Jiangtao Zheng

Bio: Jiangtao Zheng is an academic researcher from China University of Mining and Technology. The author has contributed to research in topics: Lattice Boltzmann methods & Porous medium. The author has an hindex of 12, co-authored 23 publications receiving 468 citations. Previous affiliations of Jiangtao Zheng include Lawrence Berkeley National Laboratory & Tsinghua University.

Pore-Scale Modeling of Spontaneous Imbibition Behavior in a Complex Shale Porous Structure by Pseudopotential Lattice Boltzmann Method

Abstract: Spontaneous imbibition of fracturing fluid into shale matrix is one of the primary reasons for the low flowback rate in shale gas wells after the hydraulic fracturing. This leads to concerns of impacts on both environment and shale gas production. A direct pore-scale simulation is crucial to gain a deep understanding of spontaneous imbibition behavior and its impacts. The porous structures in the shale matrix are characterized by not only a geometrical complexity but also a mixed wettability, which bring great challenges to simulation methods. An improved pseudo-potential lattice Boltzmann method is proposed to simulate the spontaneous imbibition behavior in a reproduced threedimensional porous structure of shale. The results show that the nanoscale hydrophilic pores provide the driving force and a storage place for the residual treatment fluid. The pore size and wettability heterogeneity lead to the nonuniform menisci propagation and fracturing fluid distribution in the model. Specifically, the fracturing fluid imbibed quicker in the larger pores at the early stage and gradually migrated into the smaller pores during the process. With a limited volume of the fracturing fluid, a portion of the larger pores was finally reopened. The analysis of saturation and apparent gas permeability data during the spontaneous imbibition process showed a great recovery of the model permeability along with the reopened pores. These results provide direct evidence of the residual fracturing fluid migration pattern in the shale reservoir and its influence on shale gas production. Plain Language Summary Hydraulic fracturing is widely employed to stimulate a shale gas reservoir for a better production. After fracturing, a substantial amount of the fracturing fluid stays in the reservoir even after a period of flowback operations. It is believed that the residual treatment fluid is imbibed into the surrounding shale matrix. However, whether these trapped fracturing fluids impact the shallow aquifers and the shale gas production rates remains to be fully understood. A pore-scale simulation is performed in this work to directly investigate the migration of the fracturing fluid in a reproduced porous structure of shale. The results showed that (1) the nanoscale pores in the shale matrix provide the driving force, namely, the capillary force, and the storage space for holding the fracturing fluids. (2) The nonuniform spontaneous imbibition in the complex shale porous structure results in the fracturing fluid-filled small pores and a portion of reopened large pores. The reopening of the large pores leads to a great recovery of the model transport property of the shale gas. These results can be served as evidence for answering the engineering related problems, such as the low flowback rate of the fracturing fluid and the relatively high gas production after the shut-in operation.

VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Abstract: The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

Interfacial tension of alkane + water systems

Abstract: Interfacial tension was measured for hexane + water, heptane + water, octane + water, nonane + water, decane + water, undecane + water, and dodecane + water, using the emergent drop experimental technique with a numerical method based on a fourth degree spline interpolation of the drop profile. The experimental equipment used to generate the drop consists of a cell with a stainless steel body and two Pyrex windows. The inner cell was previously filled with water. A surgical needle (at the bottom of the cell) was used to introduce the organic phase into the cell (forming the emergent drop). Water was used to keep the temperature constant inside the cell (between 10 °C and 60 °C). The cell was illuminated from the back using a fiber optic lamp and a diffuser. A video camera (with a 60 mm microlens and an extension ring) was located at the front window. The emergent drop image was captured and sent to the video recording system. The cell and the optical components were placed on an optical table with vibration isolation legs. A new correlation was found to predict interfacial tension (γ) as a function of temperature (t) and the number of carbon atoms (n) with a deviation of less than 0.05% from experimental values.

Wettability control on multiphase flow in patterned microfluidics

Abstract: Significance The simultaneous flow of multiple fluid phases through a porous solid occurs in many natural and industrial processes—for example, rainwater infiltrates into soil by displacing air, and carbon dioxide is stored in deep saline aquifers by displacing brine. It has been known for decades that wetting—the affinity of the solid to one of the fluids—can have a strong impact on the flow, but the microscale physics and macroscopic consequences remain poorly understood. Here, we study this in detail by systematically varying the wetting properties of a microfluidic porous medium. Our high-resolution images reveal the fundamental control of wetting on multiphase flow, elucidate the inherently 3D pore-scale mechanisms, and help explain the striking macroscopic displacement patterns that emerge. Multiphase flow in porous media is important in many natural and industrial processes, including geologic CO2 sequestration, enhanced oil recovery, and water infiltration into soil. Although it is well known that the wetting properties of porous media can vary drastically depending on the type of media and pore fluids, the effect of wettability on multiphase flow continues to challenge our microscopic and macroscopic descriptions. Here, we study the impact of wettability on viscously unfavorable fluid–fluid displacement in disordered media by means of high-resolution imaging in microfluidic flow cells patterned with vertical posts. By systematically varying the wettability of the flow cell over a wide range of contact angles, we find that increasing the substrate’s affinity to the invading fluid results in more efficient displacement of the defending fluid up to a critical wetting transition, beyond which the trend is reversed. We identify the pore-scale mechanisms—cooperative pore filling (increasing displacement efficiency) and corner flow (decreasing displacement efficiency)—responsible for this macroscale behavior, and show that they rely on the inherent 3D nature of interfacial flows, even in quasi-2D media. Our results demonstrate the powerful control of wettability on multiphase flow in porous media, and show that the markedly different invasion protocols that emerge—from pore filling to postbridging—are determined by physical mechanisms that are missing from current pore-scale and continuum-scale descriptions.