Core (Reservoir Rock Sample)

Key Takeaways

• Core preservation from wellsite to laboratory is critical for maintaining representative fluid saturations and mechanical properties — when core is brought to surface, it is exposed to temperature and pressure changes that cause gas expansion, fluid evaporation, and stress relief that permanently alter the rock's properties; wellsite core handling procedures designed to minimize these changes include sealing core sections in plastic or aluminum tubes immediately after retrieval, wrapping in moisture-preventing material, refrigerating to slow evaporation, and shipping promptly to the laboratory; "preserved" or "native state" core handling specifically attempts to maintain original fluid saturations and wettability by avoiding water-based preservation fluids that alter oil-wet surfaces to water-wet; the difference between preserved and non-preserved core analysis results can be significant for wettability-sensitive properties like relative permeability and residual oil saturation — properties that directly govern waterflood recovery factors and therefore reserves estimates.
• Routine core analysis (RCA) provides the fundamental rock property measurements used in reservoir characterization — standard RCA on cleaned, dried core plugs measures porosity (usually helium porosimetry or calculated from grain density and bulk density), permeability (using gas under Darcy flow conditions, with corrections to liquid equivalent permeability), and grain density for a representative sample of plugs cut from the full core at 6-12 inch intervals; RCA data are the anchor measurements against which wireline log-derived porosity and permeability transforms are calibrated, providing the ground truth that logging data is never quite able to achieve directly; the permeability range from RCA typically spans many orders of magnitude within a single cored interval (from nanodarcy shale matrix to millidarcy or darcy-level sand streaks), characterizing the internal heterogeneity of the reservoir that governs fluid flow behavior far better than any wireline measurement.
• Special core analysis (SCAL) provides the multiphase flow properties that control recovery efficiency — while RCA characterizes single-phase flow properties, SCAL measurements on carefully preserved samples under reservoir conditions (temperature, net confining stress) provide relative permeability curves (how effective permeability to oil and water changes as their saturations change during flooding), capillary pressure curves (governing fluid distribution at the pore scale and initial water saturation), wettability indices (quantifying whether pore surfaces prefer oil or water contact), and residual oil saturation after waterflood; these measurements require weeks to months of laboratory time and specialized high-pressure, high-temperature apparatus, making SCAL one of the most expensive categories of reservoir characterization data; yet they are irreplaceable: relative permeability curves from SCAL are the single most influential inputs in waterflood simulation models that guide billions of dollars of water injection infrastructure investment decisions in mature fields.
• Geomechanical core testing provides the mechanical properties needed for wellbore stability and completion engineering — triaxial compression tests on core plugs under simulated reservoir stress conditions measure unconfined compressive strength (UCS), Young's modulus, Poisson's ratio, and internal friction angle that are the mechanical earth model (MEM) inputs used for wellbore stability analysis, casing design, hydraulic fracture height prediction, and sand production risk assessment; the relationship between the static mechanical properties measured on core and the dynamic mechanical properties derived from full waveform sonic logs (using different measurement frequencies and boundary conditions) is quantified through core-log transforms that allow MEM construction for all wells in a field from log data once the core-log relationship is established in one or more core wells; without core-calibrated MEMs, wellbore stability analysis relies on uncalibrated log-derived mechanical properties that may be systematically in error in ways that lead to the wrong mud weight or casing program decisions.
• Full core CT scanning before any cutting or plugging reveals internal structure and guides sample selection — computed tomography (CT) scanning of whole core before sampling reveals the internal sedimentary structure, fractures, vugs, laminations, and lithological variations that are invisible from the core surface; CT scans identify the best sample locations for routine and special core analysis (avoiding cracks, fractures, and heterogeneous zones that would give unrepresentative results), locate preserved intervals for wettability-sensitive SCAL work, and document core damage (micro-fracturing from stress relief during coring and retrieval) that must be accounted for in mechanical property measurements; the full core CT scan has become essentially standard practice in any coring program where the core will be used for SCAL or geomechanical testing, because sample selection guided by visual inspection alone misses the internal structure that determines whether a given plug is representative of the formation interval or an artifact of the coring process itself.