Ultra-deep Wells Research Articles

Analysis and countermeasures of drilling challenges in complex ultra-deep exploration wells in thrust structural belt with multiple salt layers

The deep and ultra-deep zones of Bokhtar oilfield in Tajikistan are the critical blocks for future exploration and development, which are imbedded between two salt layers, accompany with four reservoir and cap assemblages of middle and upper Jurassic, Lower Cretaceous, Upper Cretaceous and Paleogene. The reservoir and cap assemblages of pre-salt carbonate and salt-gypsum cap have great potential. The complicated geological structure, uncertain formation and non-developed pre-salt formations made the critical challenges for drilling operation, including: (1) The difficult casing program design caused by peripheral compression and thrust of the basin, the complicated geological structure, the large geological difference above and below the salt layer, the unfaithful prediction of multiple sets of salt layer, great formation pressure uncertainty, and many sealing points; (2) The upper salt formations are featured by developed thrust belt and large dip angle, the contradiction between deviation prevention and drilling acceleration in high steep structure is prominent, and the wellbore quality is difficult to control; (3) Due to the mud loss in upper salt layer, loss coexisted with blow out in the salt layer and inaccurate prediction of salt bottom, the drilling complexes such as mud loss, overflow and well collapse are occurred frequently; (4) Ultra-deep well depth (over 7000m), HTHP (180°C,140MPa), multiple salt layers and long open hole section made high requirements on high temperature resistance of drilling fluid, cementing slurry and downhole tools, as well as high drilling risk and cost. It is necessary to carry out the drilling difficulties investigation of ultra-deep exploration well under the consideration of pertinence, safety, applicability and economy, then formed the suitable drilling engineering plan for Bokhtar Block through the optimization of domestic and foreign advanced and mature deep and ultra-deep well technologies, which is used to provide the technical support for the first pre-salt ultra-deep exploration well in this Block to be drilled successfully.

Combined use of fly ash and silica to prevent the long-term strength retrogression of oil well cement set and cured at HPHT conditions

The long-term strength retrogression of silica-enriched oil well cement poses a significant threat to wellbore integrity in deep and ultra-deep wells, which is a major obstacle for deep petroleum and geothermal energy development. Previous attempts to address this problem has been unsatisfactory because they can only reduce the strength decline rate. This study presents a new solution to this problem by incorporating fly ash to the traditional silica-cement systems. The influences of fly ash and silica on the strength retrogression behavior of oil well cement systems directly set and cured under the condition of 200 °C and 50 MPa are investigated. Test results indicate that the slurries containing only silica or fly ash experience severe strength retrogression from 2 to 30 d curing, while the slurries containing both fly ash and silica experience strength enhancement from 2 to 90 d. The strength test results are corroborated by further evidences from permeability tests as well as microstructure analysis of set cement. Composition of set cement evaluated by quantitative X-ray diffraction analyses with partial or no known crystal structure (PONKCS) method and thermogravimetry analyses revealed that the conversion of amorphous C-(A)-S-H to crystalline phases is the primary cause of long-term strength retrogression. The addition of fly ash can reduce the initial amount of C-(A)-S-H in the set cement, and its combined use with silica can prevent the crystallization of C-(A)-S-H, which is believed to be the working mechanism of this new admixture in improving long-term strength stability of oil well cement systems.

Study on the coaxial electrode in electrohydraulic shockwave drilling technology

In recent years, electrohydraulic shockwave drilling technology has been widely studied to improve the drill rate of deep and ultra-deep wells. However, few studies have been conducted on coaxial electrodes that are more suitable for drilling. Therefore, it is of great significance to study the discharge characteristics of coaxial electrodes and the influence of various factors on their discharge effects. Firstly, the electric field models of the coaxial electrode were established and simulated using the CST software. The results showed that the electrical breakdown occur randomly on the circular edge of the coaxial electrode. Abnormal electrical breakdown may occur at the junction of anode and insulation layer, which may damage the insulation layer, and this has also been verified in the experiments. Secondly, the influence of various parameters on the discharge effect was further studied based on the model. The results showed that higher voltage and liquid dielectric constant can effectively enhance the electrical breakdown ability. Larger or smaller electrode spacing and insulation concave depth can reduce the discharge effect or more damage the insulating layer. A smaller anode diameter has a stronger electrical breakdown ability, but the final shock wave becomes smaller. Finally, electrical breakdown experiment and rock breaking experiment were conducted to verify the simulation results. In summary, this study conducted in-depth research on coaxial electrodes through CST electric field simulation and experiments, which provided a basis for coaxial electrode design and can effectively promote the practical application of electrohydraulic shockwave technology.

Research and Evaluation of Efficient Drilling Speedup Supporting Technologies and Cementing Processes for Ultra-deep Wells

This paper conducts research and evaluation on the supporting technology for efficient drilling and acceleration in ultra-deep wells, as well as cementing processes. By reviewing the latest research achievements and technological advancements both domestically and internationally, various technical means related to drilling engineering design, fluid systems, equipment, and operation management are summarized. The applicability and benefits of different techniques are deeply studied through the analysis of multiple practical case studies. Additionally, a comparative analysis of various cementing processes used in ultra-deep well development is conducted to assess their strengths, weaknesses, and applicability. Finally, optimization recommendations for efficient drilling and acceleration technology, as well as cementing processes in ultra-deep wells, are proposed, along with future prospects for the development of ultra-deep well drilling technology. This study provides a scientific basis and technical support for the efficient development of ultra-deep oil and gas resources.

Prediction models of burst strength degradation for casing with considerations of both wear and corrosion

Casing wear and casing corrosion are serious problems affecting casing integrity failure in deep and ultra-deep wells. This paper aims to predict the casing burst strength with considerations of both wear and corrosion. Firstly, the crescent wear shape is simplified into three categories according to common mathematical models. Then, based on the mechano-electrochemical (M-E) interaction, the prediction model of corrosion depth is built with worn depth as the initial condition, and the prediction models of burst strength of the worn casing and corroded casing are obtained. Secondly, the accuracy of different prediction models is validated by numerical simulation, and the main influence factors on casing strength are obtained. At last, the theoretical models are applied to an ultra-deep well in Northwest China, and the dangerous well sections caused by wear and corrosion are predicted, and the corrosion rate threshold to ensure the safety of casing is obtained. The results show that the existence of wear defects results in a stress concentration and enhanced M-E interaction on corrosion depth growth. The accuracy of different mathematical models is different: the slot ring model is most accurate for predicting corrosion depth, and the eccentric model is most accurate for predicting the burst strength of corroded casing. The burst strength of the casing will be overestimated by more than one-third if the M-E interaction is neglected, so the coupling effect of wear and corrosion should be sufficiently considered in casing integrity evaluation.

Experimental study on the channeling leakage properties of compact-size casing-cement sheath-stratum assembly in deep and ultra-deep wells

Within deep and ultra-deep wells, the casing-cement sheath-stratum rock assembly(CCS) bears thermal, mechanical, and chemical multi-field coupling, causing some damage or creating micro-annular gaps at the interface between the casing and the cement sheath as well as the interface between the cement sheath and the stratum. Radial and axial cracks in the cement sheath occur, and even channeling-leakage channels form due to shear failure. These are critical reasons for sustained annular pressure and the risk of wellbore sealing integrity. This paper establishes an evaluation device for channeling-leakage properties of a compact-size CCS assembly. It observes the microstructure and morphology characteristics of cement sheath integrity, tests the channeling-leakage pressure and paths of two cement slurries, and explores the relationships between the channeling-leakage properties of the assembly and the damage to the cementing interfaces as well as the cracking characteristics of cement sheath. Results showed that the proposed method was suitable for evaluating unpredictable security risks of CCS assembly caused by a complex load in deep and ultra-deep wells, such as overload-induced cracking, cementing interface unsealing, and channeling-leakage of the cement sheath. The findings revealed the evolution law from deformation points to cracks and channeling-leakage channels inside the cement sheath under the action of load and proposed technical measures to avoid channeling-leakage of the cement sheath. This research provides a theoretical basis and testing method for evaluating the wellbore sealing integrity and formulating technical safety measures for deep and ultra-deep wells.

Structural Design and Numerical Analysis of an All-Metal Screw Motor for Drilling Applications in High-Temperature and High-Pressure Environments in Ultra-Deep Wells

It is difficult to adapt traditional screw motors with rubber stators to the high-temperature and high-pressure conditions in ultra-deep wells, where rubber stators age, deform and carbonize, resulting in motor failure. In this study, the goal is to develop volumetric power drilling tools that can be used to drill at depths of 10,000 m. To meet this goal, an all-metal screw motor that can be applied in ultra-deep wells is designed, then its deformation and structural design are studied. Through numerical simulation, the change in clearance of the motor after expansion in high-temperature environments and the characteristics of deformation in ultra-deep well environments are analyzed. The results show that the metal screw motor has good performance and stability. The maximum deformation is less than 0.3 mm at depths of 9000~15,000 m in ultra-deep wells. The minimum design clearance should be greater than 0.2 mm to ensure that the stator and rotor remain engaged. The results of this research are expected to provide theoretical guidance for the design of all-metal screw motors for applications in ultra-deep well drilling projects to meet the demand for deep earth resource development.

Transient prediction method for flow temperature at wellbore bottom

For the issue of an increase in the flow temperature at the bottom of the wellbore during the production in ultra-deep gas wells, the current completion design, the strength checks of tubing strings, the selection and combination of downhole tools, and theoretical prediction research do not take into account the micro-scale thermal effects of the formation. Thus, by coupling the temperature and pressure fields of the formation and the wellbore through source sink terms, a transient prediction model of the flow temperature at the bottom of the wellbore is established based on the Darcy seepage mechanism and the micro-scale thermal effects. Then, the influences of formation parameters on the flow temperature and the mechanical behavior of tubing strings are assessed. The results indicate that the gas production rate has the largest influence on the flow temperature, followed by the reservoir height, the initial formation pressure, and the initial formation temperature. Due to the influence of the micro-scale thermal effects, the flow temperature can be increased to 34 ℃ at a gas production rate of 400 × 104 m3/day, which is significantly higher than the rated working temperature of downhole tools. The safety factor of tubing strings is lower than the traditional design of tubing strings when the micro-scale thermal effects are taken into account. Finally, the research presented in this paper provides a theoretical prediction tool for a change in the flow temperature due to the influence of the micro-scale thermal effects.

Predicting Rate of Penetration in Ultra-deep Wells Based on Deep Learning Method

Predicting Rate of Penetration in Ultra-deep Wells Based on Deep Learning Method

"Usprawnienie technologii rurowania odwiertu poprzez wiercenie z rozszerzalną kolumną rur okładzinowych "

This article discusses new (innovative) well completion technologies that reduce economic costs and construction time, including casing drilling, expandable casing and monodiameter technology. The advantages and disadvantages of these technologies are considered. Casing drilling is one of the most advanced well construction methods, which prevents complications in the wellbore, due to simultaneous drilling and casing of the wellbore with casing pipes directly in the process of work. Due to the dimensions of the casing, there is constant contact with the well wall, which leads to mechanical clogging. When drilling deep, ultra-deep wells and wells with a large quantity of waste, there is a need for a large number of casing strings of different diameters, and there is not always enough varieties of pipe to provide them. One of the technological solutions for maintaining the diameter of casing strings when designing a well is the use of expandable casing pipes. Of all the advantages of using expandable tubular products, only one of them has the greatest potential – a well with a single bore diameter (Monodiameter). Monodiameter technology reduces the telescopic effect inherent in traditional designs. The possibility of combining two technologies into one, namely drilling with expandable casing pipes will lead to obtaining a well of one bore diameter (monodiameter). It is shown that the use of an expandable casing instead of a drill string is subject to greater risks than drilling with an expandable liner. Based on this, expandable liner drilling is the best option for using expandable casing.

Research on a Typical Casing Failure during Drilling of Cement Plugs in Ultradeep Wells

Summary Among the casing failures in 46 ultradeep wells in a particular block, 11 ultradeep wells had the same casing failure forms occur during the drilling of cement plugs. The casing failure forms are local bending deformation and local internal pressure damage, which eventually led to a local burst failure of the casing in the 11 typical ultradeep wells. Therefore, a casing strength checking model is established to investigate the primary reasons for the local burst failure of the casing and the local burst failure mechanism of the casing. A case study is performed on casing in typical ultradeep wells with force conditions to evaluate the safety state of the casing and make necessary recommendations to improve its safety. The obtained results indicate that an abnormally high dogleg (AHD), a decrease in the temperature at the bottom of the casing, and an annulus trap space are the characteristics of typical ultradeep wells, and the local burst failure of the casing is caused by the coupled superposition impact of a decrease in the annulus pressure buildup (APB), the casing temperature effect, and the magnification effect of the casing bending stress. The safety factor of the casing can be increased by changing the operation parameters and the material properties of the casing. To address the casing failure problems during the drilling of cement plugs in typical ultradeep wells, the protection and monitoring of the seal quality of the cement sheath should be improved to avoid expansion of the well diameter and formation of the annulus trap space. The casing strength and the injection temperature of the drilling fluid can be increased, and the drilling fluid displacement can be decreased.

Use of Highly Dispersed Mixed Metal Hydroxide Gel Compared to Bentonite Based Gel for Application in Drilling Fluid under Ultra-High Temperatures.

In order to solve the problem of poor dispersion and stability of mixed metal hydroxide (MMH), a kind of mixed metal hydroxide-like compound (MMHlc) gel was synthesized for use as the base mud in drilling fluid instead of bentonite gel. Na2CO3, Na2SiO3, and C17H33CO2Na were used as precipitants to form MMHlc with larger interlayer spacing and smaller particle size. MMHlc was synthesized by the coprecipitation method at 25 °C with a metal molar ratio of Mg:Al:Fe = 3:1:1. The performance evaluation of the treated drilling fluid showed that MMHlc (S2) synthesized using Na2SiO3 as the precipitant had the characteristics of low viscosity, low filtration, and a high dynamic plastic ratio at 25 °C, which fully met the requirements of oil field application, and it maintained its excellent properties after being aged at 250 °C for 16 h. Linear expansion and rolling recovery experiments showed that the S2 sample had excellent rheological properties and good inhibition. X-ray diffraction and FT-IR experiments showed that S2 had the most complete crystal structure, its interlayer distance was large, and its ion exchange capacity was strong. The thermogravimetric experiment showed that the S2 crystal was stable and the temperature resistance of the crystal could reach 340 °C. Zeta potential, particle size analysis, SEM, and TEM results showed that S2 is a nanomaterial with a complete morphology and uniform distribution. The drilling fluid of this formula had the characteristics of low viscosity, low filtration loss, and a high dynamic plastic ratio, and it met the conditions for oil field application. Considering these results, the new MMH prepared by our research institute is a drilling fluid material that can be used at ultra-high temperatures and can provide important support for drilling ultra-deep wells.

Drill string dynamic characteristics simulation for the ultra-deep well drilling on the south margins of Junggar Basin

Improper drilling parameters may cause severe vibration of drill string which leads to reduce the rate of penetration and drilling tool premature failure accidents in the drilling process of ultra-deep well. The study on dynamic characteristics of drill string plays an important role in increasing the safety of drilling tool and optimizing the drilling parameters. Considering the influences of real borehole trajectory, interaction between bit and formation, contact between drill string and borehole wall, stiction of drilling fluid and other factors, a comprehensive drill string dynamic model was established to simulate the changes of wellhead hook load, torque, equivalent stress of drill string and BHA (bottom-hole assembly) section acceleration and motion trajectory with time at different WOBs (weights on bit) and rotary speeds. The safety factor and overpull margin of wellhead drill string were calculated and the strength of drilling tool in ultra-deep well was checked using the fourth strength theory. The analysis results show that, in the drilling process of ultra-deep well, the transverse motion amplitude of the drill string near the wellhead is relatively small and vibration of drill string mainly occurs in the lower well section. As the rotary speed increases, the number of collision between lower drilling tool and borehole wall increases, wellhead transverse stress increases, change in torque is not large and change in wellhead equivalent stress is relatively small. As the WOB increases, wellhead torque will increase, axial load and equivalent stress will decrease and vibration acceleration of BHA will increase sharply. Wellhead overpull margin and safety factor will decrease with the increase of rotary speed and increase with the increase of WOB. Wellhead safety factor of S135 drilling tool in an Φ190.5mm ultra-deep well on the south margins of Junggar basin changes around 1.8. The drilling tool is safe and has relatively sufficient ability to deal with the downhole accidents if a large size high steel grade drill string (Φ139.7mm S135) is used. However, in view of BHA safety, neither rotary speed shall be too high nor WOB shall be too large.

Polymer nanocomposite ADA@SM as a high-temperature filtrate reducer for water-based drilling fluids and its filtration loss mechanism

High temperatures degrade drilling fluid performance and endanger drilling safety when drilling deep and ultra-deep wells. The development of a new high-temperature drilling fluid additive to maintain drilling fluid performance in high-temperature environments is critical. A polymer nanocomposite (ADA@SM) was synthesized from the monomers styrene (St), methyl methacrylate (MMA), acrylamide (AM), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and diallyldimethylammonium chloride (DMDAAC) through a free-radical copolymerization reaction. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (1H NMR) were used to characterize the structure of ADA@SM, and transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the microstructure of ADA@SM, indicating that it had a "star-net" structure. TGA analysis showed that the thermal decomposition temperature of ADA@SM was above 274 °C. The rheological and filtration properties of drilling fluids with ADA@SM were studied before and after aging at high temperatures (150–240 °C). The rheological model of the drilling fluid containing 2.5 wt% ADA@SM was found to be consistent with the Herschel-Buckley model. After aging at 220 °C, the filtration loss for API and HTHP was 8.4 mL and 29.8 mL, respectively. Using particle size analysis and zeta potential analysis, ADA@SM produced a drilling fluid with good particle size distribution and colloidal stability in high temperature environments. SEM analysis showed that ADA@SM adsorbed on the surface of the bentonite particles, which improved the compactness of the filter cake. The relative molecular weight (Mn) of ADA@SM decreased from 59671 to 5908 after degradation at high temperatures, and ADA@SM released polymer nanospheres, which effectively reduced the filtration loss of the drilling fluid.

Synthesis and mechanism of environmentally friendly high temperature and high salt resistant lubricants

With the exploration and development of deep and ultra-deep oil and gas, high torque and high friction during the drilling of deep and ultra-deep wells become one of the key issues affecting drilling safety and drilling speed. Meanwhile, the high temperature and high salt problem in deep formations is prominent, which poses a major challenge to the lubricity of drilling fluids under high temperature and high salt. This paper reports an organic borate ester SOP as an environmentally friendly drilling fluid lubricant. The performance evaluation results show that when 1% lubricant SOP is added to the fresh water-based mud, the lubrication coefficient decreases from 0.631 to 0.046, and the reduction rate of lubrication coefficient is 92.7%. Under the conditions of 210 °C and 30% NaCl, the reduction rate of lubricating coefficient of the base slurry with 1% SOP was still remain 81.5%. After adding 1% SOP, the wear volume decreased by 94.11% compared with the base slurry. The contact resistance experiment during the friction process shows that SOP can form a thick adsorption film on the friction surface under high temperature and high salt conditions, thus effectively reducing the friction resistance. Molecular dynamics simulation shows that lubricant SOP can be physically adsorbed on the surface of drilling tool and borehole wall through hydrogen bond and van der Waals force. XPS analysis further shows that SOP adsorbs on the friction surface and reacts with metal atoms on the friction surface to form a chemically reactive film. Therefore, under high temperature and high salt conditions, the synergistic effect of physical adsorption film and chemical reaction film effectively reduces the frictional resistance and wear of the friction surface. In addition, SOP is non-toxic and easy to degrade. Therefore, SOP is a highly effective and environmentally friendly lubricant in high temperature and high salt drilling fluid.

Investigation of the correlation between molecular structures with high-temperature and salt resistance for acrylamides polymer in drilling fluid

Deep oil and gas are essential alternative resources for future energy sources, but the lack of high temperature and high salt resistance mechanisms and agents significantly limits their development. Copolymers of six commonly used high-temperature and salt resistance monomers and acrylamide with similar molecular weights were prepared to improve oil recovery in deep reservoirs. The effects of different functional groups on the high-temperature and salt resistance of polymers were revealed by the drilling fluid and elemental analysis, TOC test, Zeta potential, particle size, and other analytical methods. Experimental results indicate that the pyrrole ring groups, quaternary amine groups, long-chain sulfonate groups, and benzenesulfonic acid groups can effectively improve the high-temperature stability of polymers in an aqueous solution. Quaternary ammonium groups and pyrrole rings can enhance the polymer adsorption with clay under high temperatures and high salinity conditions due to charge attraction. However, these functional groups hinder the clay dispersion and reduce the stability of drilling fluids. Hydration groups, such as sulfonic and carboxylic acid groups, can better maintain the colloidal stability of drilling fluids under high temperatures and high salinity, and benzene sulfonic acid groups are the optimal hydration groups. After the polymer is adsorbed on the clay surface, the benzene sulfonic acid group assists in forming a thicker hydration film on the clay surface, effectively hindering the damage of high temperature and high salinity on the clay. The findings and observations of this study provide theoretical support for the development of high-temperature and salt resistance polymers for water-based drilling fluids of ultra-deep wells.

Experimental study on stepped discharge modes of cyclic electrohydraulic shockwaves

Cyclic electrohydraulic shockwaves drilling technology is expected to solve the problem of rapid drilling in deep and ultra-deep wells. It is expected to improve the rock-breaking efficiency by continuously changing shockwaves under stepped discharge. However, there is still a lack of research on the stepped discharge of cyclic electrohydraulic shockwaves. Therefore, experiments were carried out on eight different discharge modes composed of three voltages of 24, 28, and 32 kV by using self-developed equipment. The results showed that: (1) For modes composed of constant voltage, the higher the voltage, the fewer impact times and electric energy are required to break the rock sample, but it takes longer to break the rock. However, considerable internal cracks only appear after the mechanical strength decreases under a certain number of impacts when the constant discharge voltage is 24 or 28 kV. (2) For stepped modes composed of two or three voltages, such internal cracks appear under the initial impacts, including mode 6, which is composed of 24 and 28 kV. (3) For all the discharge modes, the mechanical strength of rock samples deteriorates linearly with the impact times, and the attenuation rate is also faster under the discharge mode with faster rock breaking speed. In conclusion, the stepped discharge modes can fully combine the advantages of different voltages, and has lower impact times, electric energy, and total time required to break rock samples. Finally, the comprehensive analysis showed that mode 7 (composed of 24, 28, and 32 kV) and mode 6 (composed of 24 and 28 kV) have the best performance in different aspects. In this study, the stepped discharge modes of cyclic electrohydraulic shockwaves were studied in depth, which provides a new idea for the development of this technology and can effectively promote its field application.

Bead and necklace-like silica-based hybrid materials for controlling montmorillonite dispersion filtration

Taking the advantage of well dispersing ability and unique pore plugging capacity, nanomaterials have been extensively used as filtration control additives in water-based drilling fluids (WBDFs). However, nanomaterials are seldom robust when exposed to high temperature and salinity which restricts their applications. Herein, we synthesize two inorganic/organic hybrid nanomaterials using silica as the core and grafted with zwitterionic polymer on the surface (ZSHNM-1 and ZSHNM-2), and conduct in-depth research on their filtration performance. ZSHNM exhibits bead- and necklace-like structures, adsorbs onto and promotes the homogenous dispersion of montmorillonite irrespective of the presence of salt, and facilitates the formation of filter cake with remarkably low permeability, according to experimental results and DFT simulation results. Notably, ZSHNM-2 has significantly stronger thermal resistance up to 250 °C, salt tolerance as high as 11 wt%, which is expected to use in filed applications of ultra-deep wells. It is attributed to the more appropriate particles size distribution and longer polymer tails, efficiently plugging the pores. In addition, these nanomaterials had high polyamine rejection, whereas had little influence on KCl, which should be paid attention to selecting shale inhibitors. This study can be a guide to technically related areas where nanomaterials are used extensively.

Study and application of temporary plugging agent for temporary plugging acid fracturing in ultra-deep wells of Penglai gas field

The burial depth of Penglai gas field reservoir in the Sichuan Basin of China exceeds 7,000 m, and the reservoir temperature is 160°C. Penglai gas field belongs to the fractured reservoir, and there are many natural fractures distributed in the reservoir. Natural fractures are not only the storage place of natural gas, but also the transportation channel of natural gas. Gas wells mainly increase natural gas production through acid fracturing. In the early stage, Penglai Gas Field mainly used gelled acid fracturing technology to create an artificial fracture in the reservoir by injecting a large amount of acid fluid. However, the stimulation range of gelled acid fracturing is very small. After acid fracturing, gas wells can only produce 120,000 cubic meters of natural gas per day. To obtain higher natural gas production, this paper proposes a multi-stage temporary acid fracturing technology that can greatly improve the effect of acid fracturing. The temporary plugging agent for acid fracturing has been developed to meet the high-temperature requirements of the Penglai gas field. Based on the simulation of fracture propagation, the feasibility of acid fracturing expansion of natural fractures in the Penglai Gas Field is clarified. Fiber and granular temporary plugging agents that meet the plugging strength greater than 20 MPa are selected by using the dynamic temporary plugging instrument. MultiFracS software is used to optimize the multi-stage temporary acid fracturing process parameters. The research results in this paper have been applied to wells PS101, PS102, and PS103. After fracturing, compared with the gelling acid fracturing, the natural gas production has increased by more than three times, and the multi-stage temporary plugging acid fracturing has achieved a very good stimulation effect.

A Nano-Cleaning Fluid for Downhole Casing Cleaning

In drilling and completion projects, sludge is formed as a byproduct when barite and oil are mixed, and later sticks to the casing. This phenomenon has caused a delay in drilling progress, and increased exploration and development costs. Since nano-emulsions have low interfacial surface tension, wetting, and reversal capabilities, this study used nano-emulsions with a particle size of about 14 nm to prepare a cleaning fluid system. This system enhances stability through the network structure in the fiber-reinforced system, and prepares a set of nano-cleaning fluids with adjustable density for ultra-deep wells. The effective viscosity of the nano-cleaning fluid reaches 11 mPa·s, and the system is stable for up to 8 h. In addition, this research independently developed an indoor evaluation instrument. Based on on-site parameters, the performance of the nano-cleaning fluid was evaluated from multiple angles by heating to 150 °C and pressurizing to 3.0 Mpa to simulate downhole temperature and pressure. The evaluation results show that the viscosity and shear value of the nano-cleaning fluid system is greatly affected by the fiber content, and the cleaning efficiency is greatly affected by the concentration of the nano-emulsion. Curve fitting shows that the average processing efficiency could reach 60-85% within 25 min and the cleaning efficiency has a linear relationship with time. The cleaning efficiency has a linear relationship with time, where R2 = 0.98335. The nano-cleaning fluid enables the deconstruction and carrying of the sludge attached to the well wall, which accomplishes the purpose of downhole cleaning.