Irreducible Fluid Saturation Research Articles

A coupled fractal model for predicting the relative permeability of rocks considering both irreducible fluid saturation and stress effects

The relative permeability of rocks is an essential parameter for evaluating two-phase flow characteristics and plays an important role in engineering fields such as resource exploitation. To this end, a mathematical model for predicting relative permeability was first developed based on an equivalent capillary model and fractal theory. The proposed model considers the irreducible fluid saturation under stress and quantifies the influence of the pore structure characteristics on the relative permeability. This model was then compared with relevant experimental data and existing theoretical expressions to verify its validity. Finally, the factors affecting the two-phase seepage characteristics were discussed. The results show that the irreducible fluid saturation is intimately connected to the fractal dimensions, pore size, fluid viscosity, pressure drop gradient, and elastic modulus. Fluid properties and pore structure characteristics are the main factors affecting relative permeability. The wetting phase relative permeability is more sensitive to pore structure and irreducible fluids. Increased effective stress increases irreducible fluid saturation, reduces two-phase flow capacity, and significantly decreases the relative permeability of wetting phase fluids. Increased elastic modulus and Poisson's ratio decrease the irreducible fluid content under stress and increase the permeability.

Investigation of pore structure evolution and damage characteristics of high temperature rocks subjected to liquid nitrogen cooling shock

Liquid nitrogen (LN2) can be utilized in the development of enhanced geothermal systems, as well as for deep/ultra-deep hydrocarbon reservoir stimulation, fire suppression, and other high-temperature geological projects. It is a crucial issue in the utilization of LN2 to investigate the pore structure evolution, permeability, and damage characteristics of high-temperature rocks under the influence of LN2 cooling shock. These rocks were first slowly heated to 150∼600°C and held for 2 h, followed by LN2 or natural cooling. The evolution of pore volume in high-temperature rocks affected by liquid nitrogen cooling was quantified. T2 cutoff values were determined through centrifugal tests, while the contents of irreducible and mobile fluids were estimated. Based on the aforementioned analysis as well as changes in irreducible fluid saturation, pore throat, tortuosity, and permeability, this study examines the closure and development of pores along with permeability behavior. The findings suggest that, despite a more pronounced decrease in porosity at lower heating temperatures, LN2 cooling specimens exhibit superior pore connectivity and permeability compared to those cooled naturally. LN2 stimulation not only induces crack initiation and propagation but also results in further cooling induced densification based on heating densification. 225°C is considered to be the optimal temperature for cooling contraction induced densification in this study. At higher heating temperatures, the damage to rock cooled with LN2 is more severe than that of naturally cooled. This results in a greater increase in porosity, movable fluid content and proportion, and permeability of LN2 cooled specimens compared to naturally cooled specimens. The damage mechanism can be better understood by the constructed damage model that coordinates the pore increase/decrease and mutual pore transformation from the perspective of pore evolution.

Fractal Properties of Various Clay Minerals Obtained from SEM Images

Clay minerals significantly alter the pore size distribution (PSD) of the gas hydrate-bearing sediments and sandstone reservoir rock by adding an intense amount of micropores to the existing intragranular pore space. Therefore, in the present study, the internal pore space of various clay groups is investigated by manually segmenting Scanning Electron Microscopy (SEM) images. We focused on kaolinite, smectite, chlorite, and dissolution holes and characterized their specific pore space using fractal geometry theory and parameters such as pore count, pore size distribution, area, perimeter, circularity, and density. Herein, the fractal properties of different clay groups and dissolution holes were extracted using the box counting technique and were introduced for each group. It was observed that the presence of clays complicates the original PSD of the reservoir by adding about 1.31-61.30 pores/100 μm2 with sizes in the range of 0.003-87.69 μm2. Meanwhile, dissolution holes complicate the pore space by adding 4.88-8.17 extra pores/100 μm2 with sizes in the range of 0.06-119.75 μm2. The fractal dimension ( D ) and lacunarity ( L ) values of the clays’ internal pore structure fell in the ranges of 1.51-1.85 and 0.18-0.99, respectively. Likewise, D and L of the dissolution holes were in the ranges of, respectively, 1.63-1.65 and 0.56-0.62. The obtained results of the present study lay the foundation for developing improved fractal models of the reservoir properties which would help to better understand the fluid flow, irreducible fluid saturation, and capillary pressure. These issues are of significant importance for reservoir quality and calculating the accurate amount of producible oil and gas.

Multifractal analysis of coal pore structure based on NMR experiment: A new method for predicting T2 cutoff value

Nuclear magnetic resonance (NMR) is a commonly used method to quantitatively characterize the pore structure of coal. T2 cutoff value is closely related to the microscopic pore structure of coal reservoirs, which directly affects the calculation of porosity, permeability, movable and irreducible fluid saturation. In this study, the pore structure of four types of coal samples were studied based on NMR experiments and the multifractal characteristics of saturated coal samples were analyzed with multifractal theory. Furthermore, the correlation between T2 cutoff values and multifractal characteristic parameters were discussed. The results show that the pore structure characteristics of CZ, GD and GHS coal samples are basically the same, all of which are mainly micropores, with relatively less content of mesopores, macropores and microcracks, while PDS coal samples have relatively more content of mesopores, macropores and microcracks. Different types of coal samples have similar multifractal characteristics and all coal samples are dominated by micropores. There is an obvious linear relationship between T2 cutoff values and the multifractal characteristic parameters. Five models of T2 cutoff values changing with the multifractal characteristic parameters are obtained by linear fitting. The error range between T2 cutoff values calculated by the five models and the experimental values are within 0.1 ms, among which T2 cutoff values calculated by model 5 are highly consistent with the experimental values, indicating that model 5 can better predict T2 cutoff values. The results can provide theoretical reference for the characterization of coal pore structure and the prediction of T2 cutoff values.

Effects of Irreducible Fluid Saturation and Gas Entry Pressure on Gas Production from Hydrate-Bearing Clayey Silt Sediments by Depressurization

Gas hydrates in the Shenhu area are mainly hosted in clayey silt sediments, which have the relatively high irreducible fluid saturation and gas entry pressure. And then, they will have an impact on gas production from hydrate-bearing clayey silt sediments, which was evaluated by the numerical simulations of SH2 site in Shenhu area in this paper. The results showed that, with the increase in irreducible water saturation and irreducible gas saturation, the amount of water production and gas production was obviously reduced. When the irreducible water saturation increased from 0.10 to 0.50, the cumulative CH4 production volume decreased from 1668799 m3 to 1536262 m3, and the cumulative water production volume dropped from 620304 m3 to 564797 m3, respectively. When the irreducible gas saturation increased from 0.01 to 0.05, the cumulative CH4 production volume dropped from 1812522 m3 to 1622121 m3, and the cumulative water production volume dropped from 672088 m3 to 600617 m3, respectively. In addition, the capillary pressure increased obviously with the increase in gas entry pressure, but the effect on gas production was small and the effect on water production could be negligible. In conclusion, irreducible water and gas saturation had an important effect on the gas production from gas hydrate, whereas the effects of gas entry pressure could be ignored.

Experimental investigation on the effect of ethanol on micropore structure and fluid distribution of coalbed methane reservoir

The development of coalbed methane not only ensures the supply of natural gas but also reduces the risks of coal mine accidents. The micropore structure of coalbed methane reservoir affects the seepage of coalbed methane; improvement of pore structure is one of the effective methods to enhance the efficiency of coalbed methane exploitation. In this study, low-pressure nitrogen gas adsorption, specific surface area analysis, nuclear magnetic resonance spectroscopy, and centrifugation experiment were used to evaluate the effect of ethanol on coal microscopic pore structure and fluid distribution during hydraulic fracturing. Seven coal samples were collected from the No. 3 coal seam in Zhaozhuang Mine, Qinshui Basin. The samples are mainly composed of micropores, transition pores, and mesopores. The experimental results show that ethanol can significantly change the pore structure by increasing the pore diameter. The average specific surface area, pore volume, and pore diameter of rock samples before ethanol immersion are 1.1270 m2/g, 0.0104 cm3/g, and 14.20 nm, respectively. The three parameters of rock samples after ethanol immersion are 0.5865 m2/g, 0.0025 cm3/g, and 29.37 nm. Ethanol improves the connectivity between micropores and mesopores. The average irreducible fluid saturation of samples saturated with formation water after centrifugation is 86%, and the average irreducible fluid saturation of samples soaked in three concentrations of ethanol solution decreases. It is considered that an ethanol solution of 0.4% concentration has the best effect on improving the pore structure and fluid distribution.

A Study of the Durability of Aeolian Sand Powder Concrete Under the Coupling Effects of Freeze–Thaw and Dry–Wet Conditions

The deterioration processes of aeolian sand powder concrete under freeze–thaw and dry–wet conditions have been studied. The pore characteristics, hydration products, and micro-morphology were analyzed by nuclear magnetic resonance, x-ray diffraction, and field emission scanning electron microscopy. The results showed that the relative dynamic elastic modulus and corrosion resistance coefficient for the compressive strength of the aeolian sand powder concrete were 3.0% and 12.9% higher than ordinary concrete, respectively. Furthermore, the relative dynamic elastic modulus under the freeze–thaw plus dry–wet conditions was 2.2 times lower than that under the dry–wet plus freeze–thaw actions. The ratio of harmful pores and permeability were 7.8% and 3.7 times higher, respectively, and the irreducible fluid saturation was 13.64% lower than that of the latter. The deterioration under the freeze–thaw plus dry–wet conditions was significantly higher than that under the dry–wet plus freeze–thaw conditions, and higher than that under the single factor conditions.

Study on the Activity of Aeolian Sand Powder and Alkali Excitation Modification

Aeolian sand powder samples were made from aeolian sand which was obtained from the Kubuqi Desert of Inner Mongolia, China. With the method of boiling, the effects of different mass fraction sodium sulfate and sodium hydroxide on the dissolution of active substances of aeolian sand powder were studied. Meanwhile, based on the “alkali activation” theory, the type of activator, the quality fraction and the pre-curing temperature were shown to be variable, and the effect of aeolian sand powder modification is discussed through the test of the strength of aeolian sand powder–cement mortar. Micro-methods such as the total spectral semi-quantitative analysis, x-ray diffraction, field emission scanning electron microscopy, and nuclear magnetic resonance technology were used to study the mineral composition, microstructure and pore characteristics, and then to discuss the feasibility of aeolian sand powder as an alkali-activated material. The results showed that sodium sulfate had a better activation effect on the aeolian sand powder compared with that of sodium hydroxide. The activation rate of aeolian sand powder increases with the increase of the mass fraction of the activator, and, with the increase of the alkalinity of the solution, the dissolution of SiO2 and other active substances in the aeolian sand powder increases gradually. The effect of sodium sulfate on the aeolian sand powder is better than that of sodium hydroxide, and when the mass fraction of sodium sulfate is 2%, the volume of the aeolian sand powder is 15%. When the pre-curing temperature is 35°C, the modification effect of aeolian sand powder is better and the activity index reaches 108.2%. Under the effects of the sodium sulfate, 2.2% and 2.6% active SiO2 and CaO were, respectively, dissolved from the aeolian sand powder. Then, a polymerization reaction occurred under the combined action of the temperature, which generated the needle-shaped hydration product ettringite with a good development state. Meanwhile, the ratio of the inner 20-nm hole of the aeolian sand powder–cement mortar specimen reached 85.69%, and there were many disconnected capillary pores, the irreducible fluid saturation was as high as 94.311%, and the gelling property was good.

Correlation between porosity and permeability of carbonate rock reservoirs

ABSTRACTIt is critical to use proper definition of effective porosity, i.e., effective porosity as used in the USA minus the irreducible fluid saturation. Inasmuch as logs do not determine the irreducible fluid saturation, a lot of misinformation is spread around.

Assessment of slope stability in cohesive soils due to a rainfall

SUMMARYThe primary focus in this work is on proposing a methodology for the assessment of stability of natural/engineered slopes in clayey soils subjected to water infiltration. In natural deposits of fine‐grained soils, the presence of water in the vicinity of minerals results in an interparticle bonding. This effect cannot be easily quantified as it involves complex chemical interactions at the micromechanical level. Here, the evolution of strength properties, including the apparent cohesion resulting from initial suction at the irreducible fluid saturation, is described by employing the framework of chemoplasticity. The paper provides first the formulation of the problem; this involves specification of the constitutive relation, development of an implicit return mapping scheme, and the outline of a coupled transient formulation. The framework is then applied to examine the stability of a slope subjected to a prolonged period of intensive rainfall. It is shown that the water infiltration may trigger the loss of stability resulting from the degradation of material properties. Copyright © 2013 John Wiley & Sons, Ltd.

Utilizing NMR Mud Logging Technology To Measure Reservoir Fundamental Parameters in Well Site

Nuclear Magnetic Resonance mud logging technology (NMR mud logging) is a new mud logging technology. Mainly applies the CPMG（Carr-Purcell-Meiboom-Gill）pulse sequence to measure transverse relaxation time (T2) of the fluid. NMR mud logging can measure drill cutting, core and sidewall core in the well site, also according to the experiment results, the sample type and size has little effect to analysis result. Through NMR logging, we can obtain several petrophysical parameters such as total porosity, effective porosity, permeability, oil saturation, water saturation, movable fluid saturation, movable oil saturation, movable water saturation, irreducible fluid saturation, irreducible oil saturation, irreducible water saturation, pore size and distribution in rock samples, etc. NMR mud logging has been used nearly 10 years in China, Sudan, Kazakhstan, etc. it plays an important role in the interpretation and evaluation of reservoir and its fluids.

Irreducible fluid saturation determined by pulsed field gradient NMR

A new laboratory procedure, using a pulsed field gradient (PFG) NMR technique, for measuring the relative volumes of movable and nonmovable fluid in cores is reported. The saturations determined are in qualitative agreement with values obtained from mercury porosimetry and centrifuged gravimetric saturation measurements. An advantage of the PFG NMR method is that it is fast and does not require exchange of fluids, i.e., progressive desaturation. This may be important if wettability changes are a concern or for measurements on unconsolidated cores. The bound volume index, BVI, of some core plugs was additionally determined by CPMG NMR which was recently established as a down-hole measurement with the new generation of NMR well logging tools. Since PFG NMR is not sensitive to surface relaxivity, this technique can be used as a quick and independent check on the BVI values obtained by CPMG NMR.

Nonmobile water quantified in fully saturated porous materials by magnetic resonance relaxation and electrical resistivity measurements

Irreducible fluid saturation in porous media is the fraction of the wetting phase which remains trapped in the solid matrix after displacement with a nonwetting phase. Methods have been proposed to estimate irreducible saturation in fully water-saturated porous samples by means of proton nuclear magnetic resonance relaxation times, but the results are quite varied. Some features are discussed of the trapped fluid and it is shown that the fraction of nonmobile water can be well estimated for a varied suite of clean sandstones, using only measurements on the fully saturated samples. This is done by combining relaxation measurements with those of electrical resistivity factor F. One of several simple correlations is Swi ∝√F/T1s, where T1s is the stretched-exponential relaxation time.

Empirical expression of permeability in terms of porosity, specific surface area, and residual water saturation of carbonate rocks

Using the multi-variable linear regression analysis, the authors developed empirical expressions for permeability in terms of porosity, specific surface area, and irreducible fluid saturation for four carbonate reservoir rock areas in the USSR. The coefficient of correlation varied from 0.981 to 0.997.

Scanning electron microscope study of pore systems in rocks

Pore systems in rocks are generally affected by grain size, shape, sorting, packing, nature of cementing materials, by detrital and authigenic pore fillers, and by the previously imposed pressure and temperature history. These variables, through their influence on the pore structure, control important petrophysical properties such as porosity, permeability, irreducible fluid saturation, formation resistivity factor, specific surface area, and velocities and attenuations of elastic waves. By presentation of photomicrographs, the scanning electron microscope (SEM), with its large depth of focus and magnification, is shown to reveal details of pore spaces of rocks in three dimensions. The pore systems of 42 rock samples, obtained from surface outcrops and during the drilling of oil wells, were examined. In most cases, the details of the pore structure would not have been possible to study under a light transmission microscope. SEM micrograph sequences of typical pore structures from the more extensive study were selected and are presented for one sandstone, one limestone, one dolomite, five shale, and two granite samples. It is suggested that maps of pore structures in rocks could be quantitatively obtained from overlapping stereo pairs of micrographs through use of photogrammetric techniques.