Fracture propagation law of temporary plugging and diversion fracturing in shale reservoirs under completion experiments of horizontal well with multi-cluster sand jetting perforation

This study conducted temporary plugging and diversion fracturing(TPDF)experiments using a true triaxial fracturing simulation system within a laboratory setting that replicated a lab-based horizontal well completion with multi-cluster sand jetting perforation.The effects of temporary plugging agent(TPA)particle size,TPA concentration,single-cluster perforation number and cluster number on plugging pressure,multi-fracture diversion pattern and distribution of TPAs were investigated.A combination of TPAs with small particle sizes within the fracture and large particle sizes within the segment is conducive to increasing the plugging pressure and promoting the diversion of multi-fractures.The addition of fibers can quickly achieve ultra-high pressure,but it may lead to longitudinal fractures extending along the wellbore.The temporary plugging peak pressure increases with an increase in the concentration of the TPA,reaching a peak at a certain concentration,and further increases do not significantly improve the temporary plugging peak pressure.The breaking pressure and temporary plugging peak pressure show a decreasing trend with an increase in single-cluster perforation number.A lower number of single-cluster perforations is beneficial for increasing the breaking pressure and temporary plugging peak pressure,and it has a more significant control on the propagation of multi-cluster fractures.A lower number of clusters is not conducive to increasing the total number and complexity of artificial fractures,while a higher number of clusters makes it difficult to achieve effective plugging.The TPAs within the fracture is mainly concentrated in the complex fracture areas,especially at the intersections of fractures.Meanwhile,the TPAs within the segment are primarily distributed near the perforation cluster apertures which initiated complex fractures.

Dynamic plugging regulating strategy of pipeline robot based on reinforcement learning

Pipeline isolation plugging robot (PIPR) is an important tool in pipeline maintenance operation. During the plugging process, the violent vibration will occur by the flow field, which can cause serious damage to the pipeline and PIPR. In this paper, we propose a dynamic regulating strategy to reduce the plugging-induced vibration by regulating the spoiler angle and plugging velocity. Firstly, the dynamic plugging simulation and experiment are performed to study the flow field changes during dynamic plugging. And the pressure difference is proposed to evaluate the degree of flow field vibration. Secondly, the mathematical models of pressure difference with plugging states and spoiler angles are established based on the extreme learning machine (ELM) optimized by improved sparrow search algorithm (ISSA). Finally, a modified Q-learning algorithm based on simulated annealing is applied to determine the optimal strategy for the spoiler angle and plugging velocity in real time. The results show that the proposed method can reduce the plugging-induced vibration by 19.9% and 32.7% on average, compared with single-regulating methods. This study can effectively ensure the stability of the plugging process.

A novel responsive stabilizing Janus nanosilica as a nanoplugging agent in water-based drilling fluids for exploiting hostile shale environments

Thermo-responsive nanocomposites have recently emerged as potential nanoplugging agents for shale stabilization in high-temperature water-based drilling fluids(WBDFs). However, their inhibitory properties have not been very effective in high-temperature drilling operations. Thermo-responsive Janus nanocomposites are expected to strongly interact with clay particles from the inward hemisphere of nanomaterials, which drive the establishment of a tighter hydrophobic membrane over the shale surface at the outward hemisphere under geothermal conditions for shale stabilization. This work combines the synergistic benefits of thermo-responsive and zwitterionic nanomaterials to synchronously enhance the chemical inhibitions and plugging performances in shale under harsh conditions. A novel thermoresponsive Janus nanosilica(TRJS) exhibiting zwitterionic character was synthesized, characterized,and assessed as shale stabilizer for WBDFs at high temperatures. Compared to pristine nanosilica(Si NP)and symmetrical thermo-responsive nanosilica(TRS), TRJS exhibited anti-polyelectrolyte behaviour, in which electrolyte ions screened the electrostatic attraction between the charged particles, potentially stabilizing nanomaterial in hostile shaly environments(i.e., up to saturated brine or API brine). Macroscopically, TRJS exhibited higher chemical inhibition than Si NP and TRS in brine, prompting a better capability to control pressure penetration. TRJS adsorbed onto the clay surface via chemisorption and hydrogen bonding, and the interactions became substantial in brine, according to the results of electrophoretic mobility, surface wettability, and X-ray diffraction. Thus, contributing to the firm trapping of TRJS into the nanopore structure of the shale, triggering the formation of a tight hydrophobic membrane over the shale surface from the outward hemisphere. The addition of TRJS into WBDF had no deleterious effect on fluid properties after hot-treatment at 190℃, implying that TRJS could find potential use as a shale stabilizer in WBDFs in hostile environments.

A Gel-Based Solidification Technology for Large Fracture Plugging

Fault fractures usually have large openings and considerable extension. Accordingly, cross-linked gel materials aregenerally considered more suitable plugging agents than water-based gels because the latter often undergo contaminationvia formation water, which prevents them from being effective over long times. Hence, in this study, aset of oil-based composite gels based on waste grease and epoxy resin has been developed. These materials havebeen observed to possess high compressive strength and resistance to the aforementioned contamination, therebyleading to notable increase in plugging success rate. The compressive strength, thickening time, and resistance toformation water pollution of these gels have been evaluated indoors. The results show that the compressivestrength of the gel can reach 11 MPa;additionally, the related gelation time can be controlled to be more than3 h, thereby providing a safe construction time;Invasion of formation water has a small effect on the gel strengthand does not shorten the thickening time. All considered performance indicators of the oil-based gel confirm itssuitability as a plugging agent for fault fractures.

Plugging behaviors of temporary plugging particles in hydraulic fractures

Using the visualized experimental device of temporary plugging in hydraulic fractures, the plugging behaviors of temporary plugging particles with different sizes and concentrations in hydraulic fractures were experimentally analyzed under the conditions of different carrier fluid displacements and viscosities. The results show that the greater the carrier fluid viscosity and displacement, the more difficult it is to form a plugging layer, and that the larger the size and concentration of the temporary plugging particle, the less difficult it is to form a plugging layer. When the ratio of particle size to fracture width is 0.45, the formation of the plugging layer is mainly controlled by the mass concentration of the temporary plugging particle and the viscosity of the carrier fluid, and a stable plugging layer cannot form if the mass concentration of the temporary plugging particle is less than 20 kg/m^(3)or the viscosity of the carrier fluid is greater than 3 mPa·s. When the ratio of particle size to fracture width is 0.60, the formation of the plugging layer is mainly controlled by the mass concentration of the temporary plugging particle, and a stable plugging layer cannot form if the mass concentration of the temporary plugging particle is less than 10 kg/m^(3). When the ratio of particle size to fracture width is 0.75, the formation of the plugging layer is basically not affected by other parameters, and a stable plugging layer can form within the experimental conditions. The formation process of plugging layer includes two stages and four modes. The main controlling factors affecting the formation mode are the ratio of particle size to fracture width, carrier fluid displacement and carrier fluid viscosity.

Complex fracture propagation model and plugging timing optimization for temporary plugging fracturing in naturally fractured shale

In this paper,a viscoelasticity-plastic damage constitutive equation for naturally fractured shale is deduced,coupling nonlinear tensile-shear mixed fracture mode.Dynamic perforation-erosion on fluid re-distribution among multi-clusters are considered as well.DFN-FEM(discrete fracture network combined with finite element method)was developed to simulate the multi-cluster complex fractures propagation within temporary plugging fracturing(TPF).Numerical results are matched with field injection and micro-seismic monitoring data.Based on geomechanical characteristics of Weiyuan deep shale gas reservoir in Sichuan Basin,SW China,a multi-cluster complex fractures propagation model is built for TPF.To study complex fractures propagation and the permeability-enhanced region evolution,intersecting and competition mechanisms between the fractures before and after TPF treatment are revealed.Simulation results show that:fracture from middle cluster is restricted by the fractures from side-clusters,and side-clusters plugging is benefit for multi fractures propagation in uniformity;optimized TPF timing should be delayed within a higher density or strike of natural fractures;Within a reservoir-featured natural fractures distribution,optimized TPF timing for most clustered method is 2/3 of total fluid injection time as the optimal plugging time under different clustering modes.

Numerical Analysis of Dynamical Effects Associated with a Plugging String in a Horizontal Well

The finite element method has been applied to simulate the dynamics of a water plugging string in a complex horizontal well of a low-permeability oilfield.The force associated with the pipe string and the packer has been determined under the sucking action of the oil well pump.Such analysis has been conducted for a real drilling well,taking into account the process of lifting,lowering,unblocking and water plugging.Comparison between field measured data and simulation data indicates that the model is reliable and accurate.The packer creep effect under different pressure differences has also been investigated in the framework of the same model.

Structural formation and evolution mechanisms of fracture plugging zone

A coupled CFD-DEM method is used to simulate the formation process of fracture plugging zone.A photo-elastic system characterizing mesoscale force chain network developed by our own is used to model the pressure evolution in fracture plugging zone to reveal the evolution mechanism of the structure of fracture plugging zone.A theoretical basis is provided for improving the lost circulation control effect in fractured reservoirs and novel methods are proposed for selecting loss control materials and designing loss control formula.CFD-DEM simulation results show that bridging probability is the key factor determining the formation of fracture plugging zone and fracture plugging efficiency.Critical and absolute bridging concentrations are proposed as the key indexes for loss control formula design.With the increase of absolute bridging concentration,the governing factor of bridging is changed from material grain size to the combination of material grain size and friction force.Results of photo-elastic experiments show that mesoscale force chain network is the intrinsic factor affecting the evolution of pressure exerting on the fracture plugging zone and determines the macroscopic strength of fracture plugging zone.Performance parameters of loss control material affect the force chain network structure and the ratio of stronger force chain,and further impact the stability and strength of fracture plugging zone.Based on the study results,the loss control formula is optimized and new-type loss control material is designed.Laboratory experiments results show that the fracture plugging efficiency and strength is effectively improved.

Experimental study of the temporary plugging capability of diverters to block hydraulic fractures in high-temperature geothermal reservoirs

The effective plugging of artificial fractures is key to the success of temporary plugging and diverting fracturing technology,which is one of the most promising ways to improve the heat recovery efficiency of hot dry rock.At present,how temporary plugging agents plug artificial fractures under high temperature remains unclear.In this paper,by establishing an improved experimental system for the evaluation of temporary plugging performance at high temperature,we clarified the effects of high temperature,injection rate,and fracture width on the pressure response and plugging efficiency of the fracture.The results revealed that the temporary plugging process of artificial fractures in hot dry rock can be divided into four main stages:the initial stage of temporary plugging,the bridging stage of the particles,the plugging formation stage,and the high-pressure dense plugging stage.As the temperature increases,the distribution distance of the temporary plugging agent,the number of pressure fluctuations,and the time required for crack plugging increases.Particularly,when the temperature increases by 100℃,the complete plugging time increases by 90.7%.

Optimization of the Plugging Agent Dosage for High Temperature Salt Profile Control in Heavy Oil Reservoirs

After steam discharge in heavy oil reservoirs,the distribution of temperature,pressure,and permeability in different wells becomes irregular.Flow channels can easily be produced,which affect the sweep efficiency of the oil displacement.Previous studies have shown that the salting-out plugging method can effectively block these channels in high-temperature reservoirs,improve the suction profile,and increase oil production.In the present study,the optimal dosage of the plugging agent is determined taking into account connection transmissibility and inter-well volumes.Together with the connectivity model,a water flooding simulation model is introduced.Moreover,a non-gradient stochastic disturbance algorithm is used to obtain the optimal plugging agent dosage,which provides the basis for the high-temperature salting-out plugging agent adjustment in the field.

Formation mechanisms of fracture plugging zone and optimization of plugging particles

As formation mechanisms of plugging zone and criteria for fracture plugging remain unclear,plugging experiments and methods testing granular material mechanical properties are used to study the formation process of the plugging zone in fractured formations,analyze composition and ratios of different sizes of particles in the plugging zone,and reveal the essence and driving energy of the formation and damage of the plugging zone.New criteria for selecting lost circulation materials are proposed.The research results show that the formation of the plugging zone has undergone a process from inertial flow,elastic flow,to quasi-static flow.The plugging zone is composed of fracture mouth plugging particles,bridging particles and filling particles,and the proportion of the three types of particles is an important basis for designing drilling fluid loss control formula.The essence of the construction of the plugging zone is non-equilibrium Jamming phase transition.The response of the plugging zone particle system to pressure is driven by entropy force;the greater the entropy,the more stable the plugging zone.Lost circulation control formula optimized according to the new criteria has better plugging effect than the formula made according to conventional plugging rules and effectively improves the pressure-bearing capacity of the plugging zone.The research results provide a theoretical and technical basis for the lost circulation control of fractured formations.

Fatigue Life Analysis of Coiled Tubing in Deep Well Jet Plugging Removal

The coiled tubing plugging has become the main means of plugging in gas Wells in Xinjiang. These Wells are deep and have high pressure, which can easily affect the fatigue life of the operating coiled tubing. In order to improve the life of coiled tubing in high-pressure gas Wells, this paper studies the plugging conditions of coiled tubing in high-pressure ultra-deep Wells. Firstly, the cross section deformation of coiled tubing under high internal pressure is analyzed. Secondly, the factors influencing the fatigue life of coiled tubing and the influence of surface damage on the fatigue life of coiled tubing were studied. Finally, the mechanism of furrow damage caused by coiled tubing and the main measures to reduce furrow damage are analyzed. The following suggestions are made to improve the life of coiled tubing: select the right material and the right size coiled tubing;Use appropriate measures to prevent premature coiled tubing failure and reduce operating costs.

Study on the Plugging System of the Lower Ancient High Loss Zone in Changqing Gas Field

At present, in the workover operations of old gas wells in the Changqing Gas Field, due to years of exploitation in the production layer, pressure deficits, and general leakage during the well killing, the well must be plugged before the well can be killed. In particular, the fracture-cavity type fractures of the lower paleocarbonate reservoirs have serious leakage. Traditional plugging materials and traditional plugging materials and methods have some limitations in dealing with leakage problems. Therefore, a composite plugging system using polymer solution as the base carrier fluid, multi-particle size rigid particles, expandable particles and modified fibers is developed. Its formula is: water + 2% polyacrylamide + 0.1% N, N-methylene bisacrylamide polymer gel is the base carrier fluid;the formulation of the particle plugging agent was continuously optimized through the CDL-II high temperature and high pressure dynamic and static plugging ability to plug the core. The final formulation of the plugging agent is: 1) 20% rigid particles (5 mm, 1 mm, 0.5 mm CCP-3 ratio 4:2:1) + 1% 3 mm expanded particles SMK-1 + 1% 1 ~ 2 mm fiber SRXW-10;2) 20% rigid particles (3 mm, 0.1 mm, 0.05 mm CCP-3 ratio 3:2:1) + 1% 3 mm expanded particles SMK-1 + 1% 1 ~ 2 mm fiber SRXW-10. The water swelling multiples of the granular plugging agent in salt water are all above 7 times, which meets the requirements of leak-proof plugging operations under high salinity;3% HCl is selected as the gel breaker, and the plugging system has a gel breaking rate of 95%;through the water plugging and profile control experimental system, the sand-filled pipe is used to simulate the plugging effect under high temperature and high pressure, and the plugging system can be sealed for 5 days at a high temperature of 110°C and a high pressure of 20 MPa to achieve a good plugging of the formation. It is expected that the developed plugging system has a good application prospect in future workover operations.

Synthesis and Evaluation of a Water-Swelling Polymer Plugging Agent SWL-1

In this paper, to solve the problems of low water absorption and weak pressure bearing capacity of current plugging materials, acrylic acid (AA), acrylamide (AM), and sodium allyl sulfonate (SAS) are used as monomers, and soluble starch is used as grafting. Material, the use of free radical aqueous solution method to synthesize the plugging agent SWL-1 with good water absorption and strong salt resistance. The monomer ratio is set as n (AA):n (AM):n (SAS) = 5:2:1, the experimental synthesis temperature is 70°C, and the orthogonal experiment is used to determine the experimental synthesis conditions as AA neutralization degree 80%, The amount of initiator is 0.60%, the amount of crosslinking agent is 0.50%, the ratio of monomer to starch is 7:1, and the amount of calcium carbonate is 13%. The comprehensive evaluation of the performance of the plugging agent SWL-1 shows that the water absorption performance at 60°C is the best 268.78 g/g, and the water absorption performance in 8% NaCl, 0.9% CaCl2 and composite brine is 21.5 g/g, 12.5 g/g and 24.9 g/g, indicating good resistance to sodium and calcium, the water retention rate is still higher than 30% after 15 days at 160°C. The sand bed plugging test found that the maximum pressure of SWL-1 was 7 MPa, and the crack plugging test found that when the crack width was 1 mm, 2 mm, and 3 mm, the maximum compressive strength of SWL-1 was 10 MPa. The acid solubility test showed that the maximum pressure was 10 MPa. The highest rate is 86.38%.

Development and Application of TL-1 New High Temperature and High Pressure Plugging Instrument

In view of the fact that most of the filter media of existing plugging evaluation instruments use steel plates or steel balls, which cannot truly simulate the characteristics of the formation, a new type of TL-1 high temperature and high pressure plugging instrument has been developed. The filter medium of the instrument is the natural stone with different cracks and quartz sand beds with different particle sizes, which can better simulate the adsorption and hang-up effect of the formation on the plugging agent. It is possible to evaluate the plugging effect, pressure-bearing capacity and return pressure of the plugging slurry for different cracks and sand beds of different sizes under different temperatures and pressures. Using new and old instruments to evaluate the plugging effect of the same plugging slurry, it is found that the plugging effect is different from the distribution of the plugging material on the crack surface, the plugging effect and the pressure bearing capacity. The instrument is stable and reliable in operation and simple in operation. It is a new instrument for conducting research on anti-leakage plugging materials and anti-leakage plugging drilling fluid systems.

Combination of Plugging and Acidization Improves Injection Profile

Plugging agent treatment and acid stimulation have completely different mechanisms for improving injection profiles. In this paper, a hybrid procedure is introduced to reduce the damage of the plugging agent to low and medium permeability zones and the penetration radius of acid into high permeability zones. The procedure is: First inject plugging agent to block high permeability zones, and then inject acid to remove plugging agent damage from the low and medium permeability zones and stimulate them. To perform this procedure successfully, three kinds of plugging agents, namely strong strength plugging agent for the wells with fractures or high permeability streaks, weak gel for those with thick layer in which serious heterogeneity exists, temporary plugging agent for those in which the absorption ability of high permeability zones needs maintaining, were screened out for use in different reservoirs. Several acid systems were evaluated to be compatible with the three kinds of plugging agents. The objectives of this paper are:(1)To show the screen results about the compatible plugging agent and acid; 2 To show how to optimize the operation process;(3)To tell some experience gained ( ) from the oilfield applications of this technique. From Jan. 2001 to Dec. 2002, 46 operations using this procedure were carried out in Weicheng and Mazhai Oilfields of SINOPEC. Results show that the average benefit/cost ratio is over 3.5. Experience acquired from these applications was summarized in the paper.

LEAKAGE-PLUGGING WHILE DRILLING AND PLUGGING AGENTS

This paper describes a series of plugging agents which have been developed these years by the authors. The features of these agents can bestated as: (1) Stopping losses without stopping drilling; (2) High ratio of success; (3) Improving fluids properties; (4) Suiting to many types of losses, even large cracks and caves; (5) Perfect efficiency in different conditions of pollution such as calcium, salt pollution and high temperature; (6) Sufficient resources of raw materials, nopoison, noharm, no pollution to the environment; (7) Convenience in use; (8) Lower cost. The properties, mechanisms and applications of these agents are also described in this paper.

New method for plugging channeling in oil field by nitrogen foam

In order to present a new method for plugging channeling in oil field,the injection modes and validity period of foam system which plugged the formation water layer were studied by means of the experimental model which simulated the real conditions of oil wells existing channeling.Above all,the influence factors including reservoir pressure,permeability,oil saturation and gas-to-liquid ratio were studied through dynamic experiment.Then,in light of the technology characteristics of foam injection in oil field,the comparison between gas-liquid and liquid-gas injection modes was studied.The result shows that the gas-liquid injection mode can ensure the foam injectivity and plugging performance.The plugging validity of nitrogen foam injected into the formation water layer was evaluated in different plugging pressure gradients by the dynamic method which is more reasonable than the static evaluation method in laboratory.The research demonstrates that the plugging validity period of foam decreases with plugging pressure gradient increasing.If the plugging pressure gradient is 0.15 MPa/m,the validity period is 160 h.Finally,a empirical equation and a plate about the plugging validity and the plugging pressure gradient were obtained for forecasting the validity period of foam.

Study on Plugging Technology in Oil Test

In view of the shortcomings of poor temperature resistance, poor pumpability and poor pressure-bearing capacity of commonly used gel plugging materials, polyacrylamide (HPAM) and N,N-methylene bisacrylamide (BIS) were selected for crosslinking and compounding to make a new type of gel plugging material with high temperature and pressure resistance. The compressive strength and yield stress were measured by inverted observation method to evaluate the gel strength. The anti-temperature, anti-pressure, anti-dilution and gel-breaking properties of the gel were evaluated. Finally, 71 type high temperature and high pressure water loss instrument and high temperature and high pressure filter with slit plate were used to evaluate the plugging capacity of gel plugging agent. The experimental results show that the new gel plugging system between 100°C - 120°C, gelation time can be controlled at about 5 h;it has strong temperature resistance, compression resistance, dilution resistance and gel breaking performance. In the face of permeability and fracture leakage simulation experiments, when the ambient temperature below 120°C, pressure within 5 MPa, the filter loss of gel plugging agent is far less than the market two conventional plugging agent, has excellent plugging performance.

Thermo-sensitive polymer nanospheres as a smart plugging agent for shale gas drilling operations

Emulsifier-free poly（methyl methacrylate-styrene） [P（MMA-St）] nanospheres with an average particle size of 100 nm were synthesized in an isopropyl alcoholwater medium by a solvothermal method. Then, through radical graft copolymerization of thermo-sensitive mono- mer N-isopropylacrylamide （NIPAm） and hydrophilic monomer acrylic acid （AA） onto the surface of P（MMA- St） nanospheres at 80 ℃, a series of thermo-sensitive polymer nanospheres, named SD-SEAL with different lower critical solution temperatures （LCST）, were prepared by adjusting the mole ratio of NIPAm to AA. The products were characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, thermogravimetric analysis, particle size distribution, and specific surface area analysis. The temperature-sensitive behavior was studied by light transmittance tests, while the sealing performance was investigated by pressure transmission tests with Lungmachi Formation shales. The experimental results showed that the synthesized nanoparticles are sensitive to temperature and had apparent LCST values which increased with an increase in hydrophilic monomer AA. When the temperature was higher than its LCST value, SD- SEAL played a dual role of physical plugging and chemical inhibition, slowed down pressure transmission, and reduced shale permeability remarkably. The plugged layer of shale was changed to being hydrophobic, which greatly improved the shale stability