Clay Expansion Research Articles

Modelling the effects of water chemistry and flowrate on clay erosion

Clay buffer is a key component of the engineered barrier system (EBS) for geological disposal of higher activity radioactive waste. Experimental observations indicate the possibility of buffer erosion at the interface with host rock due to interactions with groundwater. Existing models for clay erosion are very limited in terms of addressing the hydro-chemical effects, while the assessment of the long-term performance of clay buffer requires robust predictive models covering expected environment conditions and multiphysics phenomena involved in the erosion process. The work presented here is a step towards such a predictive capability, which considers clay expansion, detachment of clay particles and transport of detached particles by groundwater within a single modelling framework. The effects of solution chemistry and flowrate on the penetration, extruded mass and particle release rate of clay buffer are investigated in this paper. A series of experimental data are used to validate the swelling and erosion model developed in this study. The results show that the extrusion distance, which is controlled by both clay swelling and detachment processes, is nearly linearly dependent on the water flowrate irrespective of the water chemistry. Similar linear dependence of the erosion rate on water flowrate is observed for flowrates less than 10−5 m/s. Higher water flowrates are shown to induce nonlinearly increasing erosion rates in accordance with experiments. A flowrate threshold is found above which the erosion behaviour of compacted bentonite can be significantly affected. A concept of tolerance time for clay buffer is introduced as a failure criterion. The results indicate that the coupled effect of water chemistry and velocity requires further investigation for ionic concentrations below 1 mM.

Monitoring of Expansive Clays over Drought-Rewetting Cycles Using Satellite Remote Sensing

New capabilities for measuring and monitoring are needed to prevent the shrink-swell risk caused by drought-rewetting cycles. A clayey soil in the Loire Valley at Chaingy (France) has been instrumented with two extensometers and several soil moisture sensors. Here we show by direct comparison between remote and in situ data that the vertical ground displacements due to clay expansion are well-captured by the Multi-Temporal Synthetic Aperture Radar Interferometry (MT-InSAR) technique. In addition to the one-year period, two sub-annual periods that reflect both average ground shrinking and swelling timeframes are unraveled by a wavelet-based analysis. Moreover, the relative phase difference between the vertical displacement and surface soil moisture show local variations that are interpreted in terms of depth and thickness of the clay layer, as visualized by an electrical resistivity tomography. With regard to future works, a similar treatment relying fully on remote sensing observations may be scaled up to map larger areas in order to better assess the shrink-swell risk.

Improved prediction of clay soil expansion using machine learning algorithms and meta-heuristic dichotomous ensemble classifiers

Soil swelling-related disaster is considered as one of the most devastating geo-hazards in modern history. Hence, proper determination of a soil’s ability to expand is very vital for achieving a secure and safe ground for infrastructures. Accordingly, this study has provided a novel and intelligent approach that enables an improved estimation of swelling by using kernelised machines (Bayesian linear regression (BLR) & bayes point machine (BPM) support vector machine (SVM) and deep-support vector machine (D-SVM)); (multiple linear regressor (REG), logistic regressor (LR) and artificial neural network (ANN)), tree-based algorithms such as decision forest (RDF) & boosted trees (BDT). Also, and for the first time, meta-heuristic classifiers incorporating the techniques of voting (VE) and stacking (SE) were utilised. Different independent scenarios of explanatory features’ combination that influence soil behaviour in swelling were investigated. Preliminary results indicated BLR as possessing the highest amount of deviation from the predictor variable (the actual swell-strain). REG and BLR performed slightly better than ANN while the meta-heuristic learners (VE and SE) produced the best overall performance (greatest R2 value of 0.94 and RMSE of 0.06% exhibited by VE). CEC, plasticity index and moisture content were the features considered to have the highest level of importance. Kernelized binary classifiers (SVM, D-SVM and BPM) gave better accuracy (average accuracy and recall rate of 0.93 and 0.60) compared to ANN, LR and RDF. Sensitivity-driven diagnostic test indicated that the meta-heuristic models’ best performance occurred when ML training was conducted using k-fold validation technique. Finally, it is recommended that the concepts developed herein be deployed during the preliminary phases of a geotechnical or geological site characterisation by using the best performing meta-heuristic models via their background coding resource.

Synergistic Coupling of CO2 and H2O during Expansion of Clays in Supercritical CO2-CH4 Fluid Mixtures.

We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na+-, NH4+-, and Cs+-montmorillonites (SWy-2) in supercritical fluid mixtures of H2O, CO2, and CH4. Building on our prior work with Na-clay that demonstrated that H2O facilitated CO2 intercalation at relatively low RH, here we show that increasing CO2/CH4 ratios promote H2O intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO2 intercalated and expanded the Cs-clay even in the absence of H2O, while increasing fluid CO2/CH4 ratios inhibited H2O intercalation. The NH4-clay displayed intermediate behavior. By comparing changes in the HOH bending vibration of H2O intercalated in the Cs-, NH4-, and Na-clays, we posit that CO2 facilitated expansion of the Na-clay by participating in outer-sphere solvation of Na+ and by disrupting the H-bond network of intercalated H2O. In no case did the pure CH4 fluid induce expansion. Our experimental data can benchmark modeling studies aimed at predicting clay expansion in humidified fluids with varying ratios of CO2 and CH4 in real reservoir systems with implications for enhanced hydrocarbon recovery and CO2 storage in subsurface environments.

Zirconium-modified natural clays for phosphate removal: Effect of clay minerals

Excessive amount of phosphate entering water bodies may cause eutrophication and have detrimental effects on ecosystems. Clay-based materials have been drawing attractive attention in mitigating phosphate release to aquatic environment. In this study, we prepared a series of zirconium (Zr)-modified clays to investigate the effect of clay structure and expansion property on phosphate adsorption. Kaolinite, montmorillonite, and vermiculite were selected as three representative natural clays for Zr modification, and the resulting Zr-modified clays were characterized using various techniques that included powder X-ray diffraction, scanning electron microscopy, and zeta potential measurement. Different Zr-modified clays exhibited substantially different phosphate adsorption behaviors, which may be related to the distinct structural and expansion properties of each clay substrate. Particularly, Zr-modified montmorillonite had fastest phosphate adsorption kinetics and highest phosphate adsorption capacity among all Zr-modified clays, which may be attributed to the good expansion property of montmorillonite that favored the uniform intercalation of Zr species, making the adsorption sites easily accessible by phosphate. Furthermore, all Zr-modified clays showed robust performance for phosphate adsorption under various water chemistry conditions. Combined aqueous sorption and solid characterization analyses suggested that formation of inner-sphere surface complexes may be the primary mechanism for phosphate adsorption by Zr-modified clays.

A new permeability model for argillaceous porous media under stress dependence with clay swelling

This paper derives a novel analytical model for stress dependent permeability of argillaceous porous media (APM) considering clay swelling using fractal theory. The model shows that the stress dependent permeability is a function of effective stress, clay content, clay expansion coefficient, initial irreducible water saturation and rock lithology. It is validated by the available test data from experimental studies and is shown to perform well. The presented model is then invoked to study the effects of relevant parameters on normalized permeability and irreducible water saturation. Under a given effective stress, the normalized permeability decreases with increasing clay content or increasing clay complete expansion coefficient. This work provides a comprehensive study on stress-dependent permeability in APM considering clay swelling, which is beneficial to analyzing the effect of clay content on permeability behavior of APM under stress dependence and clay swelling. In addition, the proposed model serves as a foundation for estimating physical parameters by inverse modeling.

The Application of an Expansion Turbine in the Production of Expanded Aggregates

The research into the expansion of Cypris clay from the overburden rocks of the Družba brown coal open-pit mine in North Bohemia has proven that targeted expansion enables good use of the thermal energy of hot aggregates, which has not yet been fully exploited. The integration of an electricity production expansion turbine into the production line and the use of residual gas heat in the turbine cycle can reduce production costs. The article assesses two possible solutions for the integration of the expansion turbine into the technological line, namely with an open and closed working cycle using various gaseous media. In both cases, the proportion of the energy usable for electricity generation in the expansion turbine cycle has been calculated along with the possible fuel savings in the rotary kiln by using combustion air preheated to a high temperature by the residual heat of the gas from the turbine gas cycle.

Effect of adsorbed moisture on the pore size distribution of marine-continental transitional shales: Insights from lithofacies differences and clay swelling

The variation in pore water distribution within gas shale reservoirs has a significant effect on gas content, and thus on resource evaluation. However, the characteristics of water micro-distribution and its effects on pore parameters are still not well understood due to the mixed wettability of shale and the complexity of the pore structure. In this study, six lower Permian transitional shale samples from the southern North China Basin, humidified at four levels up to a relative humidity of 98%, were selected for moisture-equilibrated experiments and low-pressure N2 gas adsorption measurements. The results indicate that the adsorbed moisture in transitional clay-rich shales can be divided into capillary condensation water in the micropores and monolayer–multilayer adsorbed water in the non-micropores. Moreover, thermal maturity (VRo), total organic carbon, clay, and carbonate are positively correlated with the adsorbed moisture and micro-/mesopores, indicating that water in shales could be hosted in inorganic pores as well as in organic pores. Furthermore, the distribution of adsorbed moisture is mainly controlled by the VRo, component wettability (i.e., organic matter, clay, pyrite, and carbonate), and pore structure (micro-/mesopore distribution). In addition, a subtle adsorbed moisture may significantly reduce both the pore volume (PV) and specific surface area (SSA) of micropores, and the effect on micropores and SSA is more pronounced than that on the respective non-micropores and PVs. Additionally, the mechanism of clay swelling and pore expansion in clayey shale can provide certain insights for water–methane competitive adsorption, identifying clay type and pore size, and the formation of organo-mineral complexes.

The Study to Improve Oil Recovery through the Clay State Change during Low Salinity Water Flooding in Sandstones.

Low salinity water flooding is a low-cost enhanced oil recovery (EOR) technology. The mechanism of EOR in a sandstone reservoir is still controversial, and there are many influencing factors. In this study, the effects of salinity (2000, 4000, 8000, and 100,000 ppm), pH (5.5 acidic, 7.0 neutral, and 8.0 alkaline), cation type (Na+ and Ca2+), and clay content (A rock 6.04%, B rock 11.94%) on zeta potential and recovery related to clay swelling were studied. The results showed that the absolute value of zeta potential increased with the decrease of salinity, cation changes from divalent to monovalent, and an increase of the pH value or clay content. The results of the SEM test before and after displacement and the continuous increase of displacement pressure after low salinity water injection show that low salinity water will cause clay swelling and the absolute value of zeta potential increased. The extreme value of recovery appears in the rocks with a high clay content: In neutral and alkaline NaCl solutions, RI and PEOR of rock B first increase and then decrease with the decrease of salinity. When the salinity is 4000 ppm, RI and PEOR were 8.16 and 34.13% in the neutral state, and 8.50 and 25.00% in the alkaline state, respectively. RI and PEOR of other experimental groups increased with the decrease of salinity. The study showed that the displacement pressure increases with the decrease of salinity, which indicates that the proper expansion of clay can improve the recovery of a sandstone reservoir, while the excessive expansion of clay will damage the reservoir and reduce the recovery. Based on the experimental results, the factors and indexes involved in the experiment were analyzed by multiple variance analysis. The result showed that the salinity, cationic type, and pH value have a significant effect on the zeta potential. All factors in the experiment have a significant effect on RI, salinity, and cationic type, and the clay content have a significant effect on PEOR. The conclusion of this study could guide the design of low-salinity water flooding technology in oil fields.

Bentonite homogenisation during the closure of void spaces

In a geological repository, the disposal of radioactive waste will result in the creation of engineering voids. Bentonite is commonly proposed as a sealing material as a result of its high swelling capacity. As the bentonite expands, the non-uniform development of porewater pressure and its coupling to total stress within the bentonite, may impair homogenisation. In this study we present results from five laboratory tests performed on sodium- and calcium-based bentonites to examine their swelling potential and capacity to homogenise over extreme bentonite-to-void ratios. Results demonstrate that even under these extreme ratios, the bentonite is able to swell and ultimately fill each void, creating a small swelling pressure. The swelling pressure development is spatially complex and time-consuming, and does not appear to be influenced by friction. Instead, it is characterised by plastic yielding of the clay with 70%–80% of the volume change associated with clay expansion adjacent to the void. This leads to heterogeneity illustrated by the presence of persistent differential stresses and the non-uniform distribution of moisture contents. Increases in the moisture content were measured but did not always correlate with the development of swelling pressure. This disequilibrium of the system is likely a reflection of the test durations and the slow evolution in the rates of change in swelling and porewater pressure beyond 130 days. Given the length of the experimental tests presented here, the time required to achieve full homogenisation of the clay is likely to be many years, if it occurs at all. Gravity segregation was also present in horizontal tests, further impairing clay homogenisation. However, as presented in this paper, it is possible to define functional relationships describing the bentonite swelling potential across engineering voids of differing size. This information will assist in establishing a safety case for bentonite usage in geological radioactive waste disposal.

Fracture behavior of Longmaxi shale with implications for subsurface applications

Fracture mechanical properties of shales have gained continued interest recently due to their critical role in many shale-related applications such as unconventional petroleum systems and subsurface carbon and waste disposal. However, due to their strong reactive nature to fluids, fracture mechanical properties of shales have not been extensively studied like other rock types. A more comprehensive understanding of fracture system development in shales under reactive fluids is needed for subsurface applications. We have measured fracture mechanical properties of Longmaxi shale outcrop samples in the Sichuan Basin, China, under different environments of ambient air, deionized water, saline fluids of NaCl and KCl at 0.5 M concentration, and acidic HCl fluid at pH of 5. All aqueous fluids tested show strong weakening effects on fracture propagation compared to the air environment, with the fracture toughness reduced by 75% and the subcritical fracture growth index reduced by 50%. Microstructural analysis reveals the predominantly grain boundary opening cracking mode for all tests, but the fracture traces branch more in reactive aqueous fluids. Natural fractures are comparable with artificial fractures in morphology. Bedding-perpendicular opening mode fractures with multiple-stage fracture fillings of calcite, quartz, and organic matters develop well in natural fractures. Our results suggest that clay mineral hydration and expansion are the main cause for the fluid weakening effect in Longmaxi shale, which has substantial implications for subsurface shale failure processes.

The role of KCl in cationic Gemini viscoelastic surfactant based clean fracturing fluids

KCl, as a kind of counter-ion salt, is often used in cationic viscoelastic surfactant (VES) clean fracturing fluid systems. The interaction between Cl− and VES cationic hydrophilic groups is used to promote the formation of VES micelles, which significantly improves the viscoelasticity, temperature resistance, and shear resistance of cationic VES fracturing fluid. At the same time, KCl is also a widely used inorganic salt clay stabilizer. K+ enters the interlayer of the clay and reduces the hydration expansion of clay by charge attraction. To determine whether Cl− and K+ in KCl can simultaneously play the role of counter-ion and clay stabilizer, we have evaluated the performance of KCl on viscoelasticity, temperature, shear resistance of cationic VES fracturing fluid and clay stability. The interaction of KCl and VES in clay stability was also studied. Results revealed that VES thickener has a clay stabilization effect in cationic VES fracturing fluid system, and the higher the concentration, the better the clay stabilization effect. KCl can not only effectively enhance the viscoelasticity and temperature resistance of cationic VES fracturing fluid, showing a counter-ion performance, but also acted as a clay stabilizer. However, VES and KCl can restrain each other in clay stability and produce antagonistic effects.

A Comparison of Gas and Water Permeability in Clay‐Bearing Fault and Reservoir Rocks: Experimental Results and Evolution Mechanisms

AbstractTo better understand the discrepancy between gas and water permeability and the mechanism of permeability in response to the presence of water in clay‐bearing rocks, we performed transport property experiments on synthetic quartz‐clay mixtures and natural fault gouge, as well as clay‐bearing sandstones for comparison purpose. The experiments were performed on a fluid flow apparatus with effective pressures (Pe) cycling between 5 and 105 MPa. Each sample was subjected to nine Pe cycles, along which permeability and porosity of either the gas‐saturated (nitrogen) or water‐saturated samples were measured. The results show that the permeability of all samples investigated decreases with increasing Pe. Gas permeability exhibits strong pore pressure dependence, which can be explained by the slippage (Klinkenberg) effect. Permeability results measured by water are commonly lower than the gas results. The decrease in permeability after the addition of water can be generally explained by the evolution of porosity, as can be determined from the bulk volume and solid mineral volume measurement results. The loss of permeability is associated with the hydration expansion of clay minerals that causes the reduction of porosity. A conceptual model for the evolution of porosity that incorporates clay expansion and grain rearrangement is proposed to describe the mechanism for water permeability in clay‐bearing rocks. Based on these results, we discuss the effect of clay hydration on fluid transport and mechanical behaviors in clay‐bearing fault zones and reservoir formations following fluid injection.

Effect of water intrusion on the characteristics of surface morphology and pore fracture spaces in argillaceous meagre coal

Dry coal samples and coal samples with different water invasion times (DWITs) were prepared, and X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and low-pressure nitrogen adsorption (LPNA) experiments were carried out to analyze the pore fracture volume and distribution. The fractal dimension of the aperture surface was calculated with the MIP and LPNA data of these coal samples. We found that the pore diameter of macropores decreases, and the fractal dimensions of macropores and mesopores change greatly, on the score of the dispersion of clay minerals and clay expansion; in addition, water intrusion has little effect on the pore size and fractal dimension of minipores and micropores with increasing water invasion time. We conclude that long-term water invasion greatly damages the pore diameter and surface morphology of macropores and mesopores and reduces the permeability of the fractures in coal seams. Long-term water invasion causes little damage to the pore diameter and surface morphology of minipores and micropores. Water plays an important role in sealing off gas and competing for gas adsorption position in minipores and micropores. In addition, by combining the fractal dimension data of MIP and LPNA, we found that water invasion for 20 days was an effective time point to change the pore structure and surface morphology of coal particles. We anticipate that the research conclusions could save coal seam water injection time, and the conclusions could improve the efficiency of natural gas extraction and improve coal and gas outburst prevention engineering.

Strength Softening Characteristics of Shale Clay Mineral Expansion

In recent years, the exploration and development of shale oil and gas reservoirs has become a hot topic in various countries around the world. The mechanical properties of shale are an important basis for evaluating the mining effect. Previous studies have shown that shale swells with water, which greatly effects its mechanical properties. Based on the analysis of shale composition, a clay mineral swelling test and 3D compression test were carried out to analyze the influence of clay minerals on the mechanism of shale swelling and softening in water. The results show that hydration of clay minerals results in interlayer and intergranular swelling. Furthermore, an expansion force will be exerted on the rock skeleton. The clay minerals expansion will cause a decrease in the uniaxial compressive strength, decrease in the elastic modulus, and increase in the Poisson's ratio, both in normal and parallel directions to the shale plate bedding. The brittleness index calculated based on the elastic modulus and Poisson's ratio values also decreases. This work is significant for further engineering development of shale reservoirs.

PERMEABILITY MUTATION MECHANISM OF HYDROGEOCHEMICAL CONDITIONS IN SEA AND FRESHWATER TRANSITIONAL ZONE

Taking the sandy aquifer intruded by seawater in the lower reaches of Dagu River as the research object, the samples of underground freshwater, seawater and aquifer media were collected, and their composition and properties were determined. The seepage device was used to simulate the displacement process of seawater and freshwater. The relationship between permeability change of aquifer and hydrologic-geochemical action was studied. The mechanism of water sensitivity was explained by a hydrologic-geochemical process for the first time. The results show that the exchange and migration of multi-component ions occur in the process of mutual displacement of saline and freshwater, accompanied by clay release and complex hydrologic-geochemical processes. The reason for clay release is not the enhancement of diffusion and dispersion, but the enhancement of clay expansion and diffusion by hydrologic-geochemical action, which results in the decrease of permeability of the aquifer.

Experimental study of the pomelo peel powder as novel shale inhibitor in water-based drilling fluids

The hydration and swelling of shale is a persistent challenge in the drilling of oil and gas wells. Many methods of reducing shale hydration and swelling have been developed; however, most of them are high-cost or release pollutants. In this study, we explored the use of pomelo peel powder as a novel additive to water-based drilling fluids for inhibiting shale hydration swelling in an environmentally sustainable manner. We compared the performance of the drilling fluid containing pomelo peel powder to that of traditional shale inhibitors, such as potassium chloride and polyamine. Moreover, hydration inhibition, bentonite precipitation dynamic linear expansion, rolling recovery, and adsorption experiments were conducted to investigate the inhibitory effects of the pomelo peel powder on shale. The results show that the pomelo peel powder solution with a mass fraction of 1% and an optimised particle size of over 160 mesh was acidic, could prevent shale collapse, and could reduce mud loss by filtration. The rolling recovery of shale cuttings reached 95% with the addition of pomelo peel powder, and the powder could also inhibit the hydration of bentonite, prevent clay minerals from dispersing in a solution, and reduce the expansion of bentonite. The inhibitory effect of the powder was slightly worse than that of potassium chloride and polyamine; however, the difference was not significant. The anti-swelling mechanism of pomelo peel powder was then analysed, and we found that fresh pomelo peel powder contains a high number of active substances that reduce the filtration of mud, improve its rheological properties, and hinder the hydration and expansion of clay. Pomelo peel is available worldwide and is easy to obtain as a shale inhibitor. Thus, using pomelo peel powder can effectively alleviate ecological pressure and reduce environmental pollution.

Phase Prediction of Supercritical Carbon Dioxide and its Application in Fracturing Oil Wellbores

In recent oil and gas exploration, the most reservoirs are low permeability with abundant reserves. Conventional mining of low permeability reservoir is commonly utilizing the hydraulic fracturing technology, whereas, it encounters various technical issues, such as clay expansion and water lock damage. Using the fluid of supercritical carbon dioxide (S-CO2) to exploit the low permeability oil and gas reservoirs is attracting more attention. The implementation of S-CO2, without liquid phase, can help avoid the aforementioned problems. Nevertheless, the phase change of CO2 during fracturing is complicate, and it is difficult to accurately predict the CO2 phase transition. In this work, first, the physical properties of S-CO2 were analyzed by the Span-Wagner model and Vesovic model. Next, S-CO2 was applied to a typical oilfield, and an unsteady coupling model of heat transfer and pressure drop was developed. Then the staggered grid method and iteration procedures were used for numerical solutions, and the temperature and pressure distributions of wellbores were investigated. The results indicate that the temperature control of a wellbore is the key to the phase prediction of S-CO2; CO2 within the single-diameter pipeline below 2300 m can maintain the supercritical state, while CO2 within the stepped pipeline can maintain the supercritical state at the depth of 2280 m. Moreover, compared with the single-diameter pipeline, the bottom pressure of the stepped pipeline is lower and the bottom temperature is higher. By analyzing the flow and heat transfer of S-CO2 in the wellbores, the phase state of S-CO2 was well predicted, which is helpful to improve the exploring performance of low permeability oil and gas reservoirs.

Inverse engineering approach to determine the elastic properties of lightweight expanded clay

Abstract Lightweight Expanded Clay (LECA) particles are aggregates processed by high-temperature gas infiltration and expansion of solid clay, being used to fill concrete and aluminum composites. Although, the properties of these composites are thoroughly researched, there are scarce studies on the properties of LECA itself. The referred studies are commonly based on indirect measurements and homogenization techniques. This study reports an approach, based on inverse engineering, to determine the elastic properties of these cellular materials. FEA routines are used to determine the fundamental elastic constants of modeled LECA spheres in uniaxial compression. These routines are posteriorly run in updating routines and compared with experimental data to estimate values of Young’s Modulus and Poisson’s ratio. It is reported that particle diameter has a relevant role due to the variation on the ratio between outer skin thickness and porous inner structure. The Young’s Modulus of particles with 5.08, 7.59 and 10.65 [mm] with diameter are determined to be, respectively, 200, 280 and 640 [MPa]. The Poisson’s ratio of the referred particles is estimated to be 0.34, 0.36 and 0.36.

Analyzing Tight Sand Characteristics and Its Influence on Aqueous Phase Removal by Gas Displacement

Tight sand gas is an important unconventional natural gas. Liquid filtration plays a main role in the formation damage. Flowback rate is closely related to the formation permeability recovery. Usually, the more flowback rate is, the better the permeability recovery is. In order to research the factors that influence the liquid flowback, the tight sand characteristics and gas displacement have been investigated. Some experiments were carried out, including porosity, permeability, XRD, casting thin section, SEM, stress sensitivity, rock expansion, and gas displacement. The results are shown as follows. Taiyuan formation has average high clay content than H8 formation. Illite is the main clay in Taiyuan formation; chlorite is the main clay in H8 formation. From the casting thin section, the rock has strong compression. Taiyuan formation has large pore. However, H8 formation mainly has small cracks. Through the SEM, there are many micro- to nanocracks as well as some pores in these two formations. In some cases, cracks penetrate the clays. From the permeability stress sensitivity, these tight sands have strong stress sensitivity. Permeability decreases quickly with increasing confining pressure. In the rock expansion experiment, the tight sand has less expansion rate compared with shale gas rock and volcanic gas rock. Rock expansion rate has positive relationship with clay content. In the gas displacement experiment, usually the higher the clay content, the lesser the liquid flowback. Some samples have high clay content. However, it has high flowback rate, which may account for good pore connection. Through the above study, the flowback rate has negative relationship with clay content and positive relationship with pore size, pore connection, and displacement pressure. In order to increase tight gas production in the study area, the formation should have high flowback rate and less liquid retention. Therefore, clay expansion preventing additives should be added into the fracturing liquid to reduce the liquid retention. The quick flowback should be implemented after hydraulic fracturing for reducing the interaction between liquid and rock. At the same time, the drawdown pressure should keep in a certain value to reduce stress sensitivity.