Core Sample Preparation

(adsbygoogle = window.adsbygoogle || []).push({}); Learning Objectives After completing this topic "Core Sample Preparation", you will be able to: Describe the preparation of core samples. Summarize the cutting and cleaning of core plugs. Discuss how core plugs are dried before analysis. Introduction The analysis of rock cores (Figure 1) plays a major role in helping geoscientists and reservoir engineers to determine a reservoir’s hydrocarbon storage capacity and theoretical production performance. To be most effective, such core testing and analysis should measure the representative reservoir rock properties, which entails minimizing any impact of the core preparation and testing equipment (Figure 2), and analysis procedures possibly causing alteration to the rock’s properties. Figure 1: Typical core samples Figure 2: Rock core testing equipment (shown is the Advantest 9 system for uniaxial and triaxial tests on rock cores) (adsbygoogle = window.adsbygoogle || []).push({}); To ensure that the analysis of the core material genuinely reflects the actual rock properties, the core plugs and sidewall core samples need to be very carefully prepared for testing and analysis, and then examined as shown in Figure 3. Many studies worldwide have demonstrated that different lithologies can be altered in different ways during the cutting, often with a diamond saw as shown in Figure 4, cleaning and drying of core plugs, which reduces the accuracy and effectiveness of the reservoir data expensively acquired by coring. Figure 3: Core plug analysis Figure 4: Diamond saw for cutting core samples (adsbygoogle = window.adsbygoogle || []).push({}); Cutting and Cleaning Core Plugs and Sidewall Core Samples (adsbygoogle = window.adsbygoogle || []).push({}); Core plug samples, used for a range of different types of core analyses, are often first cut from whole cores as long cylinders, as shown in Figure 1, and then trimmed to create right cylinders, as shown in Figure 2. Figure 1: Long cylinders cut from whole cores Figure 2: Core plug right cylinders Plug samples are usually cut from a full diameter core with a diamond core bit both parallel and orthogonal to the main bedding planes, and trimmed so as to create a cylindrical plug from the center of the core where the minimum amount of mud filtrate flushing and invasion of drilling mud solids is likely to have occurred. The objective of coring is to recover representative samples of the formation under study. To be fully representative, the samples should not be severely damaged or invaded by non-reservoir fluids. Therefore, sample plugs taken from the center of the core have the highest chance of being the most representative of undisturbed reservoir rock. Vertical permeability samples are drilled at right angles to the bedding planes, while horizontal permeability samples are cut parallel to the bedding planes. Although primarily used for bedded sandstone reservoirs, this technique is also satisfactory for the more homogenous, non-fractured and non-vuggy carbonates. (adsbygoogle = window.adsbygoogle || []).push({}); The cutting and trimming of such core plugs can affect the subsequent petrophysical measurements. This cutting and trimming process creates “rock flour”, some of which is deposited onto the surface of the core plug. Such a thin layer of debris on the core plug sample tends to have an effect on the subsequent measurements. Remedial techniques have included brushing the core plug dry, brushing it under a liquid, blowing it with air or nitrogen, and conducting the trimming while flowing fluids through the plug ends. These techniques are not universally successful. Brushing may just redistribute the flour into different places on the samples or, even worse, push it deeper into the pore network. Trimming under fluid flow can cause migration of the fines. Some practitioners, as well described by Byrne and Patey in 2004, have returned to a traditional stained acetate peel technique and modified it successfully to remove the vast bulk of the rock flour from both sandstone and carbonate plug end faces and curved surfaces. The acetate peel technique is also used to remove the contaminating layer of debris from preserved or fully saturated samples. Removal of the rock flour creates a fresh and uncontaminated surface that allows more accurate absolute and relative permeability measurements. Cleaning https://youtu.be/Bp7wHP-lVnM In order to conduct the testing and analysis of core samples, they need to be cleaned of residual solids, as shown in Figure 3, and fluids. The cleaning of rock samples can be achieved using many different techniques. For many core analysis measurements, samples are cleaned of pore fluids and contaminants and then measured, either in a dry state or with partial or full re-saturation. Figure 3: Cleaning residual solids from core samples The measurement of permeability and porosity using Boyle's law, as shown by a graphical representation in Figure 4, and re-saturation techniques requires that the residual fluids be removed, and then the cores be cleaned and dried. The solvents used to remove the oil must not react with the rock or damage delicate minerals such as fibrous clays. Effective solvents include toluene and xylene. Typically, water is removed by heating the rock, with subsequent vaporization, and trapping of the resultant water vapor. Although not part of routine core analysis, samples may be leached with water or methanol to remove any salt left from the vaporized interstitial or filtrate water. (adsbygoogle = window.adsbygoogle || []).push({}); Figure 4: Graphical representation of Boyle’s law There are several methods for cleaning core samples. Core Sample Cleaning Methods 1. Cold static cleaning Cold static cleaning tends to be the least damaging method for cleaning cores; but the very long time required for the process can be a significant issue, as the core analysis results are often required with short deadlines. 2. Soxhlet and Dean-Stark apparatus Continuous immersion Soxhlet cleaning is one of the best techniques. This method 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. In summary, 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 (Figure 5) 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. Figure 5: Dean-Stark apparatus (A), and Soxhlet apparatus (B) 3. Centrifugal flushing (adsbygoogle = window.adsbygoogle || []).push({}); Flushing in a centrifuge, as shown in Figure 6, is limited to plug-size cores and requires that the mechanical strength of the core be sufficient to withstand the centrifugal forces. Figure 6: Centrifuges used for flushing core samples 4. Pressure flushing Equipment for pressure flushing of solvent through the core is shown in Figure 7. This apparatus is a core cleaning system based on controlled sequential solvent displacement under moderate pressure. Extraction of the hydrocarbons and salts from formation rocks is achieved by injecting one or more solvents into the core sample under pressure and moderate temperature. The amount of pressure applied depends on the sample permeability. The core is considered clean when the effluent is clean. Figure 7: Controlled sequential solvent displacement under moderate pressure 5. Repeated pressure cycling with CO2 and toluene Repeated pressure cycling of the core with a carbon dioxide and toluene mixture is a relatively fast process (not suitable for poorly consolidated sand or chalky limestone, but a good technique for full diameter cores). Figure 8 shows a typical computer-controlled system designed to clean plug size and full diameter core samples with CO2 saturated toluene in programmed cycles to remove the residual hydrocarbons. The system normally includes a stainless steel core cleaning pressure chamber capable of accommodating core samples up to 4 1/2" in diameter. Computer software controls the cleaning process and monitors the temperature and pressure of the system. Figure 8: Equipment for pressure cycling of cores with a carbon dioxide and toluene mixture 6. Vapor soaking Vapor soaking, with condensed toluene dripping on the core is more applicable for non-clay and non-gypsum bearing formations. (adsbygoogle = window.adsbygoogle || []).push({}); Cleaning Clays (adsbygoogle = window.adsbygoogle || []).push({}); Certain clays (particularly montmorillonite, as shown in Figure 9) will dehydrate at temperatures lower than 180° F (82° C) if the relative humidity is reduced to zero during the cleaning process. It is important to avoid this because the water on the clay surface is chemically bound to the clays in the reservoir and so reduces the pore space available for hydrocarbons. Data indicate that reservoir clays at moderate depths retain two molecular layers of water on their surfaces. Removal of this water in the laboratory results in an increase of approximately 3.3 porosity percentage points for each 10% of montmorillonite present. Removal of this water will also result in an increase in the measured grain density. Figure 9: Montmorillonite (adsbygoogle = window.adsbygoogle || []).push({}); The basic Dean-Stark and Soxhlet cleaning techniques may not be suitable for clay-bearing rock, since samples have been known to crack during the cleaning process when these techniques have been used. The Soxhlet technique can be used in some circumstances by cooling the toluene prior to the time that it contacts the samples. Low temperature solvent flushing by centrifuging is recommended, as it is relatively fast and has proven to be a reliable technique when clays are troublesome. Gypsum poses similar problems and also requires special analytical techniques that expose the samples to temperatures no greater than 147°F (64ºC). Various studies have found that the presence of 10% gypsum in a rock sample will increase the porosity by 4.7 percentage points if all the water of hydration is removed from this material. (adsbygoogle = window.adsbygoogle || []).push({}); Drying Core Plugs and Sidewall Core Samples (adsbygoogle = window.adsbygoogle || []).push({}); Drying poses no significant problems in most stable rocks, and temperatures of 240º F (115º C) can be maintained with no real damage to the core samples. Formations containing hydratable clays, as shown in Figure 1, can be dried in a humidity drying oven set at 45% relative humidity and 145° F (64° C). These conditions leave two molecular layers of water on the clay surfaces, an amount that clay chemists believe to be appropriate. Observation of samples taken from depths of 1000 ft (305 m) or less shows that some cores crack while in the humidity oven even under these conditions. This would indicate that more than two molecular layers are actually present on the clay surfaces at these lesser depths. In the humidity drying technique, water bound to the sand grains reduces both the porosity and the grain density. Clay bearing samples subsequently dried in a regular oven will show both an increase in porosity and an increase in grain density (an example is given in Table 1). When interpreting core analysis data, it is always important to know which core analysis drying technique was used. Figure 1: Hydratable clays PropertyProperty Oven dried @ 240° FHumidity Dried @ 145° F and 45%relative humidityBulk volume, cm310.010.0Pore volume, cm32.62.27Porosity, %26.022.7Grain volume, cm37.47.73Dry weight, g19.6119.94Grain density, g/cm32.652.58Table 1: Example of Porosity and Grain Density Variations Between Humidity Dried and Oven Dried Clay Bearing Rock Samples High permeability samples can sometimes be dried within several hours, and this time framework can be reduced by the application of a vacuum to the samples as the heat is applied. As permeability decreases, the required drying time increases. In some cores, with permeabilities of less than one millidarcy, 48 hours have been required in non-vacuum ovens. As the sample size increases, the drying time increases. Unconsolidated rock samples enclosed within metal or plastic jackets will require longer drying times than those samples exposed on all surfaces. (adsbygoogle = window.adsbygoogle || []).push({}); Assignment Core Sample Preparation (adsbygoogle = window.adsbygoogle || []).push({}); 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 (adsbygoogle = window.adsbygoogle || []).push({}); 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 environmentC .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 .TemperatureC .PressureD .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 heterogeneityC .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 .WaterC .DieselD .Xylene Correct ✔ Recommended for You (adsbygoogle = window.adsbygoogle || []).push({}); Permeability Measurement Porosity Measurement Introduction to Core Analysis (adsbygoogle = window.adsbygoogle || []).push({});