PP core analysis requirements
- Basic rock properties (porosity, permeability, fluid saturations, grain density)
- Saturation from capillary pressure
- Effect of stress and reservoir (production-induced) compaction/subsidence)
- Electrical properties and CEC
- Acoustic properties for AVO calculations
Geological core analysis requirements
- Core description
- Facies analysis (also for SCAL sampling)
- Mineral identification
- Diagenesis and clay typing
- Depositional information
- Formation age
- Microscopic and x-ray analysis
RE core analysis requirements
- Relative permeability parameters
- Capillary pressure curves
- Critical gas saturation
- Pore volume compressibility
- Flooding tests (such as hot water or steam flooding)
PT core analysis requirements
- Well injectivity
- Sand control parameters
- Rock mechanical properties for fracture design
- Sieve analysis for gravel sizing
- Mineralogy for acid stimulation
RCA measurements
- Residual fluid saturations at surface
- Atmospheric porosity
- Air permeability: horizontal and vertical
- Klinkenberg-corrected permeability (also have to correct for clay swelling using Hill-Shirley-Klein equation)
- Grain density
- Lithology
SCAL measurements
- Stressed porosity and permeability
- m and associated FRF
- n and associated RI
- CEC or Qv
- Capillary pressure
- Wettability
- Relative permeability
- Residual oil saturation
- Mechanical rock properties such as compressibility
- Waterflood sensitivity for injectivity and well performance
- Acid solubility
Wellsite considerations
- Coring bit type (low invasion face discharge coring bits are recommended especially for unconsolidated cores)
- Core type: conventional coring, containerized coring, preservation coring or wireline coring
- Liner type: aluminium (low friction coefficient, does not bend easily), fiberglass (low coefficient of friction, but cannot be used above 180degC and cannot be used in all drilling fluids), etc.
- Mud; SOBM results in best core recovery (good lubricity and shale stability)
- Drilling parameters (ROP, WOB, RPM)
- Overbalance controls; minimize invasion
- Core barrels
- Core jamming
- Bringing the core to surface
- Laying the core down; avoid core bending by using core cradle
- Core marking: red to the right when looking from bottom to top
- Core handling/preservation: freezing, resination, wax sealing?
Core screening techniques
- Natural GR scan (pseudo GR)
- Spectral GR (U, Th, K concentrations)
- GR attenuation (pseudo bulk density)
- Probe permeametry (pseudo permeability)
- CT scan
- X-ray fluoroscopy (examination of sleeved unconsolidated material)
Core screening purpose
- Depth matching
- Estimate degree of material homogeneity for sample selection
Core sampling for RCA and SCAL
RCA: one per foot
SCAL: lithology-based
Fluids used in plug drilling
- Fresh (or tap) water for clean sands and carbonates
- Brine for cores from high salinity environments
- Kerosene (or petrofree) for shales and halite-bearing samples
- Air for fluid saturation determination studies
- Liquid nitrogen for shales and when consolidation is questionable
Note: drilling using water will disturb the clays
Name the technique used to clean consolidated samples
Removal of oil with hot solvent extraction (Soxhlett) technique. Salt is then removed by using methanol.
Solvent: azeotropic mixture (for e.g. toluene, chloroform)
Describe the principle of cleaning unconsolidated samples
Frozen sample is mounted in a core holder and a confining stress of 30-50 bar is applied. The sample is allowed to thaw. After thawing, the sample is cleaned by cold solvent flushing with chloroethene and toluene alternately (or just chloroform), followed by methanol for salt removal.
Describe oven drying of cores
Samples are dried in a vacuum oven at 95degC until constant weight is obtained.
What is the purpose of Critical Point Drying (CPD)?
To preserve the structure of the clays in the pores. Delicate clay minerals such as fibrous illite could be damaged by oven drying, profoundly affecting (increasing) the air permeability of samples.
Note: This is not a real issue for samples from the oil leg because the irreducible water holds the clay in place already.
Fluid saturations by Dean-Stark extraction
- Weigh sample + fluids, Wt
- Distillation of water from boiling solvent (toluene - immiscible with water)
- Chloroethene extraction to remove remaining oil
- Weigh dry core, Wd
- Water weight measured directly, Ww
- Calculate Wo: Wt = Wd+Ww+Wo
For OBM and above transition zone such that Sw doesn’t change. For WBM, can put tracer (deuterium) to quantify amount of invasion.
What are needed to calculate atmospheric porosity?
Bulk volume, pore volume and grain volume (2 of 3)
How to measure bulk volume?
- Buoyancy in mercury
- Mercury displacement (measuring the rise in mercury level)
- Caliper measurements
How to measure pore volume?
Compare weights of dry and liquid-saturated sample
How to measure grain volume?
- Buoyancy in chloroethene
- Helium expansion using Boyle’s Law
How is grain density measured?
By using a pycnometer
How is air permeability measured?
- Ruska permeameter (routine)
- Mini-permeameter
- Pressure pulse permeameter (for low permeabilities: <1mD)
How is RI measured?
- Porous plate (No longer recommended - many manual handling steps)
- Continuous injection (Shell method - majority of pores is filled with oil in ~4 weeks. Can also yield oil-brine cap curve.)
- Pressure equilibrium (Considered most accurate but time-consuming (~6 weeks per sample). Also yields oil-brine cap curve.)
Note: Want sample to be oil-wet to better represent the reservoir (not always the case). Position of water in the pores depends on wettability, and resistivity depends on position (connectivity) of water, affecting n parameter.
How is CEC measured?
- Membrane potential (relatively fast and very accurate)
- Multiple salinity (Co-Cw) (slow and expensive, but very accurate)
- Conductometric titration (not recommended - sample is crushed; likely to expose more CEC sites than in-situ)
