Study on the Seepage Force-Induced Stress and Poroelastic Stress by Flow Through Porous Media Around a Vertical Wellbore

Fluid flowing through reservoir pores not only generates poroelastic stress but also exerts seepage force on rock skeleton. However, the mechanism of seepage force is not clear. Traditional methods of analyzing wellbore stability and hydraulic fracture initiation are mainly focused on the poroelastic stress without the effects of seepage force. Based on the linear elasticity and consolidation theory, this paper analyzed the mechanism of seepage force and poroelastic stress, and presented an analytical solution for seepage force-induced stress around a vertical wellbore. It also introduced how to calculate poroelastic stress by exerting hypothetical body force and surface force. Through comparison and superposition of stress fields, this paper studied the change characteristics of the poroelastic and seepage force-induced stress under different borehole pressures and the effects of seepage force on the wellbore tensile failure. Numerical simulation results show that when fluid flows through the rock, using traditional models without considering, the effect of seepage force to calculate the borehole pressure-induced stress will result in lower calculation results. Compared with the traditional model, seepage force-induced circumferential tensile stress is larger, and the seepage force significantly reduces the formation breakdown pressure. Rocks near the borehole wall with lower permeability and larger Poisson’s ratio have a greater action of seepage force. When fluid flows through the reservoir, the effects of seepage forces cannot be ignored in the analysis of hydraulic fracturing and wellbore stability.

Characteristics of Organic Matter Pores and the Relationship with Current Pressure System of Lower Silurian Longmaxi Shales in Dingshan Field, Southern Sichuan, China

Organic matter pores (OMP) provide significant storage space for hydrocarbons in lower Silurian Longmaxi shales in the Dingshan field of southern Sichuan, China. The distributions of organic matter and the different OMP structure parameters were characterized through Ar-ion polishing, scanning electron microscopy (SEM), and image analysis software for shale samples of different wells. The research results indicated that organic matter has been divided into two categories based on its occurrence, location, and its relationship with authigenic minerals: organic matter in situ and migrated organic matter. OMP for organic matter in situ are mainly micropores mostly arranged isolatedly, while in migrated organic matter pores show larger sizes and higher roundness. The development of OMP in samples is predominantly controlled by the formation pressure. The existence of overpressure alleviated the stress on the rock skeleton, causing the compaction of some migrated organic matters to lag or decrease. This played a positive role in protecting the development of pores in the interior and edge of the rock skeleton, and it can also induce the development of microfractures in shale. The protective effect of formation pressure on organic pores was provided for understanding the exploration and exploitation of Longmaxi shales in the study area.

The Influence of Rate of Change in Confining and Pore Pressure on Values of the Modulus of Compressibility of the Rock Skeleton and Biot’s Coefficient

This work discusses the results of a study of the influence of rates of change of confining pressure on the result of a drained compressibility tests intended to determine the modulus of compressibility of a rock skeleton Ks. A series of cyclical compressibility tests was performed on samples of sandstone soaked in kerosene, for various rates of compression and decompression of the pressure liquid filling the cell and the pore volume of the sample. The studies showed that the deformability of the tested sample was directly proportional to the rate of change of the confining pressure. As a consequence, the value of the Ks modulus and Biot coefficient α decreased with increasing sample load rate. This phenomenon should be attributed primarily to equilibration of the liquid pressure inside the high-pressure cell with the liquid pressure in the sample pore space, caused by filtration of the pore liquid. These phenomena prove that the filtration process impacts the values of the modulus of compressibility of the rock skeleton Ks and of Biot coefficient α determined on the basis of the experiment. This is significant in the context of the use of Biot equations as constitutive equations for a porous rock medium.

Soil-Rock Slope Stability Analysis under Top Loading considering the Nonuniformity of Rocks

Soil-rock slopes are widely distributed in central or western China. With the development of transportation, many subgrades are being built on mountainsides and therefore, slope stability has to be estimated under high loadings. To obtain better estimation results, a new rock contour establishing algorithm was developed, capable of considering interlock effect between rocks. Then, computed tomography (CT) and unconfined triaxial tests with ring top loadings were conducted. Based on rock distribution characteristics (obtained by CT photos) and the appearance of shear failure surfaces in slopes under ring top loadings, four rock skeleton status and five shear failure surface developing models were introduced. Based on the developed rock contour establishing algorithm, ten groups (twelve models per group) were established and calculated by finite element method (FEM). After this, normalized ultimate loading increasing multiple N, which was the ultimate loading ratio of rock-containing slope to uniform soil slope, was introduced to evaluate the influence of rock distributions on slope stability. The value of N was increased with the increase of rock content due to rock skeleton status. The values of N in slopes with angular rocks were about three times higher than those with round rocks which was due to complex geometric shape and distribution characteristics of angular rocks. Then, considering different slope angles (50°–60°), rock contents (0%–60%), and rock shapes (round and angular), the ultimate loading increasing multiple N of soil-rock slopes under high loadings was calculated and suggested for engineering designs. Finally, based on the failure surfaces of numerical modes, three typical failure modes were developed, which could be reference for designers to deal with slopes.

Effects of Pore Structure on Sandstone Mechanical Properties Based on Micro-CT Reconstruction Model

As porous, heterogeneous, and anisotropic material, the microscopic structure of the rock has a significant influence on its mechanical properties. Rare studies were devoted to this area using pore scale modeling and simulations. In this paper, different types of sandstones are imaged using micro-CT technology. The rock porosity is obtained by filtering, binarization, and threshold segmentation. The texture coefficient (TC) and the tortuosity of the rock skeleton are calculated by open source program, where the tortuosity of the rock skeleton is firstly used to characterize the microscopic structure of the rock. Combining with the rock mechanics parameters obtained in the laboratory, the simulation of uniaxial compression is performed on the reconstructed pore scale rock finite element mesh model by ANSYS software. Young’s modulus, compressive strength, yield strength, shear modulus, and other related parameters obtained by numerical simulation are adopted to determine the optimal representative volume element (RVE) size. Moreover, the effects of microscopic structure characteristics on the mechanical properties of the rock are studied quantitatively. The results indicate that the averaged von Mises stress distribution, displacement field, and plastic strain field of rocks show anisotropy and heterogeneity. The stress concentration and the X-shaped conjugate plastic shear zone are investigated. The samples of S1∼S4 reach the elastic limit and enters the plastic yield state, when the strain is about 0.5%. And the critical yield strain of samples S5300-1∼S5400-2 is about 1%. Then, the quantitative relationships between porosity, TC, tortuosity of rock skeleton and rock mechanics parameters of digital rock samples are established and analyzed. The tortuosity of the rock skeleton is highly correlated with the mechanical parameters of the rock, i.e., Young’s modulus (R2 = 0.95), compressive strength (R2 = 0.94), yield strength (R2 = 0.92), and shear modulus (R2 = 0.94), which is believed to be more feasible to reveal the impacts of the microstructure of the rock on its mechanical properties.

Effect of Salinity on the Capturing CO2 Capabilities of Brine in Two-Dimensional Porous Media

CO2 sequestration in deep saline aquifers is being intensively studied as a strategy to mitigate CO2 emission. When CO2 is injected into the saline aquifers, a series of physical and chemical reactions will take place with the brine and rock skeleton under the multiple effects of salinity, temperature, pressure, hydrodynamic and chemistry to achieve the long-term underground storage of CO2. Therefore, we used an etched homogenous glass micromodel to investigate the impact of salinity on the brine-saturated reservoirs at the CO2 injection rate of 0.05 ml·min−1, temperature 25 °C, and pressure 0.1 MPa. Five brine concentrates were set in our experiment: 0 mol/l, 1 mol/l, 2 mol/l, 3 mol/l, and 4 mol/l to represent different types of saline aquifers. Based on the experimental results, a detailed discussion about the mode transformation of displacement, CO2 saturation and wettability variation, differential pressure change between inlet and outlet was made. The major contribution of salinity is to change the viscosity of brine, which will then affect other physicochemical properties to furtherly change the behavior of microfluidics. The effect of salinity on the drainage process was analyzed specifically in this study. It was found that as the salinity improved, the capillary number increased to make the displacement mode change from capillary fingering to viscous fingering. When the viscous force was dominant, the saturation of residual brine became bigger and the variation of wettability was not obvious. At the same time, the maximum pressure promoting the displacement finished needed to be bigger as the salinity improved to overcome more viscous force. Finally, it was found that in the brine with higher salinity, salt precipitation was more obvious.

Experimental Study on the Permeability of a Soil-Rock Mixture Based on the Threshold Control Method

The study of the permeability of soil-rock mixtures is important in supporting theories behind reclamation mechanisms for open-pit mines. To avoid the influence of differences in the spatial distribution of rock within the same sample on the permeability of a soil-rock mixture in laboratory tests, a numerical method for modelling the soil-rock mixture based on the threshold control method was proposed. Through the statistical results of 297 CT (computed tomography) cross sections of soil-rock mixture samples, the threshold values of pores, soils, and rocks are obtained, and a numerical model representing the reactions of the samples to real-world conditions is obtained. A numerical model was used that could vary with different rock block proportions (RBP) and porosities. Based on Darcy’s law, it is concluded that macroscopic voids greatly increase the permeability of the sample due to their depth of penetration. The higher the stone content, the closer the permeability will be to the permeability of the rock skeleton. Therefore, during the reclamation process of the open-pit mine, the water-retaining layer below the humus should be compacted, and RBP should be increased to lower permeability and achieve better water retention.

Numerical Analysis on Flow and Solute Transmission during Heap Leaching Processes

Based on fluid flow and rock skeleton elastic deformation during heap leaching process, a deformation-flow coupling model is developed. Regarding a leaching column with 1 m height, solution concentration 1 unit, and the leaching time being 10 days, numerical simulations and indoors experiment are conducted, respectively. Numerical results indicate that volumetric strain and concentration of solvent decrease with bed’s depth increasing; while the concentration of dissolved mineral increases firstly and decreases from a certain position, the peak values of concentration curves move leftward with time. The comparison between experimental results and numerical solutions is given, which shows these two are in agreement on the whole trend.