Insight into the Imbibition Behavior of Low-Frequency Artificial Vibration Stimulation in Tight Sandstone.

Spontaneous imbibition is the primary mechanism responsible for the enhanced oil production in a tight reservoir after hydraulic fracturing. In this article, a low-frequency artificial vibration physics stimulation method was employed to evaluate the effect of low-frequency vibration on imbibition recovery in tight sandstones. Furthermore, a high-precision in situ computed tomography (CT) scan was employed to investigate the effect of low-frequency vibration on the distribution of remaining oil micro-occurrence dynamic alterations in pore space. The findings of the study show that (1) low-frequency artificial physical vibration stimulation has been found to be highly effective in enhancing imbibition recovery in tight sandstone. The sensitivity of the vibration parameters on imbibition recovery from highest to lowest is vibration frequency, vibration intensity, and vibration time. The optimum vibration parameters for this process are a vibration frequency of 30 Hz, a vibration intensity of 2.0 m/s2, and a vibration time of 30 h. (2) Under the optimum low-frequency vibration, the imbibition recovery of tight sandstone with various physical properties can reach between 13.6 and 28.3%. This is remarkably higher than the spontaneous imbibition recovery, which ranges from 9.4 to 17.1%. Additionally, core samples with higher permeability and better pore structure show a more significant increase in imbibition recovery under the vibration treatment. Furthermore, low-frequency vibration stimulation effectively shortens the imbibition completion time, reducing the completion time from 81 h to approximately 55 h. (3) After the spontaneous imbibition process, the initial continuous oil phase present in the pore space is dispersed by the water phase imbibition process. The remaining oil is dominant in the form of a network type, which is concentrated in the central pore space area of the core. Low-frequency vibration treatment can effectively promote a positive imbibition process. The network remaining oil saturation in the core can be further dispersed, especially closer to the surface of the core area after frequency vibration treatment. Then, the cluster remaining oil type with a more dispersed and simpler individual structure has become the new dominant remaining oil micro-occurrence form in the pore space. The findings of this research investigate a novel technological approach to enhance the imbibition efficiency of a tight sandstone reservoir.

Gelation and Plugging Performance of Low-Concentration Partially Hydrolyzed Polyacrylamide/Polyethyleneimine System at Moderate Temperature and in Fractured Low-Permeability Reservoir.

HPAM/PEI gel is a promising material for conformance control in hydrocarbon reservoirs. However, its use in low-permeability reservoirs is limited by the high polymer concentrations present. In this study, the gelation performance of an HPAM/PEI system with HPAM < 2.0 wt.% was systematically investigated. The gelation time for HPAM concentrations ranging from 0.4 to 2.0 wt.% varied from less than 1 h to 23 days, with the highest gel strength identified as grade H. The hydrodynamic radius manifested the primary effect of HPAM on the gelation performance. Branched PEI provided superior gelation performance over linear PEI, and the gelation performance was only affected when the molecular weight of the PEI varied significantly. The optimal number ratio of the PEI-provided imine groups and the HPAM-provided carboxylic acid functional groups was approximately 1.6:1~5:1. Regarding the reservoir conditions, the temperature had a crucial effect on the hydrodynamic radius of HPAM. Salts delayed the gelation process, and the order of ionic influence was Ca2+ > Na+ > K+. The pH controlled the crosslinking reaction, primarily due to the protonation degree of PEI and the hydrolysis degree of HPAM, and the most suitable pH was approximately 10.5. Plugging experiments based on a through-type fracture showed that multi-slug plugging could significantly improve the plugging performance of the system, being favorable for its application in fractured low-permeability reservoirs.

Study and Field Trials on Dissolvable Frac Plugs for Slightly Deformed Casing Horizontal Well Volume Fracturing.

In view of the need for slightly deformed casing horizontal well fracturing, a new dissolvable frac plug has been designed using mechanical and material methods. The results of simulation analysis, laboratory research, and field test verification show that: (1) the integral half-split inlaid tooth slip and implicit-shaped single sealing element are adopted. Under the same pressure resistance index, the outer diameter and the total length of the frac plug are small, the inner diameter is big, and the pressure resistance downhole passing performance are better. (2) Using dissolvable magnesium-based powder, hydrogenated butadiene-acrylonitrile rubber, fluororubber polyglycolic acid, and the cellulose fiber and coating process, the solubility of the frac plug is observed to be more reliable. (3) Using the structural modular design, the frac plug has outstanding advantages in terms of on-site installation, preventing early seat sealing, self-separation after release, less dissolvable matter, and so on. (4) The finite element simulation and physical simulation experiments show that 121.36 mm ID casing with the frac plug can tolerate deformation in the range of 15 mm, which meets the needs for the casing usage. The temperature increases from 60 to 98 °C, and the maximum sustainable pressure differential decreases from 7.6 to 11.2 h at 70 MPa without any leakage. The frac plug can be completely dissolved within 300 h, and the final size of the residual solid is less than 0.3 mm. (5) In field trials of eight wells, through the minimum set of change inner diameter of 107 mm, the maximum wellhead fracturing pressure is 83.9 MPa in the formation, with the temperature ranging from 36 to 110 °C, and the plug can be completely self-dissolved in 16 days (the shortest time that has been found). After the fracturing operation with this dissolvable frac plug, the average daily oil production is maintained as high as 10.45 t with 22,000 m3 gas production. A new dissolvable frac plug tool solves the problem of continuous fracturing in slightly casing variable wells, restores the full bore of the wellbore after staged fracturing, and realizes the production of oil and gas wells without drilling plug and the smooth implementation of follow-up measures. It has a wider application range, higher efficiency, safety, and economy. It is of great significance for industrialized staged fracturing of unconventional oil and gas horizontal wells and platform wells.

Parameter Optimization of Asynchronous Cyclic Waterflooding for Horizontal-Vertical Well Patterns in Tight Oil Reservoirs.

Tight oil resources in China are mainly exploited by staged-fractured horizontal wells; horizontal wells face the problems of the rapid decline rate and low primary oil recovery. Pilot tests on the asynchronous cyclic waterflooding for the horizontal-vertical well pattern were carried out in recent years and achieved good performance. However, there are few studies on the influencing factors and parameter optimization of asynchronous cyclic waterflooding, which limits its wide application. This work took the tight oil reservoir in Yanchang formation, Fuxian area, Ordos Basin as its object, and the oil recovery mechanisms of asynchronous cyclic waterflooding for the horizontal-vertical well pattern were analyzed first. Then, the operation parameters of asynchronous cyclic waterflooding were optimized by the numerical simulation method. Among them, the injection proportion was optimized by the fuzzy synthetic evaluation method. Finally, the oilfield test was carried out based on the optimized parameters. The results showed that pressure disturbance and streamline deviation are the main oil recovery mechanisms of asynchronous cyclic waterflooding. The asynchronous mode of the diagonal well row is better than other asynchronous modes. For the injection time interval, injection-production ratio, and the injection and shut-in time, the cumulative oil production all show the trend of increasing first and then decreasing with the increase in these parameters. The optimal injection time interval and injection-production ratio are 0.5 T and 1, respectively. The optimal injection and shut-in time can be calculated by empirical formulas. Ultimately, the fuzzy synthetic evaluation model was established to optimize the injection proportion. Field practices showed that the average daily oil production of horizontal wells was increased from 1.7 to 3.0 m3 with the optimized parameters, which further verified the accuracy of the optimized parameters. This research can provide theoretical support for the effective development of tight oil reservoirs.

Mechanism of Ultrasonic Physical-Chemical Viscosity Reduction for Different Heavy Oils.

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.

Gemini Surfactant with Unsaturated Long Tails for Viscoelastic Surfactant (VES) Fracturing Fluid Used in Tight Reservoirs.

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.

Study on frequency optimization and mechanism of ultrasonic waves assisting water flooding in low-permeability reservoirs.

Water flooding is one of widely used technique to improve oil recovery from conventional reservoirs, but its performance in low-permeability reservoirs is barely satisfactory. Besides adding chemical agents, ultrasonic wave is an effective and environmental-friendly strategy to assist in water flooding for enhanced oil recovery (EOR) in unconventional reservoirs. The acoustic frequency plays a dominating role in the EOR performance of ultrasonic wave and is usually optimized through a series of time-consuming laboratory experiments. Hence, this study proposes an unsupervised learning method to group low-permeability cores in terms of permeability, porosity and wettability. This grouping algorithm succeeds to classify the 100 natural cores adopted in this study into five categories and the water flooding experiment certificates the accuracy and reliability of the clustering results. It is proved that ultrasonic waves can further improve the oil recovery yielded by water-flooding, especially in the oil-wet and weakly water-wet low-permeability cores. Furthermore, we investigated the EOR mechanism of ultrasonic waves in the low-permeability reservoir via scanning electron microscope observation, infrared characterization, interfacial tension and oil viscosity measurement. Although ultrasonic waves cannot ameliorate the components of light oil as dramatically as those of heavy oil, such compound changes still contribute to the oil viscosity and oil-water interfacial tension reductions. More importantly, ultrasonic waves may modify the micromorphology of low-permeability cores and improve the pore connectivity.

Experimental and mechanism study: Partially hydrolyzed polyacrylamide gel degradation and deplugging via ultrasonic waves and chemical agents.

Partially hydrolyzed Polyacrylamide (PHPAM) crosslinked by Cr+3 is frequently applied to plug thief zone for the better water management in matured oil reservoir. However, PHPAM gel may certainly cause inevitable formation damage nearby the wellbore. Although various kinds of chemical agents, such as hydrogen peroxide (H2O2), sodium hypochlorite (NaOCl), and chlorine dioxide (ClO2) were employed to mitigate the nearby wellbore damage. But, huge financial investment, poor degelation efficiency, environmentally insecure, corrosion problem, and long time span requirement persuade researchers to look for other effective technique. In this connection, ultrasonic waves is characterized by reliable, environment friendly, and cost effective technology. Current work involves comparative study of PHPAM gel degradation by the individual means of chemical agent and ultrasonic waves. Subsequently, the best-performed ultrasonic parameters and well-performed chemical agent were used independently and then simultaneously to deplug (PHPAM gel) the core sample. Results showed that 20 KHz frequency (1000 W) effectively reduced gel viscosity from original (2495 mPa.s) to 1.37 mPa.s after 10 min irradiation. This degradation is attributed to cavitation, heat energy, and hydroxyl radical (HO∙). However, after 2 min further exposure, the viscosity grew back to 3.29 mPa.s (18 KHz), 1.42 mPa.s (20 KHz), and 3.74 mPa.s (25 KHz). This adverse behavior is owing to hydroxyl radical (HO∙) annihilation. In chemical treatment, H2O2 among other chemicals efficiently degelled the PHPAM gel's original viscosity to 2.64 mPa.s after 24 h reaction. Similarly, NaOCl and ClO2 brought down original viscosity to 6.5 mPa.s and 159 mPa.s respectively. SEM of the samples before and after treatment was performed for the better understanding of PHPAM gel morphology. Considering dynamic experiment, maximum 23.5% and 19.80% damaged permeability recovery (30 × 10-3 µm2 gas permeability) were obtained by applying ultrasonic waves (20 KHz, 1000 W, and 100 min irradiation) and chemical agent (H2O2) respectively. Permeability recovery was further increased to 40.90% by the simultaneous application of ultrasonic waves and chemical agent.

A New Method of Plugging the Fracture to Enhance Oil Production for Fractured Oil Reservoir using Gel Particles and the HPAM/Cr3+ System.

Due to the strong heterogeneity between the fracture and the matrix in fractured oil reservoirs, injected water is mainly moved forward along the fracture, which results in poor water flooding. Therefore, it is necessary to reduce the water cut and increase oil production by using the conformance control technology. So far, gel particles and partially hydrolyzed polyacrylamide (HPAM)/Cr3+ gel are the most common applications due to their better suitability and low price. However, either of the two alone can only reduce the conductivity of the fracture to a certain extent, which leads to a poor effect. Therefore, to efficiently plug the fracture to enhance oil recovery, a combination of gel particles and the HPAM/Cr3+ system is used by laboratory tests according to their respective advantages. The first step is that the gel particles can compactly and uniformly cover the entire fracture and then the fracture channel is transformed into the gel particles media. This process can enhance the oil recovery to 18.5%. The second step is that a suitable HPAM/Cr3+ system based on the permeability of the gel particles media is injected in the fractured core. Thus, the fracture can be completely plugged and the oil in the matrix of the fractured core can be displaced by water flooding. This process can enhance oil recovery to 10.5%. During the whole process, the oil recovery is increased to 29% by this method. The results show that this principle can provide a new method for the sustainable and efficient development of fractured oil reservoirs.

Permeability recovery of damaged water sensitive core using ultrasonic waves.

It is imperative to recover the well productivity lose due to formation damage nearby wellbore during variant well operations. Some indispensable issues in conventional techniques make ultrasonic technology more attractive due to simple, reliable, favorable, cost-effective, and environment friendly nature. This study proposes the independent and combined use of ultrasonic waves and chemical agents for the treatment of already damaged core samples caused by exposure to distilled water. Results elucidate that ultrasonic waves with optimum (20kHz, 1000W) instead of maximum frequency and power worked well in the recovery owing to peristaltic transport caused by matching of natural frequency with acoustic waves frequency. In addition, hundred minutes was investigated as optimum irradiation time which provided ample time span to detach fine loosely suspended particles. However, further irradiation adversely affected the damaged permeability recovery. Moreover, permeability improvement attributes to cavitation due to ultrasonic waves propagation through fluid contained in porous medium and thermal energy generated by three different ways. Eventually, experimental outcomes indicated that maximum (25.3%) damaged permeability recovery was witnessed by applying ultrasonic waves with transducer #2 (20kHz and 1000W) and optimum irradiation timeframe (100min). This recovery was further increased to 45.8% by applying chemical agent and optimum ultrasonic waves simultaneously.

Reconstruction of a digital core containing clay minerals based on a clustering algorithm.

It is difficult to obtain a core sample and information for digital core reconstruction of mature sandstone reservoirs around the world, especially for an unconsolidated sandstone reservoir. Meanwhile, reconstruction and division of clay minerals play a vital role in the reconstruction of the digital cores, although the two-dimensional data-based reconstruction methods are specifically applicable as the microstructure reservoir simulation methods for the sandstone reservoir. However, reconstruction of clay minerals is still challenging from a research viewpoint for the better reconstruction of various clay minerals in the digital cores. In the present work, the content of clay minerals was considered on the basis of two-dimensional information about the reservoir. After application of the hybrid method, and compared with the model reconstructed by the process-based method, the digital core containing clay clusters without the labels of the clusters' number, size, and texture were the output. The statistics and geometry of the reconstruction model were similar to the reference model. In addition, the Hoshen-Kopelman algorithm was used to label various connected unclassified clay clusters in the initial model and then the number and size of clay clusters were recorded. At the same time, the K-means clustering algorithm was applied to divide the labeled, large connecting clusters into smaller clusters on the basis of difference in the clusters' characteristics. According to the clay minerals' characteristics, such as types, textures, and distributions, the digital core containing clay minerals was reconstructed by means of the clustering algorithm and the clay clusters' structure judgment. The distributions and textures of the clay minerals of the digital core were reasonable. The clustering algorithm improved the digital core reconstruction and provided an alternative method for the simulation of different clay minerals in the digital cores.