随着油气勘探开发不断深入,钻井技术逐渐向深井、超深井和小井眼方向发展,对钻井液脉冲信号处理提出了更高的要求.通过分析脉冲位置调制编码的基本原理,提出了一种基于优化变分模态分解和互相关的钻井液脉冲信号处理方法,并利用在苏北地区某页岩油井采集的钻井液脉冲信号验证了该方法的可行性.基于优化变分模态分解算法,实现了在低信噪比条件下有用信号的有效提取;基于同步头相关器对去噪后的信号进行互相关处理,实现了数据帧起始位置的可靠计算;基于数据块相关器对数据块内波形进行互相关处理,实现了码值的准确获取.与传统的钻井液脉冲信号处理方法相比,上述方法具有可靠性高和误码率低的特点,能够很好地满足复杂井眼环境下钻井液脉冲信号处理的需求.

ABSTRACT Leaking of drilling fluid problem due to leaking of well during drilling and completion process has dramatically affected the cost of well. In leaking well, plugging operations for achieving good pressure bearing ability and preventing lost cement slurry are required treatments before the cementing work. In order to avoid plugging operations and reduce cost, a new balanced cementing by using foam cement slurry was proposed. In addition, a high-temperature foam cement slurry system is developed and corresponding operation method is carried out. Foam cement slurry contains the advantages of low density, high strength, expansibility, good mud cake cleaning, high flow resistance in pores and fractures of formation. Which can effectively mitigating loast of cementing slurry and improving the cementing quality. This technology has been used in field and the results show that it dramatically improves the excellent rate of cementing. With no plugging operations, the time of well completion can be reduced by 7 days in average. It has important reference values for cementing in leaking wells with no pressure-bearing plugging. INTRODUCTION The density of foam cement slurry with independent tiny bubbles can be much more lower than normal low density cement slurry (Wang et al., 2022). The foam cement rock is also in high strength and good expansibility (Hu et al, 2012). Foam cement slurry plays an excellent cementing performance in oil and gas wells with complex conditions (Xiao et al., 2016). Its high viscosity can increase displacement of drilling fluid and cleaning of the mud cake on well wall. Cementing with foam cement slurry can significantly improve the cementing quality and sealing ability of annulus (Lu et al, 2017). Foam cementing technology is widely used in shale gas wells in Europe and North America. In the Eldfisk Oilfield and Chichimene Oilfield, cementing with foam cement slurry is mainly used to improve the displacement efficiency of drilling fluid and sealing capacity wells with hydraulic fracturing operations (Green et al, 2003; Vela et al., 2020). The work conducted by Odden et al., (2020) and Ahmady et al. (2020) show that foam cement slurry preventing shrinkage of volume during the curing process and increasing the sealing ability of at interfaces of cement sheath. Fidan et al. (2003) studied the advantage of foam cement slurry in preventing channeling of coal bed gas in annulus.

川渝地区浅表层破碎带溶洞发育,漏垮塌并存导致无法正常钻进,严重影响机械钻速,成为区域拦路虎.针对传统水泥塞密度高、滞留性差与溶洞漏失量大、难以填充的难题,利用泡沫密度低与压缩膨胀特性,开发超低密度泡沫水泥浆快干封堵体系,密度范围为0.7～1.0 g/cm3,候凝时间短,24 h封堵强度高于2 MPa.建立氮气泡沫挤注封堵溶洞工艺方法,不仅提高了溶洞填充效果,而且起到了固结井壁的作用.现场应用5口井,封堵成功率为100％,一次性解决了漏、垮、塌并存的重大技术难题,一开钻完井周期缩短了72％,保证了后续优快钻进,为浅表层破碎带漏垮塌治理及同平台随钻堵漏提供了技术借鉴.

Inadequate hole cleaning is one of the main reasons for inefficient operations in extended-reach drilling. The mechanism of cuttings transport under the back reaming operation, which is frequently adopted to remove the cuttings, has been investigated in this study. To this end, a coupled layering-sliding mesh method with the Eulerian-Granular approach has been established innovatively. The dynamic layering method has been employed to simulate the axial motion of the pipe, whereas the sliding mesh method has been used to simulate the pipe rotation. The back reaming operation of a connector-furnished pipe has been simulated, and the sensitive parameter analysis has been conducted. The results thus obtained demonstrate that the increase in the initial bed height, inclination, and the diameter and length of the connector causes a significant increase in the cuttings concentration. In addition, the cuttings concentration is observed to decrease significantly with the pipe rotation speed. Furthermore, two main factors contribute towards the cuttings accumulation around the connector, namely, the difference in the cross-sectional area and the pushing effect of the connector—like a “bulldozer”. The “bulldozer” effect of the connector dominates when the tripping velocity is significant compared to the velocity of the cuttings. Conversely, the effect of the difference in the cross-sectional area becomes the leading factor for cuttings accumulation. The “bulldozer” effect of the connector causes a more severe impact on hole cleaning. In both cases, increasing the tripping velocity only mildly affects the cuttings concentration. It is therefore suggested that the tripping velocity should be slower than that of the sand during the back reaming operation. Furthermore, increased fluid velocity might lead to a higher accumulated cuttings concentration around the connector when the cuttings bed has not entirely passed through the connector. A significant flow rate can be safely applied after the cuttings have passed through the connector furnished with a large diameter, such as the bottom hole assembly. This exploration serves as an essential guide to predicting and controlling tight spots while back reaming.

The Annulus Pressure Buildup (APB) problems in shale gas wells is mainly caused by the crack at casing-cement interface. The stresses on casing-cement interface and cement-formation interface are calculated by the established model which the constrain of formation rock is considered. The elastic modulus of cement sheath has great effect on the stress on casing-cement and cement formation interface. The radial stress on interfaces changed linearly with fluid pressure. The cement rock is treated as a porous elastic material and the residual strain in cement sheath is mainly caused deformation or collapse of pore structure in the cement rock due to the large radial stress on interfaces. Ultimately, micro-annulus will be generated on the casing-cement interface due to the residual strain of cement sheath. High flexibility cement rock is proposed to decrease the radial stress on the interfaces which can reduce the residual strain of cement sheath. On the other hand, PABP is proposed to offset the decreased radial stress due to residual strain. A model for calculating the stress on interface by considering residual strain of cement sheath and PABP is established. The radial stress on the interfaces caused by PABP is quantitative calculated. The mechanism on how the method for solving APB problems in shale gas wells is clearly explained. The field application certificated the validity of method for solving the APB problems and it also proved the correctness of the calculation method.

This study investigates the cuttings transport while tripping with a connector. First, a transient two-layer model is enhanced to simulate the cutting transport while tripping with a connector. Two mechanisms are thus analyzed regarding the cutting's pile up around the connectors: on the one hand, the difference in the cross-sectional area around the connectors and the corresponding change in the fluid velocity may cause cuttings to accumulate behind the connectors; on the other hand, the connector will push or pull the bed layer and suspension layer, like a "bulldozer", which make the cuttings bed pile up behind the connector. Second, the backreaming operation can be further analyzed by combing the circulation and pipe rotation. The results demonstrate that the cuttings pile up behind the connectors while tripping, and the stacking height increases with initial bed height, the tripping velocity, the diameter, and the length of the connector, and decreases with the flow rate. Moreover, the stacking height behind the connector decreases with the drill pipe rotation speed significantly while backreaming, especially when the rotation speed is more than 120 rpm. Furthermore, the safety of the tripping procedure is highly related to the remaining cuttings bed. Thus the total circulation time to remove the residual wavy cuttings bed is optimized, which increases with the flow rate, ROP, the well depth, and initial bed height significantly; however, the total number of bottom-up operations changes very little, basically around 2 to 3. This can serve as a guide to reduce the high-risk tight spots, especially for an extended-reach well.

We investigated the hydraulic efficiency of spark-generated bubbles under conditions of different discharge polarities and different electrode numbers. A positive polarity discharge can generate a larger spark-induced bubble than a negative polarity discharge generated using the same electrical and electrode parameters. The hydraulic efficiency of both the positive and negative polarity discharges decreases with the increasing number of electrodes. Increasing the number of electrodes leads to faster energy injection times and higher average powers. We found that the hydraulic efficiency increases when energy injection time increases but that it decreases when the average power increases. When the energy injection time is longer than 100 µs and the average power is less than 100 kW, hydraulic efficiencies of more than 15% and 10% were realized for the positive and negative polarity discharges, respectively. These results indicate that longer energy injection times will be helpful in gaining higher hydraulic efficiency for these bubbles.

Shale gas wells under complex conditions are faced with some cementing problems such as lost of circulation, low displacement efficiency and poor sealing effect. Mechanical nitrogen filling foam cementing technique is an important method to solve these problems. Therefore, based on the compressibility of nitrogen bubbles, the methods to calculate and control the foam cement slurry density were studied, and the technique of foam cement slurry to solve the cementing problem of shale gas wells was developed and put into field application. The field application shows that: ① in shale gas wells with shallow leakage and gushing, the self-expansion property of low density foam cement slurry can effectively prevent the leakage and gas invasion of cement slurry; ②in the wells with leakage of ultra-low density drilling fluid, foam cementing realizes the under-density cementing of oil-based mud, which prevents the leakage of cementing slurry and has excellent cementing quality; ③in solving the problems of shallow surface leakage and borehole wall collapse, foam cement slurry can realize the cement slurry return and consolidating borehole wall and plugging the leakage with a density lower than that of clear water, which shortens the well construction period of shallow surface leakage wells by more than 50% and has remarkable economic benefits. This technique has guiding significance and provides reference for solving the cementing problem of shale gas wells under complex working conditions.

Imbibition controls mass transfer in the complex pore-fracture network in shale, which may change the porefracture network and lead to a low efficient flowback of fracturing fluid. Thus, it is necessary to accurately characterize the pore-fracture network alteration during the imbibition process. In this study, forced and spontaneous imbibition tests were conducted under the confining pressure on selected shale core samples with induced fractures, collected from Longmaxi Formation, Sichuan Basin, China. The low-field nuclear magnetic resonance (NMR) spectrometer was employed to monitor the variation of pores and micro-fractures in the shale core samples during the dynamic imbibition process. In addition, optimal surface relaxivities, ranging from 0.019 to 0.033 mu m/ms, were determined by comparing the NMR T2 distributions with the pore size distributions (PSDs) measured via high-pressure mercury intrusion (HPMI) tests. Then, the measured dynamic T2 spectra with three distinct peaks were converted into the corresponding PSDs to quantitatively analyze the number, size, and connectivity changes of small pores, large pores, and micro-fractures in shale. Results show that the total porosities of the four shale core samples are increased by 3.5%, 10.2%, 32.9%, and 36.3% after the imbibition tests. The forced imbibition leads to more remarkable improvements in the pore volume of large pores with radius between 0.2 and 3.6 mu m. In contrast, the spontaneous imbibition results in more significant increases in the number and size of small pores with radius between 0.0004 and 0.36 mu m. It is also found that the total porosity increment is primarily an outcome of small pore alteration during the imbibition. Moreover, the enlarged pores and micro-fractures are mainly categorized as the capillary bounded fluid pores and movable fluid pores, which significantly affect the efficiency of oil and gas transfer in shale. The findings of our study demonstrate the comprehensive effects of capillary force, clay hydration, osmotic potential, confining and pore pressures, and creep deformation and failure on the pore-fracture network alteration in shale and advance the understanding of the mechanisms behind the forced and spontaneous imbibition processes.

An oil spill accident will cause serious harm to marine ecology and the environment. Rapid response and effective prevention methods are required to minimize the damage of oil spill accidents. The critical problems that marine emergency rescue teams face are when the spilled oil reaches the sea surface, the extent of the spilled oil, and how far they are from the drilling platform. However, there is no reliable model to predict the diffusion distance of spilled oil. Accurately predicting the diffusion characteristics of underwater spilled oil can provide timely and accurate information for the treatment of oil spill accidents and guide the correct implementation of emergency treatment. In this paper, the computational fluid dynamics (CFD) method was used to establish a two-phase flow model for the diffusion of a submarine oil spill. The volume-of-fluid (VOF) technique was implemented to track the interface between oil–water phases. The effects of different parameters on leakage and diffusion characteristics were investigated by adjusting spilled oil velocity, ocean current velocity, crude oil density, and crude oil viscosity. The logarithmic velocity profile was adopted for ocean currents to conform to the actual flow near the sea surface. A user-defined function (UDF) was developed and applied for CFD modeling. The focus was on analyzing the diffusion range (rising height Hp and lateral migration distance Wp) from full-field data. The results indicate that the oil spill velocity, ocean current velocity, crude oil density, and crude oil viscosity impact the viscous shear force, the oil spill’s inertia force, and the current shear effect. The formula for calculating the lateral migration distance of spilled oil under different working conditions was obtained by fitting. The results of this study can provide a scientific basis for formulating an emergency treatment plan for offshore oil spill accidents and minimizing the harm to marine ecology and the environment.

The highly efficient development of shale gas is a research hotspot in the world oil and gas industry. When drilling long horizontal wells in shale gas formation with water-based drilling fluid, the problems of wellbore collapse always happened. In this article, the Longmaxi formation shale in Sichuan Province was taken as the research object. Through the friction experiment, it was found that the lubricity of the fracture wall would be enhanced after aqueous solution intrusion into the shale fractures. Compared to pH 7, the maximum static friction coefficient of shale decreased by 23.6% and the root mean square roughness decreased from 41.9 nm to 25 nm after soaking in aqueous solution with pH 13. With the addition of viscosity modifiers CMC-Na and PAC, the friction coefficient even reduced by 37.5% in our experiment and the CMC-Na is more suitable for the stability of shale than PAC. The aqueous solution with 4% bentonite slurry makes the friction coefficient reduced by 26%. The increase of the lubricity of the fracture wall will make the maximum static friction coefficient between shale fracture and the time of the static friction stage decreased, resulting in the decrease of the friction force on the fracture wall. The shear slip on the fracture wall surface is more likely to happen with the decrease of the friction force, ultimately lead to the collapse of wellbore, which is not conducive to the stability of fractured shale formation.

复杂工况下页岩气井固井存在漏失、 顶替效率低和密封效果差等难题,机械充氮泡沫水泥浆固井技术是解决复杂工况下固井难题的重要手段.为此,基于氮气气泡可压缩性,研究了泡沫水泥浆井内密度计算方法和控制方法,建立了泡沫水泥浆解决页岩气井固井难题的工艺和方法,并投入现场应用.现场应用结果表明:①在浅层漏涌同存的页岩气井中,低密度泡沫水泥浆的自膨胀性能有效防止固井水泥浆漏失及气侵;②在超低密度钻井液漏失井中,泡沫水泥浆固井实现了油基钻井液欠密度固井,防止了固井水泥浆漏失,固井质量更优;③在解决浅表层漏失及井壁垮塌问题井中,泡沫水泥浆以低于清水的密度实现水泥浆上返并固结井壁和堵漏,使浅表层漏失井建井周期缩短50％以上,经济效益显著.该技术对解决页岩气井复杂工况下的固井难题具有指导意义.

We study the impact of reservoir heterogeneity on fault slip in shale reservoirs subject to hydraulic stimulation. We construct a two-dimensional numerical model representing the horizontal cross-section of a faulted heterogeneous shale formation and simulate coupled hydro-mechanical processes in the system during and after high-pressure fluid injection. We conceptualize the natural fault as a geological composite consisting of a fault core and a damage zone that involves a set of subsidiary fractures parallel to the fault strike. We couple the solid deformation and fault displacement with the Darcy flow based on poro-elasticity principles and hydro-mechanical constitutive relations. The heterogeneous nature of the permeability field of the shale reservoir is mimicked as a random field governed by a log-normal probability density function and prescribed correlation lengths. We elucidate the linkage between fault slip and fluid flow field, permitting the capture of the spatio-temporal evolution of preferential flow channels and their consequences on fault slip and induced seismicity. We report a significant role of reservoir heterogeneity in fault reactivation when the hydraulic fracture is away from the fault but at a distance smaller than the correlation length. The results of our research have important implications for many fluid injection-related geoengineering activities.

Pre-applied annular back pressure cementing successfully increases the sealing ability of cement sheath and reduce the annulus pressure buildup problems. In order to accurately understand the mechanical mechanism of its function, the process of cementing operation is considered. In addition, elastic mechanics and the continuous condition of interfacial displacement is used to establish a model for calculating the the radial stress on casing-cement interfaces and the mechanism of pre-applied annular back pressure cementing method improving the sealing ability of cement sheath double casing well is studied. The accuracy of the theoretical model established in this paper is verified by experimental results. The results show that the higher applied annular back pressure during cementing work can significantly increase the radial compressive stress on the interfaces and the sealing ability of cement sheath will be improved. The radial stress on interfaces increase linearly with the increase of applied annular pressure. The sealing ability of inner casing-cement sheath interface is greater improved by pre-applied annulus back pressure cementing than the cement-outer casing interface. The higher elastic modulus of cement sheath cause larger radial stress on interface. The results show that the pre-applied annular back pressure cementing can significantly increase the stress on the interfaces and the established model in this paper provides quantitative calculation method for the radial stress of the interface. The research in this paper is useful for the design and operation of annular pressure cementing technology in oil and gas wells.

The seal failure of cement sheath caused by periodic variation of casing internal pressure has seriously affected the safe and efficient development of natural gas. The current cement failure test results show that whether the existence of formation rock is considered, it will lead to the difference of cement sheath failure mode in the research results. Therefore, in this paper, the mechanical analysis model of cement sheath stress state in the presence of different formation rocks was built, and the stress state of cement sheath was calculated. The research results show that the tensile failure of cement sheath is the same as the experimental results when the formation rock constraint is not taken into account; when the formation rock constraint is considered, due to the constraints of casing and formation rock on the outer wall of cement sheath, the occurrence and development of radial cracks in cement sheath are restricted, even if the increase of casing internal pressure makes the circumferential stress of cement sheath greater than its tensile strength, radial cracks will still not occur on the cement sheath; under the action of cyclic loading, the residual strain of the cement sheath will cause the radial compressive stress of casing-cement sheath interface to decrease or even change into radial tensile stress, resulting in interfacial micro-annulus, which is the main factor leading to the seal failure of the cement sheath. The research results can provide reference for field cementing optimization design.

Wellbore instabilities frequently occur in mudstone formation with weak plane bedding because of strong anisotropies. The mechanics parameters of weak plane bedding are of vital significance to the wellbore stability analysis for mudstone formations. The conventional method for determining the mechanics parameters is to fit lots of triaxial test data due to the blindness of coring. In this paper, an evaluation method of the mechanics parameters of weak plane bedding is proposed to improve the accuracy of weak plane bedding mechanical properties. The mechanics parameters of weak plane bedding are obtained by combing the single-weak plane failure criterion with the compressive strength of rock obtained by the triaxial test of cores with different coring angles. It is seen that the new evaluation method is simple and convenient. On the other hand, a validation method of the mechanics parameters of weak plane bedding is proposed to ensure their accuracy. The compressive strength obtained from the core with the special coring angle is compared with the theoretical compressive strength for verifying the accuracy of weak plane bedding mechanical properties. It is observed that the proposed evaluation and validation methods can be used to measure the value of weak plane bedding mechanical properties precisely. The proposed methods are general and can be used for measuring the mechanical properties of fracture weak-plane and joint weak-plane.

Cost-effective development of proved oil resources is the goal of oil and gas field development workers. One of the ways to achieve this is to make the reservoir development in an optimal state based on the existing production conditions, which is also one of the core contents of intelligent oilfield. This paper proposes a new production optimization control method, which aims to maximize the net present value of reservoir development and production. By solving the mathematical model of development and production, the input and output control parameters of the reservoir are optimized in real time to obtain the optimal production plan. The conventional maximum principle method requires two time series solution calculations. In this paper, the coefficient matrix of the full implicit simulator is used to directly obtain the required adjoint equations. The efficient combination of simulation calculation and gradient solution of discrete maximum principle greatly improves the solution efficiency of the model and saves the calculation time. The reservoir examples are analyzed through theoretical research. The results show that the optimal production scheme is in line with the actual situation of the oilfield, which provides theoretical and technical support for the intelligent oilfield system.

Annulus pressure buildup (APB) problems in shale gas wells seriously affected on the safety and efficient exploitation of shale gas all around the world. The sealing failure of the cement sheath on interfaces caused by periodically changed fluid pressure in casing during hydraulic fracturing is treated as the main reason for APB in shale gas wells. Many methods are put forward to solve the APB problem in the field, and fortunately, the preapplied annulus backpressure (PABP) method shows an excellent utility. In this paper, an analytical model is established to explain the mechanism of the PABP method increasing the sealing ability of the cement sheath. The residual strain of the cement sheath and radial stress on interfaces are considered to analyze the factors that affect the effectiveness of the PABP method. In addition, based on the field data, an experimental device is established to test the validity of the PABP method and to certify the accuracy of the analytical model established in this paper. The analytical results show that the thickness of the casing has little effect on radial stress on interfaces. The outer diameter of the casing and the thickness of the cement sheath can temperately affect the radial stress. The elastic modulus of the cement sheath and the formation rock can significantly affect the radial stress. The higher elastic modulus of the cement sheath can dramatically increase the radial stress on interfaces. On the contrary, the higher elastic modulus of formation rock will induce smaller radial stress on the interfaces. In the field, the number of newly added shale gas wells with APB problems has dramatically decreased by using the PABP method. The work in this paper can be significantly useful for researchers and engineers to reduce the APB in shale gas wells.

在当前油价较低的情况下,国外油服公司仍然保持较高的专利申请量,其专利布局与主要业务市场紧密契合,业务遍及全球主要油气产区.国外油服公司专利申请比较系统,针对新技术、新产品、新方法进行全面的布局,其中以斯伦贝谢的专利布局最为全面,哈里伯顿近5年增速迅猛,年申请量超过斯伦贝谢.三大油服公司的授权专利占比50％左右,具有较好的保护效果.基于国内外技术现状,充分结合国内企业技术需求,建议建立突围式的专利布局,加强专利监测预警,借鉴、优化改进与组合相结合,实施关键技术外围包绕、差异化技术布局的策略.

As a new efficient rock-breaking technology, harmonic impact drilling technology has received great attention, but the research on its rock-breaking mechanism is insufficient, which limits its development. Based on the theory of vibration, a simplified model of high-frequency harmonic vibration impact system is established in this paper. The numerical model was solved by Matlab and the motion equations of drill bit and rock at different stages of motion are obtained, respectively. Based on the factor analysis of the mathematical model, the dynamic characteristics of the impact system under harmonic excitation are studied. Finally, numerical simulations are carried out to further analyze the drilling effect of harmonic impact drilling and verify the correctness of the simplified model. The results show that when the excitation frequency equals the resonance frequency of rock, the vibration displacement of rock reaches the peak value. The drilling speed is greatly increased by harmonic impact drilling compared with conventional drilling.