Journal of Southwest Petroleum University(Science & Technology Edition)

Sedimentary Characteristics of the Maokou Formation of the Permian in Northern Sichuan Basin and Its Petroleum Significance

ZENG Deming, XIE Xiaobin, HUANG Dong, YU Liangzhi, ZHANG Yu

2023, 45(1): 1-12. DOI: 10.11885/j.issn.1674-5086.2020.12.24.04

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Based on the fine seismic data interpretation result, combining the field sections and indoor thin-section observation, we study the depositional facies distribution characteristics, by analyzing the main rock types, paleontology, geochemical and seismic characteristics. On the basis of biological (chip) limestone, siliceous rocks and reef limestone development rule of the lithofacies and seismic stratigraphic structures and stratigraphic thickness variation characteristics, we find out that Maokou Formation has developed sedimentary facies of the open sea platform, marginal-platform shoals and slope-basin, summarize four kinds of sedimentary facies association, open sea platform with patch reefs, open sea platform with shaoal patch, the marginal-platform shoals and calcareous-siliceous basins. Land clarify the vertical evolution of sedimentary facies and plane distribution rule. The results show that the high-energy marginal-platform shoals present banding development in the Jiange——Yuanba——Longgang Area. The biological (chip) limestone develops the intercrystalline denudation pore, intergranular dissolved pore and biogenic body cavity pore, which is a high quality reservoir development zone with good physical properties. Maokou Formation (Member 4) in the western Sichuan trough and Guangyuan——Wangcang trough area sedimentary siliceous rock and mudstone is the high quality source rocks, the excellent sedimentary environment providing a favorable conditions for hydrocarbon accumulation oil source and reservoir.

Geological Conditions and Exploration Potential of Shale Gas in Dalong Formation in Northern Sichuan Basin

WANG Mingfa, WEN Hu, NI Kai, FU Xiaoping, YANG Hongfei

2023, 45(1): 13-23. DOI: 10.11885/j.issn.1674-5086.2020.11.05.01

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On the basis of the latest Well LB1 and 7 outcrops data and data of some old wells, the study evaluates the horizontal and vertical distribution law of high-quality shale of Dalong Formation in northern Sichuan Basin. It is considered that the distribution of high-quality shale is controlled by sedimentary facies and shows NW-SE trend. Basin facies have developed in Guangyuan-Wangcang Area with small shale thickness(10~20 m); deep-water shelf facies have developed on both sides of the basin facies, and the thickness of shale is relatively bigger(20~45 m); shallow-water shelf or slope facies have developed in the periphery, and the thickness of shale gradually becomes smaller(0~15 m). From slope facies to shelf facies, then to basin facies, the content of calcareous decreases and the content of silica increases gradually; the thickness of shale is bigger first (deep-water shelf facies) and then smaller (basin facies). In addition, the geochemical characteristics and reservoir microscopic characteristics of Well LB1 are studied, then compared with the Silurian shale fields in Jiaoshiba, Dingshan and Dongxi areas. It is concluded that the high quality shale of Dalong Formation has the characteristics of high organic carbon content, high gas content, good kerogen type, moderate maturity and good compressibility. It is considered that the extensive development of amorphous organic matter is the controlling factor of high organic carbon contentand the anoxic sulfide environment is the key to preservation. The characteristics of high gas content is due to a good correlation between gas content and the specific surface area, and the main contribution of the specific surface area is the content of organic matter. Finally, we have found that the resources of Dalong Formation in SINOPEC mining right have great exploration potential, which is worthy of further exploration.

Research on Parallel Inverse Q Filtering Methods for Seismic Wave Energy Compensation

ZHANG Quan, WANG Yipin, ZHANG Wei, PENG Bo, XU Lin

2023, 45(1): 24-32. DOI: 10.11885/j.issn.1674-5086.2021.02.03.03

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In seismic data processing of petroleum exploration, the inverse Q filtering method can effectively perform amplitude compensation and phase correction on seismic waves to provide more accurate information for seismic inversion and reservoir prediction. In large-scale seismic data processing, the inverse Q filtering method takes longer operation time under the CPU computing platform, which affects the efficiency of seismic interpretation. After analysis, it is found that the inverse Q filtering method consumes a lot of time in the short-time Fourier transform and calculates the amplitude and dispersion compensation terms. On the GPU platform, we first parallelizes the amplitude and dispersion compensation calculations, and accelerates the batch short-time Fourier transform with the CUFFT library, and then further optimizes the batch short-time Fourier transform and applies it to the inverse Q filtering method. The results show that compared with the CPU computing environment, the efficiency of the inverse Q filtering parallel algorithm based on the CUFFT library is improved by 3.9 times, and the optimized batch short-time Fourier transform further improves the efficiency of the parallel inverse Q filtering method by 12%.

A Study on the Characteristics of Reservoir Pore Structure Based on Nand Mercury Injection Data

ZHANG Qian, TANG Haizhong, MU Mingyang, WEI Jun, ZENG Ligang

2023, 45(1): 33-42. DOI: 10.11885/j.issn.1674-5086.2020.11.13.11

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X1 Wellblock is located in the Changshaling structural belt of Ying'er Sag, Jiuquan Basin. The rock types of this reservoir of the first member in Xiagou Formation of Lower Crataceous are mainly lithic sandstone and feldspar lithic sandstone. Reservoir heterogeneity is strong, and thin interbedded sand and mudstone are especially developed, which brings great difficulties to reservoir comprehensive evaluation. However, fluid identification, reservoir evaluation and later development measures depend on the overall understanding of pore structure of reservoir to a large extent. Taking X1 Wellblock in Changshaling as an example, the statistical analysis was carried out on the mineral composition in the study area, the pore types are classified study of electron microscopic thin section(SEM) observation, and then the key works, under the wellblock of mercury injection and nuclear magnetic resonance T₂ spectrum of test data, established the quantitative calculation methods of core pore throat radius and the T₂ relaxation time spectrum, and connecting with the nuclear magnetic resonance (NMR) logging data. A method for continuously and quantitatively characterizing the variation characteristics of reservoir pore-throat radius is developed. The quantitative characterization of pore-throat radius can provide ideas for us to understand the change of reservoir porosity and permeability, and provide more abundant information for the study of reservoir pore structure and reservoir heterogeneity.

Comprehensive Analysis of Logging and Excavation Effects in Carbonate Reservoirs

WANG Weijun, LIU Hongqi, ZHU Guangya, XIN Jionglong, WANG Youjing

2023, 45(1): 43-54. DOI: 10.11885/j.issn.1674-5086.2020.06.26.01

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Due to the large structural changes and complex lithology, there are many contradictions in fluid identification and fluid interface division of carbonate reservoir in H Oilfield. For gas identification, compensated neutron logging mining effect is effective in some blocks, but not in others. Therefore, it is necessary to analyze the influencing factors of mining effect. Firstly, the lithology and pore structure characteristics of carbonate reservoir in H Oilfield are analyzed. Then the physical characteristics of Layer KA and KB such as pore permeability are analyzed. The response characteristics of mining effects formed by neutron logging and density logging in gas reservoirs are summarized by those curves of several Wells, and the reasons of forming true and false mining effects are analyzed. The density log and neutron log correlation was expounded in theory, and two kinds of logging response correlation equation were deduced. The factors affecting the mechanism of the excavation effect were analyzed from the borehole diameter expanding permeability, pore structure, physical property and saturation. Finally, the comprehensive utilization of mining effect judgment analysis method of gas was suggested and the excavation effect of judge true and false of a few basis was put forward. The fluid identification of K Reservoir in H Oilfield is well completed with this method. Therefore, this method can provide reference for the accurate identification of fluid types in H Oilfield and similar complex carbonate reservoirs.

Research Progress, Challenge and Prospect of Nanoscale Oil-displacing Materials for Enhanced Oil Recovery

ZHANG Chenjun, JIN Xu, YUAN Bin, ZHANG Lei, ZHENG Shaojing

2023, 45(1): 55-70. DOI: 10.11885/j.issn.1674-5086.2022.04.27.03

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This review summarizes the research progress of nanoscale oil-displacing materials developed by various researchers, describes the application effect of silica nanoparticles, metal-oxide nanomaterials, polymer nanospheres and carbon nanomaterials, analyses the physical and chemical properties of existing nanoscale oil-displacing agents and their EOR mechanism in porous reservoir media, including nanometer-size effect, wettability alteration, structural disjoining pressure, mobility ratio improvement, and predicts the potential of nanoscale oil-displacing materials in high water cut reservoir, low permeability reservoirs, tight oil and shale oil in China. In addition, the review points out the challenges and research direction in the process of oilfield development, such as testing methods, accuracy of mathematical model, material versatility, economy and industrial production, and provides a theoretical and experimental basis for their large-scale application in EOR.

Progress of Researches on Self Repairing Hydrogel and Its Application in Oil and Gas Fields

TU Hongjun, ZHOU Ming, LI Tongtong

2023, 45(1): 71-80. DOI: 10.11885/j.issn.1674-5086.2020.06.10.04

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In view of the phenomenon that self-healing hydrogels can imitate the damaged parts of plants in nature to repair themselves at the molecular level, the basic principles of design and synthesis of self-healing hydrogels are summarized. Self-healing hydrogels are mainly divided into chemical self-healing hydrogels and physical self-healing hydrogels. In general, chemical self-healing hydrogels can be divided into self-healing hydrogels based on Diels——Alder addition reaction, self-healing hydrogels based on reversible disulfide bond and self-healing hydrogels based on acylhydrazone bond; physical self-healing hydrogels can be divided into self-healing hydrogels based on hydrogen bonding, self-healing hydrogels based on host guest interaction, and self-healing hydrogels based on electrostatic interaction. Self-healing hydrogel not only retains the good water absorption, water retention, biocompatibility and other properties of traditional hydrogel, but also has self-healing properties that can effectively extend the service life of materials and reduce the use cost of materials. On this basis, the potential application value and situation of self-healing hydrogel in oil and gas fields, biomedicine and other fields are analyzed and discussed, and the development trend of self-healing hydrogel in oil and gas fields is prospected.

Interpretation Model of Gas-injected Tracer for Well Groups in Fracture-vuggy Reservoirs

LIU Xueli, ZOU Ning, LIU Jianyi, XIANG Rui, LIU Zhibin

2023, 45(1): 81-88. DOI: 10.11885/j.issn.1674-5086.2020.11.15.01

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The conventional gas injection tracer interpretation method for sandstone reservoirs is based on the assumption of continuum, which is not suitable for large-scale fracture-vuggy reservoirs with discrete combinations of fractures and caves. A physical and mathematical model of gas tracer interpretation between injection and production wells in fractured-vuggy reservoirs has been established. The model considers the radial migration and diffusion of gas tracers in large-scale cave areas and the transmission of one-dimensional migration and two-dimensional diffusion in fractures. In this way, the distribution coefficient between wells, phases, and fractures are introduced. The model is solved and the theoretical recovery curve of gas tracer concentration is drawn. The parameter sensitivity analysis of the model is carried out, and the influence of the size of the cavern, the number and length of fractures, the diffusion coefficient, the gas injection rate, and the amount of tracer injection on the output curve is explored. Studies have shown that the larger the cave, the later the peak of the concentration curve appears; the longer the crack length, the lagging of the peak, the more the number of bars and the more the peak. The measured response curve of gas injection tracer in the TK425 Well Group of Tahe Oilfield is fitted and interpreted, and the volume of the karst cave area and the fracture channel between injection and production wells are calculated. The gas injection tracer interpretation model for fractured-vuggy reservoirs can effectively determine the connectivity between wells, calculate formation parameters, understand the distribution of reservoirs, and provide theoretical support for gas injection to improve recovery.

Characterization and Evaluation Method of Streamline Field in High Water Cut Reservoir

LIU Chaoyang, GUO Qi, LI Gang, HUANG Bo, WANG Zhenyu

2023, 45(1): 89-96. DOI: 10.11885/j.issn.1674-5086.2020.10.22.02

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By extracting the spatial coordinates and attribute data of particles in streamline field obtained through numerical simulation, we established a characterization method of streamline field with streamline cluster flow rate, streamline cluster potential and streamline cluster oil content. The streamline field is classified and evaluated by density peak algorithm, and the clustering effect of streamline classification is verified by S_Dbw coefficient. Finally the method of quantitative characterization and evaluation of streamline field in high water cut reservoir is formed. The results show that the three parameters can better reflect the distribution relationship and size of flow rate and potential between injection and production wells than conventional methods. Streamline strength grades in different regions can be quantitatively determined by clustering and grading of characterization parameters. The method is applied to an actual block in an eastern oilfield. The whole streamline field is divided into 14 types. The displacement intensity of each region is quite different. Through streamline field reconstruction, the streamline field grade are adjusted from 14 types to 7 types, the flow heterogeneity is weakened, and the reservoir productivity is improved obviously.

The Expansion and Application of Deviation Factor Chart of Super-high Pressure and High CO₂ Gas Reservoir

LU Ruibin, WANG Wenjuan, ZHANG Qiaoliang, HU Lin, CHEN Jian

2023, 45(1): 97-104. DOI: 10.11885/j.issn.1674-5086.2020.06.13.01

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Standing-Katz plate is one of the most common and basic methods to determine the deviation factor of natural gas. However, there are some problems, such as large error in manual reading, limited use of natural gas with super-high pressure (p_pr>15) and high CO₂ content. We design a single component deviation factor measurement experiment, and combined with the changing rule of the curve of the Standing-Katz plate, expand the p_pr to 30, the T_pr to 3.6, and digitize the plate. Through different CO₂ content deviation factor tests, and a new calculation model of pseudo critical pressure and pseudo critical temperature of natural gas considering different CO₂ content is derived. Compared with the conventional non-hydrocarbon correction model, the new model has higher calculation accuracy and wider application range. Furthermore, the new correction model is embedded in the deviation factor expansion chart, and the software is used. The deviation factor expansion chart considering different CO₂ content has a smaller error (≤3 %) in the calculation of the deviation factor of HTHP and high CO₂ content natural gas in the west of South China Sea, and the application effect is better.

A Study on Fractal Quantitative Characterization Method of Low Permeability Sandstone Pore in DLH Oilfield

SUN Qiang, SUN Zhigang, ZHANG Chao

2023, 45(1): 105-116. DOI: 10.11885/j.issn.1674-5086.2020.11.04.03

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In view of the difficulty of conventional characterization methods to accurately characterize the complexity and irregularity of pore space distribution in low-permeability sandstone reservoirs, a fractal dimension calculation method is proposed to realize the quantitative characterization of pore characteristics of low-permeability sandstone reservoirs. Based on the analysis of the difference between different fractal dimension calculation methods, the MIFA method is preferably used to solve the fractal dimension of low permeability sandstone reservoirs (between 2.042 and 2.324), with the best correlation. The displacement pressure, average pore throat radius, coefficient of variation, and mean coefficient are determined as the comprehensive characterization parameters of the complexity and heterogeneity of reservoir pore throat distribution. Based on the solution of the fractal dimension of mercury injection at a constant rate, it is found that the heterogeneity of low-permeability sandstone reservoirs presents the following characteristics:small and medium pore throats are larger than small pore throats, and throat distribution is larger than pore distribution. There is certain correlation between the fractal dimension of low-permeability sandstone reservoirs and the experimental results of starting pressure gradient and stress sensitivity damage rate. The larger the fractal dimension, the stronger the heterogeneity of the pore throat distribution and the larger the starting pressure gradient is needed and the damage to the stress sensitivity will increase. The calculation of the fractal dimension of low-permeability sandstone reservoirs can be used for qualitative prediction and judgment of laboratory experimental results, and also as an important criterion for the application of relative permeability curves in reservoir engineering.

Research on Optimization and Application of Fracturing Perforation Orientation in Directional Wells Based on Minimum Initiation Pressure

WANG Erjun, MA Lei, CAO Feng, FENG Ming, ZENG Fanhui

2023, 45(1): 117-126. DOI: 10.11885/j.issn.1674-5086.2021.01.28.02

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Hydraulic fracturing with directional wells in low permeability reservoirs is an important measure to increase production and efficiency and decrease cost in oil and gas fields. Perforation is the first procedure to open a reservoir before fracturing. The quality of perforation directly affects the productivity of directional wells. In order to reduce the fracturing pressure of reservoir and the risk of sand plugging, the perforation azimuth of directional wells fracturing is optimized. Different from previous studies, this paper considers the combined superposition of original ground stress, casing cement ring induced stress, perforation hole induced stress, wellbore injection induced stress and fluid seepage induced stress, and establishes an optimization model for directional well fracturing and perforating azimuth is established based on the minimum fracture initiation pressure(FIP) prediction and the tensile failure criterion. The reliability and rationality of the model are verified by the comparison of indoor physical simulation and theoretical model calculation results. The simulation results show that (1) FIP changes periodically in 360°perforation azimuth, there are two minimum and maximum FIP points, and with the increase of wellbore azimuth, the minimum FIP gradually increases, and the maximum FIP gradually decreases; (2) the perforation directions corresponding to the minimum FIP at different inclination angles and azimuths are quite different, and the optimal perforation direction for inclined wells should take into account the comprehensive effects of the inclination and azimuth angles at the same time; (3) factors such as horizontal principal stress difference and construction displacement have little effect on determining the optimal perforation orientation. The optimal perforation direction angle is 20°and 205°, and the corresponding minimum FIP is 45.5 MPa. Thus, it reduces the construction difficulty and provides reference for the perforation position optimization technique of low permeability reservoir reconstruction.

Effect of Nonlinear Effective Pressure on Stress Sensitivity in Fracture Sandstones

LIU Xugang, ZHOU Yongyi, ZHANG Junqiang, XIAO Wenlian, REN Xiyao

2023, 45(1): 127-135. DOI: 10.11885/j.issn.1674-5086.2020.12.03.01

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To implement the transformation of the relationships between permeability and confining pressures in changing-confining-pressure stress sensitivity experiments into the ones in changing-pore-pressure stress sensitivity experiments, and obtain the relationships between permeability and pore pressure, changing- confining -pressure stress sensitivity experiments and special stress sensitive experiments in tight sandstones with microfractures and artificial fractures are carried out. At the same time, based on the experimental data in changing-confining-pressure stress sensitivity experiments and the effective stress equations, the relationships between permeability and pore pressure are calculated, and are compared with the results in changing-pore-pressure stress sensitivity experiments. The results indicate that calculated permeabilities are larger than those measured directly, which is caused by the Klinkenberg effect, dimensionless permeabilities measured directly are equal to those calculated with the nonlinear effective principle, while larger than those calculated with Terzaghi effective principle. Both effective stress and slippage effect affect the determination of permeability in situ.

Sand Blocking Mechanisms and Sand Control Parameters Optimization Design in Fine Siltstone Reservoirs

JIA Peifeng, CUI Chuanzhi, ZHAO Yizhong, WU Zhongwei, ZHANG Yuchen

2023, 45(1): 136-144. DOI: 10.11885/j.issn.1674-5086.2020.08.04.01

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The siltstone reservoir has many vertical small layers. Well productivity is affected by sand blocking. Based on the existing productivity model of inclined vertical well, the permeability of screen pipe, annulus filling layer, squeeze filling layer and reservoir are described in detail, and the formula of skin coefficient of mechanical sand control is improved. The blocking law of sand control system is analyzed through laboratory experiments. It is considered that with the increase of displacement volume number and displacement flow, the plugging of sand control system intensifies and the maximum permeability decreases by 40%. Reservoir particles enter the casing and block the annulus gravel and sand control screen, resulting in a sharp increase in skin factor and a decrease in production capacity of more than 80%. Expanding the squeeze packing radius can block particles away from the wellbore and help to maintain oil well productivity. Through the optimization analysis of the filling radius of layered sand control wells, it is found that expanding the filling radius in medium and low permeability can improve the interlayer production effect. The filling parameters of sand control wells in typical blocks are optimized, and the filling radius of S4 layer with low permeability is expanded. With the measures, the oil well fluid volume remains stable, and the daily oil production increases by 9.56 t.

Selection and Optimization of Throttling Process for Ultra-high Pressure Gas Wells

LIU Peng, WANG Chongyang

2023, 45(1): 145-154. DOI: 10.11885/j.issn.1674-5086.2020.10.28.04

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The throttling system of ultra-high pressure gas wells plays a vital role in the life cycle of gas wells, and a reasonable throttling process can not only extend the life of gas wells, but also reduce costs. This research introduces the throttling technology used in domestic ultra-high pressure gas wells, and simulates the flow field of different throttling technology in gas wells in Northwest Sichuan Gas Mine based on computational fluid dynamics. The flow field analysis software is used to analyze the internal pressure and flow rate of single-stage and two-stage throttling process systems. The flow field analysis software is used to analyze the internal pressure and flow velocity of single-stage and two-stage throttling process systems. The reasonable CV value and opening degree of different throttling process systems were determined, and the highest flow rate of different throttling process systems was obtained. The throttling parameters selection and use suggestions under different throttling techniques on site are proposed. The research shows that the three throttling processes can meet throttling requirements and on-site production conditions. Among them, the single-stage throttling process is simple, but the flow rate is above 850 m/s, and the valve erosion is serious; one-stage fixed and two-stage adjustable throttling the maximum flow rate is between 550 m/s and 600 m/s, which is lower than the single-stage throttling, but the maximum output is limited; the maximum flow rate of the two-stage adjustable flow is similar to that of the first-stage fixed two-stage adjustable flow, and can be adjusted as needed. Production, but the cost is relatively high.

Construction and Application of Intelligent Gas Field Integrated Model Data Sharing Platform

LI Yueyang, REN Jingsi, ZHANG Su, JIN Wei

2023, 45(1): 155-162. DOI: 10.11885/j.issn.1674-5086.2020.11.19.01

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Integrated simulation model is an important basis for the construction of intelligent gas fields. How to use data-driven model in efficient operation has become a key problem to be solved urgently. In this article, in view of the data source dispersion, poor availability, low level of interaction with model, we carry out the following technical research:(1) combined with data service bus (DSB) and PI OLEDB interface technology, we realize effective integration of multi-source structured data and real-time sampling, meet production data's business requirements in different time dimension of the integration model; (2) we establish data quality rule base and automatic scanning system to realize the effective control of data quality and ensure the normal operation of the integrated model; (3) we develop data exchange tools based on data flow engine technology and has realized the automatic interaction between the comprehensive database and the simulation model. Its advantage is that when the application scene changes, the basic services flexible arrangement method can be used to efficiently reuse the results of data construction in the early stage, which greatly improves the development efficiency. Compared with similar technologies domestic and overseas, the application of data flow engine technology breaks through the interaction barrier between the integrated model and external data resources, breaks the service mode of customized data application development in this field, and lays a technical foundation for the construction of smart gas fields.

Vibration Response and Fatigue Prediction of Marine Steel Catenary Risers

ZHU Hongjun, DING Zhiqi, GAO Yue, ZHAO Honglei, HU Jie

2023, 45(1): 163-179. DOI: 10.11885/j.issn.1674-5086.2020.10.25.02

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Vortex-induced vibration (VIV) is a main factor contributing to the fatigue damage of marine risers. With the aid of Van Der Pol wake oscillator model, the present paper investigates the VIV of pinned-pinned steel catenary risers of different lengths subjected to different shear-flow profiles with the same flow rate. The second-order central difference method is employed to solve the coupled equations in both time and space domain, and the rain flow counting method is used to predict the fatigue life. The vibration presents a blended pattern of standing wave and traveling wave for the response propagation along the span. The vibration shifts from the standing-wave dominated response to the traveling-wave dominated one with the increase of the riser length. The riser experiences multi-frequency oscillation due to the influence of shear flow, and the traveling wave propagates from the riser top to its bottom. Furthermore, the propagation rate of traveling wave varies with the shear degree of flow profile and the riser length. The riser extracts less energy from ambient flow as the shear degree increases, while the energy consumption grows, resulting in the reduction of response amplitude. The fatigue damage is aggravated with the water depth at the same incoming flow profile. The same result is observed as the shear degree of flow profile increases at the same water depth.

A Study on the Mechanism and Heat Generation Law of PDC Wear Tooth Cutting

YANG Yingxin, XIE Song, CAI Can, CAO Yulei, LI Bangrun

2023, 45(1): 180-188. DOI: 10.11885/j.issn.1674-5086.2020.06.04.03

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To explore the cutting process of deep drilling PDC bit cutting teeth in the wear state, the single tooth cutting experiment is carried out by selecting PDC cutting teeth with different wear degrees on the deep drilling PDC bit. The stress state, temperature change law and crack propagation process are analyzed by stress test system, high speed camera and thermal infrared imager. The results show that the cutting force, temperature change and large volume cuttings are more obvious than the non-wear teeth, which will lead to the outburst of drill bit vibration and the sharp acceleration of wear, seriously affecting its service life. On the other hand, when cutting granite, the problem of cutting force increase and heating up is more prominent, but volume breakage is more likely to happen and produce larger block cuttings. The research results will help to re-understand the rock breaking and temperature variation law of real bottom wear teeth, and provide important reference for bit design and downhole life evaluation.

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