Jing Liu

Exploiting water-sealed gas in fully water-involved sandstone reservoirs is key to sustaining and increasing production for such gas reservoirs. Based on this, this study proposes a technology that combines formation energy replenishment with gas-water separation agent injection to disrupt gas-water interface equilibrium, thereby enabling water-sealed gas exploitation. Through static performance evaluation and microfluidic visualization experiments, high-efficiency gas-water separation agents were selected, and their mechanisms for tapping potential different types of water-sealed gases were characterized. Results show that selected agents exhibit universal applicability under experimental conditions, reducing the gas-water interfacial tension by ∼50% under reservoir conditions. Agent No. 10 achieves 63% reduction at 0.3 wt % concentration. The agents primarily exploit water-blocked gas (58.1%) and trapped gas (59.2%) in high-sweep-efficiency zones through: gas-water separation agents contacting the gas-water interface → gas-water interfacial tension progressively decreasing → internal forces exceeding external forces, inducing bubble expansion → bubble fragmentation and migration → bubble deformation and coalescence → bubble fragmentation and migration. For gases in low-sweep-efficiency zones, exploitation ratios are lower: edge water-blocked gas: 15.8%, blind-end residual gas: 2%, and edge band gas: 11.2%. This study provides crucial technical support for developing water-sealed gas in water-injected gas reservoirs.

A multiscale complex fracture and matrix damage coupled productivity prediction model under cyclic percussion of horizontal wells is established, according to the evolution of matrix permeability and the characteristics of complex fracture seepage after cyclic conflagration compression fracturing. The effects of the conflagration loading rate, cyclic percussion times, horizontal in situ stress difference, seepage interference, and wellbore pressure drop on horizontal well productivity are analyzed. The results show that the loading rate and percussion times are positively correlated with the production growth rate, but the growth through percussion has a threshold. Besides that, the length of the branch fracture is closer to that of the main fracture when there is a small stress difference, which results in a higher initial productivity and recovery degree of the gas well. Additionally, section spacing can affect the distribution of the pressure field and flow field around the well. An excessive spacing can lead to bending of flow field around the well, while a too small value is able to aggravate interjoint interference. Therefore, the critical section spacing, which can establish pressure communication between sections, is taken as the optimal section spacing. According to our work, when the fracture half-length is 5, 7, and 9 m, the optimum section spacing is 15, 25, and 30 m, respectively. Under this condition, when the horizontal length exceeds 800, 700, and 500 m, the influence of the wellbore pressure drop on the productivity of horizontal wells should be considered.

To investigate the influence of pressure difference changes on the micro start-up and percolation of heavy oil, a micro visualization displacement device was used to characterize the start-up time and oil–water percolation state of heavy oil. The mechanism of different pressure differences, as well as the frequency and amplitude of pressure difference changes, on the start-up and percolation balance of heavy oil was clarified. The results indicate that high-pressure difference and pressure difference changes can reduce the start-up time of heavy oil. A reasonable frequency of pressure difference changes effectively promotes the balance between positive and negative pressure shear and fluid–solid response. Large pressure difference changes can effectively break the viscous and adsorption resistance during heavy oil start-up; reasonable pressure difference can exert the synergistic effect of pressure difference and infiltration, achieving a balance between the water wave and the initial water film thickening process as well as the continuous percolation process of wire drawing, oil droplets, and oil columns during the medium-to-high water content period; a reasonable frequency of pressure difference variation during the high water content period can promote the superposition of inertia effects at the oil–water interface and break the balance of the oil–water interface. A large amplitude of pressure difference variation is beneficial for the strong deformation of the oil–water interface and the shear dislocation peeling of the oil–solid interface. Therefore, a relatively high amplitude of pressure difference variation and a reasonable frequency of pressure difference variation, as well as the synergistic effect of pressure difference and infiltration, are the keys to effectively start heavy oil and improving oil recovery during the ultrahigh water-cut period.

To explore the law and mechanism of enhanced surfactant flooding with a low-frequency artificial seismic wave, for single-phase fluids in porous media, a heterogeneous two-stage adsorption model for a surfactant with a low-frequency artificial seismic wave is introduced into the surfactant transport equation of a single-phase fluid. With this model, the surfactant fluid transport model in porous media with an artificial seismic wave is obtained. The model is solved using the C–N difference and chasing method. The migration law of the surfactant is simulated and quantitatively analyzed for different vibration accelerations, injection slug sizes, displacement speeds, and reservoir parameters with the action of low-frequency artificial seismic waves. The results show that artificial seismic waves can increase the effective range of the surfactants and reduce the number of chemical agents through reduced adsorption. Low-frequency vibration with the same surfactant injection rate can increase the effective range by a factor greater than one. For the same effective action distance, the dose of chemical agents can be reduced by more than 60%, and the optimal acceleration and the injection slug size are 0.3 m/s² and 0.4 PV, respectively. With the increase of the injection rate, the effect of low-frequency vibration on the diffusion and transport of the surfactant decreases. A low-frequency wave combined surfactant has a better effect on the low permeability reservoirs. The research results provide important support for further understanding of the low-frequency artificial seismic wave composite surfactant flooding law and the optimization of the field parameters.

In this study, the mechanism of physical–chemical viscosity reduction of different heavy oils under ultrasonic wave is explored. Experiments of viscosity reduction and viscosity recovery of different heavy oils under ultrasonic excitation were carried out, and the optimal ultrasonic parameters, ultrasonic physical disturbance, and cavitation viscosity reduction extent of different oil samples were determined. Based on the element analysis methods, four-component analysis, gas chromatography analysis, and formation water pH value test, the micro-mechanism of the oil chemical structure change and water samples under ultrasonic wave was analyzed. The results show that the water content, temperature, and initial viscosity of heavy oil are the key to reduce the viscosity of heavy oil. The higher viscosity of the initial oil sample, the higher water content, and the temperature were needed. Compared with the lower viscosity oil sample, the higher viscosity oil sample has higher extent of cavitation viscosity reduction and lower extent of physical disturbance viscosity reduction under ultrasonic wave. After ultrasonic treatment, the contents of heteroatoms, resins, and asphaltenes in heavy oil samples with high viscosity decreased significantly, and the conversion extent of high carbon chain to low carbon chain was greater. In addition, the pH of water in heavy oils decreased after ultrasonic treatment, and the pH of water in high viscosity heavy oil decreased more significantly after ultrasonic treatment.

The high dosage of surfactant terribly restrains the extensive application of viscoelastic surfactant (VES) fracturing fluid. In this study, a novel gemini surfactant (GLO) with long hydrophobic tails and double bonds was prepared and a VES fracturing fluid with a low concentration of GLO was developed. Because of the long tails bending near the double bonds, there is a significant improvement of the surfactant aggregate architecture, which realized the favorable viscosity of the VES fluid at a more economical concentration than the conventional VES fracturing fluids. Fourier transform infrared spectrometry (FT-IR), nuclear magnetic resonance spectrometry (¹H NMR, ¹³C NMR), and high-resolution mass spectrometry (HRMS) were employed to study the formation of the product and the structure of GLO. The designed GLO was produced according to the results of the structure characterizations. The formula of the VES fracturing fluid was optimized to be 2.0 wt % GLO + 0.4 wt % sodium salicylate (NaSal) + 1.0 wt % KCl based on the measurements of the viscosity. The viscosity of the VES fluid decreased from 405.5 to 98.7 mPa·s as the temperature increased from 18 to 80 °C and reached equilibrium at about 70.2 mPa·s. The VES fluid showed a typical elastic pseudoplastic fluid with a yield stress of 0.5 Pa in the rheological tests. It realized a proppant setting velocity as low as 0.08 g/min in the dynamic proppant transport test carried by GLO-based VES fracturing fluid. Compared to the formation water, the filtrate of the VES fracturing fluid decreased the water contact angle (CA) from 56.2 to 45.4° and decreased the water/oil interfacial tension (IFT) from 19.5 to 1.6 mN/m. Finally, the VES fracturing fluid induced a low permeability loss rate of 10.4% and a low conductivity loss rate of 5.4% for the oil phase in the experiments of formation damage evaluation.

Due to the limited temperature resistance, the deep conformance control technology of using the conventional hydrolyzed polyacrylamide (HPAM) gel failed to enhance oil recovery in high-temperature heterogeneous oil reservoirs. Therefore, it is necessary to develop a gelant with high temperature resistance to meet the demands of increasing oil production and decreasing water cut in high-temperature heterogeneous oil reservoirs. In this paper, a copolymer is first synthesized by the method of inverse emulsion polymerization using 2-acrylamide-2-tetradecyl ethyl sulfonic acid (AMC16S), acrylamide (AM), and acrylic acid (AA). The developed copolymer has a highly branching skeleton and can resist temperature up to 100 °C. And then, a gelant with high temperature resistance and good shear resistance can be formed by mixing a certain proportion of the developed copolymer and polyethyleneimine (PEI). After the controllable gelation, a copolymer gel is formed and the formed gel can maintain the stable performance for a long time in the high-temperature environment. Experimental results show that the developed gelant can be applied in the conformance control of high-temperature heterogeneous oil reservoir.