Research progress and prospect of plugging technologies for fractured formation with severe lost circulation

By reviewing the mechanisms of drilling fluid lost circulation and its control in fractured formations, the applicability and working mechanisms of different kinds of lost circulation materials in plugging fractured formations have been summarized. Meanwhile, based on the types of lost circulation materials, the advantages, disadvantages, and application effects of corresponding plugging technologies have been analyzed to sort out the key problems existing in the current lost circulation control technologies. On this basis, the development direction of plugging technology for severe loss have been pointed out. It is suggested that that the lost circulation control technology should combine different disciplines such as geology, engineering and materials to realize integration, intelligence and systematization in the future. Five research aspects should be focused on:(1) the study on mechanisms of drilling fluid lost circulation and its control to provide basis for scientific selection of lost circulation material formulas, control methods and processes;(2) the research and development of self-adaptive lost circulation materials to improve the matching relationship between lost control materials and fracture scales;(3) the research and development of lost circulation materials with strong retention and strong filling in three-dimensional fracture space, to enhance the retention and filling capacities of materials in fractures and improve the lost circulation control effect;(4) the research and development of lost circulation materials with high temperature tolerance, to ensure the long-term plugging effect of deep high-temperature formations;(5) the study on digital and intelligent lost circulation control technology, to promote the development of lost circulation control technology to digital and intelligent direction.

Plugging performance and mechanism of an oil-absorbing gel for lost circulation control while drilling in fractured formations

Lost circulation of drilling fluid is one of the most common engineering problems in the drilling process of fractured formations.In this study,an oil-absorbing polymer gel synthesized using compound monomers with rigid and flexible chains was applied to control the oil-based drilling fluid loss while drilling.The microstructure,oil-absorbing performance,and plugging performance the gel was investigated.A large number of dense pores on the surface of the gel were observed,which allowed the oil molecules to enter the internal space of the gel.The initial oil absorption capacity of the gel was fast,and it increased with the increase in the temperature and decrease in the particle size,reaching 20.93 g/g at140℃.At a high temperature of 140℃,the bearing pressure capacity of the gel formula containing particles of different particle sizes reached 7.6 MPa for a fracture of a width of 3 mm,showing that the oil-absorbing gel have excellent plugging performance at high temperature.Plugging mechanism of the gel was investigated through visualized fracture plugging experiments.Results show that the dynamic migratio n,particle-swelling,particle-bridging,particle-aggregation,deformation-filling,and compaction-plugging contribute to the whole lost circulation control process,reflecting that the plugging performance can be effectively enhanced by improving the aggregation and filling degrees of the gel with different particle sizes.

Preparation and properties of magnesium oxysulfate cement and its application as lost circulation materials

Loss of drilling fluids in large porous and fractured zones inevitably up-regulates the overall cost of drilling.As a type of acid-soluble cement,magnesium oxysulfate(MOS)cement is arousing huge attention for the less hygroscopic nature and less damaging to steel casings compared with magnesium oxychloride(MOC)cement.The present study developed MOS cement as a fast setting,high strength and acid-soluble lost circulation material to reduce the problem of losses.As suggested in this study,a higher strength of MOS cement at 70℃could be achieved by elevating M_(g)O/MgSO_(4)·7 H_(2)O molar ratio or downregulating H_(2)O/MgSO_(4)·7 H_(2)O molar ratio.Boric acid and borax could act as effective retarders.Plugging slurry based on MOS cement could effectively block the simulated porous loss zones exhibiting a diameter from 1.24 mm to 1.55 mm,as well as the fractured loss zones with a width from 2 mm to 5 mm and bearing a pressure difference up to 8 MPa.Permeability recovery test demonstrated that it facilitated future oil and gas production.The successful field application in the Junggar Basin,Xinjiang,China verified the significant plugging effect of MOS cement for severe loss problems.

Structural failure mechanism and strengthening method of fracture plugging zone for lost circulation control in deep naturally fractured reservoirs

Focused on the lost circulation control in deep naturally fractured reservoirs, the multiscale structure of fracture plugging zone is proposed based on the theory of granular matter mechanics, and the structural failure pattern of plugging zone is developed to reveal the plugging zone failure mechanisms in deep, high temperature, high pressure, and high in-situ stress environment. Based on the fracture plugging zone strength model, key performance parameters are determined for the optimal selection of loss control material(LCM). Laboratory fracture plugging experiments with new LCM are carried out to evaluate the effect of the key performance parameters of LCM on fracture plugging quality. LCM selection strategy for fractured reservoirs is developed. The results show that the force chain formed by LCMs determines the pressure stabilization of macro-scale fracture plugging zone. Friction failure and shear failure are the two major failure patterns of fracture plugging zone. The strength of force chain depends on the performance of micro-scale LCM, and the LCM key performance parameters include particle size distribution, fiber aspect ratio, friction coefficient, compressive strength, soluble ability and high temperature resistance. Results of lab experiments and field test show that lost circulation control quality can be effectively improved with the optimal material selection based on the extracted key performance parameters of LCMs.

Study on an novel composite gel material solving serious lost circulations and pressurization sealing

Lost circulations have presented great challenges to the petroleum industry, causing great expenditures of cash and time to fighting the problem. Probably the most problematic situations are the naturally fractured formations where the operator may face total loss with no mud return in the annular. The voids or large fracture encountered in this case are often far too large to be plugged with conventional Lost Circulation Material. This paper will give a detailed introduction on a novel composite gel material usable to control severe losses and pressurization sealing. The plugging mechanics of this new composite gel material, which is different from conventional lost circulation materials, were elaborated as well. In addition, the properties of the new composite gel material such as thermostability, sealing strength and bearing resistance are characterized with specific experimental devices. The experimental results proved that the breakdown pressure of the new plugging reached more than 20MPa, and the maximum degraded temperature can be exceed 130℃. The field application at 4 wells in Puguang gas field, SINOPEC, demonstrated that the new composite gel material solved the serious loss in Ordovician carbonate fractured formation successfully and guaranteed the following completion cement operation smoothly. The composite gel sealing slurries, which was easily prepared on site, gives remarkable properties regarding pumping through drill pipes, adjustment of setting time and excellent sealing strength of the lost zone sealing, additionally, the whole pressurization sealing process was complicated within only ten hours. The on-site results show that the plugging ratio of the new composite gel was reached 100%, and the success rate of sealing operation kept above 80%.Thus the new LCM can guarantee safe drilling jobs and save operation cost more effectively.

Analysis of the Lost Circulation Problem

The well-known“lost circulation”problem refers to the uncontrolled flow of whole mud into a formation.In order to address the problem related to the paucity of available data,in the present study,a model is introduced for the lost-circulation risk sample profile of a drilled well.The model is built taking into account effective data(the Block L).Then,using a three-dimensional geological modeling software,relying on the variation function and sequential Gaussian simulation method,a three-dimensional block lost-circulation risk model is introduced able to provide relevant information for regional analyses.

Bridging performance of new eco-friendly lost circulation materials

Lost circulation is one of the most important concerns of the drilling industry, causing excessive expenditure and increasing the non-productive drilling time. In this study, various lost circulation materials(LCMs) were used to control the lost circulation of two types of drilling fluids, bentonite mud and a new eco-friendly mud, named RIA-X, which has a remarkable effect on decreasing the amount of lost circulation in fractured and highly permeable reservoirs. The Bridging Material Test(BMT) apparatus was used to investigate the effectiveness of various LCMs in fractures of various sizes and to select the LCM and combination with the best performance. The use of three-dimensional fractures is one of the most notable points of this work, which makes the experimental conditions similar to those of real wells. The lost control performance of the new eco-friendly LCMs in RIA-X mud was tested in field. The outcomes show that the designed LCMs are able to control severe lost circulation that regular processes such as cementing or drilling with foam cannot deal with.

Lost circulation material for abnormally high temperature and pressure fractured-vuggy carbonate reservoirs in Tazhong block, Tarim Basin, NW China

To effectively solve the problem of lost circulation and well kick frequently occurring during the drilling of abnormally high temperature and pressure fractured-vuggy reservoirs in the Tazhong block, a rigid particle material, GZD, with high temperature tolerance, high rigidity(> 8 MPa) and low abrasiveness has been selected based on geological characteristics of the theft zones in the reservoirs. Through static pressure sealing experiments, its dosage when used alone and when used in combination with lignin fiber, elastic material SQD-98 and calcium carbonate were optimized, and the formula of a new type(SXM-I) of compound lost circulation material with high temperature tolerance and high strength was formed. Its performance was evaluated by compatibility test, static sealing experiment and sand bed plugging experiment. The test results show that it has good compatibility with drilling fluid used commonly and is able to plug fractures and vugs, the sealed fractures are able to withstand the static pressure of more than 9 MPa and the cumulative leakage is 13.4 mL. The mud filtrate invasion depth is only 2.5 cm in 30 min when the sand bed is made of particles with sizes between 10 mesh and 20 mesh. Overall, with good sealing property and high temperature and high pressure tolerance, the lost circulation material provides strong technical support for the safety drilling in the block.

Experimental study on the controlling factors of frictional coefficient for lost circulation control and formation damage prevention in deep fractured tight reservoir

Working fluids loss is a major contributor to low productivity during production process of fractured tight reservoirs.Lost circulation control effect directly related to the tribological behavior between fracture surface and lost circulation materials(LCMs).In this study,the friction coefficient(FC)was investigated using typical clastic rocks and LCMs by considering multiple effect factors divided into external condition and internal condition.The results show that normal load had a relatively high effect on sliding model.A positive correlation was observed between FC and asperities heights.FC decreased induced by particle size degradation of rigid LCMs.Elastic LCMs manifested higher FC compared with rigid LCMs.Under the lubrication condition by working fluid,FC of rigid LCMs was mainly controlled by their surface wettability.FC of organic LCMs is more sensitive to high temperature aging than inorganic LCMs.Fracture plugging experiments show that LCMs optimized based on the research results can effectively improve the efficiency and strength of fracture plugging.

Application of adaptive neuro-fuzzy inference system and data mining approach to predict lost circulation using DOE technique (case study: Maroon oilfield)

Lost circulation is the most common problem encountered while drilling oil wells.Occurrence of such a problem can cause a lot of time and cost wastes.In order to drill oil wells,a fast and profitable way is necessary to predict and solve lost circulation problem.Expert system is a method used lately for problems that deal with uncertainty.In this paper,three approaches are carried out for prediction of lost circulation problem.These approaches include design of experiments(DOE),data mining,and adaptive neuro-fuzzy inference system(ANFIS).Data of 61 wells of Maroon oilfield are selected and sorted as the feed of the systems.Seventeen variables are used as inputs of the approaches and one variable is used as the output.First,DOE is conducted to observe the effects of variables.Plackett-Burman method is used to determine the effects of variables on lost circulation.After that,data mining is conducted to predict the amount of lost circulation.The class of regression is used to determine a function to model the data and the error of the model.Then,ANFIS is applied to predict the amount of lost circulation.The chosen data are used in order to train,test,and control the ANFIS.Furthermore,subtractive clustering is used to train the fuzzy inference system(FIS)of the model.The performance of the ANFIS model is assessed through the root mean squared error(RMSE).The results suggest that ANFIS method can be successfully applied to establish lost circulation prediction model.In addition,results of ANFIS and data mining are investigated through their prediction performances.The comparison of both methods reveals that ANFIS error is much lower than data mining.

Research on conversion time between lost circulation and overflow for the fractured stratum

A new model was established based on the flow model of the drilling fluid in one-dimensional radial fracture to research the influencing factors on conversion time between lost circulation and overflow for multi-pressure system in fractured reservoirs.And the equation of the conversion time between lost circulation and overflow is obtained by solving the mathematical model.The model were validated by reproducing the field data from Sichuan oil field and the simulation results of references.The influences of consistency index,liquidity index,dynamic shear force,drilling fluid density,drilling fluid intrusion depth,pressure of lost circulation layer,crack width,decreased height of annular liquid level,pressure of high-pressure layer were analyzed.Results indicate that conversion time between lost circulation and overflow increases with the increasing consistency index,liquidity index,dynamic shear force,drilling fluid density,drilling fluid intrusion depth,pressure of lost circulation layer.Conversion time between lost circulation and overflow decreases with the increasing crack width,decreased height of annular liquid level,pressure of high-pressure layer.The time interval of“Hang's Muddingoff”operating should be appropriately less than the conversion time between lost circulation and overflow.Under the premise of ensuring wellbore safety,appropriately increasing the consistency index,liquidity index and dynamic shear force of the drilling fluid can effectively increase the conversion time between lost circulation and overflow in large fractured stratum.

A new robust predictive model for lost circulation rate using convolutional neural network:A case study from Marun Oilfield

A major cause of some of serious issues encountered in a drilling project,including wellbore instability,formation damage,and drilling string stuck e which are known to increase non-productive time(NPT)and hence the drilling cost e is what we know as mud loss.The mud loss can be prevented or at least significantly reduced by taking proper measures beforehand provided the position and intensity of such loss can be properly predicted using an accurate predictor model.Accordingly,in this study,we used the convolutional neural network(CNN)and hybridized forms of multilayer extreme learning machine(MELM)and least square support vector machine(LSSVM)with the Cuckoo optimization algorithm(COA),particle swarm optimization(PSO),and genetic algorithm(GA)for modeling the mud loss rate based on drilling data,mud properties,and geological information of 305 drilling wells penetrating the Marun Oilfield.For this purpose,we began by a pre-processing step to attenuate the effect of noise using the Savitzky-Golay method.The whole set of available data was divided into the modeling(including 2300 data points)and the validation(including 483 data points)subsets.Next,the second generation of the non-dominated sorting genetic algorithm(NSGA-Ⅱ)was applied to the modeling data to identify the most significant features for estimating the mud loss.The results showed that the prediction accuracy increased with the number of selected features,but the increase became negligible when the number of selected features exceeded 9.Accordingly,the following 9 features were selected as input to the intelligent algorithms(IAs):pump pressure,mud weight,fracture pressure,pore pressure,depth,gel 10 min/gel 10 s,fan 600/fan 300,flowrate,and formation type.Application of the hybrid algorithms and simple forms of LSSVM and CNN to the training data(80%of the modeling data,i.e.1840 data points)showed that all of the models tend to underestimate the mud loss at higher mud loss rates,although the CNN exhibited lower underestimation levels.Error analysis on different models showed that the CNN provided for a significantly higher degree of accuracy,as compared to other models.The more accurate outputs of the hybrid LSSVM model than those of the simple LSSVM indicated the large potentials of metaheuristic algorithms for achieving optimal solutions.The lower error levels obtained with the CNN model in the testing phase highlighted the excellent generalizability of this model for unseen data.The more accurate predictions obtained with this model,rather than the other models,in the validation phase further proved this latter finding.Therefore,application of this method to other wells in the same field is highly recommended.

Experimental investigation of the application of Eucalyptus bark to prevent lost circulation in pay zones with acid dissolution capability

Loss of drilling fluid is a common problem during the drilling of wells and it restricts the appropriate functionality of muds.Drilling fluid loss significantly increases drilling costs and non-productive time as well as the drilling operation risks.Various investigations have been carried out in order to find appropriate mud additives that either block fractures and pores or reduce fluid loss by improving the fluid rheology.Cheap,environmentally friendly and effective additives are still required by the drilling industry.Hence,the application of available materials in each region,to produce appropriate additives,is a challenge for the oil industry.In this study,Eucalyptus Camaldulensis(EUC)bark powder has been chosen as a new,fibrous,cheap,environmentally friendly and available material to control fluid loss,particularly in southern Iran.Different characterization tests,such as acid dissolution and fluid loss control,were carried out to study the performance of the new proposed additive.Removal by hydrochloric acid and sulfuric acid were studied at various acid concentrations and temperatures.Dynamic fluid loss was also measured at different EUC concentrations.Our study showed that EUC powder can reduce the final fluid loss by 88-97%,the initial fluid loss by 45-66%,and the total loss by 87e94%,which is a satisfactory level.

A comprehensive investigation on the performance of durian rind as a lost circulation material in water based drilling mud

Oil and gas operators worldwide are expecting service companies to deliver integrated techniques to minimize,if not prevent,drilling problems.Drilling fluids perform vital functions to ensure the success of drilling operations.The technical challenges often associated with water-based drilling fluids are loss of critical properties,such as fluid loss control and rheology,under demanding conditions,such as in drilling deeper,high-temperature and high-pressure wells.Fluid loss during drilling operations has a very significant effect in both reservoir formation damage and monetary terms.The use of durian rind(DR)as a new additive in controlling lost circulation would provide another opportunity to reduce waste and avoid pollution.Therefore,DR was used to improve the rheological properties of water-based mud,and it was prepared for use as a fluid loss additive.For a better understanding of the influence of pectin on drilling mud properties,the rheological evaluation of untreated DR was compared to that of mud samples containing treated DR.The pectin in DR was extracted using four different solvents,namely,ethanol,methanol,sodium hydroxide and hydrogen peroxide,and the most effective solvent to remove the pectin was then determined.The Fourier transform infrared spectroscopy(FTIR)results showed that NaOH was the best solvent for removing pectin from DR.Thermogravimetric analysis(TGA)was used to determine the thermal stability of DR before and after treatments.The TGA results demonstrated that the treated DR had improved thermal stability compared to untreated DR.The sizes of DR used were coarse,medium,and fine.The untreated DR presented better rheological properties than the treated DR.The experimental investigation showed that a concentration of 20 lb/bbl of intermediate-sized DR was the best concentration among the tested samples.

Data–driven decision–making for lost circulation treatments: A machine learning approach

Lost circulation is an expensive and critical problem in the drilling operations.Millions of dollars are spent every year to mitigate or stop this problem.In this work,data from over 3000 wells were collected from multiple sources.The data went through a processing step where all outliers were removed and decision rules were set up.Multiple machine learning methods(support vector machine,decision trees,logistic regression,artificial neural networks,and ensemble trees)were used to create a model that can predict the best lost circulation treatment based on the type of loss and the reason of loss.5-fold cross-validation was conducted to ensure no overfitting in the created model.After using all the aforementioned machine learning methods to train models to choose the best lost circulation treatment,overall,the results showed that support vector machine had the highest accuracy among the other algorithms.Thus,it was selected to train the model.The created model went through quality control/quality assurance(QC/QA)to limit the results of incorrect classification.Two treatments were suggested to treat partial loss,four to treat severe loss,and seven for complete loss,based on the reason of loss.In addition,a formalized methodology to respond to lost circulation was provided to help the drilling personnel handling lost circulation in the field.

The mechanism for fuzzy-ball working fluids for controlling & killing lost circulation

Lost circulation controlling & killing material has been a focused issue since a long time ago.A novel leak resistance fluid with the raw materials such as SDS,SDBS,HES,PAM and proprietary productions has been developed from laboratory.The experiment and application shows that the new leak resistance fluid can plug leaking passage of different sizes simultaneously.Observing with 1000 to 2000 time microscope,it is found that the working liquid contains a kind of spherical material which has the microstructure of "one core,two layers,and three membranes".When it is in a static state,the inside of the material looks like an airbag and the outside looks like some fuzzy things which has high gel strength.While when it is in a dynamic state,the fuzzy thing is cut or scattered and flow ability becomes much better.That is the reason why it is called fuzzy-ball.When the diameter or the width of the leak passage is greater than the fuzzy-ball's,the fuzzy-ball accumulates in conical shape to decompose the working fluid pressure of liquid column to achieve blocking;when the diameter or the width of the leak passage is equivalent to the fuzzy-ball's,the fuzzy-ball changes from sphere to oval-shape to increase the resistance to the leak passage and plug the holes;when the diameter or the width of the leak passage is smaller than the fuzzy-ball's,the leaks could be blocked up by the non-permeable membrane formed by the gel of high strength in the working fluid.In these cases,the leak passages of different sizes could be blocked comprehensively.

Development of a New Diagnostic Method for LostCirculation in Directional Wells

Failure to manage and minimize lost circulation can greatly increase the cost of drilling and the risk of well abandonment.Many lost circulation remedial procedures are not working as planned because the locations of loss zones are incorrectly estimated.The lack of this critical piece of information prevents treatments from being applied directly to the points of losses and, thus,resulting in low efficiency and extended NPT （non-productive time）. This paper presents an integrated method for identifying thelocations of loss zones with continuous temperature measurement data enabled by drilling microchip technology. A transient thermalmodel in predicting the temperature profiles in the wellbore and formation during mud loss is developed as a forward calculationprocedure of the loss zone mapping method. For a deep well with moderate to severe loss, there are significant changes in the mudcirculating temperature profiles as mud loss persists. Certain characteristics ofwellbore thermal behavior are evaluated and identifiedas good indicators of loss zones. Case studies are conducted to demonstrate the practical applications of the method in both onshoreand offshore drilling applications. The results from these case studies are important in setting cement plugs, applying expandabletubular systems, and spotting LCM （lost circulation material） pills. Additional uses of this method include identifying highlypermeable zones for reservoir or formation evaluation purposes. This method can be used as a routine monitoring process performedregularly without any interference of the drilling operations at the time.

钻井过程中井漏特征精细识别方法研究与应用

井漏是钻井工程中普遍存在的井下复杂问题,严重影响着钻井施工安全与钻井周期,井漏特征精细识别是高效治理井漏的关键。首先,综合测井、钻井、录井及地质等资料,结合漏层力学性质与物理机理分析,利用加权系数法,建立了基于“井漏综合指数”的井漏层位识别新方法。然后,研究了漏失通道类型及尺寸、漏失压差、漏失速度等井漏特征参数的定量分析方法,并给出计算模型,形成了多信息融合的井漏特征精细识别方法。该方法在X区块进行了实例分析,分析结果表明,X区块实际井资料的处理结果与现场实际漏失地层的层位、漏失类型及漏失速度等井漏特征基本吻合。井漏特征精细识别方法综合考虑了井漏的主要影响因素,利用其可以准确识别漏层的特征,为防漏堵漏技术优化及施工提供科学依据。

结合LSTM自编码器与集成学习的井漏智能识别方法

为了解决传统的井漏智能识别模型因井漏样本数量受限导致其识别准确率低的问题,提出了一种长短期记忆(long short-term memory,LSTM)网络与自编码器(auto-encoder,AE)相结合、集成LSTM-AE的井漏智能识别方法。首先,采用正常样本训练多个包含不同隐藏层神经元数目的LSTM-AE模型,利用重构得分筛选出识别效果较好的几个模型作为基识别器;然后,采用集成学习对多个基识别器的识别结果进行加权融合,解决单一模型因对样本局部特征过度学习导致的误报与漏报问题,提高模型的识别准确率。从某油田18口井的钻井数据中选取了6000组正常钻进状态下的立压、出口流量、池体积数据,对集成LSTM-AE模型进行训练和测试,结果表明,提出方法的识别准确率达到了94.7%,优于其他常用的智能模型的识别结果,为井漏识别提供了一种新的技术途径。

深层断溶体油气藏钻完井储层保护技术挑战与对策

深层断溶体油气藏储层裂缝发育,易导致大量工作液漏失,造成严重储层损害。以顺北区块奥陶系碳酸盐岩储层为例,考虑储层超深、超高压、超高温与断裂带特点,综合工区钻井液漏失情况、储层物性特征、矿物组成和裂缝发育情况,结合流体敏感性、应力敏感性与储层润湿性评价,揭示了深层断溶体油气藏储层损害机理。钻井液漏失是深层断溶体油气藏最重要的储层损害方式,岩体结构破碎、储层非均质性强、走滑断层裂缝缝面光滑及高温长井段循环进一步加大了储层保护的难度;通过采用控压钻井技术,结合抗高温高酸溶屏蔽暂堵技术与抗高温强滞留堵漏技术,可以达到较好的储层保护效果;根据深层断溶体碳酸盐岩油气藏裂缝发育特征,钻进全程加入高强度架桥材料,利用鳞片状岩屑作为填充材料,达到钻井过程防漏与开发过程预防裂缝应力敏感性的效果,实现漏失控制—储层保护—增产增渗的一体化目标。