Core Sample Preparation

Assignment Core Sample Preparation

You are the company petrophysicist who is responsible for the analysis of two full diameter cores cut from the reservoir section of a recent successful appraisal well. Describe to your multi-disciplinary team colleagues the various ways in which the core plug samples from this full diameter core could be cleaned prior to their laboratory analysis.

Solution

Core plug samples are often first cut from full diameter cores as long cylinders and then trimmed to create right cylinders. The core plug samples can be cleaned by any of the following methods:

• Cold static cleaning tends to be the least damaging method for cleaning cores, but the very long time required for the process can be an issue if the core analysis results are required for this important project within a limited timeframe.
• Continuous immersion Soxhlet cleaning is one of the best techniques. This uses a flask wherein the solvent is boiled. The vapor rises and passes into a condenser positioned above the core sample. Condensed solvent drips down into the sample chamber, where it is routed to the chamber base from where it percolates upwards. The chamber is heated and the solvent agitated. Core samples are placed on a perforated shelf placed a few centimeters from the chamber bottom. An outlet at the top of the sample chamber enables the solvent to run off back into the original solvent boiling chamber, allowing the continuous circulation of the solvent. This process allows the core samples to be in continuous contact with warm, clean liquid solvent which results in efficient plug cleaning without damaging the delicate minerals.
• Refluxing in Dean-Stark or Soxhlet apparatus is a fairly slow process. The Soxhlet type cleaning should not entail many rapid drying cycles, as the potential damage to the rock core thereby may be unacceptable.
• Flushing in a centrifuge is limited to plug-size cores and requires that the mechanical strength of the core be sufficient to withstand the centrifugal forces.
• Pressure flushing of solvent through the core is a core cleaning system based on controlled sequential solvent displacement under moderate pressure. Extraction of the hydrocarbons and salts from reservoir rocks is achieved by injecting one or more solvents into the core sample under pressure and moderate temperature. The pressure used depends on the sample permeability. The core is considered clean when the effluent is clean.
• Repeated pressure cycling of the core with a carbon dioxide and toluene mixture is a relatively fast process and a good technique for full diameter cores, but not suitable for poorly consolidated sand or chalky limestone. This process normally includes a stainless steel core cleaning pressure chamber capable of accommodating core samples up to 4 ½ inches in diameter. Computer software controls the cleaning process and monitors the temperature and pressure of the system.
• Vapor soaking, with condensed toluene dripping on the core is suitable for non-clay and non-gypsum bearing formations.

Assessment Core Sample Preparation

1. Which core sample preparation processes can cause alteration to the core material, reducing the accuracy and effectiveness of the reservoir data expensively acquired by coring? (Select all that apply.)

A .Drying Correct ✔
B .Cutting Correct ✔
C .Cleaning Correct ✔
D .Boxing

2. What are remedial techniques used to remove rock flour generated during the cutting and trimming of core plugs? (Select all that apply.)

A .Blowing the core plug with air or nitrogen gas Correct ✔
B .Working entirely in a vacuum environment
C .Conducting the trimming while flowing fluids through the plug ends Correct ✔
D .Brushing the core plug dry Correct ✔
E .Brushing the core plug under a liquid Correct ✔

3. Analysis of rock cores plays a major role in helping geoscientists and reservoir engineers to determine which of the following reservoir properties? (Select all that apply.)

A .Hydrocarbon storage capacity Correct ✔
B .Temperature
C .Pressure
D .Production performance Correct ✔

4. To be most effective, the core analysis results should reflect the real reservoir rock properties, which involves minimizing what potential impacts? (Select all that apply.)

A .Impact of the analysis procedures Correct ✔
B .Impact of the formation heterogeneity
C .Impact of the testing equipment Correct ✔
D .Impact of the core preparation Correct ✔

5. Which of the following can be used to effectively remove oil? (Select all that apply.)

A .Toluene Correct ✔
B .Water
C .Diesel
D .Xylene Correct ✔

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