Acid-Etched Channels in Heterogeneous Carbonates—a Newly Discovered Mechanism for Creating Acid-Fracture Conductivity

SPE Journal ◽  
2009 ◽  
Vol 15 (02) ◽  
pp. 404-416 ◽  
Author(s):  
Jianye Mou ◽  
D.. Zhu ◽  
A.D.. D. Hill
Keyword(s):  
Numerical Simulation ◽  
Fracture Surface ◽  
Flow Direction ◽  
Core Sample ◽  
Acid Etching ◽  
Fracture Conductivity ◽  
Acid Rock ◽  
Fracture Surfaces ◽  
Surface Etching ◽  
Fracturing Process

Summary In the acid-fracturing process, the fracture conductivity created by acid etching of the fracture walls is because of the surface roughness created by the acid's nonuniform dissolution of the fracture surfaces. The acid-fracture conductivity is dependent on surface etching patterns, which are determined by permeability and mineralogy distributions. That is, the spatial distribution of fracture roughness affects the fracture conductivity, which cannot be considered in laboratory measurements of acid-fracture conductivity, which use core samples that are too small to observe such macroscale heterogeneities, or in typical acid-fracture simulators, in which the gridblock size is much larger than the scale of local heterogeneities. An accurate prediction of acid-fracture conductivity necessitates the detailed description of the acid etching profiles on the fracture surfaces, which depend on acid transport in the fracture, leakoff because of local permeability, and acid/rock reactions. In this paper, we developed a 3D intermediate-scale acid-fracture model with gridblock sizes small enough (gridblock sizes comparable to the core-sample size in experiments) and total dimensions large enough (the total dimensions comparable to a gridblock size in an acid-fracture simulator) to capture local and macroscale heterogeneity characteristics. The model predicts the pressure field, the flow field, acid concentration profiles, and fracture-surface profiles as functions of acid injection volume. In the model, we use a front-fixing method (Crank 1984) to handle the irregular, moving boundaries in numerical simulation. Spatially correlated permeability and mineralogy distributions were generated by using a semivariogram model. The model was validated by comparing simulation results with experimental results from an acid-fracture conductivity cell. With the model, by extensive numerical simulation, we analyzed the relationship among fracture-surface-etching patterns, conductivities, and the distributions of permeability and mineralogy. We also illustrated the formation characteristics necessary for acid to create channel-caused high acid-fracture conductivity. We found that a fracture segment with channels extending from the inlet to the outlet of the segment has high conductivity because fluid flow in deep channels causes a very small pressure drop. Such long and highly conductive channels can be created by acids if the formation has heterogeneities in either permeability or mineralogy or both, with high correlation length in the main flow direction, which is the case in laminated formations.

Modeling Acid Fracturing Treatments with Multi-Stage Alternating Injection of Pad and Acid Fluids

2021 ◽  
Author(s):  
Rencheng Dong ◽  
Mary F. Wheeler ◽  
Hang Su ◽  
Kang Ma
Keyword(s):  
Viscous Fingering ◽  
High Viscosity ◽  
Carbonate Reservoirs ◽  
Acid Etching ◽  
Fracture Geometry ◽  
Fracture Conductivity ◽  
Acid Fracturing ◽  
Fracture Surfaces ◽  
Low Viscosity ◽  
Multi Stage

Abstract Acid fracturing technique is widely applied to stimulate the productivity of carbonate reservoirs. The acid-fracture conductivity is created by non-uniform acid etching on fracture surfaces. Heterogeneous mineral distribution of carbonate reservoirs can lead to non-uniform acid etching during acid fracturing treatments. In addition, the non-uniform acid etching can be enhanced by the viscous fingering mechanism. For low-perm carbonate reservoirs, by multi-stage alternating injection of a low-viscosity acid and a high-viscosity polymer pad fluid during acid fracturing, the acid tends to form viscous fingers and etch fracture surfaces non-uniformly. To accurately predict the acid-fracture conductivity, this paper developed a 3D acid fracturing model to compute the rough acid fracture geometry induced by multi-stage alternating injection of pad and acid fluids. Based on the developed numerical simulator, we investigated the effects of viscous fingering, perforation design and stage period on the acid etching process. Compared with single-stage acid injection, multi-stage alternating injection of pad and acid fluids leads to narrower and longer acid-etched channels.

Acid Fracturing Shales: Effect of Dilute Acid on Properties and Pore Structure of Shale

2015 ◽  
Author(s):  
Weiwei Wu ◽  
Mukul M. Sharma
Keyword(s):  
Pore Structure ◽  
Core Sample ◽  
Petrophysical Properties ◽  
Acid Dissolution ◽  
Eagle Ford Shale ◽  
Fracture Conductivity ◽  
Acid Fracturing ◽  
Microstructure Changes ◽  
Fracture Surfaces ◽  
Closure Stress

Abstract Many microfractures created during hydraulic fracturing are too small to be filled with proppants and are likely closed during production. However, for some shales that are rich in calcite (calcareous mudstones), such as the Bakken and Eagle Ford shale, dilute acids can be used while fracturing to maintain the conductivity of these microfractures under closure stress by non-uniformly etching the fracture surfaces. The mineralogy and pore structure of the shale and their evolution during acid fracturing are critical factors on the surface surface etching profile and the fluid leakoff. Therefore, understanding how acid dissolution changes the microstructure, petrophysical properties and pore structures of shale is essential in the design and application of acid fracturing in shales. In this paper changes in shale properties and pore structure by acid fracturing were demonstrated and visually observed for the first time with a scanning electron microscope. Acidized sections of a shale core sample were carefully isolated, and its microstructure, pore structure and petrophysical properties were systematically studied and compared with non-acidized sections of the core. Microstructure changes were found to be strongly dependent on mineral distribution, and several patterns were identified: channels developed in carbonate-rich regions; cavities or grooves formed in carbonate-rich islands or carbonate rings; and surface roughness was created in mixed zones of scattered carbonate and inert minerals. Inert minerals such as clay, organic matter stay relatively undisturbed in the structure, while some mineral grains can be dislodged from their original locations by dissolution of the surrounding carbonates. Many macropores with size up to 120 µm were created and mesopores mostly associated with clay gained more accessibility. Significantly increased permeability and porosity was measured in an acidized shale matrix. Brinell hardness measurements show that, as expected, the hardness of the shale was reduced by acidizing. This means that for acidizing to work effectively, it is important to not etch the fracture surfaces uniformly. Doing so will result in a reduction in the fracture conductivity under stress. The microstructure changes introduced by acid fracturing demonstrated in this study will result in the formation of surface asperities which is likely to improve the fracture conductivity of induced unpropped fractures. The acidized shale matrix close to the fracture surface with increased abundance of macropores and accessibility to mesopores may serve as a preferred pathway for fluid flow as well.

Modeling Acid Fracturing Treatments in Heterogeneous Carbonate Reservoirs

2021 ◽  
Author(s):  
Rencheng Dong ◽  
Mary F. Wheeler ◽  
Hang Su ◽  
Kang Ma
Keyword(s):  
Reactive Transport ◽  
Transport Model ◽  
Viscous Fingering ◽  
Carbonate Reservoirs ◽  
Acid Etching ◽  
Fracture Conductivity ◽  
Acid Fracturing ◽  
Fracture Surfaces ◽  
Etching Pattern ◽  
Viscous Fingers

Abstract The goal of acid fracturing operations is to create enough fracture roughness through non-uniform acid etching on fracture surfaces such that the acid fracture can keep open and sustain a high enough acid fracture conductivity under the formation closure stress. A detailed description of the rough acid-fracture surfaces is required for accurately predicting the acid-fracture conductivity. In this paper, a 3D acid transport model was developed to compute the geometry of acid fracture for acid fracturing treatments. The developed model couples the acid fluid flow, reactive transport and rock dissolution in the fracture. We also included acid viscous fingering in our model since the viscous fingering mechanism is commonly applied in acid fracturing to achieve non-uniform acid etching. Carbonate reservoirs mainly consists of calcite and dolomite minerals but the mineral distribution can be quite heterogeneous. Based on the developed model, we analyzed the effect of mineral heterogeneity on the acid etching process. We compared the acid etching patterns in different carbonate reservoirs with different spatial distributions of calcite and dolomite minerals. We found that thin acid-etched channels can form in carbonate reservoirs with interbedded dolomite layers. When the reservoir heterogeneity does not favor growing thin acid-etched channels, we investigated how to utilize the acid viscous fingering technique to achieve the channeling etching pattern in such reservoirs. Through numerical simulations, we found that thin acid-etched channels can form inside acid viscous fingers. The regions between viscous fingers are left less etched and act as barriers to separate acid-etched channels. In acid fracturing treatments with viscous fingering, the etching pattern is largely dependent on the perforation spacing. With a proper perforation design, we can still achieve the channeling etching pattern even when the reservoir does not have interbedded dolomite layers.

A New Method of Reproducing Rock Samples with Rough Surfaces for Testing Conductivity: A Case Study on Shale Propped Fractures

2021 ◽  
Author(s):  
Chi Chen ◽  
Shouxin Wang ◽  
Cong Lu ◽  
Kun Wang ◽  
Jie Lai ◽  
...  
Keyword(s):  
Fracture Surface ◽  
Surface Morphology ◽  
Shale Gas ◽  
Gas Production ◽  
New Method ◽  
Fracture Conductivity ◽  
Fracture Surface Roughness ◽  
Rock Samples ◽  
Fracture Surfaces ◽  
Effective Development

Abstract Hydraulic fracturing technology provides a guarantee for effective production increase and economic exploitation of shale gas wells reservoirs. Propped fractures formed in the formation after fracturing are the key channels for shale gas production. Accurate evaluation of local propped fracture conductivity is of great significance to the effective development of shale gas. Due to the complex lithology and well-developed bedding of shale, the fracture surface morphology after fracturing is rougher than that of sandstone. This roughness will affect the placement of the proppant in the fracture and thus affect the conductivity. At present, fracture conductivity tests in laboratories are generally based on the standard/modified API/ISO method, ignoring the influence of fracture surface roughness. The inability to obtain the rock samples with the same rough morphology to carry out conductivity testing has always been a predicament in the experimental study on propped fracture conductivity. Herein, we propose a new method to reproduce the original fracture surface, and conductivity test samples with uniform surface morphology, consistent mechanical properties were produced. Then, we have carried out experimental research on shale-propped fracture conductivity. The results show that the fracture surfaces produced by the new method are basically the same as the original fracture surfaces, which fully meet the requirements of the conductivity test. The propped fracture conductivity is affected by proppant properties and fracture surface, especially at low proppant concentration. And increasing proppant concentration will help increase the predictability of conductivity. Due to the influence of the roughness of the fracture surface, there may be an optimal proppant concentration under a certain closure pressure.

scholarly journals Study of the Effect of Centrifugal Force on Rotor Blade Icing Process

2017 ◽  
Vol 2017 ◽  
pp. 1-9 ◽  
Author(s):  
Zhengzhi Wang ◽  
Chunling Zhu
Keyword(s):  
Numerical Simulation ◽  
Centrifugal Force ◽  
Rotor Blade ◽  
Flow Direction ◽  
Three Dimensional ◽  
Simulation Method ◽  
Two Phase ◽  
Numerical Simulation Method ◽  
Calculation Results ◽  
Flow Field Characteristics

In view of the rotor icing problems, the influence of centrifugal force on rotor blade icing is investigated. A numerical simulation method of three-dimensional rotor blade icing is presented. Body-fitted grids around the rotor blade are generated using overlapping grid technology and rotor flow field characteristics are obtained by solving N-S equations. According to Eulerian two-phase flow, the droplet trajectories are calculated and droplet impingement characteristics are obtained. The mass and energy conservation equations of ice accretion model are established and a new calculation method of runback water mass based on shear stress and centrifugal force is proposed to simulate water flow and ice shape. The calculation results are compared with available experimental results in order to verify the correctness of the numerical simulation method. The influence of centrifugal force on rotor icing is calculated. The results show that the flow direction and distribution of liquid water on rotor surfaces change under the action of centrifugal force, which lead to the increasing of icing at the stagnation point and the decreasing of icing on both frozen limitations.

Research on Anisotropic Fracture Mechanical Behavior of Cortical Bone with Combined Method of Fractal and Gray Level Co-Occurrence Matrix

2021 ◽  
Vol 11 (1) ◽  
pp. 67-75
Author(s):  
Dagang Yin ◽  
Bin Chen ◽  
Huifen Zhou
Keyword(s):  
Mechanical Properties ◽  
Fracture Surface ◽  
Cortical Bone ◽  
Fracture Mechanisms ◽  
Gray Level ◽  
Combined Method ◽  
Characteristic Parameters ◽  
Fracture Surfaces ◽  
Anisotropic Fracture ◽  
Occurrence Matrix

The irregular fracture surface of cortical bone, which is caused by complex multilevel micro-nanostructure, reflects the mechanical properties and fracture mechanisms. It is of great significance to characterize some characteristic parameters from the fracture surfaces of bone. In this research, anisotropic fracture mechanical properties of bovine femoral cortical bone along transverse, longitudinal and radial direction are firstly obtained by three-point bend experiment. Then the fracture routes and fracture surfaces are observed by scanning electron microscope. The observation shows that the formed fracture surfaces, which are caused by different crack routes, are extremely rough and have complex textures. Lastly, the combined method of fractal and gray level co-occurrence matrix are adopted to describe the morphology of fracture surface of cortical bone objectively and quantitatively. It is shown that the fracture surface of cortical bone has obvious fractal characteristics and four statistical texture feature parameters (contrast,angular second moment, correlation and entropy) of GLCM of fracture surfaces can describe a certain fracture texture character. The relationship between the characteristic parameters and macroscopic mechanical properties are established. The quantitative analysis and automatic class identification for the fracture surfaces of cortical bone can be achieved.

Scanning Auger and Xps Studies of Fracture Surfaces of B4C Hot Pressed with Excess Carbon

1989 ◽  
Vol 153 ◽  
Author(s):  
Benjamin M. DeKoven ◽  
Eric A. Ness ◽  
David D. Hawn
Keyword(s):  
Fracture Surface ◽  
Boron Carbide ◽  
Carbon Content ◽  
Low Dose ◽  
Xps Spectra ◽  
Excess Carbon ◽  
Fracture Surfaces ◽  
Four Point Bending ◽  
Hardness Values

AbstractA series of boron carbide materials was hot pressed with 0-7% excess carbon. The strength of each material was determined by four point bending, and found to decrease from about 600MPa to 300MPa as the carbon content increased from 0% to 7%. Diamond indentation yielded hardness values that decreased from 28.3 to 25.OGPa and toughness values that increased from 3.5 to 4.5 MPa√mover the same carbon range. Each sample was fractured in situ in ultrahigh vacuum (UHV) and examined by scanning Auger microanalysis (SAM) and XPS to determine both the elemental and chemical state distributions. For the samples with excess carbon, localized carbonrich regions are observed on fracture surfaces by SAM. XPS reveals a 50% enhancement of excess carbon on the fracture surface compared to the bulk for the sample with 7% excess carbon. A correlation was observed between surface carbon composition and the bulk toughness and hardness. The C(ls) XPS spectra were utilized to determine the nature of carbon in B4C on freshly fractured and Ne+ bombarded surfaces. Two distinct peaks were observed in the C(ls) region. Low dose ion bombardment resulted in a single broad C(ls) peak at the midpoint of the two initial peaks. It can be inferred from this data that there are C-C-C intericosahedral linkages in B4C.

New Computational Procedures for Evaluation of Fracture Surfaces of the Drop Weight Tear Test of the Piping Steel

2011 ◽  
Author(s):  
Pavel Zˇidli´k ◽  
Petr Ferfecki ◽  
Bohumi´r Strnadel
Keyword(s):  
Fracture Surface ◽  
Ductile Fracture ◽  
Unstable Fracture ◽  
Fracture Surfaces ◽  
Drop Weight ◽  
Highly Sensitive ◽  
Drop Weight Tear Test ◽  
Computational Procedures ◽  
New Procedures

Drop weight tear test (DWTT) is one of the standard methods for evaluation of the ductility of large-dimensional structural components, such as pipelines used for gas and/or oil transportation. In general, the pipelines are even used in places with temperatures close to −40 °C, and in such environments, it is necessary to guarantee the resistance of the material used for pipeline against the initiation of unstable fracture. Currently, the percentage portion of the ductile fracture of the DWTT specimen is determined by an expert evaluator. The objective of this paper is to introduce new procedures working on the principle of deterministic, statistical and fractal description of the fracture surface. For the proposed computational procedures, the fracture surface of the test specimen is scanned at the macroscopic level using the 3D-Cam scanner. The newly investigated procedures show highly sensitive to the determination of the percentage portion of the ductile fracture on the tested DWTT specimens. The developed procedures to assess the fracture surfaces of the DWTT specimens contributes to making the results of this test more correct, objective and also increases the reliability and safety of the manufactured pipelines.

Self-affine Fracture Surface Parameters and Their Relationship with Microstructure in a Cast Aluminum Alloy

2002 ◽  
Vol 17 (6) ◽  
pp. 1276-1282 ◽  
Author(s):  
M. Hinojosa ◽  
J. Aldaco
Keyword(s):  
Aluminum Alloy ◽  
Fracture Surface ◽  
The Self ◽  
Cast Aluminum Alloy ◽  
Cast Aluminum ◽  
Force Microscopy ◽  
Fracture Surfaces ◽  
Atomic Force ◽  
Roughness Exponent

The possible role of microstructural features in determining the self-affinity of the fracture surface of a cast aluminum alloy is explored in this work. Fracture surfaces generated both in tension and impact tests were topometrically analyzed by atomic force microscopy, scanning electron microscopy, and stylus profilometry. The roughness exponent exhibited the “universal” value ζ ≈ 0.78, and the correlation length ζ was of the order of the grain size. The brittle intermetallic compounds known to be important in crack initiation did not show any correlation with the self-affine parameters of the resulting fracture surfaces in this particular case.

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