Dominant Fracture Research Articles

After Years of Ever-Longer Fracturing Stages, Some Engineers Say Shorter Can Be Better

The trend in fracturing designs has been longer stages with more perforation clusters, which save time and money. Based on papers at this year’s SPE Hydraulic Fracturing Technology Conference and Exhibition, the thinking that has sharply reduced the number of perforations per cluster and improved the effectiveness of fracturing is becoming the industry norm. But there are a couple of companies questioning the consensus by fracturing wells with shorter stages and fewer clusters to see if they deliver more oil and gas production. ConocoPhillips has been considering “going backward to fewer clusters per stage,” said Dave Cramer, a senior engineering fellow at the company. “In some areas, we are testing out fewer clusters per stage based on fiber-based observations in offset wells that (indicate) far-field treatment uniformity is improved as a result,” he said, adding “another advantage of reducing stage length is that injection rate into the hydraulic fractures is increased, which leads to increased fracture width and improved proppant transport.” The offset-well observations mentioned were in a recent paper by Devon Energy that reported on a test which concluded that stages with fewer clusters were more efficiently fractured than longer stages with more of them (SPE 212340). The 36-page paper was based on extensive testing at Phase 3 of the Hydraulic Fracturing Test Site 1 in the Eagle Ford, where a well fractured using a wide range of clusters per stage was used to find a way to most effectively refracture productive rock missed by an old fracture design. The data were shared with companies backing the private-public partnership with the US Department of Energy, which included ConocoPhillips. The paper’s authors wrote, “Generally, cluster efficiencies are higher for stage designs with fewer clusters.” Cluster efficiency depends on the entry holes in a cluster getting enough fluid at a high enough rate to create a productive fracture. Fracture length is the result of other choices, including the pump rate, the entry-hole numbers, diameter, and placement. The thinking behind these designs—as with most fracturing nowadays—is based on the limited-entry method. This technique ensures the pumping rate and fluid volumes are enough to properly stimulate all the perforations, which are sized and placed to ensure all the entry holes have a chance of developing a fracture. The industry’s focus on equalizing treatment goes back to early studies using fiber optics that revealed that the first clusters passed nearest the heel side of the lateral were taking in the lion’s share of the fluid and developing dominant fractures, leaving many later clusters under stimulated. Based on recent papers from Devon and Hess, limited-entry ensures most clusters are stimulated, but dominant clusters are still getting more than their share because the high flow rates that favor them with more fluid also ensure they erode faster, allowing them to take in more fluid.

Inconel 718 produced by hot pressing: optimization of temperature and pressure conditions

This paper aims to act as a useful engineering tool for researchers who are studying the production of well-densified IN718 parts by uniaxial vacuum hot pressing. To the best of the authors’ knowledge, there is no relevant information on literature about densification of IN718 parts by this technique. This work is focused on understanding the influence of uniaxial vacuum hot pressing sintering conditions (temperature and pressure) on Inconel 718 (IN718) powder densification, microstructural, fracture mode, and hardness properties. The optimization of temperature and pressure sintering conditions are presented as well as its influence on the densification, microstructural features, and hardness properties. The sintering conditions included temperatures of 1000, 1068, 1150, and 1200 °C; pressures of 50 and 60 MPa; and a dwell time of 60 min.The results showed an increase in the grain size (GS) of the compacts with the processing temperature and a change on the fracture mode from intergranular dominant fracture to fully dimple ductile fracture. Regarding the microstructural properties, the results showed that γ′(Ni3(Al, Ti)) intermetallic precipitate originated from IN718 powders was retained in the sintered specimens. The hardness results revealed that the sintering temperature of 1000 °C is not enough to promote accurate densification. The optimum hardness results were achieved at 1200 °C (327 HV) with high levels of densification and pure intragranular fracture mode. In future studies, shear and tensile strength test should be performed in order to properly evaluate the mechanical behavior of hot-pressed IN718 specimens.

Parametric sensitivity analysis to select the relevant natural fractures in reservoir rock cells based on the equivalent permeability response

Over the last few years, several works have been conducted to improve the productivity of unconventional highly fractured reservoirs, in which the fracture network strongly affects the hydraulic behavior of the porous medium. However, fracture networks can present a complex geometry making difficult its spatial discretization. This work proposes to use a statistical method to select a group of natural fractures that reproduces the equivalent permeability response. An adapted Box-Behnken experimental design together with steady state fluid flow analyses is carried out in order to determine the relevance of each fracture, which is instrumental to select the dominant fractures based on the statistical significance. The results of the model with only few dominant fractures were validated and verified against the ones from the original model, which contains all natural fractures. The time evolution of pressure and the accumulated fluid curve obtained by means of transient analyses showed that a model with the selected fractures is able to properly represent the hydraulic behavior and main trends associated with the original cell.

Study on the Fracture Propagation in Multi-Horizontal Well Hydraulic Fracturing

Multi-horizontal well hydraulic fracturing is a widely employed and highly effective method for stimulating tight and shale reservoirs. However, most existing studies primarily focus on investigating the impact of intra-well interference on fracture propagation while neglecting the influence of inter-well interference. Here, a multi-well hydraulic-fracture-propagation model is established to examine the effects of inter-well interference on fracture propagation within a multi-well system. In this study, based on the bilinear T-S criterion, the stiffness degradation is used to describe the damage and evolution process of fracture, the coupling process of fluid flow and solid damage and deformation is realized, and the dynamic distribution of inter-fracture flow is realized by using Kirchhoff function on the basis of the cohesive zone method (CZM) finite element model. Finally, the fracture-propagation model of multiple horizontal wells is established. Based on this model, the mechanism of inter-well interference on fracture propagation is studied, and the influence law of Young’s modulus and fracture displacement on fracture propagation in multi-wells is investigated. The results show that the reservoir can be divided into self-influence area, tension area and compression area according to the stress distribution state in the hydraulic fracture propagation of multi-wells. The propagation rate of hydraulic fractures in horizontal wells is significantly accelerated when they propagate to the local tension area generated by the fracture tip of neighboring wells, and rapidly decreases as the hydraulic fractures continue to propagate to the compression area of neighboring wells. Rocks with a lower Young’s modulus tend to be more plastic, forming hydraulic fractures with usually lower fracture lengths and usually larger fracture widths. The hydraulic fracture has an inhibitory effect on the propagation of fractures closer to each other in neighboring wells, and this inhibitory effect gradually increases as the distance decreases. The dominance of the dominant fracture to propagate in the self-influence area gradually decreases under inter-well and intra-well interference. As the dominant fracture propagates into the tension and compression areas of the neighboring well fractures, the feed fluid will show a brief rise and then eventually stabilize. This study quantifies the effect of inter-well interference on fracture propagation and lays the foundation for treatment optimization of small well spacing hydraulic fracturing.

Structural Classification, Discharge Statistics, and Recession Analysis from the Springs of the Gran Sasso (Italy) Carbonate Aquifer; Comparison with Selected Analogues Worldwide

The relative importance of karst conduits and fractures in driving groundwater flow affects the discharge of springs and the long-term availability of water resources. Applying statistics to the hydrographs of the discharge and studying the recessions provide information on the degree of reliability and variability of the springs and, therefore, the flow regime within the saturated part of the carbonate aquifers. This approach was applied to six springs at the Gran Sasso aquifer in Central Italy. These springs were divided into three structural geological groups that determined the position of the permeability thresholds. The type of tectonic structures and the pattern of the permeability thresholds allow a correlation with the computed statistics. The studied springs were associated with the presence of thrusts, overturned drag folds, and a normal fault. The computed statistics describe a general scenario of reliability and steadiness for the springs. The Flow Duration Curves for the springs show limited groundwater flow through the conduits through a comparison with analogues in Slovakia. Joints and bedding plane fractures dominate the groundwater flow, fitting both the relative steadiness of the discharges and the pattern of the Flow Duration Curves. The recessions are also characterized by more gentle slopes with respect to nearby areas fitting a conceptual model of dominant fracture flow. This mathematical scenario depicts groundwater resources, which have limited exposure to episodes of summer droughts. The proposed approach is a holistic combination of structural geology and hydrologic elements and can be successfully exported to other tectonized carbonate areas for the sustainable management of groundwater resources worldwide.

Effect of welding speed and shielding gas composition on the properties of laser welded EN AW 6061-T6 al alloy using 5356 filler metal

In this study, EN AW 6061-T6 Al alloy having a thickness of 6 mm was joined with a 5000 W fiber laser welding machine using different welding speeds (20, 25, and 35 mm/s) and shielding gas compositions (100% Ar, % 50 Ar + % 50 He, % 25 Ar + 75 He). In order to reduce the strength decrease in the weld zone, 5356 filler metal with a diameter of 1.2 mm was used with the cold wire feeding method at a constant speed of 2 m/min. Structural analyses of the joints were performed by macroscopic and microscopic examination, and hardness profiles and strength values of the joints were determined by mechanical tests. The findings showed that (i) the width of the seam increased due to the increase in the amount of He in the gas composition, and the number of pores decreased accordingly, (ii) although no significant change was observed in the hardness profiles, the highest tensile strength value (242 MPa) was obtained using a gas composition of 50% He + 50% Ar and welding speed of 20 mm/s, (iii) shear failure was the dominant fracture, and the porosities played a decisive role in the fracture.

Failure mechanisms of hybrid metal–composite joints with different protrusion densities under a tensile load

The mechanical behaviors of hybrid metal–composite joints with different protrusion densities were investigated by combining numerical and experimental methods. High-fidelity finite element models that considered the failure modes of all components were developed, and specimens based on metal additive manufacturing technology were tested under quasi-static tensile load to verify the numerical calculations. The results showed that the load capacity and the dominant fracture mode of joints were significantly affected by the metal protrusion density. The failure mechanisms of joints under different protrusion conditions exhibited a clear difference, which proved the possibility of an optimal and functional joint design.

Experimental Study of Mechanical Properties and Fracture Characteristics of Conglomerates Based on Mohr–Coulomb Criteria

Gravel is one of the main factors affecting the mechanical properties of conglomerates, which plays a decisive role in crack propagation. In this paper, taking the conglomerate of the Baikouquan Formation in Mahu Sag of Xinjiang as the research object, a three-dimensional model of the conglomerate is constructed by the discrete element numerical simulation method, and the triaxial compression experiment under different confining pressures is simulated. The mechanical properties and fracture morphology of conglomerate are analyzed with gravel content as a variable and verified by laboratory tests. In this simulation, with the increase of gravel content, the compressive strength of the conglomerate decreases, angle of internal friction decreases, and the fractures show different forms. The results show that the gravel morphology, spatial location, and gravel content have an impact on the mechanical properties of the conglomerate. The gravel content affects the formation process of the dominant fracture surface by controlling the distance between gravels so as to control the internal friction angle and it is the main controlling factor for the mechanical properties of the conglomerate. Gravel cracks initiate at the edge of gravel. Stress controls the formation of main cracks under low gravel content, and the influence of gravels under high gravel content makes cracks more discrete and complex.

Investigation on the microstructure and mechanical properties of gum metal TNTZO titanium alloy prepared by selective laser melting

In this study, a high-dense multi-functional gum metal Ti36Nb2Ta2.7Zr0.3O (mass%, TNTZO) alloy was successfully produced using selective laser melting (SLM). The as-built components exhibited epitaxially grown β grains with a slight deviation from the building direction, resulting in a cubic-like texture where (001)β was oriented along the building direction. Additionally, nanoscale ω precipitates were found embedded in the β matrix. The uniaxial tensile tests revealed that the as-built TNTZO alloy had repeatable strength with a yield strength of 756 MPa. Dislocation slip was the dominant mechanism responsible for the deformation of the as-built TNTZO alloy, with no phase transformation or deformation twin observed. The interaction between the ω variants with the dislocations during plastic deformation led to the formation of “ω-free channels” due to the different orientation relationship of ω variants with β matrix. The post-mortem analysis revealed that ductile fracture is the dominant fracture mode.

Four-dimensional characterization of a PFOA-contaminated fractured rock aquifer (FRA) in Bennington, Vermont, U.S.A

Between 2016 and 2019, the Vermont Department of Environmental Conservation discovered that groundwater from over 50% (355/628) of wells and springs in the Bennington area had perfluorooctanoic acid (PFOA) concentrations > 20 parts/trillion (ppt) (Vermont action level). The distribution of contaminated wells was complex, with very different PFOA concentrations in closely-spaced wells (<100 m apart). PFOA was introduced into the environment by a factory that produced Teflon™-coated fabrics from about 1978–2002. Manufacturing involved PFOA expulsion from smokestacks followed by atmospheric transport, deposition, and downward leaching to the aquifer system. The fractured rock aquifer was characterized through physical components (geologic mapping, spatial analysis of wells, and geophysical logging) to build a Three-dimensional conceptual model and then map the spatial distribution of chemical groundwater tracers (PFOA, major and trace elements, stable isotopes, and and recharge ages), adding a fourth temporal dimension. The field area consists of four imbricated metamorphic rock slices bounded by thrust faults. The dominant fracture sets that overprint these slices control the N-S and E-W topographic “grain.” Geophysical logging divided wells into those completed internal to or on the margins of thrust slices. Major and trace element analysis shows that groundwater can be spatially discriminated into carbonate (TDS, Ca, Mg, HCO3), shaly (Sr, U, SO4), and siliceous (K, Si) groups. Depleted H2O stable isotope signatures characterize the siliceous group. Recharge-ages determined from CFC11, 12, and 113 and tritium correlate with geochemical groups and were divided into “older” (~1953–73) and “younger” (1963–88), which predate and are synchronous with factory operation, respectively. Our conceptual model shows recharge from highlands to the west and east flowing toward the Bennington valley where it is locally tapped by wells along thrust faults or fracture zones. This water has older recharge ages and anomalously low PFOA concentrations (<6.7 ppt in 12 of 15 samples, mean = 7.9 ppt) compared to the majority of wells in this region, which are completed in the internal portions of thrust slices. These wells contain groundwater with younger recharge ages and high PFOA concentrations (mean = 290 ppt) and occurrence (>20 ppt in 28/36 samples), indicating mixing of younger and older waters.

An efficient inversion method to interpret distributed temperature measurement of horizontal wells in shale gas reservoirs

In order to obtain accurate production profile and fracture parameters of horizontal wells in shale gas reservoir from distributed temperature sensing (DTS) data, an efficient inversion method based on improved dichotomy (ID) algorithm has been developed. It has realized quantitative interpretation of production profile and fracture parameters. The synthetic case indicates the inversion calculation of the ID inversion model is 9 times faster than that of conventional inversion algorithm. Finally, the ID inversion model has been applied to a field case to inverse the DTS data. 69 effective artificial fractures have been identified. The production profile and fracture half-length of the field well are obtained. The interpreted fracture half-length indicates that the extension of the hydraulic fractures is extremely uneven. There are obvious dominant fractures in several fracturing stages of which the fracture half-length is more than 120 m, however, the average fracture half-length of the field case is only 61.21 m. The inversion simulated temperature profile matches the measured DTS data well. The interpreted production profile is in good agreement with the PLT testing results that verifies the reliability of the developed ID inversion model.

Reloading Mechanical Properties and Particle Flow Simulation of Pre-Peak Confining Pressure Unloading Sandstone

The excavation-unloading damage effects of western high-geostress slopes on rock were explored by testing the pre-peak confining pressure unloading sandstone reloading mechanical properties. The deformation and failure mechanisms were studied from a mesoscopic perspective using the particle discrete-element method. (1) Approaching the unloading failure, confining pressure increased the specimen bearing capacity attenuation. (2) The confining pressure unloading promoted microdefect propagation and development; the specimens increased rapidly to the damage stress value after reaching the initiation stress value. The penetration fracture zone was more evident and expansive in the model, and the distribution of the dense crack areas was more concentrated in the fracture zone and area. (3) The average interval of the tangential contact force was the largest in the direction of crack expansion and propagation. The strong force chains were shown to primarily bear external loads, whereas the weak force chains played a key auxiliary role in maintaining stability. (4) The number of cracks developing in the confining pressure unloading damage process indicated that the loading process did not cause damage to the specimens. The fracture zones further propagated and formed on the dominant fractures based on the damage caused by the confining pressure unloading disturbance.

Coupled hydromechanical analysis of sparsely fractured crystalline rocks

This paper addresses the issue of evolution and coalescence of localized damage zones in sparsely fractured crystalline rocks. The approach incorporates a constitutive law with embedded discontinuity, which is phrased in terms of both the hydraulic and mechanical response. The formulation takes into account the hydromechanical interaction in regions intercepted by discontinuities. An internal length scale parameter is employed in the definition of equivalent hydraulic conductivity and the tangential stiffness operators, and the onset of newly developed macrocracks is detected by the bifurcation analysis. An enhanced mixed u-p finite element formulation is derived which considers the effect of progressive evolution of the fracture aperture in the weak statements of balance equations. Fully implicit temporal discretization is employed, and the finite element formulation is stabilized by invoking the Polynomial-Pressure-Projection (PPP) technique. A coupled FE analysis is conducted examining the response of Luc du Bonnet granite, with pre-existing fracture network, subjected to plane strain compression that triggers the crack propagation and coalescence. The approach is first verified on some benchmark problems that involve the presence of a dominant fracture. The results of simulations are compared with those obtained using a very fine mesh incorporating interface elements. Later, a series of coupled analyses are carried out examining the hydromechanical response in the presence of multiple fractures.

Fracture toughness of PA6/POE‐g‐MA/TiO2 ternary nanocomposites according to the essential work fracture method

AbstractPolymer‐based nanocomposites can be used in a wide variety of applications in the industrial, electronics, and energy segments. In order to attain the application‐specific properties that are desired, good mechanical and fracture efficiency is frequently required. Polyamide 6 (PA6)/polyethylene octene grafted with maleic anhydride (POE‐g‐MA)/titanium dioxide (TiO2) nanocomposites' fracture characteristics were investigated utilizing the essential work of fracture (EWF) approach in this work. Four levels of POE‐g‐MA (0, 10, 20, and 30 wt%) and three levels of TiO2 (0, 2, and 4 wt%) are therefore utilized. The reliability of the EWF theory is demonstrated via the self‐similarity of the force‐displacement curve and Hill's analysis. Results showed that EWF and non‐essential work of fracture (non‐EWF) were improved by 73% and 54%, respectively, by increasing POE‐g‐MA up to 30 wt%. The improvement of EWF value confirmed the role of POE‐g‐MA as an impact modifier. Nevertheless, by increasing TiO2 up to 4 wt%, EWF, and non‐EWF decrease by 20% and 25%, respectively. Adding 30 wt% POE‐g‐MA reduced tensile strength and enhanced strain at the break by 45% and 109%, respectively. Moreover, the tensile strength was enhanced up to 10% by adding 4 wt% of TiO2 content. However, the strain at break was decreased by 44% by increasing 4 wt% of TiO2 nanoparticle. In addition, the dominant fracture mechanism in polyamide‐based blends and nanocomposites is shear‐yielding and fibrillation structures.

Cooling-induced permeability enhancement for networks of microfractures in superhot geothermal environments

Recent researches have proposed the use of enhanced geothermal system reservoirs consisting of dense networks of microfractures, created by hydraulic and/or thermal fracturing in superhot/supercritical geothermal environments, because of their suitability for thermal energy harvesting. During fracturing and energy extraction, the fracture networks are exposed to cooling due to the injection of cold fluid into the reservoirs. Previous studies showed such cooling enhanced reservoir permeability in conventional geothermal environments. However, the cooling may result in a higher risk of seismicity, owing to decreased normal stress on the fractures. Nevertheless, it is unclear whether cooling-induced permeability enhancement and a higher risk of seismicity occurs within networks of microfractures which consist of numerous interconnected microfractures at various orientations to the in situ triaxial stress. Thus, no dominant fractures have the possibility to cause permeability enhancement/induced seismicity. In this study, results are presented for borehole cooling experiments on a dense network of microfractures in granite, at 400 °C, under true triaxial stress. Permeability and acoustic emissions were measured with decreases in borehole temperature (up to ~ 90 °C). Results showed that permeability increased with increasing temperature drop at relatively low stress levels (15 and 20 MPa). The permeability enhancement occurred without intensive failure, and was reversible. However, permeability was almost constant at a higher stress level (65 MPa). Results showed that permeability enhancement required a thermal stress equivalent to the mean stress, so that the normal stress was reduced to near-zero, for a considerable amount of the microfractures. Additionally, the permeability of dense microfracture networks can be increased by cooling primarily through thermo-elastic deformation (without intensive failure), which may be useful to compensate for the reduction in injectivity due to cooling-induced fluid property changes.

The role of porosity in the dynamic disturbance resistance of water-saturated coal

Porosity significantly affects the dynamic response of water-saturated coal and the safety of coal mine engineering in water-rich areas. For mastering the dynamic response of water-saturated coal to the porosity, this study used the coal with four levels of the porosity for dynamic tests. In this process, the dynamic compressive tests were operated on these coal specimen in both of water saturation and dry states, and the reduction factor of dynamic mechanical parameters of the coal in water-saturated state to these in dry state were focused to eliminate the effect of self dynamic properties of the coal. The results indicate that the dynamic mechanical properties non-linear positively correlate with the porosity, and its sensitivity to porosity gradually weakens as with the increase of the porosity. This effect is intuitively expressed in the dominant fracture propagation features recorded by a high-speed camera, and microscopically attributed by the size and number of microfractures around the dominant fracture. The dynamic stress intensity factor model of the dominant fracture was further established with consideration of total cohesive force for the equivalent particle on the fracture surface. It well reveals the effect mechanism of porosity on the meso-deterioration and macroscopic mechanical properties of the coal around the roadway. On this basis, the applicable conditions and suggested methods were put forward to improve the dynamic stability of water-saturated coal with various porosity.

Structure and fracture characterization of the Jizan group: Implications for subsurface CO2 basalt mineralization

The coastal region of southwest Saudi Arabia contains a thick sequence of Late Oligocene basalts in the Jizan Group, which accumulated along the continental rift that preceded the opening of the Red Sea. These basalts are targeted for the disposal of CO2 emitted from industrial sources by subsurface carbon mineralization processes. The disposal potential of the Jizan Group basalts depends on having adequate permeability along fracture networks capable of conducting injected fluids away from the wellbores. The basalts in the Jizan Group generally lack primary permeability due to hydrothermal alteration, but are cross-cut by a dense network of fractures. In this paper, we describe and interpret the structural geology of the area based on field and geophysical data, and characterize the fracture development in the Jizan Group. The Jizan Group in the area comprises a bimodal suite of 30–21 Ma volcanic and volcaniclastic rocks and lacustrine sediments that accumulated in a continental rift valley similar to the East African rift. It consists predominantly of basaltic lavas that were fed by dense swarms of sheeted basalt dikes intruded parallel to the rift axis. Structurally the area is composed of half grabens bounded from the west by antithetic normal faults, and from the east by a megaflexure. Fractures in the Jizan Group were characterized by ground and aerial digital photogrammetry of outcrops. Mean P21 fracture intensities from 12 scattered meter scale outcrops are in the range 5–54 m−1, which demonstrates that the Jizan Group is highly fractured. Fracture directions are multimodal. The dominant fracture trend is 140–160 N, which is parallel to the sheeted dike swarms and normal faults, and therefore parallel to the paleo-rift axis. Additional conjugate and orthogonal fracture sets are also recognized. The presence of pervasive fracture-based permeability in the Jizan Group will facilitate the injection and mineral carbonation of carbon dioxide in the mafic volcanic rocks in this region.

Effect of Mo2C on the microstructure and properties of (W, Mo)C-10Co cemented carbides

Mo2C doped WC-10Co alloys were prepared via an in-situ synthesis method composed of steps of spray drying, roasting, carbothermal pre-reduction and carbonization-vacuum sintering. The effects of Mo2C content on the microstructure and mechanical properties of cemented carbides were studied in detail. The results showed that WC and Mo2C formed a complete solid solution by the current method. With the increase of Mo2C content from 0 wt% to 0.5 wt%, the effects of solution strengthening and fine grain strengthening were obvious, and the grain size was decreased from 0.72 ± 0.32 μm to 0.62 ± 0.27 μm, while the hardness was improved from 1316 ± 20 HV30 to 1549 ± 14 HV30. However, the (W, Mo)C solid solution would deteriorate the fracture toughness of the alloys. With the further increase of Mo2C content to 9 wt%, the lattice distortion in the solid solution became more serious. Meanwhile, the dissolution of more Mo2C in the liquid Co further weakened the dissolution-precipitation process of the solid solution, thus inhibiting the growth of grains and increasing the hardness of the alloys. In addition, with the increase of Mo2C content, the dominant fracture mode of the alloy changed from transgranular fracture to intergranular fracture.

Simulation-based evaluation of the effectiveness of fiber-optic sensing in monitoring and optimizing water circulation in next-generation enhanced geothermal systems

The success of multi-stage hydraulic fracturing technology in unconventional oil and gas reservoir development has attracted much attention from the geothermal community. Some pilot field experiments have been conducted to investigate the multi-stage hydraulic stimulation along horizontal or high-deviated wellbores in geothermal reservoirs (denoted as next-generation enhanced geothermal systems). One common output of such stimulation is that the created hydraulic fractures show various conductive capabilities, which could potentially lead to undesirable flow localization, early thermal breakthrough, and low cold water sweep efficiency. Monitoring the water circulation between horizontal wellbores connected through hydraulic fractures becomes important in avoiding early severe producing temperature reduction in practical operations. In this study, we investigate the effectiveness of distributed temperature sensing (DTS) and distributed strain sensing (DSS) in identifying dominant flow paths in geothermal reservoirs with multi-stage hydraulic stimulation and horizontal well completion designs. A coupled thermo-poro-elastic model for general-purpose geothermal reservoir simulation is developed and solved using a combined finite volume/finite element method, together with a fully implicit sequential coupling scheme. The temperature and strain responses along horizontal production wells are simulated and analyzed during water circulation. Reservoir temperature reduces in the area that is close to the water flow paths, which induces thermal contraction. These thermoelastic characteristics can be monitored by fiber-optic sensing, forming distinct signatures for flow-localization identification. DTS data may be dominantly influenced by the fractures with higher conductivity, ‘masking’ the signals related to fractures in the downstream, but DSS data can provide additional information for accurate identification of high conductive fractures. Successful mitigation strategies, for example plugging the dominant fractures illustrated in this study, could substantially increase the producing fluid temperature. The analyses in this study show that fiber-optic sensing is an effective technology to monitor and optimize water circulation in next-generation enhanced geothermal systems.

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept

In this study, fracture orientations in carbonate reservoirs were determined using a multicomponent velocity analysis based on shear wave splitting. The analysis is based on the estimated velocities of large seismic events with different polarizations. In a fractured zone with a dominant orientation, weak amplitude split shear events, including shear noise, result in shear waves that are polarized toward the symmetry and anisotropy axes and propagate with a common fast and slow velocity, respectively. Thus, a velocity stack should show high coherency anomalies in directions parallel and orthogonal to the fracture strike. Furthermore, because the analysis is applied locally at a specific depth range, it is less susceptible to the effects of overburden anisotropy and noise. The dominant fracture orientations from carbonate reservoirs of four oilfields were compared to those interpreted from fullbore microimager and core data. Fractures in two offshore reservoirs strike NNE-SSW and NW-SE, which are related to Zagros stress. Fractures in two onshore reservoir strikes NE-SW, while in deeper onshore reservoir fractures are aligned with N-S direction. The findings of this study are promising, particularly for the fractured reservoirs especially those located in Abu Dhabi, which are characterized by high heterogeneity and complex fracture network related to complex tectonic history. In order to obtain geometrical parameters of fractures at seismic scale, it is recommended to implement the analysis adapted in this study after acquiring three component zero-offset vertical seismic profiling. Cited as: Diaz-Acosta, A., Bouchaala, F., Kishida, T., Jouini, M. S., Ali, M. Y. Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept. Advances in Geo-Energy Research, 2023, 7(2): 99-110. https://doi.org/10.46690/ager.2023.02.04