Materials Characterization Solutions to Determine Unconventional Gas Reservoir Potential
Until recently, tight shales were not considered an economically feasible option for hydrocarbon production. Logically, their minimal permeability and porosity made for small amounts of oil, water, and gas content that were hard to extract. However, abundance of tight shale in the earth’s crust, along with a desire to lessen import energy dependence have made the development of this non-conventional gas play very attractive.
Shales are complex reservoirs and present significant challenges to petrophysical characterization and physical core analysis. Key reservoir parameters for shale deposits include: thermal maturity, reservoir thickness, total organic carbon (TOC) content, adsorbed gas fraction, free gas fraction within the pores and fractures, and transport properties. Thermal maturity and reservoir thickness are routinely measured. The remaining four parameters require a creative approach that can utilize a number of petrophysical rock core measurement techniques.
Micromeritics provides key analytical tools for evaluating rock cores and shale reservoir potential.
- Reservoir performance evaluation
- Gas storage capacity of shale and kerogen
- Correlation between geological conditions and physical properties
- Organic and non-organic matrix porosities and pore distributions
Micromeritics also provides computational and modeling tools for modeling and predicting shale reservoir potential.
Rock Storage Properties
It is of vital importance to determine a reservoir’s capacity for adsorption of hydrocarbon gas. Often, pores smaller than 2 µm remain undetected in routine core analysis. The majority of the pore space in shales is mesoporous. Pore volume provides a measurement of the capacity of organic and non-organic components of the shale to store gas. Through the application of sub-critical nitrogen or carbon dioxide gas adsorption, the acquired analytical data will indicate capacity as well as reveal information about pore volume, area, and pore distribution.
Gas adsorption analysis by sub-critical gas adsorption is used to characterize core samples to determine:
- Free gas stored within pores
- Adsorbed gas on the surface and within the organic matter
- Dissolved gas in pore fluids
- Mesoporosity, microporosity, and total pore volume character
- Desorption kinetics for estimating the rate of gas production
The HPVA II Series of gas adsorption instruments uses the static volume method to obtain high-pressure adsorption and desorption isotherms utilizing gases such as nitrogen, hydrogen, methane, argon, oxygen and carbon dioxide.
Methane Capacity of Shale at Specific Pressure and Temperature
Many shale gas formations are over pressured. Super critical gas-adsorption parameters are needed to estimate total amount of gas in the system. Measuring the shale capacity for adsorption of hydrogen gas at pressures and temperatures that may exist at depths is important to the evalutaion of a reservoir. High-pressure methane can be dosed onto shale sample to generate adsorption and desorption isotherms. This provides the methane capacity of the shale at specific pressures and temperatures. The adsorption isotherm can be used to calculate the Langmuir surface area and volume of the shale. The Langmuir volume is the uptake of methane at infinite pressure - the maximum possible volume of methane that can be adsorbed to the surface of the sample.
- Determine Langmuir surface arfea at simulated shale depth conditions
- Provide kinetic data to show the rate of adsorption and desorption
The AutoPore IV Series uses mercury intrusion and extrusion to determine total pore volume, pore size distribution, percent porosity, density, compaction/compression, and fluid transport properties.
Shale Pore Throat Size, Pore Volume, and Pore Size Distribution
Mercury intrusion porosimetry is a valuable method for characterizing relative pore space dimensions. This dynamic technique is based on the intrusion of mercury into a porous structure under stringently controlled pressures. Mercury intrusion porosimetry permits the calculation of numerous sample properties such as pore size distributions, total pore volume, total pore surface area, median pore throat diameter, and sample densities (bulk and skeletal).
Capillary behavior and permeability are critical to reservoir behavior. Porosimetry is among the few techniques capable of probing the fine connected pore space of shales. This method is particularly useful in evaluation reservoir quality variations within a shale play and in petrotyping shale (Kale et al., 2010, Society of Petroleum Engineer).Various empirical transforms can be applied to estimate permeability from mercury intrusion data. While not quantitative, these estimates can provide a relative assessment of the variation in permeability. Mercury intrusion, used in conjunction with pycnometer data, provides estimates of the fraction of pore space that is connected and as such producible. Mercury intrusion also provides information in terms of tortuosity, or the loss of interconnectivity of micro channels under depth conditions, that will lower permeability, as well as gas and water flowability. Combined with gas adsorption data, our instruments can provide the full range of pore size distributions in natural shale samples.
The AccuPyc High Pressure pycnometer is a fully automatic pycnometer that provides high-speed, high-precision volume measurements and density determinations on intact or crushed shale core samples
CorePyc - designed for the specific needs of operations that require pore volume knowledge of intact sample cores.
Grain Density Metals Measurement
Grain density measurement of core samples is an important parameter that is used to determine the gas storage potential of rock reservoirs. Gas pycnometry is performed on intact cores or crushed samples to measure grain volume which, with the initial mass, yields grain density. Measured bulk and grain density are combined to deduce porosity. With properly crushed and cleaned sample, an absolute porosity can be determined. It is important to keep in mind that effective cleaning, drying, and crushed particle diameter will have a strong influence on the density and porosity measurements. With circular core cylinders, simple physical measurements of its diameter and length allow the envelope volume to be calculated. A gas displacement pycnometer, with a larger sample chamber designed specifically to accept intact drill cores, provides a low-cost, time-saving, non-destructive technique for measuring the skeletal volume. Knowing the envelope and skeletal volumes of a score sample allows the total pore volume percent porosity of the sample to be determined.
- Determine shale reservoir effective porosity
- Provide measurement of grain densities
- Calculate the saturation of free, or combination of free and bound fluids and water
- Estimate of water and volatile hydrocarbons in pore space
Transition Metals Measurement
The MA-1040 Magnetic Analyzer detects and measures low levels of metalic iron, nickel, or cobalt content in sample materials.
The use of NMR for geophysical characterization of core samples is well accepted. NMR provides information for diffusive characterization of shale samples in terms of fluid mobility, effective porosity, and to determine kerogen conversion.
Core samples contain significant amounts of metallic minerals that can contaminate NMR measurements and skew results. It is prudent to measure the metallic content of the core samples to make the analyst aware of any possible contamination. The experimental approach can then be adjusted as required.
- Effectively measure metallic iron group metals contamination as possible interference to NMR results
- NMR is a non-destructive, simple operation for measurement of core samples