Multistage Hydraulic Fracturing Research Articles

Design and Analysis of Improved Swelling and Degradable Diverting Agent for Multistage Hydraulic Fracturing

Summary Fracturing fluid diversion plays a crucial role in maximizing the well productivity of multistage fractured wells. Proper sizing and material design of diverting agents are key elements to effectively bridge and plug perforations and fractures during treatment. In this study, we present an improved design for a water-soluble diverting agent that can cover a range of fracture widths. In addition, a wellbore flow model that predicts the swelling and dissolution behaviors of diverting agents flowing from the surface to the fractures was developed for field applications. Butenediol vinyl alcohol copolymer (BVOH), which has elastic and sticking properties in water, is used as a diverting agent. Cylindrical pellets and smaller sized powder, made from the polymer, were mixed to bridge and plug hydraulic fractures. BVOH diverting agents were evaluated for slit widths of 1–4 mm with different pellet geometries using a high-pressure and high-temperature filtration apparatus. The swelling and dissolution rates depend on many parameters such as the temperature, dissolution time, crystallinity degree, and geometry. In this study, empirical correlations that predict the swelling and dissolution rates of BVOH polymers for various formulations were developed and implemented in a wellbore flow model that simulates fluid flow and heat transfer during pumping operations. A theoretical case study of a multistage hydraulic fracturing treatment was also presented to demonstrate the applicability and effectiveness of the treatment. The filtration test results with various diverting agent designs indicate that the length and diameter of the pellets affect the performance and effectiveness of the bridging and plugging fracture-like slits. Moreover, an optimum pellet size exists for different slit sizes. With modified pellet size and diameter ratios, wider slits of 3–4 mm can be effectively plugged by diverting agents with reduced leakoff volumes. The swelling and dissolution models correlated well with the experimental data, considering the temperature, dissolution time, and crystallinity degree. The case studies presented in this study illustrate that the models can predict the time required for the diverting agents to dissolve under field conditions and determine whether the diverting agents pumped into the well provide sufficient conditions for diversion. Furthermore, the study results indicate that the pump rate and injection conditions before pumping the diverting agent are key controlling factors in determining the dynamic downhole temperatures and thus affect the time required for the degradation of the diverting agent. In addition, a field trial result is presented to demonstrate the effectiveness of the swellable, BVOH diverting agent at low downhole temperatures, and hydraulic fracturing treatment in the Permian Basin. Swelling diverting agents exhibit a more elastic behavior than existing particulate diverting agents. Swelling polymers are less abrasive and thus reduce the risk of equipment damage during preparation and pumping. The wellbore flow simulator developed in this study helps stimulation engineers optimize material types, particle-size distribution, and concentration of diverting agents for various field applications.

A Numerical Multistage Fractured Horizontal Well Model Concerning Hilly-Terrain Well Trajectory in Shale Reservoirs with Natural Fractures

Multistage hydraulic fracturing is one of the most prevalent approaches for shale reservoir development. Due to the complexity of constructing reservoir environments for experiments, numerical simulation is a vital method to study flow behavior under reservoir conditions. In this paper, we propose a numerical model that considers a multistage fractured horizontal well with a hilly-terrain trajectory in a shale reservoir with the presence of natural fractures. The model was constructed based on the MATLAB Reservoir Simulation Toolbox and used the Embedded Discrete Fractured Model (EDFM) to describe the interrelationship between the matrix, fractures, and wellbore. The model was then applied to an actual condensate gas well producing from a shale reservoir, and the effects of reservoir parameters on the simulation data were studied based on this well case. The simulation results were highly consistent with the actual production data, which validates the accuracy of this model and proves its potential for predicting future production trends. We extended the discussion to two examples with extreme well trajectories by reviewing the inflow contribution of each hydraulic fracture with respect to fracture pressure, and the changes in static pressure with time. In conclusion, the proposed model is capable of providing simulation results close to reality and thus guiding field design and operation in the fracturing and drilling process.

Performance of CO2 Foam Huff and Puff in Tight Oil Reservoirs

The challenges associated with unconventional reservoirs are related to their intrinsic nature: extremely low porosity and permeability. Combinations of horizontal wells and multistage hydraulic fracturing techniques have been developed to overcome the production obstacles and unlock the vast amount of oil in place in such formations. However, oil production still exhibits a sharp decline within the first 2 years after the stimulation, leading to an oil recovery of less than 15%. Thus, enhanced oil recovery methods need to be investigated to further increase the production rates and the recovery. In this study, laboratory experiments and numerical simulations were conducted to evaluate the performance of the CO2 foam huff and puff process and its impacts on oil recovery in tight oil formations. More specifically, the foam half-life was measured as a function of surfactant concentration and followed by the foam drainage ratio and its rheological properties in the subsequent tests. Reservoir simulations were conducted using the lab data and the field data collected from Cardium formation. Sensitivity analyses were finally carried out to investigate the effects of controlling variables on the CO2 foam performance. Experimental results revealed that the optimal surfactant concentration was found to be 0.2%, which is the critical micelle concentration point. Simulation results show that CO2 foam huff and puff can increase the oil recovery by more than 11% compared to that of the primary production. Moreover, sensitivity analyses show that the production time, injection time, and soaking time are the main effecting parameters, while the injection rate and the incremental injection rate are less important.

Effective Stress Factor Analysis of Proppant for Multi-stage Fracturing in Horizontal Wells

In order to determine the effective stress factor of proppants during the multi-stage hydraulic fracturing operation in horizontal wells and select the appropriate proppant type, the analysis of the effective stress characteristics of proppants is carried out. This analysis considers the geomechanics and long-term production characteristics of multi-stage fractured horizontal wells in reservoirs like Xinjiang Mahu shale oil and Southwest China shale gas. The analysis of influencing factors is also carried out. The results show that, during the multi-stage hydraulic fracturing operation in horizontal wells, the stress on proppants is not only related to geological factors such as reservoir closure stress, but also closely related to total injection volume, cluster spacing, liquid type, injection displacement and post fracture management. First, increasing injection intensity, reducing fracture spacing, using low viscosity fracturing fluid, adopting high injection displacement and utilizing reasonable flowback system can effectively supplement reservoir energy, postpone the effective stress peak of proppants, and increase the effective conductivity of proppants. Second, the production performance analysis results of nearly 300 horizontal wells (from Xinjiang oil field, Erdos tight oil reservoir, Sichuan shale gas reservoir, etc.) shows that: the effective stress of proppants in horizontal wells is only 50–60% of that in vertical wells, which results in a different proppant selection criterion in the volume stimulation of horizontal wells and provides the geomechanics basis of replacing ceramsite proppant with quartz sand to reduce cost and increase efficiency. Based on the above conclusions, the field test of replacing ceramsite proppant with quartz sand was carried out. The proportion of quartz sand increased from less than 30% in 2014 to 69% in 2019. Without any impact on production, the annual investment cost was decreased by more than 1 billion yuan, which set a great example for the promoted low-cost development of unconventional oil and gas reservoirs under low oil price background.

Completion Designs in Horizontal Well with Transverse Hydraulic Fractures for Enhanced Recovery in Unconventional or Tight Reservoirs

Summary Low recovery, 2 to 15%, in unconventional plays (including tight reservoirs and source rocks) has long been recognized as a business deterrent. The industry applies enhanced oil recovery (EOR) techniques, along with hydraulic fractures in tight/unconventional plays, to improve the recovery. To maximize matrix sweep, the fractures are aligned in a face-to-face assembly. Such an arrangement can be achieved using a vertical or longitudinal hydraulic fracture on horizontal wells, but these, generally, do not provide as effective reservoir contact (hydraulic fracture surface area) as horizontal wells with multistage transverse hydraulic fractures. The multistage transverse hydraulic fracture, however, comes at the costs of conformance issues with early water breakthrough from short-circuiting and inability to achieve fracture face-to-fracture face alignment of the injection and production fractures. The vast majority of wells drilled in unconventional plays are in the transverse configuration; hence, there is a need for an optimal solution for transverse fractures combined with improved oil recovery (IOR)/EOR approaches. In this work, we introduce the multistage enhanced recovery (MS-ER) techniques that enable face-to-face alignment for optimal enhanced hydrocarbon recovery (EHR)/IOR/EOR in horizontal wells with multistage transverse fractures, thereby enabling optimal recovery and mitigating the key risk of fracture short-circuiting.

Impacts of Proppant Flowback on Fracture Conductivity in Different Fracturing Fluids and Flowback Conditions.

Multistage hydraulic fracturing is used in horizontal wells to increase the production of tight oil. Fracturing fluids are used in hydraulic fracturing to ensure proppants are suspended, but fluid residuals can cause formation damage and reduce rock permeability; meanwhile, fracture conductivity can be further reduced due to the flowback of proppants during the early stage of production. In this study, steel plates and hydraulically fractured reservoir rocks are tested in a modified API cell to understand the impacts of flowback rate, fracturing fluid, and closure stress on proppant flowback and fracture conductivity. When the closure stress increased from 21 to 30 MPa, retained permeability decreased by slickwater from 35.71 to 29.84% in steel plates; during the flowback, more than 47% of proppants flowed back, and the fracture conductivity increased by 10 times under 21 MPa, which shows the limitation of the API method on the study of proppant flowback. When shale plates are used, the critical flow rate that prevents the proppant flowback was found to be 5.5 × 10–4–1.6 × 10–3 m/s for the 30/50 mesh sands (around 55–340 m3/d for a typical horizontal well), and the retained permeability decreased from 23.33 to 22.86% due to an increase of closure stress from 21 to 30 MPa. Results of this study can guide the optimizing of the flowback scheme in the field that minimizes the proppant flowback in different fracturing fluids.

Predicting variations of the least principal stress with depth: Application to unconventional oil and gas reservoirs using a log-based viscoelastic stress relaxation model

Knowledge of layer-to-layer variations of the least principal stress, S hmin, with depth is essential for optimization of multi-stage hydraulic fracturing in unconventional reservoirs. Utilizing a geomechanical model based on viscoelastic stress relaxation in relatively clay rich rocks, we present a new method for predicting continuous S hmin variations with depth. The method utilizes geophysical log data and S hmin measurements from routine diagnostic fracture injection tests (DFITs) at several depths for calibration. We consider a case study in the Wolfcamp formation in the Midland Basin, where both geophysical logs and values of S hmin from DFITs are available. We compute a continuous stress profile as a function of the well logs that fits all of the DFITs well. We utilized several machine learning technologies, such as bootstrap aggregation (or bagging), to improve the generalization of the model and demonstrate that the excellent fit between predicted and observed stress values is not the result of over-fitting the calibration points. The model is then validated by accurately predicting hold-out stress measurements from four wells within the study area and, without recalibration, accurately predicting stress as a function of depth in an offset pad about 6 miles away.

Особенности проведения гидроразрыва пласта в отложениях с трудноизвлекаемыми запасами

Introduction. When developing productive deposits containing hard-to-recover oil reserves, the main difficulty is associated with low values of the reservoir properties of the reservoirs and their significant heterogeneity. The use of hydraulic fracturing technology allows solving these problems by increasing the well drainage area, and also as a means of intensifying production. Materials and methods of research. The study of the process of hydraulic fracture development in the deposits of the Bazhenov formation in a vertical well showed that when water is injected, long narrow fractures are formed in the formation, their filling with subsequent supply of a fracturing fluid with proppant leads to an increase in the width and height of the created fracture. Due to the hydrophobic properties and increased brittleness of the rocks of the Bazhenov formation, an effect similar to hydraulic fracturing is also possible during the injection of water into the formation at high pressures, for example, when organizing a waterflooding system. Another direction is multi-zone hydraulic fracturing. The advantage of this method is the greater distribution of fractures over the area compared to point hydraulic fracturing operations. Results and Discussion. On the deposits of the Bazhenov formation, pilot work was carried out to create artificial fracturing by pumping water. The positive effect was expressed in a twofold increase in oil recovery in the pilot area. As a negative side effect, a sharp increase in water cut was noted. The experience of using multi-zone hydraulic fracturing in the conditions of the Bazhenov formation was considered on the example of the deposits of the Surgut arch. The dynamics of flow rates and cumulative oil production by wells during operation is presented. Also, a comparison was made of the main performance indicators of wells with multi-stage hydraulic fracturing with the average indicators of directional wells of the same fields. Conclusion. The pilot work conducted on water injection into the Bazhenov formation deposits unambiguously showed that additional fracturing is formed in the reservoir and the volume of additional fractured capacity is directly proportional to the volume of injected water. The experience of using multi-zone hydraulic fracturing revealed the advantages of this technology compared to directional wells. Under conditions of reservoir reservoir properties uncertainty, the creation of artificial conductivity using hydraulic fracturing may be the only way to ensure well productivity. MSHF in horizontal wells forms a system of fractures in a large volume and seems to be more effective in such conditions compared to conventional hydraulic fracturing.

Downhole Microseismic Monitoring Using FOSS and Its Field Test Comparison With Moving-Coil Geophone

We report downhole microseismic monitoring field test results of a fiber-optic-based seismic sensor (FOSS) array in a multistage hydraulic fracturing stimulation and present, for the first time, its systematic comparison with the conventional moving-coil geophone array deployed on-site. Perforation shots' analysis demonstrates that the FOSS has ~7.5 dB higher narrowband signal-to-noise ratio and, thus, 2.3 times smaller azimuth calibration error than the commercial moving-coil geophone. These benefits are mainly attributed to the intrinsic immunity to electromagnetic interference and a higher frequency resonance of FOSS. Field test results show that the FOSS can identify P- and S-waves of microseismic events, the temporal and spatial distributions of which are consistent with the geophone. In addition, the FOSS is found preferable to detect microseismic events with higher frequencies from some hundreds of Hz-to-kHz range. This superior ability contributes to distinct signatures in the identified P- and S-waves, including a shorter event duration and more concentrated frequency band, comparing to those collected by the geophone. The fracture interpretation results validate the application of fiber-optic seismic sensors on downhole microseismic monitoring.

Results of the development system optimization and increasing the efficiency of carbonate reserves extraction of the turney stage of the Chetyrmansky deposit

The article is devoted to the issue of optimizing the development system and increasing the efficiency of carbonate deposits of the Tournaisian stage of the Chetyrmanskoye field developing, and the formation of a strategy for their additional development. As a result of the horizontal drilling, the rate of withdrawal from current recoverable reserves in the main area in terms of reserves increased from 0.3 to 5%, which confirms the high efficiency of horizontal wells drilling with multi-stage hydraulic fracturing in reservoirs with high stratification and heterogeneity degree of the productive section in order to increase the rate of reserves production and achieve the approved oil recovery factor, as well as the high efficiency of the proposed methodological approach in the design of the facility development by a system of horizontal wells, the correct choice of the facility development strategy in the design solutions formation. Keywords: oil fields development; carbonate deposits; development of reserves; multi-stage hydraulic fracturing; horizontal well.

Numerical Simulation Research on Influencing Factors of Post-Fracturing Flowback of Shale Gas Wells in the Sichuan Basin

The horizontal well multistage hydraulic fracturing technology is the most effective way to exploit shale gas resources. Compared with conventional reservoir fracturing, the flowback rate of a fracturing fluid in a shale reservoir is extremely low, and a large amount of fracturing fluid remains in the formation. Therefore, the research on the mechanism of shale reservoir fracturing fluid flowback process will contribute to laying a theoretical foundation for improving the effect of the innovation for increasing output of shale gas wells. Based on the shale in the Sichuan Basin, this study first describes basic experiments on physical properties such as the porosity, permeability, mineral composition, wettability, and microstructure. The physical properties of shale reservoirs were also analyzed, which laid the foundation for subsequent modeling. Second, CMG software is used to establish a numerical model that fits the characteristics of the flowback process. The effect of reservoir properties, fracturing parameters, drainage–production system, chemical permeability on gas and water production in the flowback process and their mechanisms are also analyzed. According to most numerical simulation results, the lower cumulative gas production will be with the higher cumulative water production which means the higher flowback rate. The pursuit of only a high flowback rate is not advisable, and the development of the drainage–production system requires reasonable control of the fracturing fluid flowback rate. This study provides a theoretical basis for the optimization of shale gas drainage–production system after hydraulic fracturing.

Numerical modeling of fluid flow in tight oil reservoirs considering complex fracturing networks and Pre-Darcy flow

Multistage hydraulic fracturing for horizontal wells is extensively applied in the exploitation of tight oil reservoirs. Complex fracture networks with different scales are created and hydrocarbon flowing in the matrix demonstrates a Pre-Darcy characteristic due to a strong interaction of rock and fluids in nanoscale pores. The modeling of complex fracture networks and a Pre-Darcy effect is required in numerical simulation of tight oil reservoirs. In this paper, stochastic fracture networks are employed to mimic actual fracturing networks according to the corresponding probability density functions (PDFs), and microseismic events are used to constrain the locations of fractures. The numerical simulation for tight oil reservoirs is developed by considering complex fracture networks and the Pre-Darcy effect based on an embedded discrete fracture model (EDFM) framework. Finally, the influences of fracture network properties including isolated fractures, connected fracture networks and the Pre-Darcy effect on well production are investigated via integrating the developed fracture network generation and tight oil numerical simulator. The results indicate that for isolated fractures with no direct connection with a wellbore, there is little influence on well performance. The influence becomes more obvious when isolated fractures connect to a wellbore, and an increase in fracture length and conductivity can enhance well performance especially when fractures lie off streamline lines. Similarly, for a connected fracture network, an increase in fracture network properties (number, length and conductivity) can promote well performance and fractures perpendicular to a horizontal wellbore can generate a maximal flow rate compared with other fracture orientations. The fracture network connectivity plays an important role in the fluid flow, and there are good relationships between well production and two proposed parameters, which can be treated as candidates to characterize the flow characteristics in fractured reservoirs.

Supercritical CO2-Shale interaction induced natural fracture closure: Implications for scCO2 hydraulic fracturing in shales

Multi-stage hydraulic fracturing has been identified as a must to develop shale gas reservoirs by increasing the stimulated reservoir volume (SRV). Supercritical CO2 (scCO2) has been studied as an alternating fracturing fluid due to its tendency to solve numerous problems associated with conventional aqueous based hydraulic fracturing such as formation damage, clay swelling, water scarcity and ground water contamination. However, its consequences to the host rock are not well understood. It has been recognized that scCO2-shale interaction alters the petrophysical properties during the long-term exposure of shale into scCO2, far little attention has been paid to understand the impact of this process for the short term. Thus, laboratory fracturing experiments using scCO2 on cubic shale samples (50 × 50 × 50 mm) in true triaxial stress cell (TTSC) were conducted. X-ray computed tomography (CT) imaging and low-pressure N2 adsorption were also performed to gain a deeper understanding of the fluid-rock interactions on the studied shales at a short-time process. Post-fracturing x-ray CT scans revealed a significant reduction, in the range of 14% to 46%, in the aperture of the natural fractures, indicating towards a possible scCO2 induced swelling. Mechanical compression test on the sample results in around 12% reduction in the fracture aperture, ruling out the possibility of confining stress being the key factor behind the fracture closure observed during fracturing. scCO2 soaking and N2 adsorption experiments showed the narrowing down of the macropores after scCO2 treatment implying the adsorption swelling as one of the controlling factors for the reduction of fracture aperture. Taken together, our results suggest that scCO2-shale interactions during the short term process of hydraulic fracturing can contribute to decreasing the conductivity of pathways between matrix and hydraulic fractures and hence adversely affecting the post-fracturing productivity of the rock.

The efficiency of gas injection into low-permeability multilayer hydrocarbon reservoirs

The efficiency of gas injection for developing terrigenous deposits within a multilayer producing object is investigated in this article. According to the results of measurements of the 3D hydrodynamic compositional model, an assessment of the oil recovery factor was made. In the studied conditions, re-injection of the associated gas was found to be the most technologically efficient working agent. The factors contributing to the inefficacy of traditional methods of stimulating oil production such as multistage hydraulic fracturing when used to develop low-permeability reservoirs have been analyzed. The factors contributing to the inefficiency of traditional oil-production stimulation methods, such as multistage hydraulic fracturing, have been analysed when they are applied to low-permeability reservoirs. The use of a gas of various compositions is found to be more effective as a working agent for reservoirs with permeability less than 0.005 µm2. Ultimately, the selection of an agent for injection into the reservoir should be driven by the criteria that allow assessing the applicability of the method under specific geological and physical conditions. In multilayer production objects, gas injection efficiency is influenced by a number of factors, in addition to displacement, including the ratio of gas volumes, the degree to which pressure is maintained in each reservoir, as well as how the well is operated. With the increase in production rate from 60 to 90 m3 / day during the re-injection of produced hydrocarbon gas, this study found that the oil recovery factor increased from 0.190 to 0.229. The further increase in flow rate to 150 m3 / day, however, led to a faster gas breakthrough, a decrease in the amount of oil produced, and a decrease in the oil recovery factor to 0.19 Based on the results of the research, methods for stimulating the formation of low-permeability reservoirs were ranked based on their efficacy.

Production Decline Analysis and Hydraulic Fracture Network Interpretation Method for Shale Gas with Consideration of Fracturing Fluid Flowback Data

After multistage hydraulic fracturing of shale gas reservoir, a complex fracture network is formed near the horizontal wellbore. In postfracturing flowback and early-time production period, gas and water two-phase flow usually occurs in the hydraulic fracture due to the retention of a large amount of fracturing fluid in the fracture. In order to accurately interpret the key parameters of hydraulic fracture network, it is necessary to establish a production decline analysis method considering fracturing fluid flowback in shale gas reservoirs. On this basis, an uncertain fracture network model was established by integrating geological, fracturing treatment, flowback, and early-time production data. By identifying typical flow-regimes and correcting the fracture network model with history matching, a set of production decline analysis and fracture network interpretation method with consideration of fracturing fluid flowback in shale gas reservoir was formed. Derived from the case analysis of a typical fractured horizontal well in shale gas reservoirs, the interpretation results show that the total length of hydraulic fractures is 4887.6 m, the average half-length of hydraulic fracture in each stage is 93.4 m, the average fracture conductivity is 69.7 mD·m, the stimulated reservoir volume (SRV) is418×104 m3, and the permeability of SRV is5.2×10−4 mD. Compared with the interpretation results from microseismic monitoring data, the effective hydraulic fracture length obtained by integrated fracture network interpretation method proposed in this paper is 59% of that obtained from the microseismic monitoring data, and the effective SRV is 83% of that from the microseismic monitoring data. The results show that the fracture length is smaller and the fracture conductivity is larger without considering the influence of fracturing fluid.

Transient Pressure Behavior of Complex Fracture Networks in Unconventional Reservoirs

Unconventional resources have been successfully exploited with technological advancements in horizontal-drilling and multistage hydraulic-fracturing, especially in North America. Due to preexisting natural fractures and the presence of stress isotropy, several complex fracture networks can be generated during fracturing operations in unconventional reservoirs. Using the DVS method, a semianalytical model was created to analyze the transient pressure behavior of a complex fracture network in which hydraulic and natural fractures interconnect with inclined angles. In this model, the complex fracture network can be divided into a proper number of segments. With this approach, we are able to focus on a detailed description of the network properties, such as the complex geometry and varying conductivity of the fracture. The accuracy of the new model was demonstrated by ECLIPSE. Using this method, we defined six flow patterns: linear flow, fracture interference flow, transitional flow, biradial flow, pseudoradial flow, and boundary response flow. A sensitivity analysis was conducted to analyze each of these flow regimes. This work provides a useful tool for reservoir engineers for fracture designing as well as estimating the performance of a complex fracture network.

Advancement of Hydraulic Fracture Diagnostics in Unconventional Formations

Multistage hydraulic fracturing is a technique to extract hydrocarbon from tight and unconventional reservoirs. Although big advancements occurred in this field, understanding of the created fractures location, size, complexity, and proppant distribution is in its infancy. This study provides the recent advances in the methods and techniques used to diagnose hydraulic fractures in unconventional formations. These techniques include tracer flowback analysis, fiber optics such as distributed temperature sensing (DTS) and distributed acoustic sensing (DAS), tiltmeters, microseismic monitoring, and diagnostic fracture injection tests (DFIT). These techniques are used to estimate the fracture length, height, width, complexity, azimuth, cluster efficiency, fracture spacing between laterals, and proppant distribution. Each technique has its advantages and limitations, while integrating more than one technique in fracture diagnostics might result in synergies, leading to a more informative fracture description. DFIT analysis is critical and subjected to the interpreter’s understanding of the process and the formation properties. Hence, the applications of machine learning in fracture diagnostics and DFIT analysis were discussed. The current study presents an extensive review and comparison between different multistage fracture diagnostic methods, and their applicability is provided. The advantages and the limitations of each technique were highlighted, and the possible areas of future research were suggested.

A comprehensive review of recent advances on surfactant architectures and their applications for unconventional reservoirs

The depletion of conventional resources and steady increase in the global oil demands has led to the shift of attention to the unconventional resources which are difficult to extract due to poor permeability, low porosity and extreme heterogeneity of their reservoirs. For the unconventional reservoirs to produce at an economic rate, the invention of stable oilfield chemicals tailored for drilling, stimulation by multi-stage hydraulic fracturing, and enhanced oil and gas recovery activities should be scaled up. The extreme reservoir conditions of high temperature, complex heterogeneity, salinity, and high-water saturation pose a challenge to the use of currently available chemicals. In this review work, the latest surfactant formulations with enhanced properties are presented. A detailed review of surfactant's application on unconventional resources are included. A comprehensive description of various surfactants architect, applications, and their corresponding influence on the interfacial tension reduction (IFT), wettability alteration, emulsion and microemulsion phase behaviors, surfactant adsorption are also included. By using several surfactants, laboratory experimental results and field study data obtained, useful conclusions have been drawn. The correlation between the experimental data and the isotherm models suggested that the Freundlich model could certainly predict the loss of surfactants due to high adsorption behavior. This revealed that the adsorption of surfactants occurs on the heterogenous surfaces and showed multi-layered behavior. On the other hand, the kinetic behavoir of the surfactant adsorption exhibited pseudo-second order kinetic model. Their conclusions on Freundlich and pseudo-second order models were supported by the correlation values (R2) which were 0.9815and 0.98583 respectively. The detailed discussion on this work is intended for researchers and professionals in the relevant field to gain insights on the existing progress in surfactants application of unconventional resources and pave the future path of current methodologies. The outcomes of this study will help the oil industry practioners to expand their understanding of surfactant selection based on the structures, adsorption behavior and various reservoir conditions and optimize the oil recovery during surfactant flooding.

Comparative study on heat extraction performance of geothermal reservoirs with presupposed shapes and permeability heterogeneity in the stimulated reservoir volume

The stimulated reservoir volume (SRV) is critical to the heat extraction from an enhanced geothermal system (EGS). Being different from previous simple cube-shape SRV, this study proposes a new shape SRV with heterogeneous permeability to evaluate the heat extraction performance of an EGS. First, the new shape SRV is generated based on the location and density of microseismic events during multi-stage hydraulic fracturing. Other two SRVs with simple cube or complex shape but same volume are generated for comparison. Second, a fractal permeability is developed for the heterogeneous SRV domain. Third, a thermal-hydraulic (TH) coupling model is implemented for the heat transfer and water flow in these three SRVs. Finally, the EGS performances of these three SRVs with a stimulated volume of 0.405 km 3 are comparatively evaluated through numerical simulations. Numerical simulations reveal that the EGS with cube-shape SRV overestimates both output temperature and energy extraction. The heterogeneous permeability in the SRV domain leads to a dramatic increase of reservoir impedance. This reduces energy extraction efficiency and breakthrough time, thus shortening the lifetime of the EGS system. • A new shape SRV with heterogeneous permeability in HDR reservoir is proposed. • An evaluation system for EGS is established based on a TH coupling model. • EGS performance of the three presupposed SRVs is comparatively analyzed. • The feasibility of a megawatt EGS power plant in Gonghe Basin is discussed.

Effect of Silica Nanoparticles on Polymer Adsorption Reduction on Marcellus Shale.

Polymers play a major role in developing rheology of fracturing fluids for multistage hydraulic fracturing horizontal wells in unconventional reservoirs. Reducing the amount of polymer adsorbed in the shale formation is essential to maintain the polymer efficiency. In this study, the ability of silica nanoparticles to minimize polymer adsorption in Marcellus shale formation at reservoir temperature was investigated. Partially hydrolyzed polyacrylamide polymers of varying molecular weights (1–12 MD), salinities (2500–50,000 ppm), polymer concentrations (100–2000 ppm), and silica nanoparticle concentrations (0.01–0.1 w/w) were used in the static adsorption experiments. Adsorption of the polymer in the Marcellus shale samples was contrasted with and without the silica nanoparticles at a Marcellus formation reservoir temperature of 65 °C, showing a significant polymer adsorption reduction of up to 50%. The adsorption and adsorption reduction were more sensitive to the variation of the polymer concentration than to the variation of the salinity within the tested conditions. The highest adsorptions were reported at the higher molecular weight of 10–12 MD. In addition, silica nanoparticles significantly improved polymer rheology at elevated temperatures. The results indicate that nanoparticles can play a significant role in reducing polymer adsorption in the fracturing fluid and improve its rheological properties and its efficiency, which will reduce the number of issues caused by the polymers in the fracturing fluid and making it more cost effective.