An Experimental Study on Stress Sensitivity of Tight Sandstones with Different Microfractures

Aiming at the stress sensitivity problem of tight reservoirs with different microfractures, the cores of H oilfield and J oilfield with different microfractures were obtained through the fractures experiment, so as to study the change of gas permeability in tight sandstone core plug during the change of confining pressure. Besides, we use the nuclear magnetic resonance (NMR) spectra of the core before and after saturation to verify whether the core has been successfully fractured. Based on Terzaghi’s effective stress principle, the permeability damage rate (D) and the stress sensitivity coefficient (Ss) are used to evaluate the stress sensitivity of the core, which show consistency in evaluating the stress sensitivity. At the same time, we have studied the petrological characteristics of tight sandstone in detail using thin section (TS) and scanning electron microscope (SEM). The results show that the existence of microfractures is the main factor for the high stress sensitivity of tight sandstone. In addition, because of the small throat of the tight reservoir core, the throat closes when the overlying stress increases. As a result, the tight sandstone pore size is greatly reduced and the permeability is gradually reduced. Therefore, in the development of tight reservoirs, we should not only consider the complex fracture network produced by fracturing, but also pay attention to the permanent damage of reservoirs caused by stress sensitivity.

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Study on the impact of confining pressure on gas permeability of tight sandstone cores

Journal of Physics Conference Series ◽

10.1088/1742-6596/2076/1/012006 ◽

2021 ◽

Vol 2076 (1) ◽

pp. 012006

Author(s):

Jianxiang Tong ◽

Hengyang Wang ◽

Yuyi Wang ◽

Ya Zhang ◽

Xiaohe Huang

Keyword(s):

Power Function ◽

Rate Coefficient ◽

Gas Permeability ◽

Confining Pressure ◽

Experimental Sample ◽

Tight Sandstone ◽

Change Rate ◽

Shengli Oilfield ◽

Confining Pressures ◽

The Impact

Abstract Taking the tight sandstone core of Shengli Oilfield as the experimental sample, this paper studies the permeability variation of the tight sandstone under different confining pressures. The experimental results show that when the pore pressure is constant, the measured gas permeability of core decreases with the increase of confining pressure. Power function is more reasonable to describe the influence of confining pressure on permeability of tight sandstone between power function and exponential function. Analyze the impact of confining pressure on gas permeability of tight sandstone cores by using permeability change rate coefficient D and coefficient S.

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Seepage Behaviour of Fractured Rock Mass Infilling Using Different Transfixion Rates under Cyclic Loading

Geofluids ◽

10.1155/2021/9995090 ◽

2021 ◽

Vol 2021 ◽

pp. 1-15

Author(s):

Shuren Wang ◽

Jiyun Zhang ◽

Zhichao Li ◽

Yongqiang Yu

Keyword(s):

Rock Mass ◽

Cyclic Loading ◽

Fractured Rock ◽

Confining Pressure ◽

Sensitivity Coefficient ◽

Stress Sensitivity ◽

Fractured Rock Mass ◽

Penetration Length ◽

Permeability Changes ◽

Cycle Loading

It is very important to determine the seepage behaviour of fractured rock mass infilling to evaluate the stability of the surrounding rock. The joint transfixion rate is the ratio of the unpenetrated length to the penetration length of a joint in a sample. Samples of the fractured rock mass infilling using different transfixion rates were prepared, and a TCQT-III low-permeability coal-rock triaxial seepage device was used to conduct three cycles of confining pressure-seepage coupling tests. Results show that the permeability is a power function in the confining pressure of the sample, and the permeability changes most significantly with the confining pressures. The permeability of the sample increases exponentially with the joint transfixion rate. The permeability loss is positively correlated with the plastic deformation of the sample; the permeability changes most significantly during the first cycle loading. There is over 60% recovery of the permeability of the sample under cyclic loading for loads that do not exceed the strength of the infilling. The stress sensitivity coefficient decreases as the confining pressure increases and is higher during the unloading stage than that during the loading stage for samples with an incomplete transfixion rate. The conclusions obtained in this study can serve as a reference for grouting applications.

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Experimental Investigation on the Stress-Dependent Permeability of Intact and Fractured Shale

Geofluids ◽

10.1155/2020/8897911 ◽

2020 ◽

Vol 2020 ◽

pp. 1-16

Author(s):

Yongxiang Zheng ◽

Jianjun Liu ◽

Yichen Liu ◽

Di Shi ◽

Bohu Zhang

Keyword(s):

Pore Pressure ◽

Confining Pressure ◽

Fracture Network ◽

Permeability Evolution ◽

Matrix Permeability ◽

Reservoir Permeability ◽

Slippage Effect ◽

Before And After ◽

Shale Reservoir ◽

Stress Dependent

The permeability of shale is extremely low. Therefore, the shale reservoir needs fracturing. The fracture network by fracturing can increase the permeability in a stimulated shale reservoir. To understand the permeability evolution in the stimulated shale reservoir, this study measured the permeability of intact and fractured shale samples with different pore pressure and confining pressure by the transient pulse test. And the differences between the two kinds of samples in permeability were analyzed. The results show that permeability magnitude of fractured shale is increased by 5 orders compared to the intact shale. It means that fracture networks after fracturing can effectively improve the permeability. Besides, the change in matrix permeability is the result of the combined effect of slippage effect and matrix deformation. At low pore pressure, the influence of slippage effect is more significant. Based on the results, an improved exponential function was established to describe the relationship between permeability and effective stress of shale matrix. Moreover, the permeability of fractured shale is still bigger than that of the shale matrix when the confining pressure is larger than pore pressure. This paper provides theoretical guidance for studying the evolution of reservoir permeability before and after fracturing.

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Pore Structure and Limit Pressure of Gas Slippage Effect in Tight Sandstone

The Scientific World JOURNAL ◽

10.1155/2013/572140 ◽

2013 ◽

Vol 2013 ◽

pp. 1-7 ◽

Cited By ~ 6

Author(s):

Lijun You ◽

Kunlin Xue ◽

Yili Kang ◽

Yi Liao ◽

Lie Kong

Keyword(s):

Porous Media ◽

Pore Structure ◽

Gas Flow ◽

Gas Permeability ◽

Flow In Porous Media ◽

Slip Effect ◽

Tight Sandstone ◽

The Core ◽

Core Samples ◽

Limit Pressure

Gas slip effect is an important mechanism that the gas flow is different from liquid flow in porous media. It is generally considered that the lower the permeability in porous media is, the more severe slip effect of gas flow will be. We design and then carry out experiments with the increase of backpressure at the outlet of the core samples based on the definition of gas slip effect and in view of different levels of permeability of tight sandstone reservoir. This study inspects a limit pressure of the gas slip effect in tight sandstones and analyzes the characteristic parameter of capillary pressure curves. The experimental results indicate that gas slip effect can be eliminated when the backpressure reaches a limit pressure. When the backpressure exceeds the limit pressure, the measured gas permeability is a relatively stable value whose range is less than 3% for a given core sample. It is also found that the limit pressure increases with the decreasing in permeability and has close relation with pore structure of the core samples. The results have an important influence on correlation study on gas flow in porous medium, and are beneficial to reduce the workload of laboratory experiment.

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Influence of CO2-brine-rock Interactions on the Physical Properties of Tight Sandstone Cores via Nuclear Magnetic Resonance

IOP Conference Series Earth and Environmental Science ◽

10.1088/1755-1315/898/1/012021 ◽

2021 ◽

Vol 898 (1) ◽

pp. 012021

Author(s):

Hanbin Liu ◽

Chengzheng Li ◽

Zhenfeng Zhao ◽

Guangtao Wang ◽

Changheng Li ◽

...

Keyword(s):

Nuclear Magnetic Resonance ◽

Physical Properties ◽

Oil Recovery ◽

Injection Pressure ◽

Low Permeability ◽

Pore Throat ◽

Tight Sandstone ◽

The Core ◽

Before And After ◽

Enhance Oil Recovery

Abstract For sandstone reservoirs with extra-low permeability, CO2 injection is regarded as a valid method to enhance oil recovery. When CO2 injection is implemented in such reservoirs, the physical properties of the formation could be altered owing to the interactions between CO2, water, and rock. In this study, the influence of CO2–brine–rock interactions on the physical properties of tight sandstone cores was analyzed by comparing the obtained T 2 spectrum before and after CO2 injection. The results revealed that the T2 spectrum after CO2 injection was significantly different from the original T2 spectrum. CO2 injection changed the pore size distribution of the core samples. When the injection pressure was low, the pore volume decreased from micropores to macropores leading to a decrease in both permeability and porosity. As the injection pressure increasing, the dissolution of CO2 in the micropores was enhanced thus improving the pore-throat connectivity; which ultimately improved the reservoir physical properties.

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Nilai-Nilai Pendidikan Akhlak dalam Al-Qur’an

Al-Riwayah Jurnal Kependidikan ◽

10.32489/al-riwayah.1 ◽

2018 ◽

Vol 10 (2) ◽

pp. 269-295

Author(s):

Sri Waluyo

Keyword(s):

Moral Education ◽

Primary Source ◽

Secondary Data ◽

Primary Data ◽

The Core ◽

Primary Data Source ◽

Before And After ◽

Data Source

This paper discusses the content of Q.S. al-Baqarah ([2]: 67-73). The data used in the preparation of this paper is the data that is primary and secondary. The primary source is data obtained from the core source. In conducting a study of a verse, it is clear that the primary data source is derived from the Qur'an,precisely on Q.S. al-Baqarah ([2]: 67-73). Secondary data is dataobtained from other sources that are still related to the problemand provide interpretation of the primary source. The method usedin analyzing this paper is the tahlili method. This method describesthe meaning contained by the Qur'an, verse by verse, and letterafter letter according to the order in the Mushaf. The descriptionincludes the various aspects which the interpreted verses contain,such as the meaning of the vocabulary, the connotation of thesentence, the background of the verse down, its relation to otherverses, both before and after. And do not miss the opinion that hasbeen given regarding the interpretation of these verses, whetherdelivered by the Prophet, companions, the tabi'in, as well as othercommentators. This study shows that in Q.S. (2): 67-73) there arevalues of moral education which include: 1) morals in asking, (2)morals to parents, (3) patience of educators, (4) educator honesty,and (5) obedience of learners.

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Deep fluids and their role in hydrocarbon migration and oil deposit formation exemplified by supercritical СO2

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s1755691021000013 ◽

2021 ◽

pp. 1-11

Author(s):

Sara LIFSHITS

Keyword(s):

Supercritical Fluid ◽

Oil And Gas ◽

Gas Permeability ◽

Sedimentary Rocks ◽

Source Rocks ◽

Reservoir Properties ◽

Hydrocarbon Migration ◽

Geological Processes ◽

Before And After ◽

Oil Source

ABSTRACT Hydrocarbon migration mechanism into a reservoir is one of the most controversial in oil and gas geology. The research aimed to study the effect of supercritical carbon dioxide (СО2) on the permeability of sedimentary rocks (carbonates, argillite, oil shale), which was assessed by the yield of chloroform extracts and gas permeability (carbonate, argillite) before and after the treatment of rocks with supercritical СО2. An increase in the permeability of dense potentially oil-source rocks has been noted, which is explained by the dissolution of carbonates to bicarbonates due to the high chemical activity of supercritical СО2 and water dissolved in it. Similarly, in geological processes, the introduction of deep supercritical fluid into sedimentary rocks can increase the permeability and, possibly, the porosity of rocks, which will facilitate the primary migration of hydrocarbons and improve the reservoir properties of the rocks. The considered mechanism of hydrocarbon migration in the flow of deep supercritical fluid makes it possible to revise the time and duration of the formation of gas–oil deposits decreasingly, as well as to explain features in the formation of various sources of hydrocarbons and observed inflow of oil into operating and exhausted wells.

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Gas permeability and fracture compressibility for proppant-supported shale fractures under high stress

Journal of Natural Gas Science and Engineering ◽

10.1016/j.jngse.2021.104157 ◽

2021 ◽

pp. 104157

Author(s):

Tianyu Chen ◽

Yanji Fu ◽

Xia-Ting Feng ◽

Yuling Tan ◽

Guanglei Cui ◽

...

Keyword(s):

Gas Permeability ◽

High Stress

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Experiment on Mechanical Properties and Seepage Capability of Deep Tight Sandstone Under True-Triaxial Stresses Environment

10.2118/208082-ms ◽

2021 ◽

Author(s):

Jiaying Li ◽

Chunyan Qi ◽

Ye Gu ◽

Yu Ye ◽

Jie Zhao

Keyword(s):

Mechanical Properties ◽

In Situ Stress ◽

Stress Sensitivity ◽

Stress Conditions ◽

Tight Sandstone ◽

Gas Reservoir ◽

True Triaxial ◽

Strain Behavior ◽

Triaxial Stresses

Abstract The characteristics of seepage capability and rock strain during reservoir depletion are important for reservoir recovery, which would significantly influence production strategy optimization. The Cretaceous deep natural gas reservoirs in Keshen Gasfield in Tarim Basin are mainly buried over 5000 m, featuring with ultra-low permeability, developed natural fractures and complex in-situ stress states. However, there is no comprehensive study on the variation of mechanical properties and seepage capability of this gas reservoir under in-situ stress conditions and most studies on stress-sensitivity are conducted under conventional triaxial or uniaxial stress conditions, which cannot truly represent in-situ stress environment. In this work, Cretaceous tight sandstone in Keshen Gasfield was tested under true-triaxial stresses conditions by an advanced geophysical imaging true-triaxial testing system to study the stress-sensitivity and anisotropy of rock stress-strain behavior, porosity and permeability. Four groups of sandstone samples are prepared as the size of 80mm×80mm×80mm, three of which are artificially fractured with different angle (0°,15°，30°) to simulate hydraulic fracturing. The test results corresponding to different samples are compared to further reveal the influence of the fracture angle on rock mechanical properties and seepage capability. The samples are in elastic strain during reservoir depletion, showing an apparent correlation with fracture angles. The porosity decreases linearly with stress loading, where the decrease rate of effective porosity of fracture samples is significantly higher than that of intact samples. The permeabilities decrease exponentially and show significant anisotropy in different principal stress directions, especially in σH direction. The mechanical properties and seepage capability of deep tight sandstone are successfully tested under true-triaxial stresses conditions in this work, which reveals the stress-sensitivity of anisotropic permeability, porosity and stress-strain behavior during gas production. The testing results proposed in this paper provides an innovative method to analyse rock mechanical and petrophysical properties and has profound significance on exploration and development of tight gas reservoir.

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Application of machine learning algorithms to predict permeability in tight sandstone formations

Nafta-Gaz ◽

10.18668/ng.2021.05.01 ◽

2021 ◽

Vol 77 (5) ◽

pp. 283-292

Author(s):

Tomasz Topór ◽

Keyword(s):

Machine Learning ◽

Oil And Gas ◽

Learning Algorithms ◽

Confining Pressure ◽

Machine Learning Algorithms ◽

Core Material ◽

Confining Stress ◽

Tight Sandstone ◽

Oil And Gas Exploration ◽

Better Than

The application of machine learning algorithms in petroleum geology has opened a new chapter in oil and gas exploration. Machine learning algorithms have been successfully used to predict crucial petrophysical properties when characterizing reservoirs. This study utilizes the concept of machine learning to predict permeability under confining stress conditions for samples from tight sandstone formations. The models were constructed using two machine learning algorithms of varying complexity (multiple linear regression [MLR] and random forests [RF]) and trained on a dataset that combined basic well information, basic petrophysical data, and rock type from a visual inspection of the core material. The RF algorithm underwent feature engineering to increase the number of predictors in the models. In order to check the training models’ robustness, 10-fold cross-validation was performed. The MLR and RF applications demonstrated that both algorithms can accurately predict permeability under constant confining pressure (R2 0.800 vs. 0.834). The RF accuracy was about 3% better than that of the MLR and about 6% better than the linear reference regression (LR) that utilized only porosity. Porosity was the most influential feature of the models’ performance. In the case of RF, the depth was also significant in the permeability predictions, which could be evidence of hidden interactions between the variables of porosity and depth. The local interpretation revealed the common features among outliers. Both the training and testing sets had moderate-low porosity (3–10%) and a lack of fractures. In the test set, calcite or quartz cementation also led to poor permeability predictions. The workflow that utilizes the tidymodels concept will be further applied in more complex examples to predict spatial petrophysical features from seismic attributes using various machine learning algorithms.

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