Low Density Fractures Research Articles

Structural geology of El Peñón karst system along an anticlinorium of the northern Andes, Colombia

Structural control is one of the most important factors in developing the karstic system since the fractures facilitate the infiltration and circulation of groundwater. This accelerates the mineral dissolution processes responsible for the formation of endo and exokarstic geomorphological features, such as sinkholes and underground cavities, among other landforms that characterize this environment. This study provides information on the structural geology of the El Peñón karstic system, considered one of the most significant and extensive karstic regions in Colombia. The karst is tectonically controlled by joints with a preferential NW-SE orientation and cleavage planes with an NE-SW trend, guiding the development of drainage systems, closed depressions, and underground cavities. The stress tensor obtained with a NW-SE direction (127°) explains the deformation in the area and its influence on the development of the karstic system. It was also established that the potential groundwater flow occurs through open fractures with a direction of 120°–130° (parallel to the stress tensor) and along the cleavage planes (40°–50°). The fracture patterns are characterized by intermediate intensity values, predominantly low fracture density, minimal lengths, and good connectivity between fracture planes.

Sequential Bayesian seismic inversion for fracture parameters and fluid indicator in tilted transversely isotropic media

A rock permeated by tilted aligned fractures, which is common in the earth, can be considered as a tilted transversely isotropic (TTI) medium under the assumption of the long wavelength. We develop a feasible method to predict fracture parameters (namely weakness parameters and dip angle) and fluid type in TTI rock using azimuthal seismic data. Based on an approximate stiffness matrix, we first deduce a linear reflection coefficient of the gas-bearing TTI medium in terms of the new fluid indicator and fracture parameters. The reflection coefficient is then rewritten in the form of the Fourier series to decouple the fracture and skeleton-fluid information. Next, the sequential Bayesian inversion method is proposed, which consists of three steps. The first two steps conquer inversion instability owing to coupling of the fracture parameters by constructing a linear relationship between the second- and fourth-order Fourier coefficients. The last step aims at estimating the skeleton-fluid parameters. The sequential Bayesian inversion method alleviates the inversion ill-posedness that is caused by large differences in contributions of the fracture and skeleton-fluid parameters to the reflection coefficient. Synthetic and field cases prove our method is stable and rational in fracture and fluid detections. Finally, we draw the following conclusions from numerical experiments. The approximate stiffness coefficients and derived reflection coefficient are of satisfactory accuracy for the gas-bearing reservoir with low fracture density. The new fluid indicator is sensitive to fluid type but very weakly dependent on mineral composition and porosity. Deconvolution processing can improve the accuracies of different seismic components calculated using the discrete Fourier transformation.

Relationship between the heterogeneity of low-permeability reservoirs and the dynamic evolution of fractures under uniaxial compression conditions by CT scanning: A case study in the jiyang depression of Bohai Bay Basin, China

Based on an analysis of core fractures and computed tomography scanning by X-ray under uniaxial compression tests with loading and unloading, this study investigated the three-dimensional evolution of fracture initiation and propagation in different heterogeneity conditions of low-permeability reservoirs. Reservoir heterogeneity affected the 3D dynamic development of fractures. With increasing loading value, fractures with strong heterogeneity in low-permeability reservoirs followed the development sequence from shallow to deep depth in the profile. When the fracture tip reached the incompetent layer, propagation stopped. Subsequently, when the deformation continued to increase, the fracture crossed the incompetent layer and initiated a new fracture at the front end. Therefore, the across-bedding and through-going macro fractures formed in conjugate sets or were Y-shaped, with roughly equal spacing. In the plane, the fractures initiated along the pre-existing bedding parallel fracture and further propagated into large fractures. Consequently, they formed a net-shaped fracture from the edge to the center, with a long development period. These fractures intersected to form a complex, interconnected, and high-fracture-density system in three dimensions. Under the same loading value, owing to uniform lithology, the formation of micro-fractures in the weak heterogeneity of low-permeability reservoirs is difficult. Furthermore, long shear fractures were only produced in a relatively short formation period with a low fracture density. In the plane, the fractures stepped in an en-énchelon and terminated with horse-tailing microfractures. Moreover, the strong heterogeneity of low-permeability reservoirs was more favorable for producing fractures. The fracture volume was 13.17% of the total sample volume compared to 0.67% in the weak-heterogeneity reservoir under the same loading compression. Furthermore, a better fracturing effect was achieved by fracturing along the direction >50° with the natural fracture and under the condition of small differential horizontal stresses.

A Numerical Study of Elastic Wave Arrival Behavior in a Naturally Fractured Rock Based on a Combined Displacement Discontinuity-Discrete Fracture Network Model

The arrival behavior of elastic waves in a naturally fractured rock is studied based on numerical simulations. We use the discrete fracture network method to represent the distribution of a natural fracture system and employ the displacement discontinuity method to compute the propagation of elastic waves across individual fractures. We analyze macroscopic wavefield arrival properties collectively arising from the interaction between elastic waves and numerous fractures in the system. We show that the dimensionless angular frequency ῶ = ωZ/κ exerts a fundamental control on the arrival behavior of a plane wave traveling through the fractured rock, where ω, Z, and κ are the angular frequency, seismic impedance, and fracture stiffness, respectively. An asynchronous arrival phenomenon of the wave energy occurs and becomes more significant with an increased ῶ. Two regimes are identified according to the two-branch dependency of the fractal dimension D of the FFAW on ῶ, where the wave arrival behavior is within a non-fractal regime for ῶ smaller than the critical frequency ῶc ≈ 1.0, and enters the fractal regime for ῶ ≥ ῶc. The self-affine properties of the FFAW, i.e., the roughness exponent α and the correlation length lc, both linearly decrease as a function of the exponent ξ (with ῶ = 10ξ) in the fractal regime. Early breakthrough of wave transport occurs in regions with relatively low fracture density, while late-time arrival happens in regions of high fracture density.

Azimuthal Investigation of a Fractured Carbonate Reservoir

Summary Oil production and enhanced oil recovery in carbonate reservoirs in Abu Dhabi, UAE, are largely affected by fracture systems that control the fluid path and the permeability of reservoirs. Most fracture properties, such as fracture orientations and density, are obtained by interpreting petrophysical data acquired at the wellbores, whereas fracture properties between wells are typically derived from nonzero offset seismic data. However, deriving fracture properties from seismic data is challenging, as it requires a robust methodology and a careful seismic processing procedure. In the current case study, we used the azimuthal amplitude vs. offset (AVAz) method on 3D seismic data acquired in onshore Abu Dhabi, to generate maps of fracture orientation and density in a carbonate reservoir. A sophisticated processing series was carefully performed to increase signal-to-noise ratio (SNR) and preserve seismic amplitudes. The main parameters controlling the AVAz method were investigated and optimized before being applied to the 3D seismic data. The reservoir has a high fracture density in the lower regions, but a low fracture density in the upper parts, indicating a weaker anisotropy. The resulting dominant fracture directions span from north-northwest/south-southwest to north-northeast/south-southwest, as well as from northwest/southeast to east/west, which is consistent with the primary fracture orientations determined from the interpretation of fullbore formation microimager (FMI) data acquired at well locations. These fracture systems are the result of the Late Cretaceous obduction of the Semail ophiolite, which was oriented east/west and northeast/southwest, followed by the south/north to southwest/northeast trending Late Oligocene-Miocene continent-continent collision of the Arabian and Central Iran plates along the Zagros orogenic front.

Regolith and host rock influences on CO2 leakage: Active source seismic profiling across the Little Grand Wash fault, Utah

Understanding carbon dioxide (CO2) reservoir to surface migration is crucial to successful carbon capture and sequestration approaches; especially fault/reservoir interactions under injection pressure. Through seismic imaging, we explore regolith and shallow stratigraphy across the Little Grand Wash fault. The presence of natural CO2 seeps, travertine and tufa deposits confirm modern and ancient fault-controlled CO2 leakage. We consider this an analogue for a long-failed sequestration site. We estimate bulk porosity and fracture density for host rock, regolith, and fault zone from petrophysical relationships. When combined with existing geochemical and geological data, we characterize a 60 m wide damage zone that represents the primary surface delivery channel for CO2 originating from reservoir depths. Within this damage zone, low seismic velocities suggest sediments have formed through host rock chemical dissolution or mechanical weathering. In contrast, velocities within the adjacent host rock are consistent with low fracture density clastic rocks. We measure anomalously high seismic velocities within the fault zone along one profile that best represents a sealed (cemented/plugged) low permeability, relic flow channel. This suggests that shallow fault zone permeability varies along strike. While regional stress changes may account for decadal- to millennial-scale changes in CO2 pathways, we speculate that the total fluid pressure has locally reduced the fault's minimum horizontal effective stress; thereby producing both low- and high-permeability fault segments that either block or promote fluid migration. Studying CO2 migration in this system can inform potential risks to future sequestration projects and guide monitoring efforts.

An improved response equation of dual laterolog in dual porosity reservoirs and its solution scheme and applications in fractured reservoirs

Many fractured reservoirs are dual porosity reservoirs including matrix pores and fractures, which both contribute to the porosity. However, previous studies either ignored the effect of pores or fractures on dual laterolog, or had special application conditions. Considering the joint contribution of pores and fractures, a response equation of dual laterolog in dual porosity reservoirs is established. First, based on a borehole model with a vertical fracture in dual porosity reservoirs, fracture porosity equations are established in the investigation depths of deep and shallow laterologs, respectively. Using Archie's equations, the response equations of deep and shallow laterolog resistivities are derived, respectively. Then, the response equation, which is similar to the traditional equation of dual laterolog in fractured tight reservoirs, is established. It contains three unknown parameters, i.e. fracture width, fracture cementation exponent and matrix cementation exponent. To solve the equation, a virtual core model is proposed to estimate the fracture cementation exponent by fracture width. The estimation method of matrix cementation exponents is established by empirical formulas. Then, the response equation can be transformed into a nonlinear equation with one unknown parameter of fracture width, and conventional iterative methods can be employed to solve it and obtain the fracture width. Using the response equation and the solution scheme, the change characteristics of fracture cementation exponents in different fractured reservoirs are first analyzed. It shows that the fracture cementation exponents increase with the fracture widths, and they decrease with the increase of the ratio of formation water resistivity to unfractured reservoir resistivity. The applications indicate that the equation and solution scheme can obtain relatively accurate fracture widths quickly in dual porosity reservoirs, and they are recommended for dual porosity reservoirs with high fracture angel (>70°) and low fracture density (<1/3.78 m-1). The findings of this study can help better understanding of the different contributions of fractures and pores to dual laterolog responses in dual porosity reservoirs, and they can provide an alternative solution for evaluating dual porosity reservoirs.

Evaluation of Open Fractures-Sonic Velocity Relation in Fractured Carbonate Reservoirs

Summary Rock physics evaluation of heterogeneous fractured reservoirs where such fractures are the main fluid conduits is necessary for production optimization and field development. In this regard, fracture parameters (i.e., aperture, density, etc.) play a significant role in acoustic wave velocities and propagation, which should be investigated. However, the effect of fracture parameters on sonic wave velocities and their propagation is relatively vague in fractured reservoirs. This study aims to evaluate the mentioned relation by integration of velocity deviation logs, image logs, conventional logs, sonic waveforms, core data, and thin sections in the four studied wells. The results of the velocity deviation logs and sonic waveforms indicate that fracture density and fracture aperture directly control their fluctuations. For example, results showed that zones with higher fracture aperture and lower fracture density, mainly in the dolomitic facies, exhibited more deviations (&amp;lt;−500) compared with the zones with higher fracture density and lower fracture aperture. Hence, the negative deviations on the velocity deviation log are not directly controlled by fracture density, and fracture aperture is the most effective factor on the negative deviation of the velocity deviation log. So, it seems that this parameter controls sonic waves scattering and reservoir behavior. It was found that open fractures and zones with higher porosity (&amp;gt;10%) would have a great impact on sonic wave velocities, attenuation, and other rock physics properties. Ultimately, it was confirmed that a simple but robust combination of the velocity deviation log with sonic waveforms can be used to delineate fractured zones with high fracture aperture (&amp;gt;0.1 mm) in the wells where core samples or image logs are not available.

A fast numerical method and optimization of 3D discrete fracture network considering fracture aperture heterogeneity

A novel numerical method was developed to evaluate the effect of the aperture heterogeneity of individual fractures on fluid flow through three-dimensional discrete fracture networks (DFNs). First, the rock fractures are modelled as circular discs with sizes, orientations, and positions following exponential, Fisher, and uniform distributions, respectively. Isolated fractures (clusters) that make no contribution to fluid flow through the DFN were then removed by optimization programming using breadth optimization and deep optimization algorithms. Subsequently, individual fractures with heterogeneous apertures were developed using successive random addition algorithms. After that, fracture networks were triangulated, and the Reynolds equations were solved for fluid dynamics computations. The verification showed that the developed method provides advantages in computational accuracy and efficiency. Finally, the influence of the aperture heterogeneity of individual fractures on fluid flow through a DFN was evaluated with the proposed method. For comparison, DFN models with identical mechanical apertures (DFN-I) and another model with heterogeneous apertures (DFNH) were developed. The results show that a preferential flow phenomenon appeared in both the DFN-I and DFNH models. However, owing to the aperture heterogeneity of the individual fractures, the results of the DFNH model were more substantial than those of the DFN-I model. This preferential flow phenomenon would be weakened by an increase in either the mechanical aperture or the fracture density. The results also showed that the permeability difference between the DFN-I and DFNH models was obvious for DFN models with small apertures and low fracture densities, but this permeability discrepancy decreased with increasing mechanical aperture or fracture density. Therefore, a critical mechanical aperture exists for a DFN model with a specified fracture density. Beyond this threshold, the influence of aperture heterogeneity within individual fractures on fluid flow can be neglected, and the DFN-I model can be selected to replace the DFNH model for efficient modelling and computation.

3D finite-element modeling of effective elastic properties for fracture density and multiscale natural fractures

AbstractNatural fractures are usually multiscale in size, orientations and distribution, resulting in complex anisotropic characteristics. Analytical methods for quantifying the associated effective elastic properties are based on some assumptions, such as dilute fracture concentration and regular-shaped fractures, which do not occur in actual reservoirs. Here, we conduct anisotropic finite-element modeling of effective elastic properties of complex fractured rocks using the least-square fitting method. The algorithm is developed for a 3D case and validated by classical effective medium theories for models with different fracture densities. The results of the 3D numerical method agree well with the theoretical predictions at low fracture density. The model also considers the interactions among fractures to calculate equivalent stiffness tensors at high fracture density. Three 2D fracture models are simulated to demonstrate the basic behavior of stress interactions and their effect on the overall elasticity under different fracture densities. We applied the developed model to 3D natural fractures built from a real outcrop, and we found that the fracturing pattern significantly affects the effective anisotropy properties. The resultant P-wave phase velocities as functions of the incidence angle and frequency are anisotropic. This study provides a great potential to calculate equivalent stiffness tensors and anisotropic properties of 3D multiscale natural fractures.

Effective permeability of three-dimensional fractured rock with low fracture densities

Effective permeability of three-dimensional fractured rock with low fracture densities

Denudation Rate Changes Along a Fast‐Eroding Mountainous River With Slate Headwaters in Taiwan From 10Be (Meteoric)/9Be Ratios

AbstractThe novel 10Be (meteoric)/9Be system, where 10Be is delivered by precipitation and stable 9Be is released by weathering, provides denudation rates over weathering‐erosion timescales. The new tool is applicable to quartz‐poor lithologies, for example, mafic rock and claystone, which are not readily accessible by the commonly used in situ‐produced 10Be in quartz. We provide a first application of this proxy to a tectonically active mountainous river, the Zhuoshui River in Taiwan. Taiwan Rivers supply a disproportionately high suspended and dissolved flux to the oceans and are often underlain by fine‐grained shale/slate. 10Be (meteoric)/9Be‐derived denudation rates (Dmet) from the Zhuoshui Catchment are highest in the slate‐dominated headwaters (4–8 mm/year), and much lower (1–2 mm/year) along the midlower reaches with mixed lithologies. At the basin‐wide scale, we find a poor correlation between Dmet and basin‐averaged channel steepness despite a small climatic gradient. Because large lithological heterogeneities exist in this basin, we invoke a lithological effect to explain this poor correlation. Relying on a revised stream power incision model that incorporates rock erodibility, the resulting lithology‐ and runoff‐adjusted ksn (kLrsn) can be reconciled with denudation rates with the highest erodibility predicted to prevail in the Miocene slate of low metamorphic grade and high fracture density. This model suggests that the lithological heterogeneity can alter the coupling between surface denudation and channel morphology. On a broader perspective, the successful application of the 10Be (meteoric)/9Be proxy shows its applicability as a tracer for erosion and sediment transport processes in fast‐eroding mountain belts underlain by slate lithologies.

Lessons Learned from Stimulation of Dibei Tight Gas Reservoir with Strong Heterogeneity in Tarim Basin

The Dibei gas field is a naturally fractured tight sandstone gas reservoir in the Tarim Basin, West China. It is characterized by large depth (4600–5200 m), huge thickness (100–200 m), low abundance, low matrix permeability, high pressure (80–95 MPa), high temperature (140 – 150°C) and strong heterogeneity. Different stimulation techniques were applied to improve well production. For the wells with good development of natural fractures, matrix acidizing, acid fracturing or conventional hydraulic fracturing is used. For the wells with low fracture density and huge thickness, separate-layer acid-fracturing, massive separate-layer hybrid fracturing or composite diverting agent is applied to improve stimulation efficiency. The low-damage high-temperature weighted fracturing fluids are used to reduce the effect of reservoir sensitivity and the treatment pressure, and high-strength proppant is used at the same time to keep high fracture conductivity. Meanwhile, large-diameter fracturing string with 3-1/2″ and 4-1/2″ connections is used to decrease pump pressure and increase pump rate. The above technologies were applied in more than twenty treatments in seven wells, of which more than half witnessed production increase. The matrix acidizing or conventional hydraulic fracturing is well performed for the wells with natural fractures, but inefficient for the extremely tight gas wells. In Well A without fractures, the massive separate-layer hybrid fracturing was used. In the treatment, a total of 1,328 m3 fracturing fluid was used, with 50 m3 proppant; the pump rate was 8 m3/min and the maximum pump pressure was 92 MPa. Unfortunately, the post-frac production was not as expected, mainly due to the damage of killing well. In Well B with fractures, high gas production was achieved by acid fracturing, but the gas rate dropped from 258,000 m3 to 51,000 m3 after killing well, and the gas rate never recovered even by acid fracturing, hydraulic fracturing and relieving damage with methanol. The development of natural fractures is the key controlling factor for gas production. Well C, less than 800 m from well B, The development of natural fractures is quite different from Well B, and 98,000 m3 gas rate was acquired after hydraulic fracturing with more than 1,000 m3 low damage high-temperature weighted fracturing fluid and 30 m3 proppant. The development of natural fractures is the key controlling factor for gas production and killing well damage is the fatal blow to the gas production, so the stimulation techniques and working fluid system should be prudently optimized.

A novel radial jet drilling stimulation technique for enhancing heat recovery from fractured geothermal reservoirs

In this study, the application of the Radial Jet Drilling (RJD), a novel stimulating technique for enhancing productivity in the existing wells in deformable naturally fractured reservoirs was investigated using a robust three-dimensional finite element DFM (discrete fracture-matrix) model. Results showed that the RJD laterals were more effective in enhancing injectivity/productivity in cases with lower fracture density, i.e. lower equivalent permeability, while they had no significant effect on the heat production in these cases. In higher fracture density cases, the RJD laterals improved the heat production while had no significant effect on the injectivity/productivity. Results also showed that in reservoirs with very low permeability matrix, the RJD laterals can be used to connect the wells to the fracture network and hence enhance the well performance. The sensitivity analysis on the average net energy production rate with respect to the length of the RJD laterals showed that in the situations where the wells were not connected directly to the fractures, the length of RJD laterals played a crucial role in enhancing the average net energy rate. However, the 100 m laterals almost removed the dependency of the average net energy production rate on the well placement for low, medium and high fracture density cases.

Experimental investigation on water flooding and continued EOR techniques in buried-hill metamorphic fractured reservoirs

Fractured reservoirs are complicated due to the coexistence of matrix and fracture systems. In particular, the matrix of buried-hill metamorphic fractured reservoirs (BHMFRs) has plenty of micro-fractures and a few dissolution pores rather than common pores in sandstone. The storage and heterogeneity features, such as the storage ratio between matrix and fractures, and the distribution of fractures, significantly affect the performances of waterflooding and EOR techniques. With the objective of finding effective waterflooding mode and EOR techniques in such reservoirs, the BHMFR of JZ25-1S in Bohai Bay in China was taken as an example. First, static imbibition experiments using a novel imbibition cell were conducted. Then, 3D large-scale models of BHMFRs using outcrops were designed based on the similarity criterion and employed to conduct the experiments of waterflooding and followed EOR techniques. The results show that co-current imbibition is principal and gravity is the dominant force of imbibition for BHMFRs. Because imbibition is slow, most of the yield comes from the macro-fractures. Both water-free recovery and waterflooding recovery increase as the decrease of the storage ratio of matrix to fractures. Pulsing cyclic water-injection is better than intermittently cyclic water-injection. In BHMFRs with heterogeneity of fracture density, injection in the area with low fracture density and production in the area with high fracture density can achieve better effects than the reverse scenario for both waterflooding and followed EOR techniques. If gas flooding is selected as the EOR technique, producer should be perforated at the lower layers of the reservoir. Suitable gel particles not only get a good EOR effect by effective plugging, but also can create a more favorable condition for surfactant flooding. In our experiments, another 2–4% of OOIP was produced by PPG flooding. Surfactant can enhance the oil recovery by both removing the adsorbed oil on the fracture surface and promoting imbibition in matrix. 3–5% of OOIP was recovered by surfactant flooding and soaking in our experiments.

Numerical analysis of production rate based on interaction between induced and natural fractures in porous media

Hydraulic fracturing in reservoirs with low or high fracture density could have a significant impact on the reservoirs’ production rate. In the present study, having determined the fracture direction through the finite difference method according to the induced-stress distributions, this operation simulated fracturing in two reservoirs with high or low levels of natural fracturing, using the distinct-element method. The goal was to investigate the effect of hydraulic fracture length and aperture on the production rate from both reservoirs. Sensitivity analysis indicated that in a reservoir with more natural fractures, hydraulic fracture length plays a more significant role, and in a reservoir with fewer natural fractures, both hydraulic fracture length and opening are highly influential. Also, factors such as fracture coalescence and intersected fractures, which influence the production rate, were examined. Moreover, the results revealed that when the fracture length and aperture increase, the production rate fluctuates.

Prediction of fracture density using genetic algorithm support vector machine based on acoustic logging data

Existing methods of well-logging interpretation cannot be applied accurately for the exploration and evaluation of carbonate reservoirs because of the fracture development. Based on the fracture density obtained by core analysis in a carbonate reservoir located in the Ordos Basin, in northwest China, three types of fracture density (low fracture density, medium fracture density, and high fracture density) of the target formation were identified. We investigated the effect of fractures on acoustic logging signals in the time and frequency domains by the Hilbert-Huang transform (HHT) and extracted 11 features in the time domain and nine features in the frequency domain. Then, we reduced the features in the time and frequency domain to three principal components by principal component analysis. Finally, a new prediction model of genetic algorithm-support vector machine method based on HHT of acoustic logging data was reported to predict the fracture density. The results indicate that the fracture density has a greater effect on the attenuation of intrinsic mode function 2 (IMF2) and IMF3 components for three different types of formation by empirical-mode decomposition analysis. The energy of the Stoneley wave and S-wave has higher sensitivity than the P-wave. Compared with the time domain, the distribution in the high-frequency domain has a greater correlation with fracture density by the Hilbert spectrum and marginal spectrum. The correlation coefficients between the fracture density and nine features in the frequency domain ([Formula: see text]) are better than the coefficients with 11 features in the time domain ([Formula: see text]). The core analysis and interpretation of resistivity image logging support the validity and effectiveness of our model. The prediction accuracy using the features in the frequency domain can reach to 82%–90%, which is much higher than using the features in the time domain with accuracy of 52%–59%. The application with more information of original acoustic logging data in our model not only avoid the error in velocity picking but also point the direction for the future prediction.

Quantitative Prediction of 3-D Multiple Parameters of Tectonic Fractures in Tight Sandstone Reservoirs Based on Geomechanical Method

Conventional fracture characterization model for low-permeability tight reservoirs cannot be directly built through data statistical method and simplified strain energy density method under high confining pressures. In this research, we make an attempt to establish a series of geomechanical models for prediction of fracture distribution in brittle reservoirs, especially for tight sandstone reservoirs. First, we emphasize that energy generated by tectonic stress on brittle sandstone can be distinguished into accumulating elastic strain energy, fracture surface energy, and residual strain energy and natural fractures can be interpreted or inferred from geomechanical-model-derived strains. For this analysis, we confirm 0.85 $\sigma _{c} $ as the key threshold for mass release of elastic strain energy and bursting of macrofractures, then deduce and build a physical relation model between fracture volume density and strain energy density under uniaxial stress state based on theory of geomechanics and CT scanning. Then on this basis, using combined Mohr-Coulomb criterion and Griffith’s criterion and considering the effect of filling degree in fractures, we continue to modify and deduce the mechanical models of fracture parameters under complex stress states. Finally, all the geomechanical equations are planted into the FE platform to quantitatively simulate the present-day 3-D distributions of fracture density, aperture, porosity and occurrence based on paleostructural restoration of Keshen anticline. It is concluded that the evolutionary model, the Keshen structure, present to us is a distinct and intuitive top-pop-up fold with low amplitude and relatively low-density fractures in the top, has only experienced pre-folding stage and strong compression stage, and has not yet been developed to rapid uplift stage. Its predictions also agreed well with present in-situ core observations and FMI interpretations.

Quantitative prediction of fracture distribution using geomechanical method within Kuqa Depression, Tarim Basin, NW China

Understanding and predicting the development and distribution of tectonic fractures in the super-deep tight sandstone reservoir are important for both gas exploration and exploitation activities in Kuqa Depression. We analyzed the characteristics of regional structural evolution and paleotectonic stress field setting. A reasonable geological model of the research area was built based on an interpretation of the geological structure, a test for rock mechanics, and experiment on acoustic emission. Thereafter, a three-dimensional paleotectonic stress field during the Late Himalayan movement was simulated by the finite element method. Composite rock failure criterion and geomechanical models between fracture density, aperture, strike and strain energy density were used to determine the quantitative development of fractures and predict zones that are prone to fracture development. Areas with well-developed tectonic fractures are mainly controlled by the tectonic stress field and elastic strain energy density, typically located in the fold limbs, fault zones and locations of changes in the orientation of fault. In turn, factors of faults, the lithology, depth and curvature control the development and distribution of tectonic fractures in the tight sandstone. The overall simulated results match well with core observation and FMI (Formation MicroScanner Image) results both in the whole geometry and fracture distribution. This evolutionary model of the structures present to us is a distinct and intuitive top-popup fold with low amplitude and relatively low-density fractures in the top, has only experienced pre-folding stage and strong folding stage, and has not yet been developed to vertical uplift stage.

Seismic dispersion and attenuation in saturated porous rocks with aligned fractures of finite thickness: Theory and numerical simulations — Part 1: P-wave perpendicular to the fracture plane

When a seismic wave travels through a fluid-saturated porous reservoir containing aligned fractures, it induces oscillatory fluid flow between the fractures and the embedding background medium. Although there are numerous theoretical models for quantifying the associated seismic attenuation and velocity dispersion, they rely on certain assumptions, such as infinitesimal fracture thickness and dilute fracture concentration, which rarely hold in real reservoirs. The objective of this work is to overcome some of these limitations and, therefore, improve the applicability of the available theoretical models. To do so, we extend existing models to the finite fracture thickness case for P-waves propagating perpendicular to the fracture plane using the so-called branching function approach. We consider three types of fractures, namely, periodically and randomly spaced planar fractures, as well as penny-shaped cracks. The extended unified model is then tested by comparing with corresponding numerical simulations based on Biot’s theory of poroelasticity. We consider two cases of 2D rock samples with aligned elliptical fractures, one with low fracture density and the other with high fracture density. The results indicate that the influence of the finite fracture thickness on seismic dispersion and attenuation is small at low frequencies when the fluid pressure has enough time to equilibrate between the fractures and background medium. However, this effect is significant at high frequencies when there is not sufficient time for the fluid pressure equilibration. In addition, the theoretical predictions of the penny-shaped crack model are found to match the numerical simulation results very well, even under relatively high fracture density. Analyses of stress distributions suggest that the small discrepancies found between theoretical predictions and numerical simulations are probably due to fracture interactions. In a companion paper, we will extend the analysis for considering the full stiffness matrix and anisotropic properties of such rocks.