Hydraulic Fracturing Research Articles

Coupled THMC model-based prediction of hydraulic fracture geometry and size under self-propping phase-transition fracturing

The Self-Propping Phase-transition Fracturing Technology (SPFT) represents a novel and environmentally friendly approach for a cost-effective and efficient development of the world’s abundant unconventional resources, especially in the context of a carbon-constrained sustainable future. SPFT involves the coupling of Thermal, Hydraulic, Mechanical, and Chemical (THMC) fields, which makes it challenging to understand the mechanism and path of hydraulic fracture propagation. This study addresses these challenges by developing a set of THMC multifield coupling models based on SPFT parameters and the physical/chemical characteristics of the Phase-transition Fracturing Fluid System (PFFS). An algorithm, integrating the Finite Element Method, Discretized Virtual Internal Bonds, and Element Partition Method (FEM-DVIB-EPM), is proposed and validated through a case study. The results demonstrate that the FEM-DVIB-EPM coupling algorithm reduces complexity and enhances solving efficiency. The length of the hydraulic fracture increases with the quantity and displacement of PFFS, and excessive displacement may result in uncontrolled fracture height. Within the parameters considered, a minimal difference in fracture length is observed when the PFFS amount exceeds 130 m3, that means the fracture length tends to stabilize. This study contributes to understanding the hydraulic fracture propagation mechanism induced by SPFT, offering insights for optimizing hydraulic fracturing technology and treatment parameters.

Experimental study on the influence of horizontal stress difference coefficient on coal rock fracture

Hydraulic fracturing is a pressure-relief and permeability-enhancing technique widely used in the mining of low-permeability coal seams. The horizontal stress difference coefficient is an important factor that affects the fracture propagation during the fracturing. To study the mechanism of the hydraulic fracturing process, in this paper, the initiation and propagation of cracks under horizontal stress difference coefficient are studied by using the developed true triaxial hydraulic fracturing system. The fracture surface morphology was obtained by 3D scanner. The experimental results are verified by the mathematical model. The results show that the fracture pressure and the peak values of AE count rate and b value increase and the cumulative AE count decreases with the increase of the horizontal stress difference coefficient. After the first pressure drop, the “pressure built-up” in the specimen increases and the peak value of AE count rate decreases with the increased horizontal stress difference coefficient. The 3D scanning results show that when the horizontal stress difference coefficient is larger, the hydraulic fracture propagation is straighter and the fracture surface area is smaller. The experimental results are in good agreement with the theoretical values. The experimental results provide a basis for engineering practice.

Numerical Simulations of the Hydraulic Fracture Propagation in Poroelastic Media Using the Coupled Hydro-Mechanical Field-Enriched Finite Element Method

Numerical Simulations of the Hydraulic Fracture Propagation in Poroelastic Media Using the Coupled Hydro-Mechanical Field-Enriched Finite Element Method

Source apportionment of airborne volatile organic compounds near unconventional oil and gas development

Abstract Oil and natural gas (ONG) extraction activities have been associated with the release of volatile organic compounds (VOCs), including hazardous air pollutants (HAPs) that are known to have adverse health effects and other VOCs that contribute to ozone formation. This study investigated the impact of ONG operations on HAP and other VOC concentrations during the development of multi-well ONG pads in suburban Broomfield, Colorado. Weekly VOC measurements were collected at 18 sites across the community, including locations near the well pads, in adjacent neighborhoods, and at a regional background site. The measurements, from October 2018 to December 2020, encompassed different stages of well pad development, including well drilling, hydraulic fracturing, flowback, and production. Measured concentrations were analyzed using Positive Matrix Factorization (PMF) to characterize VOC sources. Six PMF source factors were identified, including factors associated with combustion, background/biogenic sources, light alkanes, complex alkanes, drilling activities, and ONG acetylene. The factors related to local ONG activities showed clear temporal and spatial correlations with Broomfield well development activities. A benzene source apportionment analysis revealed important source contribution gradients across the community, with ONG-related sources exhibiting the largest influence at locations near the well pads, especially during pre-production activities. Weekly average benzene concentration contributions from ONG-related sources ranged from 9% to 63% at a community background site and from 18% to 89% at a neighborhood site close to a well pad.

The fracture propagation law of axially symmetric intersecting precut hydraulic fracturing in mine rock-breaking

As a mine rock-breaking technique, hydraulic fracturing technology can reduce the amount of explosives used, which enhance safety and reduce environmental pollution in mines. After precutting along the borehole axis, hydraulic fractures will expand along the precutting direction within a certain range and reduce initiation pressure. These hydraulic fractures cut through the rock mass, reducing its integrity and weakening its mechanical properties. Hydraulic fracturing with axially symmetric intersecting precut fractures not only controls the multi-directional propagation of fractures but also increases the fractures within rock mass. The lattice method simulated the hydraulic fracturing process, focusing on the parameters like angles between precut fractures and the minimum horizontal principal stress, the maximum horizontal principal stress, and angles between intersecting precut fractures. Results indicate that the hydraulic fractures propagate along intersecting precut fractures, forming main and interconnected secondary fractures. The directional cutting effect is influenced by the number of secondary fractures. With the increase in the angle between precut fractures and the minimum horizontal principal stress, the maximum horizontal principal stress, the angle between precut fractures, the area of secondary fractures decreased, and the expansion extent of main fractures along the precut fractures increased, indicating better directional effects. The study identifies relationships between initiation time, initiation pressure, and parameters. These findings provide valuable technical guidance for designing on-site construction plans for hydraulic fracturing projects involving intersecting precut fractures.

Experimental Study on Plugging Behavior of Multitype Temporary Plugging Agents in Hydraulic Fractures

Summary Various types of temporary plugging agents are used in hydraulic fracturing to promote the uniform propagation of multicluster hydraulic fractures and increase the complexity of hydraulic fractures. However, the plugging behavior of these agents in hydraulic fractures has not yet been fully clarified, making the optimization of temporary plugging formulas challenging. In this study, dozens of plugging experiments were carried out to reveal the plugging behavior of pure fiber, pure particle, and fiber-particle combination materials in hydraulic fractures. The results indicate that the high fiber concentration and long fibers are beneficial to obtaining high maximum plugging pressures. However, the low tensile strength of fibers makes it difficult to form stable plugging layers under high pressure, especially for wide fractures. For particle plugging agents, the high rigidity of the particles prevents them from compacting tightly within the plugging layer, resulting in high permeability and low temporary plugging pressure. Excessive particle diameter and concentration tend to cause rapid blockage at the fracture entrance, leading to poor plugging performance. In contrast, the fiber-particle composite plugging scheme can form a stable and tight plugging layer at lower concentrations of both fibers and particles. Moreover, replacing single-size particles in composite with multisize particles can further enhance the plugging effect, allowing for a higher plugging pressure with a lower dosage of temporary plugging agents. Comprehensively considering the effects of material concentration and size on the plugging effect, the critical plugging quantitative characterization equations for pure fiber, pure particle, and fiber-particle combination plugging schemes are established respectively, with fracture width as the independent variable and the product of material concentration and size as the dependent variable. The temporary plugging schemes for various hydraulic fracture widths can be preliminarily determined using these equations. Based on the principle of economic optimization, the optimal temporary plugging schemes with consideration of the plugging pressure requirements were selected, which have shown good field application performance.

A National Study of the Associations between Hormonal Modulators in Hydraulic Fracturing Fluid Chemicals and Birth Outcomes in the United States of America: A County-Level Analysis.

Risk of preterm birth (PTB) and low birth weight (LBW) due to hydraulic fracturing (HF) exposure is a growing concern. Regional studies have demonstrated links, but results are often contradictory among studies. This is the first US national study to our knowledge linking fracturing fluid ingredients to the human hormone pathways targeted-estrogen, testosterone, or other hormones (e.g., thyroid hormone)-to assess the effect of HF ingredients on rates of PTB and LBW. We constructed generalized linear regression models of the impact of HF well density and hormone targeting chemicals in HF fluids (2001-2018) on the county-level average period prevalence rates of PTB and LBW (2015-2018) with each outcome measured in separate models. Our data sources consisted of publicly available datasets, including the WellExplorer database, which uses data from FracFocus, the March of Dimes Peristats, the US Census Bureau, the US Department of Agriculture, and the Centers for Disease Control and Prevention. We conducted additional stratified analyses to address issues of confounding. We used stratification to address issues regarding outcomes in rural vs. urban communities by assessing whether our models achieved similar results in nonmetro counties, as well as farming and mining counties. We also stratified by the year of the HF data to include HF data that was closer to the time of the birth outcomes. We also added covariate adjustment to address other important factors linked to adverse birth outcomes, including the proportion of the population belonging to various racial and ethnic minority populations (each modeled as a separate variable); education (bachelor's degree and high school); use of fertilizers, herbicides, and insecticides, acres of agricultural land per square mile; poverty; insurance status; marital status; population per square mile; maternal care deserts; and drug deaths per 100,000 people. We found that the density of HF wells in a county was significantly associated with both PTB and LBW rates (percentage of live births) in our fully adjusted models. We report the results from our more restrictive stratified analysis with a subset including only the 2014-2018 data, because this resulted in the most meaningful time frame for comparison. Across all models, the magnitude of effect was highest for wells with ingredients that include estrogen targeting chemicals (ETCs), testosterone targeting chemicals (TTCs) and other hormone targeting chemicals (OHTCs), and, finally, all wells grouped regardless of chemical type. For every unit increase in well density per square mile of wells that use chemicals that include an ETC, we observed a 3.789-higher PTB rate (95% CI: 1.83, 5.74) compared with counties with no ETC wells from 2014 to 2018 and likewise, we observed a 1.964-higher LBW rate (95% CI: 0.41, 3.52). Similarly, for every unit increase in well density per square mile of wells that use TTC, we observed a 3.192-higher PTB rate (95% CI: 1.62, 4.77) compared with counties with no TTC wells. Likewise, for LBW, we found a 1.619-higher LBW rate (95% CI: 0.37, 2.87). We also found that an increase in well density per square mile among wells that use chemicals that include an OHTC resulting in a 2.276-higher PTB rate (95% CI: 1.25, 3.30) compared with counties with no OHTC wells, and for LBW, we found a 1.244-higher LBW rate (95% CI: 0.43, 2.06). We also explored the role of HF well exposure in general (regardless of the chemicals used) and found that an increase in total well density (grouped regardless of hormonal targeting status of the chemicals used) resulted in a 1.228-higher PTB rate (95% CI: 0.66, 1.80) compared with counties with no wells, and for LBW, we found a 0.602-higher LBW rate (95% CI: 0.15, 1.05) compared with counties with no wells. We found similar results in our primary analysis that used all data without any exclusions and the statistical significance did not change. Our findings reinforce previously identified regional associations between HF and PTB and LBW, but on a national scale. Our findings point to dysregulation of hormonal pathways underpinning HF exposure risk on birth outcomes, which warrants further exploration. Future research must consider the specific ingredients used in HF fluids to properly understand the differential effects of exposure. https://doi.org/10.1289/EHP12628.

Utilization law of shale gas in hydraulic fracturing with the Changning–Weyuan block in China as an example

The process of extracting shale gas, particularly the study of the utilization law of shale reservoirs modified by hydraulic fracturing technology, is crucial for understanding the effective development of shale gas. This understanding can significantly influence the estimated ultimate recovery of reserves. Our study focused on Longmaxi Formation shale in the Changning–Weiyuan block. To investigate the hydraulic fracturing expansion mechanism and utilization law, we employed shale reservoir physical and mechanical characterization, indoor triaxial hydraulic fracturing experiments, and numerical simulations. The findings are as follows. The rupture pressure of hydraulic fractures increases with the peripheral pressure, and high stress is conducive to the formation of internal microfractures. Additionally, a high rate of water injection enhances fracture extensions along the direction of fluid injection. Under varying injection pressures, the length of crack extensions and number of bond damages in shale are positively correlated with the injection pressure. Conversely, under different differential ground stresses, the length of crack extensions and number of bond damages are negatively correlated with the injection pressure. The six main factors affecting the effective utilization of shale gas are as follows: the stimulated reservoir volume, number of hydraulic fracture clusters, fracture length, fracture height, number of fracture stages, and cluster spacing, accounting for 27.13%, 14.74%, 10.31%, 9.58%, 9.53%, and 9.29%, respectively, with a cumulative contribution rate of 80.58%. This study clarifies the hydraulic fracturing mechanisms of shale reservoirs and the laws governing shale gas production, providing technical support for the development of shale gas reservoirs.

Interaction behavior between hydraulic fractures and interface in coal-rock complex rock layer: Experimental study and application exploration

To achieve long-term and efficient gas extraction in soft, low-permeability coal seams, this study conducted hydraulic fracturing experiments on coal-rock complexes under true triaxial conditions. The pattern of hydraulic fractures (HFs) was reconstructed based on the fractal dimension concept. The results indicate that the tendency of the complex rock layers to initiate fractures toward the coal weakens the trend of increasing fracture initiation pressure with rising geostress. When HFs interact with the interface, the extension pressure significantly decreases. With the lateral pressure coefficient decreasing, HFs tend to extend toward the coal and be captured by the interface, transitioning from a single-wing to a double-wing shape and approaching a symmetrical conjugate state. Only when the vertical principal stress is sufficiently large can HFs separate from the interface. Based on the derived distribution function of induced stress in the coal-rock matrix around the HFs, the displacement conditions of the coal, rock, and interface were examined. The interaction process of rock layer HFs and the interface was divided into three stages: deflection, capture, and separation. The applicability of this study to high-gas soft coal seams was discussed, and a gas management plan involving roof fracturing and full-period extraction was proposed, with the aim of providing a theoretical foundation for the co-extraction and efficient utilization of coal and gas in mines.

Far-field reactivation of natural fractures by stress shadow effect

Hydraulic fracturing is the most important means to achieve the reservoir stimulation of tight formations like shale, where the reservoir permeability is enhanced after the interaction between hydraulic fracture (HF) and natural fractures (NFs). Apart from the widely known direct intersection behaviors between HF and NFs (i.e., the HF meets the NFs), the NFs may also be reactivated by the stress shadow effect of HF, but the failure mechanism and reactivation modes are still unclear. To address this, the far-field reactivation of NFs under the stress shadow effect is described based on a hybrid phase field model (PFM) with the frictional contact criterion, where the open or slip conditions can be quantitatively calculated using the Mohr-Coulomb yield function along the contact interface of NFs. The zone with reactivated NFs is divided into mode II and mixed mode dominated sub-zones respectively, and the phase diagram of reactivation behaviors is summarized as variations of the location and angle of the NF. The NF reactivation modes are discussed under different geological conditions (strength of NFs, distribution angle of NFs, and initial stress difference). It could be found that the higher initial stress ratio leads to more mode II reactivated NFs, and NFs strengths and angles influence both mode II and mixed mode zones notably. The NF reactivation tends to take place with lower strength, higher injection rate, higher initial stress ratio, and special angle (e.g., HF are perpendicular to NFs), during which the permeability enhancement and reservoir stimulation are evaluated by the phase field model.

Surrogate Model of Shale Stress Based on Plackett-Burman and Central Composite Design

Summary Multifactor analysis and accurate prediction of dynamic stress in shale reservoirs are of great practical significance for designing hydraulic fracturing. In this paper, a surrogate model for the rapid prediction of shale stress is proposed by considering the geomechanical heterogeneity and multiscale seepage mechanism of shale gas. The Plackett-Burman method is used to compare the influence of different parameters on shale stress, and significant parameters are selected as decision variables for establishing a surrogate model. The surrogate model for predicting stress is obtained by central composite design fitting, and the interaction of significant factors on shale stress is studied. The results show that after considering the heterogeneity, the minimum horizontal stress fluctuation range is 20.25 to 44.03 MPa and the maximum horizontal stress fluctuation range is 26.46 to 49.77 MPa in the area controlling hydraulic fracture. The initial reservoir pressure, as well as the length and width of hydraulic fractures, are the key factors influencing reservoir stress. The analysis of variance demonstrates that the proposed method is effective for predicting shale stress. The research results are helpful for gaining a deeper understanding of the evolution mechanism of dynamic stress fields in shale reservoirs and provide guidance for treatment design and dynamic optimization of gas wells.

Imbibition mechanism of water in illite nanopores of deep shales by molecular simulation

Hydraulic fracturing is an important technique to develop deep shale gas. After hydraulic fracturing, a large amount of fracturing fluid fails to flow back and retains in deep shales. This phenomenon leads to fracturing fluid imbibition and significantly impacts on gas production. However, the understanding of imbibition mechanism in deep shales is very limited. In this work, the microscopic mechanism of fracturing fluid imbibition into illite pores of deep shales was studied using molecular simulation. The factors affecting imbibition were also analyzed. Results show that water imbibition in illite pores can be divided into three stages. In stage 1, the hydrogen bonds between water molecules and illite pore walls are formed, which forces water to enter the pores and becomes a water layer. In stage 2, the hydrogen bonds between water molecules and the water layer are formed, and gas begins to be displaced out of the pores by water. In stage 3, water molecules no longer enter the pores, and the system becomes equilibrium. As for the influencing factors, fugacity pressure mainly affects the imbibition time. The imbibition time becomes longer when fugacity pressure rises. Pore size plays a significant role in imbibition. Gas in illite pores can be totally displaced if the pore size is smaller than 5 nm. The imbibition is also affected by the amount of water. More gas tends to be displaced out of the pores when the ratio of water volume to pore volume (RWP) increases.

The law of fracture propagation and intersection in zipper fracturing of deep shale gas wells

In response to the unclear understanding of fracture propagation and intersection interference in zipper fracturing under the factory development model of deep shale gas wells, a coupled hydro-mechanical model for zipper fracturing considering the influence of natural fracture zones was established based on the finite element – discrete element method. The reliability of the model was verified using experimental data and field monitoring pressure increase data. Taking the deep shale gas reservoir in southern Sichuan as an example, the propagation and interference laws of fracturing fractures under the influence of natural fracture zones with different characteristics were studied. The results show that the large approaching angle fracture zone has a blocking effect on the forward propagation of fracturing fractures and the intersection of inter well fractures. During pump shutdown, hydraulic fractures exhibit continued expansion behavior under net pressure driving. Under high stress difference, as the approaching angle of the fracture zone increases, the response well pressure increase and the total length of the fractured fracture show a trend of first decreasing and then increasing, and first increasing and then decreasing, respectively. Compared to small approach angle fracture zones, natural fracture zones with large approach angles require longer time and have greater difficulty to intersect. The width of fractures and the length of natural fractures are negatively and positively correlated with the response well pressure increase, respectively, and positively and negatively correlated with the time required for intersection, the total length of hydraulic fractures, and fracturing efficiency, respectively. As the displacement distance of the well increases, the probability of fracture intersection decreases, but the regularity between displacement distance and the response well pressure increase and the total length of fractures is not obvious.

Demise of fossil fuels part I: Supply and demand

According to United Nations , the world needs to reduce the emission of CO2 and other global warming gases by 45 % by 2030 and to negligible amount by 2050. According to UN, this is necessary to keep the goal of keeping earth temperature not exceeding 1.5 °C compared to pre-industrial level temperatures.To achieve this objective means reducing the fossil fuel consumption. Fossil fuels are largely responsible for emitting CO2; therefore, without significant reduction in fossil fuels consumption by 2050, the net zero emission by 2050 is not possible.One argument advanced in support of reducing fossil fuels consumption is limited supply. Using the principle of “Peak Oil Theory,” proponents state that fossil fuels is a limited resource and the world will simply run out of it. Based on the available data, we demonstrate that supply of fossil fuels is relatively abundant, and we will not run out of fossil fuels in near term. Using shale formations as an example, we demonstrate how the success of drilling horizontal wells with hydraulic fractures have significantly increased the oil and gas resource available for exploitation. To reduce fossil fuels demand, the available solutions include improving energy efficiency, reducing subsidies on fossil fuels, imposing carbon tax and life choice changes. We discuss some of those alternatives. Fossil fuel supply is relatively abundant and only possible solution to reduce the consumption is to reduce the demand. By comparing developed countries' consumption with developing countries’ consumption, we see a pathway where growth in GDP can be achieved without significant increase in fossil fuels consumption. Because of efficiency gains in energy usage, GDP in developed countries continues to grow without increase in fossil fuels consumption. Faster we reach the efficiency stage in developing countries, easier it would be reduce the fossil fuel consumption in the world.

A thermodynamics-based coupled model for multi-phase flowback in deformable dual-porosity shale gas reservoirs

Over the past decades, there has been growing interest in modelling multi-phase flowback in shale gas reservoirs during hydraulic fracturing, primarily due to the significant risk of groundwater pollution. However, the lack of fully coupled simulations and a unified theoretical model has hindered the study of coupled adsorptive hydro-mechanical behaviour and its influence on fluid flowback prediction. In this paper, we have extended the non-equilibrium thermodynamics-based Mixture Coupling Theory to derive the constitutive relationship and governing equation for adsorptive multi-phase fluid flows in deformable dual-porosity media. Helmholtz free energy density has been used to establish the relationship between solids and fluids. The interaction of adsorptive fluids in the pore and fracture space has been fully considered, leading to a new form of constitutive equation, which obtained the molecular adsorptive effect on the rock by introducing adsorption entropy production. The proposed governing equations determine the fully coupled evolution of solid deformation and multi-phase flow, with the consideration of adsorption as well as pore and fracture porosity change. Numerical simulation is then performed to study the flowback of a real shale gas well and the influence of coupled behaviour on model prediction. The simulation shows that the flowback flux is mainly by the fracture (more than 99.9 %) and the overall cumulative volume is 2427.1 m3 within 225 h which accounts for about 5.3 % of the total injected fracturing fluid, and the sensitivity analysis reveals that the manual fracture porosity is the most sensitive factor to the cumulative flowback. This work provides new insights into the flowback process of shale reservoirs in a fully coupled view and the proposed theoretical tool should contribute to the modelling of other adsorptive multi-phase flow problem in deformable dual-porosity media such as carbon storage and coalbed methane production.

Study on multi-cluster fracture interlaced competition propagation model of hydraulic fracturing in heterogeneous reservoir

The heterogeneity of the reservoir plays a crucial role in influencing the propagation of multi-cluster hydraulic fractures in horizontal wells. To explore this impact, a seepage-stress-damage coupled model was developed in this study for analyzing the competitive propagation of multi-cluster fractures utilizing the finite-discrete element method. Utilizing the model, a random assignment program has been developed to establish the coupling distribution of mechanical parameters between matrix elements and discrete elements for characterizing the reservoir heterogeneity. Simultaneously, a wellbore flow model describing the pressure drop of fracturing fluid flow is developed by introducing pipe flow element and fluid connection element, enabling the realization of dynamic flow distribution. Based on the developed model, an investigation is conducted into the impact of pumping rates, fracturing fluid viscosity, and perforation parameters on the fracture propagation of multi-cluster fracturing. The study findings suggest that as reservoir heterogeneity increases, the distribution of rock strength becomes more uneven, resulting in a more complex morphology of fracture propagation. Higher pumping rates lead to elevated pressure within fractures, thereby facilitating the uniform propagation of multi-cluster fractures, particularly in reservoir characterized by medium to high heterogeneity. Increasing the viscosity of fracturing fluid can diminish the tortuosity of multi-cluster fracture propagation, albeit with a somewhat limited enhancement in uniformity. As reservoir heterogeneity intensifies, the interference between fractures escalates, necessitating a reduced number of perforations to heighten perforation friction and enhance the balanced propagation of multi-cluster fractures. This research offers theoretical guidance and a scientific foundation for the design of schemes and optimization of parameters in staged multi-cluster fracturing technology for horizontal wells in heterogeneous reservoirs.

Experimental study on fracture properties of heat-treated granite in I-II mixed mode suffered from water and liquid nitrogen cooling methods

Hot dry rock (HDR) development is significant for solving energy problems and realizing energy conservation and emission reduction. Liquid nitrogen (LN2) fracturing and hydraulic fracturing can form cracks in the HDR and improve the efficiency of geothermal energy mining. Therefore, it is necessary to study the fracture characteristics of high-temperature granite under different cooling methods. In this study, the deterioration of the physical and mechanical properties of granite subjected to high-temperature treatment under water cooling and LN2 cooling was studied. Two I/II mixed modes (tensile orientation mode and shear orientation mode) and a pure II mode fracture characteristics of cracked straight-through Brazilian disc (CSTBD) specimens made of granite were explored. The displacement and strain fields of cracked granite specimens were measured by using a digital image correlation (DIC) method. The results show that when the temperature is 25℃, 200℃, and 400℃, the loading angle and cooling method have a great influence on the fracture mechanical characteristics of the granite. In general, the increase of loading angle and LN2 cooling will lead to the decrease of peak load. For example, at 200℃, β = 15°, the deterioration degree of water-cooled and LN2-cooled specimens is 13.77 % and 16.69 %, respectively. When β = 23°, it increases to 14.62 % and 19.91 %, respectively. At the same time, an interesting phenomenon was found in the study. At 400℃, due to the Leidenfrost effect, the peak load of LN2-cooled specimens was higher than that of water-cooled specimens, and the further increase of temperature weakened the effect. When the temperature is 600℃, the difference between the loading angle and the cooling method is weakened.

Fracture-controlled fracturing mechanism and penetration discrimination criteria for thin sand-mud interbedded reservoirs in Sulige gas field, Ordos Basin, China

Considering the problems in the discrimination of fracture penetration and the evaluation of fracturing performance in the stimulation of thin sand-mud interbedded reservoirs in the eighth member of Shihezi Formation of Permian (He-8 Member) in the Sulige gas field, a geomechanical model of thin sand-mud interbedded reservoirs considering interlayer heterogeneity was established. The experiment of hydraulic fracture penetration was performed to reveal the mechanism of initiation–extension–interaction–penetration of hydraulic fractures in the thin sand-mud interbedded reservoirs. The unconventional fracture model was used to clarify the vertical initiation and extension characteristics of fractures in thin interbedded reservoirs through numerical simulation. The fracture penetration discrimination criterion and the fracturing performance evaluation method were developed. The results show that the interlayer stress difference is the main geological factor that directly affects the fracture morphology during hydraulic fracturing. When the interlayer stress difference coefficient is less than 0.4 in the Sulige gas field, the fractures can penetrate the barrier and extend in the target sandstone layer. When the interlayer stress difference coefficient is not less than 0.4 and less than 0.45, the factures can penetrate the barrier but cannot extend in the target sandstone layers. When the interlayer stress difference coefficient is greater than 0.45, the fractures only extend in the perforated reservoir, but not penetrate the layers. Increasing the viscosity and pump rates of the fracturing fluid can compensate for the energy loss and break through the barrier limit. The injection of high viscosity (50–100 mPa·s) fracturing fluid at high pump rates (12–18 m3/min) is conducive to fracture penetration in the thin sand-mud interbedded reservoirs in the Sulige gas field.

Integrating Petrophysical, Hydrofracture, and Historical Production Data with Self-Attention-Based Deep Learning for Shale Oil Production Prediction

Summary As the energy industry increasingly turns to unconventional shale reservoirs to meet global demands, the development of advanced predictive models for shale oil production has become imperative. The inherent complexity of shale formations, coupled with the intricacies of hydraulic fracturing, poses significant challenges to efficient resource extraction. Our study leverages a substantial data set from the Ordos Basin to develop an advanced predictive model, integrating 18 parameters that blend static petrophysical attributes and dynamic factors, including hydraulic fracturing parameters and real-time pump pressure data. This holistic approach enables our self-attention (SA) model to accurately forecast future production rates by processing the complex interplay between reservoir characteristics and operational inputs. In testing across three wells, the model achieved average accuracies of 99.28% for daily oil production (DOP) and 99.25% for daily liquid production (DLP) over 20 days, surpassing traditional long short-term memory (LSTM) and gated recurrent unit (GRU) models, proving its efficacy in fractured well production forecasting. Furthermore, using the initial 30 days of production data as input, the model demonstrated its capability to predict DOP and DLP over a one-year period, achieving prediction accuracies of 96.2% for DOP and 99.6% for DLP rates. Our model’s profound implications for the shale industry include establishing a quantifiable link between key factors and production forecasts, guiding the optimization of controllable aspects, and serving as a decision-support tool for more efficient and cost-effective oil recovery.

An improved two-phase rate transient analysis method for multiple communicating multi-fractured horizontal wells in shale gas reservoirs: Field cases study

The reduction in well spacing for multi-fractured horizontal wells (MFHWs) and the increase in infill drilling exacerbate the risk of fracture communication. This presents substantial challenges for the rate transient analysis of the parent–child well system. This study proposes an improved two-phase straight-line analysis method to evaluate the linear flow parameters of multiple communicating MFHWs in shale gas reservoirs. Considering shale gas adsorption–desorption mechanisms, nonuniform length of fractures, stress-dependent effects, and matrix shrinkage effects, two-phase productivity equation is established within the dynamic drainage area (DDA). Subsequently, we develop a three-well square root of time plot based on DDA correction and propose a rigorous workflow for evaluating linear flow parameters in single-well, two-well, and three-well systems. The straight lines on the parent well's square root of the time plot exhibit varying degrees of jumps, depending on the frequency of fracture communication. After communication, it is necessary to adjust the cumulative production and production time of the parent well to accurately recalculate the average pressure within the drainage area. Numerical simulations are employed to generate a series of cases under different reservoir conditions to validate the accuracy of the proposed model. The results show that the model estimates fracture half-lengths with errors within 8%, meeting the precision requirements for field applications. Additionally, the method exceeds numerical simulations in computational speed. Two field case studies in the WY Basin, China, further illustrate the effectiveness and practicality of the proposed method, providing theoretical support for hydraulic fracturing construction design and development planning.