Fluid Rock Research Articles

Impact of Fluids on the Mode I Fracture Toughness of Two Granites and One Sandstone

AbstractFluids affect the mechanical behavior of geomaterials, including properties such as unconfined compressive strength and brittleness. However, their impact on mode I fracture toughness (KIc) has been less explored. This study investigates the impact of saturating fluids on the KIc of three rock types: a porous siliceous sandstone (Corvio) and two high‐strength, low‐porosity granites (Blanco Mera and Blanco Alba). Pseudo‐compact tension (pCT) specimens (diameter ∼50–54 mm, thickness ∼25 mm, notch depth ∼16 mm) were saturated with seven different fluids (deionized water, methanol, NaCl‐saturated water, mineral oil, diesel fuel, an acidic HCl solution and a caustic NaOH solution) and tested under identical conditions. Results show that all fluids reduce KIc, but the extent varies with rock type and fluid properties. Aqueous fluids caused the most significant reductions, with deionized water having the greatest impact on granites (∼18%–30%) and the acid solution on sandstone (∼70%). Non‐polar hydrocarbon fluids, despite their lack of reactivity, caused moderate effects attributed to poro‐mechanical effects. Additionally, pH‐shift experiments, involving sequential exposure to alkaline and acidic solutions, mitigated fluid‐induced weakening. This behavior is hypothesized to stem from silica dissolution in the alkaline phase, followed by rapid nucleation and precipitation during the acidic phase, forming silica‐rich coatings on mineral surfaces. Fracture energy was not equally distributed, with higher post‐peak energy absorption due to crack bifurcation, grain rotation or friction. These findings underscore the interplay of lithological factors, fluid properties and chemical processes in fracture behavior, with implications for subsurface engineering and modeling of fluid‐rock interactions.

An explicit peridynamics model for hydraulic fracturing in porous media

PurposeThe purpose of this paper is to systematically investigate the hydraulic fracture branching phenomena in porous media under different loading conditions and the stepwise phenomenon. The effect of the pore pressure in hydraulic fracturing branching is studied, and more evidence for the stepwise phenomenon with the peridynamics approach is provided.Design/methodology/approachA fully coupled fluid-filled explicit peridynamics model is developed to simulate the complex evolution of crack branching and stepwise phenomena in saturated porous media. Based on the peridynamics theory, an explicit time integration scheme is used to solve the coupled equation system including rock deformation, fluid flow and fracture propagation. Using the proposed model, a series of peridynamic computational tests are performed to examine two common kinds of phenomena observed in hydraulic fracturing: the crack branching phenomenon and the stepwise phenomenon.FindingsFor crack branching phenomenon, the results obtained indicate that sufficient loading is required in order to initiate the crack branching process. Compared with the stress applied on crack surfaces condition, crack branching is more easily induced with the stress applied on boundaries. In addition, for the fluid-driven crack (stress applied on crack surfaces), the existence of pore pressure will depress the growth and branching of the crack. For stepwise phenomena, the results obtained indicate that the peridynamics is a promising tool to study the stepwise phenomenon. The stepwise phenomenon is more distinct under mechanical loading conditions due to the solid behaviour. A sudden jump or crack extension will happen when enough energy is accumulated in the hydraulic fracturing system.Research limitations/implicationsIn this study, the explicit method is used, which means it is conditionally stable, and the critical time step needs to be decided. The reason to use the explicit method is for the study purpose; the explicit method is faster and has no need for matrix inversions.Originality/valueThis study helps to understand the effect of the pore pressure in hydraulic fracturing branching and provides more evidence for the stepwise phenomenon with peridynamics.

VSP Modeling With The Reflectivity Method And Semblance Processing

The present work deals with a specific detail of the VSP technique for estimating local compressional, 𝑣𝑃, and shear, 𝑣𝑆 , wave velocities focused on reservoir characterization and seismic interpretation in sedimentary basins. This investigation was based on the presence of fluids (water, oil, gas) in porous rocks and its relationship with the 𝛾 = 𝑣𝑆 𝑣𝑃 ratio, which mainly affects the shear wave velocity. The forward modeling was based on the reflectivity method to simulate seismic sections along depth in the borehole, considering the presence of a reservoir formation with an anomalous 𝛾 ratio value. The seismic processing of the vertical, transversal, and radial components produced by VSP modeling was based on band-pass filtering for upward and downward field separation, semblance picking, and NMO correction for estimating 𝑣𝑃 and 𝑣𝑆 wave velocities. Additionally, we compare different semblance measures to validate the NMO correction. The results show an accurate estimate of the 𝛾 ratio in the target layer, even considering receivers at different depths and high noise levels in the VSP sections. Besides that, the 2D numerical experiment reveals information about the sensitivity of the 𝛾 parameter in a layered medium with velocity variation, principally around regions with the potential presence of oil and gas.

Research on mass transfer mechanisms due to short-term water-rock interactions between granite cuttings and alkaline NaCl solution and their patterns

Water-rock interactions induced by working fluids in hot dry rock (HDR) reservoirs lead to reservoir damages through mass transfer. Therefore, investigating the mass transfer mechanisms due to the interactions between working fluids and reservoir rocks contributes to minimizing HDR reservoir damages. Drilling fluid is an important working fluid, yet the interactions between drilling fluids and HDR reservoir rocks lack understandings. Here, interaction experiments were conducted using a high-temperature and high-pressure flow reactor under temperatures of 180 °C and 240 °C, a pressure of 24 MPa, and a flow rate of 0.05 mL/min for 7 days. Pure water and an alkaline NaCl solution, which is referred to a HDR reservoir drilling fluid, were prepared to interact with granite cuttings from the HDR reservoir at Qiabuqia site in Gonghe Basin, Qinghai Province, China. Mineral characteristics, mass and chemical changes in the cuttings, solution element content concentrations, micro-morphology and element contents on the cuttings surface, as well as the size of suspending fines in the solutions were determined to reveal the mass transfer mechanisms. The results indicate that the mass transfer mechanisms include mineral reactions and fines migration, and fines migration is the dominating mechanism. The mineral reactions, including feldspar dissolution, quartz dissolution and feldspar alteration, are strong at the front section, while are weak at the end section. At the middle section, the released element contents by mineral reactions form various precipitates, mainly including aluminosilicate, silicate and silica. Strong mineral reactions at the front section lead to fines migration, while the precipitation of Na-bearing minerals at the middle section inhibits fines migration through a coating effect. Suspending fines are identified in the solutions, with a diameter of 100–300 nm, and settled fines are observed at the end section, with a diameter of hundreds of nanometers to a few microns. Simulations of reaction equilibrium show that the precipitation of quartz and biotite increases with a higher NaCl concentration and a lower temperature, while albite changes from dissolution to precipitation as NaCl concentration rises from 6 wt% to 12 wt%. The formation of halite by simulations supports the coating effect. This study provides theoretical basis to minimize HDR reservoir damage induced by working fluids.

A rock physics modelling approach for time‐lapse monitoring and characterization of fluid–rock interactions in hydrocarbon reservoirs

AbstractOne of the research gaps is to understand the development of seismic characteristics of gas‐saturated rock along with the change in rock properties because of chemical reactions. We suggest a method to explain the change in elastic properties brought on by CO2 injection in a rock by capturing the physico‐chemical interactions observed in the laboratory in a theory of rock physics. To explain the laboratory‐measured physical characteristics and velocity of a dynamic rock–fluid system, we include a time‐dependent component in the existing cemented‐sand model. We demonstrate theoretically the rate of change of elastic moduli of the dry frame by incorporating the measured rate of change of cement due to chemical dissolution. We adapt the theory such that it can be applied to the field data and calibrate the theory using water‐saturated well log data from the Ankleshwar field, an established oil field in the Cambay basin, western India. Theoretical time‐lapse logs of velocity and density are then produced using the theory over a range of CO2 saturations, assuming cementing material in grain contacts and geochemical interactions comparable to those observed in the laboratory rock. Then, using theoretical logs, corresponding time‐lapse synthetic seismic data are produced for different saturation. These data clearly demonstrate that, for a uniform model, velocity decreases by up to 18% as CO2 saturation increases from 0% to 20% (ignoring the chemical effect), and that, for a specific saturation, say 20%, chemical effects result in a 17% decrease in velocity from the present to the end of 60 years. However, for the patchy model, velocity decreases maximum by 14% and 16% due to varying saturation and chemical reaction. Moreover, for a particular saturation of CO2, say 20%, velocity differs by 16% for different types of models. This research contributes to making strategy for CO2‐sequestration in a designated field.

Development of fluid pathways and associated diagenetic variations in a natural CO2 leaking fault zone.

Abstract Permeability within fault zones can vary through time due to repeated deformation events and rock–fluid interactions. Understanding the history of fault zone alteration is critical when building hydrogeologic models and evaluating the risk of mechanical rock failure during subsurface storage. Newly acquired drill core recovered within the fault damage zone and fault core of the Little Grand Wash fault (LGWF) is combined with observations from outcrop, optical petrography, computerized tomography image analysis, and ultrasonic velocity measurements to characterize the rock types and preserved structural deformation features within this fault zone. These data are used to understand the history of mechanical rock failure and mineralization associated with this fluid-charged fault system. We identify multiple structural features and use their cross-cutting relationships to understand the history of deformation and their association with changes in fault zone permeability and rock mechanical properties. At the LGWF zone, structural deformation features vary temporally and are used to recognize a decoupling between fault slip and fluid flow. The formation of most open-mode veins occurred after shear failure within the LGWF zone. Early-developed shear bands are cut by carbonate veins, that are in turn cut by shear fractures, followed by a second phase of vein formation and ultimately by folding in the fault core. This sequence of formation reflects changes in mechanical rock properties due to subsurface rock–fluid interactions and indicates that the alteration of rock matrix by secondary carbonate cement results in increased unconfined compressive strength, decreased permeability, and increased ultrasonic velocity. In this fault zone, and possibly other fault zones, the changes in rock properties associated with deformation can be detected through their geophysical response.

Pore pressure prediction based on rock physics theory and its application in seismic inversion

Formation overpressure seriously affects drilling and downhole operation. Accurate prediction on the formation pore pressure can not only reduce the probability of drilling accidents, but also quantitatively evaluate the original formation pressure of underground pore space, which provides an important reference for site selection of carbon sink projects using underground space resources such as CO2 geological storage. It is therefore necessary to set up a widely applicable method that is based on rock physics theory and conforms to the characteristics of rock mechanics and fluid mechanic. This method is suitable for both logging prediction and seismic inversion of pore pressure. The traditional method of predicting pore pressure based on P-wave velocity has multiple solutions, and the prediction based on S-wave velocity which is not sensitive to fluid has new significance. Based on the Hertz-Mindlin petrophysical model that considering pressure variation and the Gassmann fluid substitution equation that addresses the change in fluid saturation, this paper firstly derived rock physical formulas for predicting pore pressure in logging, and then derived the intrinsic power function relationship between the effective pressure (Pe) and S-wave velocity (Vs) as well as S-wave impedance (Is). Based on this, a set of geophysical methods integrating S-wave velocity prediction, pore pressure prediction in well and seismic inversion is finally established. The efficacy of this method has been well validated, with an average error of 2.35% in S-wave velocities prediction, 4.5% in single-well pore pressure prediction. The results of seismic inversion of pore pressure are consistent with the phenomenon of overpressure development in actual working area. This method can be further extended to other areas, providing invaluable reference for underground operation such as oil and gas exploration and CO2 geological storage.

Pressure Dependence of Permeability in Cracked Rocks: Experimental Evidence of Non‐Linear Pore‐Pressure Gradients From Local Measurements

AbstractUnderstanding the coupling between rock permeability, pore pressure, and fluid flow is crucial, as fluids play an important role in the Earth's crustal dynamics. We measured the distribution of fluid pressure during fluid‐flow experiments on two typical crustal lithologies, granite and basalt. Our results demonstrate that the pore‐pressure distribution transitions from a linear to a non‐linear profile as the imposed pore‐pressure gradient is increased (from 2.5 to 60 MPa) across the specimen. This non‐linearity results from the effective pressure dependence of permeability, for which two analytical formulations were considered: an empirical exponential and a new micromechanics‐based model. In both cases, the non‐linearity of pore pressure distribution is predicted. Using a compilation of permeability versus Terzaghi's effective pressure data for granites and basalts, we show that our micromechanics‐based model has the potential to predict the pore pressure distribution over the range of effective pressures expected within the brittle crust.

How Energy Dissipation Mode Controls the Evolution of Multiple Plane‐Strain Hydraulic Fractures Under Isotropic Stresses

AbstractThe geometrical prediction of multiple hydraulic fractures fed from a single fluid source is a major challenge due to transient stress interference among fractures and nonlinear coupling between rock deformation and fluid flow. Here we find that the evolution of multiple hydraulic fractures under isotropic stresses is controlled by a dimensionless toughness, which measures the ratio of the energy dissipated in rock fracturing to that dissipated in viscous fluid flow. The existence of a relation between the dimensionless toughness and the dimensionless length of the arrested fractures is demonstrated by using a bifurcation analysis. The numerical results show that the fractures tend to grow simultaneously in the viscosity‐dominated regime, and the scaling remains effective when the number of fractures varies. This study provides a quantitative and efficient tool for predicting the fracture pattern in engineered and natural fracture systems.

Mineral and fluid transformation of hydraulically fractured shale: case study of Caney Shale in Southern Oklahoma

This study explores the geochemical reactions that can cause permeability loss in hydraulically fractured reservoirs. The experiments involved the reaction of powdered-rock samples with produced brines in batch reactor system at temperature of 95 °C and atmospheric pressure for 7-days and 30-days respectively. Results show changes in mineralogy and chemistry of rock and fluid samples respectively, therefore confirming chemical reactions between the two during the experiments. The mineralogical changes of the rock included decreases of pyrite and feldspar content, whilst carbonate and illite content showed an initial stability and increase respectively before decreasing. Results from analyses of post-reaction fluids generally corroborate the results obtained from mineralogical analyses. Integrating the results obtained from both rocks and fluids reveal a complex trend of reactions between rock and fluid samples which is summarized as follows. Dissolution of pyrite by oxygenated fluid causes transient and localized acidity which triggers the dissolution of feldspar, carbonates, and other minerals susceptible to dissolution under acidic conditions. The dissolution of minerals releases high concentrations of ions, some of which subsequently precipitate secondary minerals. On the field scale, the formation of secondary minerals in the pores and flow paths of hydrocarbons can cause significant reduction in the permeability of the reservoir, which will culminate in rapid productivity decline. This study provides an understanding of the geochemical rock–fluid reactions that impact long term permeability of shale reservoirs.

Paleosol-induced early dolomitization with U[sbnd]Pb age constraints and its implications for fluid pathways in ancient sandstone aquifers

In hydrogeology, a key challenge involves identifying patterns within ancient formations to forecast the distribution of fluid pathways and barriers to permeability, as well as comprehending the palaeohydrological system and its changes over time. This study addresses two main research inquiries concerning fluid flow: firstly, the influence of dolomitization induced by paleosols on flow characteristics, and secondly, the implications for fluid flow pattern distribution in continental sedimentary units. The objectives are pursued through: (1) meticulous, high-resolution measurements of porosity and permeability coupled with well-log data from an outcrop and two boreholes; (2) investigation of petrographic features of diagenetic minerals and their ages using the UPb geochronology system; and (3) an integration of these methodologies to grasp rock properties and fluid flow at a broader scale. Findings suggest that early dolomitization in continental sequences significantly affects fluid flow properties across the basin. The development of paleosols triggered early dolomitization, supported by UPb geochronology evidence. Subsequent groundwater migration along hydraulic gradients, influenced by fluctuations in the local aquifer's water table, facilitated the vertical distribution of dolomitization. Dolomitization occurred in residual pores resulting from initial mineral alteration, early lithifying the sediment and preventing mechanical compaction, thus preserving porosity. During migration events, fluids moved vertically along local faults and horizontally through the dolomitized layers of the formation, which likely remained porous at the time, leading to substantial silica mineralization.

Modelling of water injection-induced shear stimulation and heat extraction in hot fractured reservoirs

Hydroshearing has been proposed as an effective technique for enhancing the permeability of fractured geothermal reservoirs. This approach involves complicated thermo-hydro-mechanical (THM) coupling, but the shear behaviour and its effect on flow and heat transfer are often simplified. In this study, Barton–Bandis (BB) joint behaviour is extended to include nonlinear rock joint deformation, shear dilation and post peak softening and healing behaviours. Two models of field-scale fractured geothermal reservoirs are considered. In addition, the widely used Mohr-Coulomb (MC) model is also incorporated into the THM coupled model for comparison. The results show that cold water injection-induced high-temperature rock contraction and fluid pressure increase, reducing the effective stress and promoting fracture shear deformation and irreversible slip. Moreover, shear dilation, shear softening, and nonlinear fracture opening notably alter reservoir permeability. The two models exhibit entirely different behaviours due to different pathway evolutions and consequent pressure build-up. The BB model displays a larger shear disturbance zone than the MC model, significantly enhancing the permeability and heat extraction within the fractured reservoirs. Accurate descriptions of the mechanical behaviour of rock fractures in hydroshearing models are crucial, suggesting the potential of these models to enhance fractured reservoirs and regulate fracturing for improved geothermal heat extraction.

Rock fluid interactions during seawater injection in carbonates: An experimental evaluation on dolomitization and anhydrite precipitation

Rock fluid interactions during seawater injection in carbonates: An experimental evaluation on dolomitization and anhydrite precipitation

Combined effect of rock fabric, in-situ stress, and fluid viscosity on hydraulic fracture propagation in Chang 73 lacustrine shale from the Ordos Basin

Combined effect of rock fabric, in-situ stress, and fluid viscosity on hydraulic fracture propagation in Chang 73 lacustrine shale from the Ordos Basin

Study on the Influence of Perforating Parameters on the Flow Rate and Stress Distribution of Multi-Fracture Competitive Propagation

It is of great significance to investigate the flow rate and stress distribution of multi-fracture propagation for the optimization of perforation parameters and fracture parameters. Considering the coupling of rock deformation, fracture direction and fluid flow in multi-fracture scenarios, a mathematical model and solution program for the flow and stress distribution of multiple fractures are established, and the analytical model is used for comparison and verification. The effects of perforation cluster number, cluster spacing, perforation diameter on fracture extension trajectory, fracture width, flow rate of each fracture and stress field are studied by the model. The results show that, as the number of perforating clusters increases, the inner fracture is inhibited more severely with less width, length and flow distribution, as well as lower bottom hole pressure. With the increase in cluster spacing, the stress interference between whole fractures is weakened and the flow distribution of the inner fracture is increased with lower bottom hole pressure. With the decrease in perforation diameter, the inhibition effect of inside fractures is weakened, while the inhibition effect of outside fractures, the flow distribution of inside fractures and the bottom hole pressure are increased. The uniform propagation of multiple fractures can be promoted by decreasing the perforation clusters’ number and perforation diameter or increasing fracture spacing.

Fluid inclusions in quartz of the gold-ore occurrences and gold-quartz intergrowths from placers in the Sololi uplift of the Olenyok arch (Yakutia)

Fluid inclusions have been studied in vein quartz with gold sulfide mineralization from metamorphosed sandstones of the Eekite series and metarhyolites of the Early Proterozoic, in quartz breccia from the zone of overlapping gold mineralization on the Early and Middle Permian sandstones, as well as in the gold quartz intergrowths from the Sololi River placer. It has been revealed that formation of quartz breccias occurred within a wide temperature interval from 230 to 425 ºC, with predominance of carbon dioxide and nitrogen in the vapor phase. It is suggested that the increased nitrogen content may be associated with a chemical reaction between the fluid and ammonium-containing silicates of host rocks, in which nitrogen in the form of NH4+ isomorphically replaces potassium at the regressive stage of metamorphism. At the same time, it is possible that mantle nitrogen, which was transported along the Anabar-Eekite deep fault, participated in formation of the studied breccias. The close homogenization temperatures and similar nature of the water-salt composition for the fluid inclusions of quartz veins that inject the Eekite series meta-rocks and meta-rhyolites indicate the synchronism of their formation and attribute them to the common stage of ore formation. Quartz veins with gold sulfide mineralization were the primary sources of pebbles with gold-quartz intergrowths from the Sololi River, this is evidenced by similarity of principal characteristics of fluid inclusions. Oxidizing conditions of the mineralization serve as favorable factor for the Au deposition, it is indicated by the predominant CO2 content in fluid inclusions, keeping role of a geochemical barrier and leading to an elevated gold content in quartz veins.

Estimating elastic properties of sediments by pseudo-wells generation utilizing simulated annealing optimization method

The hydrate concentration model considerably affects elastic properties, including bulk and shear modulus. Defining seismic properties of sediments, such as compressional and shear wave velocity and density, provides valuable information to identify rock facies and fluid types. This information commonly results from pre-stack seismic inversion, while post-stack seismic information provides acoustic impedance as a layer-based property. Traditionally, seismic inversion requires well logs to produce an initial guess of inversion routines and provide a low-frequency part of the amplitude spectrum. Eventually, seismic inversion methods could not be performed in the areas without well-log data, such as deep sea areas. In such cases, pseudo-well logs derived from pre-stack seismic data are a solution. Pseudo-well generation is a title used to estimate the elastic parameters of sediments in areas, such as deep marine environments, where drilled wells are absent or sparse. Metaheuristic optimization algorithms are suitable tools for minimizing the cost function as they best match real and synthetic seismic data. In this study, the SEAM earth model has been used as a reference to investigate the quality of pseudo-well generation utilizing a simulated annealing (SA) algorithm as an optimization method of property model change, which minimizes the cost function of seismic inversion. As a result, considering an initial model type of the SEAM model, simultaneous seismic inversion introduced the best compressional and shear wave velocities and density logs, which provide the best real and synthetic seismic data match when synthetic data is calculated from the simplified Zoeppritz equation.

Influences of lithofacies on fluid mobility in mixed sedimentary rocks: Insights from NMR analysis of the middle Permian Lucaogou Formation, Junggar Basin

The multi-source mixed sedimentation resulted in a unique series of mixed fine-grained sedimentary rocks evolved within the Permian Lucaogou Formation in the Jimusar Sag, located in the southeastern Junggar Basin, China. The variety of lithofacies within this series resulted in pronounced heterogeneity of pore structures, complicating the analysis of fluid occurrence space and state within reservoirs. As a result, the impact of lithofacies on fluid mobility remains ambiguous. In this study, we employed qualitative methods, such as field emission scanning electron microscopy (FE-SEM) and thin section observation, and quantitative analyses, including X-ray diffraction (XRD), total organic carbon (TOC), vitrinite reflectance (Ro), high-pressure mercury intrusion (HPMI) porosimetry, and nuclear magnetic resonance (NMR), along with linear and grey correlation analyses. This approach helped delineate the effective pore characteristics and principal factors influencing movable fluids in the fine-grained mixed rocks of the Lucaogou Formation in the Jimusar Sag, Junggar Basin. The findings indicate the development of three fundamental lithologies within the Lucaogou Formation: fine sandstone, siltstone, and mudstone. Siltstones exhibit the highest movable fluid saturation (MFS), followed by fine sandstones and mudstones sequentially. Fluid mobility is predominantly governed by the content of brittle minerals, the sorting coefficient (Sc), effective pore connectivity (EPC), and the fractal dimension (D2). High content of brittle minerals favors the preservation of intergranular pores and the generation of microcracks, thus offering more occurrence space for movable fluids. A moderate Sc indicates the presence of larger connecting throats between pores, enhancing fluid mobility. Elevated EPC suggests more interconnected pore throat spaces, facilitating fluid movement. A higher D2 implies a more intricate effective pore structure, increasing the surface area of the rough pores and thereby impeding fluid mobility. Ultimately, this study developed a conceptual model that illustrates fluid distribution patterns across different reservoirs in the Lucaogou Formation, incorporating sedimentary contexts. This model also serves as a theoretical framework for assessing fluid mobility and devising engineering strategies for hydrocarbon exploitation in mixed fine-grained sedimentary rocks.

Reservoir Prediction and Prospectivity of Omos Field Onshore Niger Delta Basin, Nigeria

Reservoir prediction and prospectivity are performance indices of the oil and gas field development planning and reserves estimation which depicts the behaviour of the reservoir in the future; its success is dependent on accurate description of the reservoir rock properties, fluid properties, rock-fluid properties and flow. Hence, the objective of this paper was to investigate and predict the reservoir and prospectivity of Omos Field, Onshore Niger Delta Basin, by integrating appropriate standard methods. Results obtained from rock physics attribute map analysis revealed very high Mu-Rho values between 13.0-16.2 Gpa, that is expected for sand presence, and this agrees with the good quality reservoir sands predicted from sequence stratigraphy study.Lambda-Rho values obtained ranges between - 1 to 9.5 GPa, with a much lower value between -1 to 4.3 in the prospective zone that indicates the presence of hydrocarbon bearing sand. These results agree with the results of structural and amplitude maps analysis. Hence the prospective reservoir has a very good hydrocarbon potential in this zone and should be a target for further appraisal for development and production purposes.

Numerical analysis of 3D nonplanar hydraulic fracture propagation in fractured-vuggy formations using a hydromechanical coupled XFEM approach

In this paper, a hydromechanical coupled XFEM approach is developed for simulating 3D nonplanar hydraulic fracturing, fracture–vug interactions and natural crack intersection problems. The extended finite element method associated with a high-order tip enrichment function is used for describing rock deformation, and the finite element method is chosen for discretizing fluid flow at the fracture surface. Rock deformation and fluid flow are coupled and solved by the Picard iteration method. The hybrid explicit–implicit representation method is adopted to represent 3D nonplanar fracture surfaces, and the displacement correlation method is chosen for extracting the mixed-mode stress intensity factor. A new mixed-mode failure criterion is introduced for determining 3D nonplanar fracture growth. Analytical solutions and experimental investigations are employed to verify the validity and effectiveness of our model. The simulations of fracture–vug interactions show that fracture surface growth on the side without vugs is constrained, while vugs can aggravate the tortuousness of the fracture surface on the side with vugs. In addition, four new intersection patterns (penetration, partial arrest, partial diversion, full arrest, and vertical propagation) are found in the 3D nonplanar fracture intersection simulations. The numerical examples highlight the considerable impacts of vertical growth and the mixed-mode effect on hydraulic fracturing.