Fractures In Tight Sandstones Research Articles

Underbalanced Coiled Tubing Approach Targets Natural Fractures in Tight Sandstones

Underbalanced Coiled Tubing Approach Targets Natural Fractures in Tight Sandstones

Research on optimization of conductivity of multilevel fractures in tight sandstone based on similar circuit principle

AbstractVolume fracturing technology is commonly used to develop tight sandstone reservoirs. During the fracturing process, it is important to not only focus on achieving a larger stimulation volume but also on creating a fracture network conductivity that is suitable for the tight reservoir. However, while there have been numerous studies on optimizing the conductivity of individual fractures, the optimization of fracture network conductivity is still incomplete. To address this issue, a reservoir numerical model is established to optimize the equivalent conductivity of the post‐fracturing fracture network, considering it as a high permeability zone, optimizing the equivalent conductivity to 9.7 μm2 cm. Utilizing the discrete element theory, a “pipe domain” discrete element model is developed to analyze fracture expansion. The findings indicate that the ratios of first‐level, second‐level, and third‐level fractures differ based on the number of clusters, such as 1:4:8 and 1:5:9 for three‐cluster and four‐cluster perforations. By applying the hydropower similarity principle, the fracture network is treated as a three‐level circuit to determine the conductivity of each fracture level. Finally, the model is fitted based on conductivity to determine the optimal fracture conductivity for different fracture number ratios, thereby enhancing the flow capability.

Controls of Multi-Scale Fractures in Tight Sandstones: A Case Study in the Second Member of Xujiahe Formation in Xinchang Area, Western Sichuan Depression

Abstract Different scales of fractures affect the reservoir quality in tight sandstone. There are more studies on macroscopic tectonic fractures but less on bedding fractures and microfractures. The control factors of multi-scale fractures are unclear. In this paper, we analyzed the types and controls of fractures in the second member of the Xinchang region in Western Sichuan. We use core and outcrops observations, imaging logging, scanning energy spectra, and rock slices. Natural fractures can be classified into tectonic, bedding, and microscopic types. The tectonic fractures are mainly low- to medium-angle tensile fractures. The bedding fractures are nearly horizontally distributed along the bedding surface, including parallel, dark mineral interface, and carbonaceous fragments interface bedding fractures. The microfractures develop intra-grain, edge-grain, and inter-grain types. The intra-grain microfractures are inside quartz or feldspar grains, whereas inter-grain types penetrate multiple grains with larger extension lengths. The tectonic fractures are related to the stress, grain size, mineral component, argillaceous content, and lithologic thickness. Parallel bedding fractures are controlled by the coupling of water depth and flow velocity. Bedding fractures at the interface are controlled by rock component. The microfractures are controlled by the length-width axis ratio of the grain, grain element content, and brittleness index. Fractures of different scales form a three-dimensional fracture system that has a substantial impact on the gas production.

Characteristics, logging identification and major controlling factors of bedding-parallel fractures in tight sandstones

More and more tight oil and gas exploration practices show that bedding-parallel fractures (BPFs) are closely related to hydrocarbon migration and accumulation. However, there is still a lack of studies on the detailed characterization, the logging identification method and the major controlling factors of BPFs. The tight sandstones in the northern Sanbian of Ordos Basin have developed numerous BPFs which are typical research objects. In this study, the development characteristics and oil-bearing of BPFs were firstly analyzed based on cores, thin sections and image logs. Then, constrained by the BPFs appearing in cores and image logs, the identification method of BPFs was established by conventional log curves. Finally, the major controlling factors for the formation of BPFs were discussed. Resultsshow that the oil-bearing of BPFs is markedly better than high-angle fractures (HAFs), which indicates that BPFs have a great significance to tight oil migration and accumulation in the northern Sanbian. The bedding-parallel fractures index (BPFI), which consists of the composite fracture index (CFI), fracture dip index (FDI) and principal component analysis (PCA), can identify the BPFs of tight sandstones, and the identification rate can reach 85% with the detection result of image logs. Fine sandstones with parallel bedding and cross bedding which easily enriched mica, underwater distributary channel sedimentary microfacies with comparatively strong hydrodynamic conditions, and a certain mudstone thickness that can produce differential compaction overpressure are the major factors affecting the development of BPFs.

Experimental study on the true triaxial fracturing of tight sandstone with supercritical CO2 and slickwater

Under the effect of the goal of carbon peaking and carbon neutrality, the use of supercritical CO2 has gradually become a research hotspot as a new waterless fracturing fluid, supercritical CO2 fracturing is expected to become an effective techniques to solve the problem of fracturing fluid backflow in low pressure sandstone reservoirs. The outcrop of the Shaximiao Formation in the Sichuan Basin was selected as the research object, the true triaxial fracturing experiments with supercritical CO2 and slickwater were carried out by the large true triaxial fracturing experimental equipment, the computed tomography (CT) scanner and three-dimensional laser scanner to investigate the initiation, propagation patterns, fracture tortuosity and fractal dimension of fractures produced in tight sandstone via supercritical CO2 and slickwater fracturing and to clarify the influences of the different fluids, displacement rates, and in-situ stress differences on the propagation of fractures in tight sandstone. The results showed that the supercritical CO2 fracturing had a lower fracturing pressure and a longer rock fracturing time than slickwater fracturing. Furthermore, the supercritical CO2 fracturing produced fractures with greater tortuosity and fractal dimension as well as a more complex spatial morphology. In addition, the increase in the displacement rate and the decrease in the in-situ stress difference for supercritical CO2 fracturing was more conducive to the multi-point initiation and reorientation of the hydraulic fractures, increasing the complexity of the reservoir fractures and improving the flow effect. It can provide the important theoretical foundation for the creating plans for supercritical CO2 fracturing of tight sandstone with the experimental knowledge.

Multi-component induction logging detection characteristics of the fracture azimuth and dip angles and the fast extraction methods of fracture distribution parameters

In the field of log interpretation of tight sandstone and shale reservoirs, it is difficult to quantitatively evaluate the fracture distribution parameters (fracture azimuth angle and fracture dip angle) near the borehole. Multi-component induction logging can measure the apparent resistivities of the reservoir in different directions by an orthogonal coil system, which lays a foundation for the quantitative evaluation of the fracture distribution parameters near the borehole. First, we derive the finite element equations for numerical calculation from the Maxwell equations for the time-harmonic field and construct a three-dimensional (3D) vector finite element numerical algorithm for the fractured formation. Second, we quantitatively analyze the multi-component induction logging detection characteristics of the fracture azimuth and dip angles. Third, we establish a fast extraction method of the fracture azimuth angle based on coordinate system rotation and an analytical calculation method of the fracture dip angle using the combination of cross components and coplanar components of multi-component induction logging. Finally, we use field data to test the fracture distribution parameter extraction method, and the fracture dip and azimuth angles calculated by multi-component induction logging are consistent with those calculated by borehole imaging logging, which realizes the fast extraction of the distribution parameters of fractures in tight sandstone and shale reservoirs.

Characterization of fractures in low‐permeability thin tight sandstones: A case study of Chang 6 Member, Yanchang Formation, western Ordos Basin

Structural fractures are formed when the underground rock mass deforms and exceeds its own strength limit, so they represent the complex geomechanical properties of rock. Small or core‐scale fractures usually extend less than 10 m in length, which affects the productivity of tight sandstone. Therefore, core‐scale fracture is the core factor for the prediction of reservoir sweet spots. In this paper, the research on the evolution and prediction of natural fractures in tight sandstone is investigated using the core, thin section, logging materials and numerical simulation methods. The results show that a large number of vertical and horizontal sliding fractures with shearing properties are developed in the tight sandstones of the Yanchang Formation. The particle size affects the compactible space inside the reservoir, which then affects the fracture development degree. Since the superimposed thin sand bodies are formed by the frequent lateral migration of the underwater distributary channels in the study area, therefore, the natural fractures in the thin sand bodies at the edge of river channels are relatively developed. According to statistics, the vertical extension distance of fractures is usually less than 2 m. Meanwhile, the fractured zones are mainly located in the cumulative sand body thickness range of 3 ~ 10 m. Moreover, the fractured zones are generally located in the high part of the structure and its wings. Regional structure and thin sand body distribution jointly affect the evolution of fractures in tight sandstones. Through this study, we found that fractures are more developed in thin sand bodies, and the controlling factors include lithology, sedimentary microfacies, sand thickness and local structure. The comprehensive evaluation of fractures in tight sandstones with high accuracy from 1D to 3D was achieved using core observation, logging interpretation, and three‐dimensional (3D) fracture modelling.

Geomechanical log responses and identification of fractures in tight sandstone, West Sichuan Xinchang Gas Field

Natural fractures provide important reservoir space and migration channels for oil and gas in tight reservoirs. Moreover, they are key factors controlling the high yield of tight oil and gas. Accordingly, methods to identify and characterize fractures are essential; however, conventional well-logging data are not ideal for such purposes. To this end, our study proposed an efficient method for identifying and characterizing fractures. First, core observations, core sample test analysis, numerical simulations, core calibration of borehole image logs, and borehole image log calibration of conventional logs were performed to identify sensitive log curves and log response characteristics of fractures. Second, we analyzed the response characteristics and differences in the log and properties of tight sandstone with and without fractures. Third, logging eigenvalue (EvF) and rock physical eigenvalue (MvF) models were constructed to determine different causes of fractures in tight sandstone. Finally, the two models were applied to identify and characterize fractures in the tight sandstone reservoirs in the West Sichuan Xinchang gas field, China. It was found that the effect of using the logging eigenvalue and rock physical eigenvalue models to identify fractures was similar to that observed using an image log. Overall, different fractures were accurately identified and characterized, indicating that the proposed method efficiently identifies and characterizes fractures in tight sandstone, ultimately advancing the research of fine reservoir evaluation and fracture theory.

Occurrence characteristics and relation to oil and gas distribution of bedding fractures in tight sandstone

Occurrence characteristics and relation to oil and gas distribution of bedding fractures in tight sandstone

Occurrence characteristics and relation to oil and gas distribution of bedding fractures in tight sandstone

The existence of bedding fractures has a very important influence on the formation and distribution of tight sandstone oil and gas reservoirs. In this paper, the occurrence characteristics of bedding fractures and their influence on oil and gas distribution were preliminarily studied. The results show that bedding fractures are generally developed in high energy hydrodynamic environments such as river channel, sea (lake) bank and beach, in which parallel bedding is the main type. The existence of dark minerals leads to weak laminar adhesion. Beddings weak the bond force of laminae due to the existing of dark minerals. The pore development condition of bedding fractures is better than that of the upper and lower sandstone laminae. Influenced by these, bedding fractures become important channels for hydrocarbon migration. [Received: April 23, 2021; Accepted: January 2, 2022]

Three-Dimensional Ultrasonic Imaging and Acoustic Emission Monitoring of Hydraulic Fractures in Tight Sandstone

Hydraulic fracturing is an important means for the development of tight oil and gas reservoirs. Laboratory rock mechanics experiments can be used to better understand the mechanism of hydraulic fracture. Therefore, in this study we carried out hydraulic fracturing experiments on Triassic Yanchang Formation tight sandstone from the Ordos Basin, China. Sparse tomography was used to obtain ultrasonic velocity images of the sample during hydraulic fracturing. Then, combining the changes in rock mechanics parameters, acoustic emission activities, and their spatial position, we analyzed the hydraulic fracturing process of tight sandstone under high differential stress in detail. The experimental results illuminate the fracture evolution processes of hydraulic fracturing. The competition between stress-induced dilatancy and fluid flow was observed during water injection. Moreover, the results prove that the “seismic pump” mode occurs in the dry region, while the “dilation hardening” and “seismic pump” modes occur simultaneously in the partially saturated region; that is to say, the hydraulic conditions dominate the failure mode of the rock.

Hydromechanical Investigations on the Self-propping Potential of Fractures in Tight Sandstones

The hydromechanical properties of single self-propping fractures under stress are of fundamental interest for fractured-rock hydrology and a large number of geotechnical applications. This experimental study investigates fracture closure and hydraulic aperture changes of displaced tensile fractures, aligned tensile fractures, and saw-cut fractures for two types of sandstone (i.e., Flechtinger and Fontainebleau) with contrasting mechanical properties, cycling confining pressure between 5 and 30 MPa. Emphasis is placed on how surface roughness, fracture wall offset, and the mechanical properties of the contact asperities affect the self-propping potential of these fractures under normal stress. A relative fracture wall displacement can significantly increase fracture aperture and hydraulic conductivity, but the degree of increase strongly depends on the fracture surface roughness. For smooth fractures, surface roughness remains scale-independent as long as the fracture area is larger than a roll-off wavelength and thus any further displacement does not affect fracture aperture. For rough tensile fractures, these are self-affine over a larger scale so that an incremental fracture wall offset likely leads to an increase in fracture aperture. X-ray microtomography of the fractures indicates that the contact area ratio of the tensile fractures after the confining pressure cycle inversely correlates with the fracture wall offset yielding values in the range of about 3–25%, depending, first, on the respective surface roughness and, second, on the strength of the asperities in contact. Moreover, the contact asperities mainly occur isolated and tend to be preferentially oriented in the direction perpendicular to the fracture wall displacement which, in turn, may induce flow anisotropy. This, overall, implies that relatively harder sedimentary rocks have a higher self-propping potential for sustainable fluid flow through fractures in comparison to relatively soft rocks when specific conditions regarding surface roughness and fracture wall offset are met.

Fracture characterization in oil-based mud boreholes using image logs: example form tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area, Kuqa Depression, Tarim Basin, China

The natural fractures are dominated factors for the gas yield in deeply buried tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area, Kuqa Depression. For the high temperature and pressure environment, boreholes were drilled in oil-based mud. To characterize fractures in ultra-deep formation, electrical imagers (High-definition Formation MicroImager (FMI-HD), Earth Imager (EI), and Oil Base MicroImager (OBMI)) and acoustic imagers (Circumferential Borehole Imaging Log (CBIL) and Ultrasonic Borehole Imager (UBI)) were run in five oil-based mud openholes in KS5 well area. In this paper, fractures in different classifications were interpreted and compared in various borehole images. Opened and partially opened fractures were, respectively, continuous and discontinuous bright sinusoidal traces in electrical images. Closed fractures should be recognized by electrical and acoustic images cooperatively. In Bashijiqike Formation, opened and partially opened medium-angle, high-angle, and network fractures were well developed, while low-angle fractures were infrequent. Bashijiqike Formation might be divided into fractures well-developed, no-developed, and less-developed intervals upwards vertically. The strikes of fractures concentrated in nearly NWW-SEE, paralleled the strikes of major faults. This paper might help in understanding the complete vertical distributional characteristics of fractures in tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area and in turn on the exploration potential of petroleum systems.

Study on non-plane propagation characteristics of fracturing fractures in tight sandstone of Qiulin block under high geo-stress

In order to study the propagation characteristics of hydraulic fracturing fractures in tight sandstone reservoirs in Qiulin block, Sichuan Basin, hydraulic fracturing of tight sandstone under high triaxial in-situ stress were carried out by using a large true triaxial fracturing physical experimental system. The fracture morphology and pump pressure curves under different in-situ stress, fracturing fluid types and displacement were obtained. The experimental results show that: (1) under high geo-stress conditions, the fracture morphology of tight sandstone after hydraulic fracturing is relatively simple, and complex fracture characteristics are less formed. The sedimentary characteristics and the development of internal natural fractures are the main control factors affecting the initiation and propagation of hydraulic fracturing fractures. (2) The characteristics of in-situ stress affect the fracture initiation and propagation direction. When the vertical in-situ stress is equal to the horizontal minimum principal stress, the horizontal fracturing fractures perpendicular to the vertical in-situ stress direction can be formed, and most of the fracturing fractures are non-plane propagation fractures. (3) When the low displacement slick water is used for fracturing, the fracturing fluid is easy to enter into the micro fractures inside the tight sandstone, which is helpful to open the original micro fractures, and the fracture morphology after fracturing is relatively complex; when the high displacement slick water or high viscosity linear gel is used for fracturing, the fractures with non-planar expansion are formed after fracturing. The research results can provide some technical support for the reservoir reconstruction of the second member of ES_2 formation in Qiulin block, Sichuan.

A new method for identifying fractures in tight sandstone of a gentle structural area using well logs

AbstractIn this paper, we conducted a systematic study on the development characteristics and logging prediction of fractures of the Upper Paleozoic tight gas sandstone reservoirs in the eastern Ordos Basin. A new fracture prediction method was proposed based on the variable scale fracture probabilistic model. The calculation steps of this method were as follows: First, the sensitivity of each conventional log series to the tight sandstone fractures was analyzed. It was found that acoustic time difference (AC), density (DEN), natural gamma ray (GR), and deep investigate double lateral resistivity (RD) have good log responses to fractures. Then, the ratio of the range (R) to the standard deviation (S) of the log data was obtained. Finally, the second derivatives of the log series AC, DEN, GR, and RD were calculated, and a new fracture index was constructed (integrated second derivative, FC) using the multiple regression method. The fracture recognition rates of this method are distributed in 64.3%‐85.7%. Therefore, the constructed integrated second derivative (FC) based on conventional logs is effective to quantitatively characterize the fracture development of tight sandstone.

Fracture identification and evaluation using conventional logs in tight sandstones: A case study in the Ordos Basin, China

Fractures are of great significance to tight oil and gas development. Fracture identification using conventional well logs is a feasible way to locate the underground fractures in tight sandstones. However, there are three problems affecting its interpretation accuracy and practical application, namely weak well log responses of fractures, a lack of specific logs for fracture prediction, and relative change omission in log responses. To overcome these problems and improve fracture identification accuracy, a fracture indicating parameter (FIP) method composed of a comprehensive index method (CIM) and a comprehensive fractal method (CFM) is introduced. The CIM tries to handle the first problem by amplifying log responses of fractures. The CFM addresses the third one using fractal dimensions. The flexible weight parameters corresponding to logs in the CIM and CFM make the interpretation possible for wells lacking specific logs. The reconstructed logs in the CIM and CFM try to solve the second problem. It is noted that the FIP method can calculate the probability of fracture development at a certain depth, but cannot show the fracture development degree of a new well compared with other wells. In this study, a formation fracture intensity (FFI) method is also introduced to further evaluate fracture development combined with production data. To test the validity of the FIP and FFI methods, fracture identification experiments are implemented in a tight reservoir in the Ordos Basin. The results are consistent with the data of rock core observation and production, indicating the proposed methods are effective for fracture identification and evaluation.

Natural fractures at depth in the Lower Cretaceous Kuqa Depression tight sandstones: identification and characteristics

AbstractThe Kuqa Depression in the northern Tarim Basin, NW China, is characterized by fault-controlled anticlines where natural fractures may influence production. Natural fractures in the Lower Cretaceous tight sandstones in the depression have been studied using seismic profiles, borehole images, cores and thin-sections. Results show that thrust faults, two types of opening-mode macrofractures and two types of microfractures are present. Thrust faults were generated during Cenozoic N–S-directed tectonic shortening and have hydraulically linked Jurassic source rocks and Cretaceous sandstones. Opening-mode fractures can be subdivided on the basis of sizes, filling characteristics and distribution patterns. Type 1 macrofractures are barren or mainly calcite-lined. They have straight traces with widths (opening displacements) that are of the order of magnitude of 10 μm, suggesting that their primary role is that of migration channels. Type 2 macrofractures are calcite-filled opening-mode fractures. They have an elliptical or tabular shape with sharply tapering tips. Transgranular microfractures are lens-shaped and open or filled mostly by calcite; maximum widths range between 0.01 mm and 0.1 mm. Intragranular microfractures are the most common microfracture type. They are filled by calcite, feldspar or quartz. The macrofractures and transgranular microfractures have regular distributions, while most intragranular microfractures are irregularly distributed owing to their inherited origin. The results imply that natural fractures in the tight sandstones were formed as tectonic, diagenetic and natural hydraulic origins. In situ stress and cementation analyses suggest that Type 1 macrofractures and their genesis-related microfractures have controlled the present flow system of the tight sandstones.

致密砂岩构造裂缝的评价标准及应用—以库车坳陷东部致密砂岩为例 The Assessment Criterion and Application of Structural Fracturesin Tight Sandstone in the Eastern Kuche Basin

近些年来，致密砂岩气和致密砂岩油成为国内外非常规油气勘探的热点，其中致密砂岩的裂缝发育程度评价是关键问题。本文从塔里木盆地北部的库车坳陷东部中下侏罗统致密砂岩岩心的构造裂缝测量入手，结合12口井的测井资料统计分析，通过致密砂岩岩心的构造裂缝面密度统计分析，提出了针对致密砂岩的构造裂缝评价标准，并利用这个评价标准对研究区致密砂岩构造裂缝发育程度进行了系统评价。认为阿合组的致密砂岩构造裂缝比阳霞组和克孜勒努尔组的更发育，依南地区致密砂岩的构造裂缝相对比其他地区更发育，尤其依南2井区的构造裂缝面密度较高，且绝大多数未充填，而依南5井区的构造裂缝面密度很低，且大多数被充填成为无效裂缝。 The tight gas and tight oil are recently paid more attention in the unconventional petroleum exploration, in which the assessment of structural fractures is key problem. The assessment criterion of structural fractures in tight sandstones is proposed based on the logging analysis and measurement of cores in 12 wells, and the criterion is applied in the assessment of tight sandstones. The structural fractures in tight sandstone are more developed in the Ahe Formation than the Yangxia and Kezilenuer Formations. The structural fractures in tight sandstone in Yinan area are more developed than the other area. The fracture density is highest in Yinan 2 well, and not filled, but the fracture density is lower in Yinan 5 well, and most of them are filled.

致密砂岩构造裂缝的评价标准及应用—以库车坳陷东部致密砂岩为例

致密砂岩构造裂缝的评价标准及应用—以库车坳陷东部致密砂岩为例

Application of Horizontal Drilling to Tight Gas Reservoirs

Vertical fractures and lithologic heterogeneity are extremely important factors controlling gas flow rates and total gas recovery from tight (very low permeability) reservoirs. These reservoirs generally have in situ matrix permeabilities to gas of less than 0.1 md. Enhanced gas recovery methods have usually involved hydraulic fracturing; however, the induced vertical hydraulic fractures almost always parallel the natural fracture and may not be an efficient method to establish a good conduit to the wellbore. Horizontal drilling appears to be an optimum method to cut across many open vertical fractures. Horizontal holes will provide an efficient method to drain heterogeneous tight reservoirs even in unfractured rocks. Although many horizontal wells have now been completed in coalbed methane and oil reservoirs, very few have been drilled to exclusively evaluate tight gas reservoirs. The U.S. Department of Energy (DOE) has funded some horizontal and slanthole drilling in order to demonstrate the applicability of these techniques for gas development. Four DOE holes have been drilled in Devonian gas shales in the Appalachian basin, and one hole has been drilled in Upper Cretaceous tight sandstones in the Piceance basin of Colorado. The Colorado field experiment has provided valuable information on the abundance and openness ofmore » deeply buried vertical fractures in tight sandstones. These studies, plus higher gas prices, should help encourage industry to begin to further utilize horizontal drilling as a new exploitation method for tight gas reservoirs.« less