net-to-gross (NTG) ratio in WCSB stratigraphically complex reservoirs like the Viking, which consists of interbedded clean shoreface sandstones and shale laminations, is the fraction of gross interval that is classified as clean sand and therefore counted in the net pay column; Viking NTG in central Alberta WCSB pools ranges from 0.25 to 0.85 depending on depositional facies, with the highest NTG in wave-dominated shoreface environments and the lowest in deltaic and estuarine settings where shale drapes between sand laminations are frequent. In WCSB AER reserves reporting under NI 51-101, the clean sand net pay volume times average porosity and (1 minus Sw) gives the hydrocarbon pore volume that is converted to recoverable reserves by multiplication by a recovery factor established from pool performance, making the clean classification a direct input to the reserves estimate with material financial consequence to the reporting company.
• Clean wellbore and clean hole conditions in WCSB drilling and completion operations: In WCSB drilling operations, a clean hole or clean wellbore is one that has been circulated until the return drilling fluid is free of significant cuttings load, gas shows are background, and mud properties (density, rheology, and filtration) are restored to their pre-drill baseline values; achieving clean hole conditions before running casing, logging, or perforating is mandatory in WCSB operations because residual cuttings beds cause logging tool sticking, wireline damage, and cement channeling around casing in the annular space. Clean circulation in WCSB horizontal wells drilled with oil-based mud requires pumping at least 1.5 to 2.0 annular volumes of mud past the bit with reciprocation and rotation of the drill string to dislodge settled cuttings beds from the low side of the inclined annulus; in extended-reach WCSB Montney horizontals with lateral lengths of 2,000 to 3,500 m, achieving a clean hole for liner running may require 3 to 4 hours of dedicated clean-out circulation at maximum allowable pump rate. Clean hole confirmation in WCSB operations is verified by low and stable shaker return solids volume (solids loading below 3 to 5 percent by volume in the return mud), consistent mud weight at the flow-line compared to the suction pit (no discernible density increase from cuttings suspension), and a shaker screen without any coarse cuttings accumulation after the annular volume has been circulated.
• Clean oil and clean gas production specifications in WCSB gathering and processing systems: Clean oil in WCSB production operations is crude oil that meets the pipeline company's basic sediment and water (BS&W) specification for delivery into the gathering and transmission system, typically less than 0.5 percent BS&W by volume measured by centrifuge test (ASTM D4007) at custody transfer; achieving clean oil specification from WCSB Cardium and Viking emulsion producers requires chemical demulsifier addition (100 to 500 ppm, typically polyoxyalkylene block copolymers optimized for the specific oil-water system), electrostatic treater or heated free-water knockout vessel residence time, and periodic cleaning of treater internals to remove paraffin and scale deposits. Clean gas in WCSB processing operations refers to sales-specification natural gas after field treatment to remove H2S (to below 4 mg/m3 for WCSB pipeline delivery by the TCPL Nova system), CO2 (to below 2 percent by volume), water vapor (dew point depressed to minus 10 degrees Celsius at pipeline pressure), and C3+ condensate recovered in the gas plant before the residue gas is sold; achieving clean gas specification from sour WCSB Montney and Duvernay wells with H2S concentrations of 1 to 10 percent requires amine treating (MDEA or hybrid amine systems) at the field gas plant, with the removed H2S either converted to elemental sulfur by the Claus process or reinjected for reservoir pressure maintenance in WCSB EOR programs.
• Clean completion and clean perforation practices in WCSB wellbore stimulation programs: In WCSB Cardium and Viking perforated completions, a clean perforation is one that has been fully opened to formation flow without crushed-zone debris or perforation tunnel fill blocking connectivity from the formation to the wellbore; achieving clean perforations in WCSB wells requires either overbalanced perforating with immediate cleanup flow after perforation (flowing well kills the perforating debris into the wellbore), underbalanced perforating (differential pressure draws debris and crushed zone material into the wellbore during the perforation event), or a post-perforation acid surge treatment that dissolves calcium carbonate and iron scale from the perforation tunnel and near-wellbore matrix. Clean fluid in WCSB hydraulic fracturing operations is fracture fluid that has been filtered to remove suspended particles above 50 microns (to prevent proppant bridging in the fracture near the wellbore) and treated to remove oxygen (below 20 ppb dissolved oxygen target to prevent polymer degradation and equipment corrosion), iron (below 50 ppm to prevent iron precipitation at reservoir pH), and bacteria (treated with glutaraldehyde or THPS biocide to prevent microorganism-induced H2S generation and polymer degradation in the proppant pack); water quality management is particularly critical in WCSB Montney slickwater completions where the large fracture fluid volume (3,000 to 10,000 m3 per well) passes through the proppant pack during flowback and must not introduce damaging fines or scale-forming species that would impair fracture conductivity.

Clean GR Baseline Correction Improving WCSB Viking Net Pay Estimation

A WCSB Viking pool study in central Alberta reviewed petrophysical interpretation across 28 wells where initial clean GR baselines had been set inconsistently, ranging from 22 to 38 API units across the pool. A standardized clean GR baseline of 24 API units was established from the cleanest (core-calibrated quartz arenite) Viking sandstone samples identified in 6 cored wells, with XRD confirming clay volume below 3 percent at GR readings of 22 to 26 API. Recalculating Vcl with the standardized baseline changed net pay thickness by 0.5 to 2.2 m (average 1.1 m) in 19 of the 28 wells; pool net pay volume increased by 8 percent and total hydrocarbon pore volume increased by 6 percent versus the prior inconsistent interpretation. The revised pool OOIP estimate was 1.82 million m3 versus the prior 1.71 million m3; reserves were rebooked upward by 85,000 m3 (proved developed producing), a material change to company reserves disclosures that was attributed entirely to consistent clean baseline methodology rather than any new drilling or production data.

Related Terms

Gamma-ray log is the primary clean formation indicator in WCSB wells; the clean GR baseline (15-30 API in Cardium and Viking sandstones) is the reference for clay volume and net pay determination. Clay content defines the boundary of clean formation quality; Vcl above 0.25-0.40 excludes intervals from WCSB net pay regardless of porosity because clay-filled pore space is not producible. Net pay is the cumulative clean, porous, hydrocarbon-saturated interval thickness; clean sand classification is the first filter before porosity and saturation cutoffs in WCSB NI 51-101 reserves booking. Effective porosity is total porosity minus clay-bound water; intervals below the clean Vcl cutoff are typically excluded from WCSB net pay even with clay-corrected porosity calculations. Basic sediment and water (BS&W) measures oil cleanliness; clean oil at less than 0.5% BS&W meets WCSB pipeline custody transfer specification.