Table of Content

10 December 2019, Volume 41 Issue 6

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Linear Coupling Seepage Model for Fractured Horizontal Wells in Shale Gas Reservoir

ZHANG Liehui, CUI Qianchen, XIE Jun, ZHENG Jian, LI Chengyong

2019, 41(6):  1-12.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.13

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In order to study the effects of threshold pressure gradient at low-velocity gas flow in shale matrix on well test curves, and to overcome the difficulties of coupling of flow mechanism of shale gas, a new cylindrical triple-porosity shale matrix model was proposed. Combined with the five-region flow model, a new linear seepage model characterizing flow process of multifractured horizontal wells was established. Laplace transformation and Green function were employed to solve the model, and the Stehfest numerical inversion algorithm was applied to plot dimensionless pseudo-pressure curves and conduct sensitivity analysis. The results show that pressure response curves of the new model could be divided into 7 periods and the threshold pressure gradient conspicuously affects the middle and later periods. The lager the threshold pressure gradient is, the larger seepage resistance is, and the higher pseudo-pressure and its derivative are. The smaller the matrix permeability is, the more difficult it is for transition flow from matrix to fractures, and the later the boundary control flow occurs.

An Experimental Study on NMR Response Characteristics of Imbibition Subjected to Pressure in Shale

XIAO Wenlian, ZHANG Junqiang, DU Yang, ZHAO Jinzhou, ZHAO Zhejun

2019, 41(6):  13-18.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.12

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The imbibition of shale samples from the Sichuan Basin are studied under different pressure cycles using the technology of NMR in order to have some insight into the influence of invading fluids in shale gas reservoirs. The results show that the T₂ curve of sample S1 (confining pressure 0) gradually expands during the process of spontaneous imbibition and changes from a single peak to bimodal ones, indicating that the amount of imbibition continuously increases and a new pore-fracture system is formed, which is observed in microstructure experiments. Meanwhile, the T₂ curve of sample S2 (confining pressure 8 MPa) has single peak and almost remain unchanged; when the confining pressure on sample S2 is unloaded, the T₂ curve turns into the bimodal ones, which is consistent with the results of sample S1, illustrating that the stress on the shale sample has an effect on the imbibition of shale. Therefore, it is suggested that the effect of stress sensitivity on shale imbibition should be considered in future study and application.

Influence of Seepage Velocity on Shale Gas Exploration by Pore Network Simulation

GUO Xiao, YANG Kairui, YANG Yurui, JIA Haowei

2019, 41(6):  19-27.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.01

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Pore network model is an effective method to predict the flow characteristics of porous media. Based on characteristic values of shale gas reservoir, a pore network was established. And gas-water phase flow was simulated in that pore network. Seepage image was drawn by solving the pressure matrix equations of the model. By simulating flow under different velocities, the water penetration and seepage path were analyzed. As the results shows, the seepage velocity was positively correlated with the displacement pressure difference. When seepage velocity is higher than critical velocity v_c, pressure drop gradient has a main effect on the seepage path, which conforms to characteristics of fractal curve, and seepage velocity has an effect on stability of oil-gas interface and destroys equilibrium of gas/water interface thus facilitates water penetration and reduce gas production when it's high. There will be a stable gas/water interface and a large amount of gas production when seepage velocity is lower than v_c.

Simulation of Methane Adsorption of Quartz with Different Wettability

WANG Zhouhua, ZHAO Jianfei, BAI Yin, GUO Ping, LIU Huang

2019, 41(6):  28-34.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.11

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As one of the important basic parameters of reservoir rocks, wettability's characteristics directly affect the micro and macro distribution characteristics of fluids in rock pores. Understanding the interaction between shale gas and minerals at the micro-level is the basis of understanding the state occurrence of shale gas. Molecular simulation was used to study the adsorption of methane in different conditions of modified quartz, and different crystal faces and wettability were studied. The conclusions of the study are as follows. In different crystal-faces models, the adsorption capacity of (100) surface is greater than that of the other models. The adsorption of methane in the modified oil-philic model is larger than that in the modified water-philic model. The adsorption of methane in different wettability modification models increases with the increase of pressure, and decreases with the increase of temperature, and the effect of temperature is less than that of aperture. Under the same conditions, the adsorption capacities of the modified models are much larger than that of the unmodified model. In the 1 nm modified models, an adsorption layer is formed at the wall surface, while multiple adsorption layers are formed in 2 nm modified models.

An Experiment Study on the Effect of High Temperature on the Acoustic Properties of the Shale

XIONG Jian, HUANG Linlin, LIU Xiangjun, ZHOU Wen, LIANG Lixi

2019, 41(6):  35-43.  DOI: 10.11885/j.issn.1674-5086.2019.08.29.02

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Taking the organic shale of Longmaxi Formation in Sichuan Basin as the research object, the variation laws of physical properties of shale rock samples after high temperature treatment are investigated. The influences of high temperature on the acoustic characteristics of shale rocks are discussed. And the variation laws of dynamic elastic parameters of shale rocks under high temperature influence are also studied. The results show that when the operating temperature is lower than 400℃, the temperature has little effect on the apparent color and physical properties of the shale, but when the operating temperature exceeds 400℃, the apparent color of the shale gradually changes from black to grayish white, and the shale quality declines rapidly, the volume expansion rate rises rapidly, the shale density decreases rapidly. As the temperature increases, the shale acoustic wave velocity decreases continuously, and the acoustic wave dominant frequency decreases, while the shale acoustic wave attenuation coefficient increases continuously, and the acoustic wave attenuation coefficient is more sensitive to the thermal damage effect of shale rock than the acoustic wave velocity. As the temperature increases, the shale damage factor increases, and the dynamic Poisson's ratio of shale also increases, while the dynamic elastic modulus, bulk modulus and shear modulus of shale rock samples decrease.

Shale Layer Correlation of Horizontal Wells and Their 3D Visual Characterization

OU Chenghua, YANG Xiao, LIANG Chenggang, HUANG Yi, ZHU Haiyan

2019, 41(6):  44-50.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.04

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Till now, it is still a technical problem to build the single-layer architecture and surface structure models for a shale gas-production area under the horizontal well factory pattern. Focusing on the single-layer shale, We analyze the structure characteristics of the single shale layers, establish the shale layer correlation model, develop the method of 3D visual multi-well correlation technology for the single-layer shale by using horizontal well logging, carry out the division and correlation of the single-layer shale, and complete the 3D single-layer architecture and surface structure models by relying on thecoupling method. A new method of 3D visual characterization of the single-layer architectures and surface structures under the horizontal well factory pattern in the shale gas-production area is finally established. By using this method, the establishment of 6 shale layer models, the visual shale layer correlation of horizontal wells and the establishment of the single layer structure and 7 single layer surface structural models in one shale gas-production area of the shale gas field W have been successfully completed, which not only proves the practicability and reliability of this method, but also provides the structural models for fine production management in the later stage of the study area.

Borehole Completion Stability of Deep Brittle Shale Horizontal Wells

MENG Yingfeng, LIU Houbin, YU Anran, HU Yongzhang, DENG Yuanzhou

2019, 41(6):  51-59.  DOI: 10.11885/j.issn.1674-5086.2019.09.18.03

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The conventional shale gas reservoir reconstruction process combines casing perforation with multi-stage fracturing reconstruction, with high construction cost and long cycle. With the horizontal well open hole completion, the cost can be reduced and the cycle can be shortened, but the horizontal open hole The stability of the section wall is a prerequisite for the implementation of open hole completion. In this paper, the physicochemical properties and mechanical properties of the Longmaxi Formation shale are studied, and the established open hole completion model is used to evaluate the stability of the open hole wall in the horizontal section of the shale in the Weiyuan-Rongxian area. The results show that the Longmaxi Formation shale is a brittle layered shale with dense rock mass, high mechanical strength and obvious mechanical anisotropy. The oil-based and water-based drilling fluids cause no obvious change to rock swelling performance and mechanical properties. The drilling fluid used in the field has a high compatibility with the formation; the well trajectory, the bedding direction, the mechanical anisotropy, etc. can affect the stability of the open hole section, resulting in a significant difference in the internal stress of the wellbore in different directions; drilling horizontal wells along the direction of minimum horizontal principal stress has the best wall stability. Under the completion condition, the equivalent density of borehole wall collapse pressure is 1.26 g/cm³. In the initial stage of completion, the well wall has a slight collapse, but the collapse range is less than 30°. The horizontal wells can be considered for open hole completion in the Longmaxi Formation shale formation.

Study on Mechanical Characteristics and Wellbore Stability of Hard Brittle Shale in Western Sichuan

LIU Houbin, CUI Shuai, MENG Yingfeng, WU Shuang, WU Ke

2019, 41(6):  60-67.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.10

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In the drilling process of hard brittle shale formation, the phenomenon of wellbore instability occurs frequently, which seriously hinders the process of field drilling. In view of the phenomenon of downhole collapse in the process of shale drilling in Xiaotangzi Formation, we carryied out systematic laboratory tests of the shale cores of Xiaotangzi Formation established the theoretical model with experimental data, and evaluated and analyzed the wellbore stability of Xiaotangzi Formation. The results showed that Xiaotangzi Formation is the typical hard brittle shale with developed bedding cracks, which can produce tensile fracture under external force or high stress. Drilling fluid immersion effect can reduce the mechanical strength of shale formation in Xiaotangzi Formation. The influence of bedding strike and weak plane effect on the mechanical properties of shale in Xiaotangzi Formation is obvious. Fracture strike, wellbore trajectory and wellbore seepage field affect wellbore stability of shale in Xiaotangzi Formation. When the angle between fracture surface and borehole axis satisfies a certain relationship, the borehole wall rock is prone to slip and collapse along the fracture surface.

Technology of In-situ Toughening of Oil Well Cement in Shale Gas Horizontal Wells

CHENG Xiaowei, ZHANG Gaoyin, MA Zhichao, LI Bin, GU Tao

2019, 41(6):  68-74.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.07

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As a non-homogeneous porous brittle composite material in the development of shale gas horizontal wells, oil well cement is difficult to withstand the heterogeneous load caused by large hydraulic fracturing, resulting in failure of the mechanical integrity of the cement sheath. Researches on oil well cement in-situ toughening technology was carried out. In the study, solid phase sintering and spherical re-quenching techniques were used to obtain microcrystalline brownmillerite (MB) as an in situ toughening material. The research indicates the prepared in-situ toughening material reduced the elastic modulus of the cement based on the improvement of mechanical properties. After 7 days of curing, the compressive strength ccould reach 24 MPa, the tensile strength was increased by 50%, and the elastic modulus was reduced to 5.55 GPa. The toughening mechanism of microcrystalline brownmillerite is: "crack deflection", "crack termination", "consumption fracture energy". The development of this study is great significance to meet the requirements of oil well cement service under shale gas mining conditions, extend the service life of shale gas wells and increase the production capacity of shale gas.

Key Technology Progress and Enlightenment in Refracturing of Shale Gas Horizontal Wells

YANG Zhaozhong, LI Yang, LI Xiaogang, LI Chengyong

2019, 41(6):  75-86.  DOI: 10.11885/j.issn.1674-5086.2019.10.15.05

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Due to the unfavorable factors of initial stimulation treatment in shale gas horizontal wells, the output is less than expected, coupled with the current downturn in the oil price background, shale gas horizontal wells refracturing technology is increasingly concerned about the development of shale gas resources. With the continuous development of fracturing technology, many shale gas fields have now successfully implemented this technique to increase production rate and to enhance ultimate gas recovery from shale gas wells. Overviewed the procedure for candidate well identification and discussed the key fracturing techniques and operation monitoring techniques. Lastly, based on the current situation of the development of different techniques, the direction of the refracturing technique of shale gas horizontal wells were prospected. Some suggestions were put forward, such as selecting wells with "big data" technology, flexible restoration of casing damaged and deformed wells, fracture monitoring based on damage mechanics, and fracturing fluid and proppant with new materials. As China is abundant in potential shale gas reserve, to accelerate the formation of supporting technology suitable for shale gas horizontal wells refracturing in China can play a role in promoting the commercial development of shale gas in China in the future.

Wellbore Fracture Initiation Mechanical Behavior in a Horizontal Shale Gas Well

MA Tianshou, PENG Nian, CHEN Ping, DENG Zhizhong

2019, 41(6):  87-99.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.05

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In order to clarify the effect of anisotropy of layered shale on the wellbore fracture initiation mechanical behavior, the uniaxial compression testing and Brazilian split testing experiments with different angle were conducted for LMX shale in the Lower Silurian Sichuan Basin. The anisotropic characteristics of elastic modulus and tensile strength were specified, and a wellbore fracture initiation mechanical model of shale gas horizontal wells was established considering the anisotropy of shale. The wellbore fracture initiation behaviors under different influencing factors are also analysed. The results show that:modulus anisotropy, in-situ stress difference, tensile strength anisotropy and pore pressure all have an obvious effect on fracture-initiation pressure but Poisson's ratio anisotropy has slightly effect on it. Modulus anisotropy and in-situ stress difference also have a significant influence on wellbore fracture initiation location and inclination angle, Poisson's ratio anisotropy, tensile strength anisotropy and pore pressure, however, have slight effects on that. Wellbore fracture initiation mechanical behaviour is closely related to wellbore azimuth; when the borehole azimuth is close to the direction of maximum or minimum horizontal principal stress, the difference of wellbore fracture initiation mechanical behavior varies slightly with borehole azimuth changing, while when the borehole azimuth is far away from the direction of maximum and minimum horizontal principal stress, the difference of wellbore fracture initiation mechanical behavior varies greatly with the changing of borehole azimuth. The research results can provide theoretical basis and reference for well leakage prevention during drilling and hydraulic fracturing design of shale gas horizontal wells.

Low Damage Supercritical CO₂ Gel Fracturing Fluid for Shale Gas

ZHOU Ming, LIAO Mao, HAN Hongchang, XIAO Yang, LI Chen

2019, 41(6):  100-105.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.07

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Considering that conventional hydraulic fracturing fluid could cause damage to the shale, easy to produce water lock, not easy to flowback, and cause problems such as water consumption and environmental pollution, low damage carbon dioxide gel fracturing, the self-made F2EU and F4EU thickeners were added to the supercritical CO₂ base solution to research the effect of the dosage of the two thickeners on the viscosity of supercritical CO₂ gel fracturing fluid. Taking into account the cost and the efficiency, the optimum thickener was 2% F4EU, which could increase the viscosity of CO₂ to 15.4 mPa·s. The effects of temperature, pressure and shear rate on the viscosity of gel fracturing fluid system was studied. The experimental results showed that the system viscosity decreased with the increase of temperature, but there was a brief rising stage in the middle and the fracturing fluid system viscosity increases with the increase of pressure; but the fracturing fluid viscosity decreased with the increase of shear rate, which proved to be a typical shear thinning pseudoplastic fluid. The average damage rate of F4EU thickened supercritical CO₂ gel fracturing fluid was 1.39%, which was much less than the damage rate of conventional fracturing fluid. The researches showed the F4EU thickened supercritical CO₂ gel fracturing fluid would have a good application prospect in shale gas production.

Water Absorption of Organic Shale with Oxidation

YOU Lijun, XU Jieming, KANG Yili, CHENG Qiuyang, ZHOU Yang

2019, 41(6):  106-116.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.08

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Shale gas reservoirs generally need to be fractured for the industrial production. However, the flowback of fracturing fluid is low. Residual fracturing fluid can induce fracture in shale gas reservoir, but it also occupy seepage channels in shale gas reservoirs. This paper takes typical organic rich shale samples from the Longmaxi Formation in the Sichuan Basin, and recoustructed the fractured shale gas reservoir by sand pack and granular shale. Through comparative experiments of water absorption of shales treated with water and oxidation liquid, we find out mechanism of oxidation effect on fracturing fluid imbibition and dispersion to pore of matrix near fracture. The experiment results show that the liquid imbibition range increase after oxidation of granular shale, free water in shale pore space decreases and water absorbtion of increases. It is concluded that the oxidation-induced dissolution of organic matter can increase the contact area between water and clay minerals, and promote the imbibition of clay minerals and the growth of micro-fractures. The water saturation of fractures decreases, and increase seepage channels of gas. It is suggested to optimize the formula of fracturing fluids and the soak system according to the specific engineering geological characteristics, and optimize the adding time and amount of oxidizing fluid. so as to improve to the fracturing capacity of the retained oxidizing fracturing fluid, promoting the dispersion of fracturing fluid, minimize the return of fracturing fluid, and to increase effective seepage channels of gas in fractures.

Shale Productivity Model Considering Stress Sensitivity and Hydraulic Fracture Azimuth

LI Yongming, LUO Ang, WU Lei, YI Xiangyi

2019, 41(6):  117-123.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.02

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The productivity prediction plays an important role in the efficient and rational development of shale gas reservoirs. At present, the researches on the productivity of multi-stage fracturing horizontal wells in shale gas reservoirs at home and abroad does not take into account stress-sensitive effects of natural fractures and hydraulic fracture morphology and seepage characteristics on productivity. Therefore, based on dual medium seepage theory, considering the adsorption, desorption and diffusion mechanisms (pseudo-steady state and transient diffusion) of shale gas in reservoirs and stress-sensitive effects of natural fractures, the shale gas reservoir seepage model is established. The limited conductivity of hydraulic fractures and hydraulic fracture azimuth are considered into the hydraulic fracture model. The source function method is used to discretize the crack, and then the hydraulic fracture model is superimposed. Finally, the two models are coupled to obtain the unstable seepage model and productivity model of the multi-stage fracturing horizontal well in the shale gas reservoir. According to the established shale gas fracturing horizontal well productivity model, the productivity characteristic curve is calculated by programming. The comparative hydraulic simulation results show that the optimal hydraulic fracture conductivity of the shale gas fractured horizontal well which is similar to the example is 15~18 D·cm, the optimal distribution mode of seam length is u-shaped with outer high and inner low, and the optimal fracture spacing distribution is equidistant. The comparison between the simulation results and the field data of shale gas wells also verifies the accuracy of the model. This research is of significance to shale gas development.

A New Method for Quick EUR Evaluation of Shale Gas Wells

ZHAO Yulong, LIANG Hongbin, JING Cui, SHANG Shaofen, LI Chengyong

2019, 41(6):  124-131.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.09

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Managed pressure drilling (MPD) is usually applied to shale gas, which can reduce reservoir stress sensitivity, improve production and increase the estimated ultimate recovery (EUR) of gas wells. However, the frequent change of working system leads to the fluctuation of production data, and brings new challenges to accurately EUR prediction. Therefore, based on the analysis of data processing methods, the method of noise reduction for data fluctuation is firstly completed, and the data can effectively converge in the double logarithmic coordinate after using pressure normalization production and material balance time. Then, the EUR model of shale gas well is established based on the data characteristics after noise reduction. Finally, a new evaluation method of EUR for shale gas wells is proposed. Compared with the results of the new method and classical method based on the field production data of the shale gas well, the new method is proved reliable. It is easier to operate and calculate than the classical method, which is difficult to fit due to multiple parameters. Therefore, this method can be applied to quick EUR evaluation of shale gas wells.

Productivity Evaluation of Fractured Wells in Shale Gas Reservoirs

HU Degao, GUO Xiao, ZHENG Aiwei, SHU Zhiguo, ZHANG Baiqiao

2019, 41(6):  132-138.  DOI: 10.11885/j.issn.1674-5086.2019.11.17.01

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Shale reservoir has nano-pores structure with self-generation and self-storage, so fracturing has become an important technology for shale gas exploitation. Based on the multi-scale non Darcy flow model, a multi-scale flow model is established to simulate various flow states including continuous flow, transition flow, slippage flow and free molecular flow. The steady-state production equation of fractured well with finite conductivity fracture is obtained. In this model, the influence of pore size on Knudsen coefficient is fully considered. The IPR curve of fractured well is obtained, and the effects of fracture half length, Knudsen diffusion coefficient, shale permeability, slippage factor, fracture penetration ratio and fracture conductivity on the production of fractured well are explored. The results show that the permeability correction coefficient has a great influence on productivity, and the productivity of fracturing well determined by multi-scale seepage model is very consistent with the actual production data. When the flow pressure in the bottom hole is less than 15 MPa, the slippage effect on production begins to increase in the fracturing well, and with the increase of slippage factor, the fracturing well production increases. The lower the permeability of shale gas reservoir, the greater Knudsen diffusion coefficient D_K and slippage effect on production.

The Finite-conductivity Fracture Networks Model in Shale Gas Reservoirs with Consideration of Induced Fractures

FANG Quantang, LI Zhenglan, DUAN Yonggang, WEI Mingqiang, ZHANG Yuyi

2019, 41(6):  139-145.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.06

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In order to analyze the influence of secondary induced fractures on the pressure response of shale gas wells, a finite conductivity fracture network flow model of shale gas reservoir with coupling multiple migration mechanism has been established, and the pressure dynamic characteristics have been studied. Firstly, the analytical solution of pressure in shale gas reservoir has been obtained by employing Laplace space source function, local coordinate transformation and superposition principle. Then, based on the finite difference method and the flow distribution transformation of intersection element, the numerical solution of fracture element has been derived. By coupling the flow of gas reservoir and fracture, the pressure response curve considering the influence of secondary fracture was drawn, and the influences of characteristic parameters (such as the number of secondary fracture groups, secondary fracture angle, and fracture conductivity, etc.) were analyzed. The results show that there are 10 typical flow stages, which can effectively characterize the influence of secondary fractures. In addition, the presented model will be helpful for understanding the transient performance of multi-stage fractured horizontal wells with consideration of induced fractures.

Research on Safety Assessment Technology of Sustained Casing Pressure in Shale Gas

ZENG Dezhi, YU Zhiming, HE Qiyao, LIU Qiaoping, SHI Taihe

2019, 41(6):  146-154.  DOI: 10.11885/j.issn.1674-5086.2019.09.18.01

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Aiming at the potential safety risks of wellbore annulus pressure caused by the integrity failure of the wellbore in the process of shale gas development, a safety risk assessment method for shale gas annulus compression wells is established. Based on the three key risk indicators of annulus pressure, cementing quality and formation integrity, this method considers the physical barrier, fluid barrier and operation management of shale gas wells based on fuzzy comprehensive evaluation theory. This method was applied to quantitative evaluation of the risk value and risk of 19 annulus pressure in a shale gas field. According to the results of risk assessment, a set of recommended practices for safety control of shale gas wells is presented from the aspects of determination of the maximum allowable value of annular pressure, diagnosis, monitoring and management of annular pressure, which provides important technical guidance for the safe production of shale gas wells.

Calculation Method of Critical Control Value of Sustained Annular Pressure in Shale Gas Well

ZHANG Zhi, DING Jian, ZHAO Yuanjin, DENG Hu, LU Qi

2019, 41(6):  155-164.  DOI: 10.11885/j.issn.1674-5086.2019.10.23.01

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Large scale fracturing of shale gas wells leads to the destruction of well integrity, a sustained annulus pressure problems, seriously affecting the safe production of shale gas wells. Aiming at the problem of annular pressure in shale gas wells, based on the calculation method of critical control value of annular pressure in API RP 90-2, considering the factors such as formation pressure, production change, corrosion and wear of shale gas wells, the change of pressure bearing capacity of each annulus component with service time in actual production is analyzed, the calculation method of critical control value of sustained annular pressure and its control chart are established in this paper, and an example is given. The results show that the critical control value of annular pressure in shale gas well is mainly affected by the pressure bearing capacity of wellhead device and technical casing in the early stage of service. In the late stage of service, with the decrease of corrosion rate and formation pressure, it is mainly affected by the external extrusion strength of weak point of tubing, and it decreases with the increase of service time and corrosion rate. When the control value of annular pressure is less than formation pressure, it is necessary to monitor the sustained annulus pressure value and take corresponding measures to ensure the safety of production on site.

Research and Application of Wellbore Transient Temperature in Deep Shale Gas Horizontal Wells

FU Jianhong, SU Yu, JIANG Wei, ZHONG Chengxu, LI Zhengtao

2019, 41(6):  165-173.  DOI: 10.11885/j.issn.1674-5086.2019.09.17.09

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Wellbore transient circulation temperature is of great importance in the selection of rotary steering tool during the deep shale gas horizontal well drilling process. A wellbore transient temperature model was established with consideration of heat exchange mechanism between the wellbore and the formation and the principle of energy conservation. The effects of circulation time, flow rate, horizontal length and drilling fluid inlet temperature on the annulus transient temperature were analyzed. The shale gas horizontal well trajectory control method was optimized, and then the measures for reducing the bottom hole circulation temperature were proposed. The results show that the annulus drilling fluid temperature in the upper part increases with the increase of circulation time and flow rate, while the drilling fluid temperature in the lower part decreases. As the horizontal length increases, the annulus drilling fluid circulation temperature increases, and the longer the horizontal length, the lower the cycle cooling effect. With the increase of the drilling fluid inlet temperature, the drilling fluid outlet temperature increase, but the drilling fluid temperature in the lower part changes little. When the vertical length is more than 4 000 m, the rotary steering drilling tool can be used with the short horizontal length, and the downhole motor with LWD measuring tool is recommended for long horizontal length and bottom hole circulation temperature is higher than 135℃. The bottom hole circulation temperature can be reduced by increasing the circulation time and flow rate and using the circulating while tripping-in, so as to ensure that and the rotary steering tool and LWD tool are within the safe working temperature.

Analysis of Pressure Characteristics in Working Cavity of Ultra High Pressure Reciprocating Pump for Shale Gas

WANG Guorong, WANG Teng, LI Rong, WANG Fei

2019, 41(6):  174-180.  DOI: 10.11885/j.issn.1674-5086.2019.09.16.08

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In order to explore the solution to improve the working performance and reliability of the pump, we took the ultrahigh pressure reciprocating pump for shale gas horizontal well fracturing as the research object, and analyzed and studied the pressure variation law and basic characteristics of the pump working chamber. According to the basic theory of mechanics, the pressure changes in the working chamber at the opening and closing time of the pump valve were qualitatively analyzed. A fullscale pressure test bench for working chamber of ultra-high pressure reciprocating pump was built, and pressure curves of pump chamber under four plunger strokes and five discharge manifold pressures were obtained. It was found that the plunger stroke and the discharge manifold pressure during the drain stroke have a significant effect on the lag time of the working chamber pressure reduction. At the same time, the pressure in the working chamber generated an "M" type pressure shock characteristic in the high pressure range near the peak pressure, which could provide a solution and experimental basis for further analysis of the causes of fatigue failure of the inner wall of the valve box and optimization of the motion mechanism of the pump valve.

Optimization of Solid Plunger's Structure

LIU Yonghui, HUANG Qijun, DU Jing, MA Hongkui, XU Zhiyu

2019, 41(6):  181-186.  DOI: 10.11885/j.issn.1674-5086.2019.10.24.01

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Plunger lift has been the key technology of liquid flowback after fracking in shale gas reservoirs. In order to enhance the seal effect of fluid lift with solid plunger, the Fluent software was employed to set up the two-dimensional geometric model with a single flow channel, and turbulent flow model was adopted to simulate the flow field condition of groove with movement of solid plunger. Based on optimizing the shapes of groove (straight angle trapezoid groove, isosceles trapezoid groove, inverse isosceles trapezoid groove, inverse right angle trapezoid groove, and rectangular groove), this paper contrasts and analyzes the flow field condition with different groove width and depth, and then carries out the physical simulation experiment to contrast the dual pad plunger commonly and optimized solid plunger. Based on the above numerical simulations and experiments, we conclude that there is a low speed zone in the center of the groove when the plunger running, in which eddies appear constantly and interfere with the normal flow of the fluid. In that case, the flow rate is reduced to achieve the sealing effect. Through the comparison of groove type, it can be seen that the seal effect of trapezoidal groove with straight angle is the best. Accounting to the common situation on site, with wider and deeper groove, the sealing effect would be better (the performance of groove with the width as 20 mm and the depth as 6 mm is much better than the others'). Physical simulation experiment proved that under the same condition of low gas velocity, the liquid lifting effect of rod plunger is better than that of liner plunger. In this study, a reliable physical model was established in combination with CFD, and the optimized solid plunger had a good lift effect, which could guide the field application.