Pore Structure Of Shale Samples Research Articles

Oil saturation quantitative evaluation in lacustrine shale: Novel insights from NMR T1-T2 and displacement experiments

Oil saturation is important for shale reservior to identify favorable sections and determine the geological sweet spots. Current oil saturation evaluation methods, including experiments and empirical formulas, are not suitable for shale reservoir because of the complex mineral, fluid components and pore structure characteristics. A novel method was proposed to accurately calculate the shale oil saturation through the morphological dynamic variation characteristics of the 1D NMR T2 spectra and the signal intensity of the different hydrocarbon-bearing components of the 2D NMR T1-T2 map of shale samples with different oil and water saturation The results (1) indicated that the NMR T2 spectrum revealed that the pore structure of shale samples is complex and highly heterogeneous, and the primary factor that determines the volume of displaced oil in the displacement experiment with slight differences in wettability is the shale’s pore size distribution. (2) shown that the NMR T1-T2 map serves as a useful tool in distinguishing different hydrocarbon-bearing components of shale samples, including kerogen, adsorbed oil, free oil, structural, adsorbed water, and free water. Typically, kerogen is found in the top left region of the T1-T2 map with 10 < T1 < 100 ms & 0.02 < T2 < 0.2 ms. Based on this, T2 threshold for free oil and adsorbed oil are 2 and 0.2 ms, and the corresponding threshold of pore radius are 40 and 4 nm according to the NMR theory. (3) found that the morphological characteristics of 1D NMR T2 spectra vary with the variation of shale oil saturation. Specifically, T2 geometric mean (T2gm) and interval porosity corresponding to the first peak (Ap1) are positively and negatively correlated with oil saturation respectively. Then, oil saturation calculation method is established using the above two parameters and the Root Mean Square Error (RMSE) between the measured and calculated results is 5.85 %, which reflecting the accuracy and validity. In general, this method allows the shale oil saturation to be accurately calculated and provides basis for the determination of favorable sections and evaluation of resource of shale oil reservoir. Moreover, it also offers a new idea for the oil saturation prediction by NMR logging.

The Multitype Pore Structure Evolution Characteristics of Ziliujing Formation Shale from the Northeastern Sichuan Basin in China: Experimental Study Using Small Angle Neutron Scattering Technology

The high complexity and heterogeneity of shale pore compositions and abundant nanopores complicate the multiscale characterization of a shale reservoir pore structure. As the degree of geological evolution changes, the pore structure and connectivity also undergo complex changes and the thermal simulation experiment is a practical method to reveal the evolution characteristics of shale within a relatively short time period using the time–temperature compensation principle. In this study, thermal simulation experiments were used to obtain shale samples of the Ziliujing Formation in Northeastern Sichuan Basin with different evolution degrees. The pore structure of the samples was quantitatively characterized by SANS combined with N2 adsorption and mercury injection capillary pressure (MICP) methods. The pore morphology was qualitatively characterized by the field emission scanning electron microscopy (FE-SEM) experiment. Finally, the influence of thermal evolution on the pore structure of shale samples was investigated. The results show that the pore size distribution of SANS is similar to that of MICP and N2 adsorption methods, and the pore volume is mainly concentrated in 5–100 nm. The porosity and pore specific surface area obtained by the three characterization methods showed the trend of SANS > MICP > N2 adsorption, and the difference in pore structure was mainly affected by fluid accessibility. With the increase in thermally simulated temperature, the total pore volume and pore specific surface area of shale samples show a trend of an increase first and then a decrease. The proportion of >10 nm pores increases gradually, and the fraction of closed pores shows a downward trend as a whole. Basically, with the increase in thermally simulated temperature, the pore connectivity is enhanced, and the sample heterogeneity is weakened, all of which are conducive to the migration of shale reservoir fluid. Overall, clarifying the evolution of closed pores in shale reservoirs is of great significance for estimating original oil and gas reserves more accurately and improving oil and gas recovery.

The role of fluid-rock interactions in permeability behavior of shale with different pore fluids

The permeability of shale is a controlling factor in fluid flow, solute transport , and overpressure development in a sedimentary basin . However, shale permeabilities measured with different fluids can be very different. To investigate and characterize the effects of fluid-rock interactions on shale permeability, a series of flow experiments on three samples collected from the Carboniferous formation in the eastern Qaidam Basin, China, with flow directions perpendicular to the bedding plane , were conducted using deionized water , liquid CO 2 and 1 mol/L NaCl solution as permeating fluids, respectively. A water permeability model under the influence of fluid-rock interactions was newly derived. The experimental results indicate that for the same sample, the liquid CO 2 permeability is the highest permeability measured, followed by that of deionized water, and that the 1 mol/L NaCl solution permeability is the lowest permeability measured. The liquid CO 2 flow obeys Darcy's law , showing a constant permeability, with values of 6.90 × 10 −19 m 2 , 3.80 × 10 −20 m 2 and 1.59 × 10 −18 m 2 . The movement of the deionized water and NaCl solution in these samples deviates from Darcy's law. The relationship between water permeability and pressure gradient follows a power function, with exponents ranging from 0.96 to 3.41 for deionized water and 0.34–3.30 for NaCl solution. The permeabilities measured with these two fluids exhibit nearly identical ranges (10 −20 –10 −21 m 2 ), but the sample permeated with NaCl solution generally shows a lower permeability under the same pressure gradient. The permeability reduction magnitude between the three liquid permeabilities and the helium absolute permeability was used to represent the fluid-rock interactions strength (FRIS). The FRIS is 0.25–0.96 for liquid CO 2 , 1.44–2.32 for deionized water and 1.89–3.09 for NaCl solution. The relations between the FRIS and mineral compositions and pore structure of shale samples indicate that the FRIS has a better correlation with the specific surface area and porosity.

Proper Experimental Parameters in N2 Adsorption: The Effects of Data Points and Equilibrium Interval Time

Evaluating pore structures of shale samples can be done with high accuracy with adsorption experiments. However, in order to obtain reliable results, proper guidelines of experimental parameters sh...

The Effect of Particle Size on the Interpretation of Pore Structure of Shale by N2 Adsorption

Gas adsorption experiments are becoming one of the most common methods to quantify and analyze the pore structures of shale samples in the petroleum industry. In this regard, particle size of the specimen plays an important role in the results that could ultimately affect the pore structure interpretation. Hence, in this study, five shale samples at different thermal maturity levels are picked, and all are crushed into different groups of particle sizes: less than 40 mesh (&lt;375 μm), less than 60 mesh (&lt;250 μm), less than 80 mesh (&lt;187.5 μm), and less than 100 mesh (&lt;150 μm). Next, N2 adsorption is used to characterize the pore structures of the samples within different particle sizes. Furthermore, to interpret the data, several attributes such as the pore volume, surface area, fractal dimension (from the fractal analysis), and heterogeneity index (from the multifractal analysis), are studied and compared between the samples and particle size intervals to provide us with the effect that particle size could have on the pore structure analysis. The results showed that as the particle size varies, the pore structures of the shale samples could get affected. Based on the comparison of the results, it is recommended that a suitable particle size for the shale pore structure characterization in N2 adsorption experiments should be less than 60 mesh (&lt;250 μm).

Multifractal characteristics of Longmaxi Shale pore structures by N2 adsorption: A model comparison

N2 adsorption is one of the most widely used techniques to assess pore structures of shale samples due to its ability for characterizing pores in nanoscale. Various models have been developed to quantify pore structures based on adsorption isotherms. In this regard, using a suitable model can give us more accurate pore structure information. The Barret, Joyner and Halenda (BJH) model along with density functional theory (DFT), two most frequently used ones for pore structures of shales, employed on Longmaxi shale samples and compared. BJH model can be divided into two sub-models: adsorption (BJHAD) and desorption (BJHDE). First, the multifractal analysis was used to quantify the heterogeneity of pore size distributions derived from these models. Second, partial least regression analysis (PLS) was employed to quantify the correlations between pore structures and rock compositions. The results showed that pore structures (volume and surface area) and pore heterogeneity derived from BJHAD, BJHDE and DFT model would differ. In addition, PLS results indicated that minerals (except dolomite and clay) and organic matter would correlate positively while clay minerals negatively with pore surface area and volume independent of the method that was used. Finally, the comparison of results from these three methods demonstrated that DFT model is superior to BJHAD and BJHDE for pore structure characterization in shale gas formations.

Pore Characteristics Analysis of Shale from Sichuan Basin, China

Pore characteristics are significant for shale gas exploration and production. In this paper, the method of field emission scanning electron Microscopes (FE-SEM) was applied to qualitatively describe minerals and pore structures of shale samples. Low pressure nitrogen adsorption-desorption and carbon dioxide adsorption were applied to analyse meso-pores and micro-pores respectively. Inter-particle pores are always associated with rigid mineral grains and intra-particle pores are mainly located in unstable minerals. The BET (Brunauer–Emmett–Teller) surface area of Longmaxi Formation (LMX) is 5.47m 2 /gr and 16.33m 2 /gr of Wufeng Formation (WF). N 2 and CO 2 adsorption shows that the diameter of micro-pores in the LMX and WF formation is approximately 1nm. Most meso-pores in WF formations range from 2nm - 20nm, while meso-pores existing in LMX formations range from 2nm - 30nm.

FRACTAL PORE STRUCTURE MODEL AND MULTILAYER FRACTAL ADSORPTION IN SHALE

The complex structure and surface property of porous media have significant impact on its accumulation and adsorption capacity. Based on the fractal theory, this paper presents a fractal pore structure model for shales. The effect of different pore structures on fractal dimension is discussed, and the influence of fractal dimension and pore size distribution on porosity is also analyzed. It is shown that the fractal dimension D decreases with the increase of structure parameter q/m for a certain pore diameter ratio, and porosity has positive relationship with fractal dimension. This paper also presents a multilayer fractal adsorption model which takes into account the roughness of adsorption surface by using fractal theory. With the introduction of pseudo-saturated vapor pressure in the supercritical temperature condition, the proposed adsorption model can be applied into a wider range of temperature. Based on the low-pressure nitrogen adsorption and methane isothermal adsorption experiments, the effect of fractal dimension on the adsorption behavior of shales is discussed. Fractal dimension has significant impact on the surface adsorption property and adsorption layer number n. The monolayer saturated adsorption volume Vm increases with the increase of D, while parameter C has the opposite variation trend. Finally, the optimal combination of fractal parameters for describing pore structure of shale samples is selected.