Optimization method of refracturing timing for old shale gas wells

Based on the elastic theory of porous media,embedded discrete fracture model and finite volume method,and considering the micro-seepage mechanism of shale gas,a fully coupled seepage-geomechanical model suitable for fractured shale gas reservoirs is established,the optimization method of refracturing timing is proposed,and the influencing factors of refracturing timing are analyzed based on the data from shale gas well in Fuling of Sichuan Basin.The results show that due to the depletion of formation pressure,the percentage of the maximum horizontal principal stress reversal area in the total area increases and then decreases with time.The closer the area is to the hydraulic fracture,the shorter the time for the peak of the stress reversal area percentage curve to appear,and the shorter the time for the final zero return(to the initial state).The optimum time of refracturing is affected by matrix permeability,initial stress difference and natural fracture approach angle.The larger the matrix permeability and initial stress difference is,the shorter the time for stress reversal area percentage curve to reach peak and return to the initial state,and the earlier the time to take refracturing measures.The larger the natural fracture approach angle is,the more difficult it is for stress reversal to occur near the fracture,and the earlier the optimum refracturing time is.The more likely the stress reversal occurs at the far end of the artificial fracture,the later the optimal time of refracturing is.Reservoirs with low matrix permeability have a rapid decrease in single well productivity.To ensure economic efficiency,measures such as shut-in or gas injection can be taken to restore the stress,and refracturing can be implemented in advance.

Study on the Impact of Massive Refracturing on the Fracture Network in Tight Oil Reservoir Horizontal Wells

Class III tight oil reservoirs have low porosity and permeability,which are often responsible for low production rates and limited recovery.Extensive repeated fracturing is a well-known technique to fix some of these issues.With such methods,existing fractures are refractured,and/or new fractures are created to facilitate communication with natural fractures.This study explored how different refracturing methods affect horizontal well fracture networks,with a special focus on morphology and related fluid flow changes.In particular,the study relied on the unconventional fracture model(UFM).The evolution of fracture morphology and flow field after the initial fracturing were analyzed accordingly.The simulation results indicated that increased formation energy and reduced reservoir stress differences can promote fracture expansion.It was shown that the length of the fracture network,the width of the fracture network,and the complexity of the fracture can be improved,the oil drainage area can be increased,the distance of oil and gas seepage can be reduced,and the production of a single well can be significantly increased.

Refracturing candidate selection for MFHWs in tight oil and gas reservoirs using hybrid method with data analysis techniques and fuzzy clustering

The selection of refracturing candidate is one of the most important jobs faced by oilfield engineers. However, due to the complicated multi-parameter relationships and their comprehensive influence, the selection of refracturing candidate is often very difficult. In this paper, a novel approach combining data analysis techniques and fuzzy clustering was proposed to select refracturing candidate. First, the analysis techniques were used to quantitatively calculate the weight coefficient and determine the key factors. Then, the idealized refracturing well was established by considering the main factors. Fuzzy clustering was applied to evaluate refracturing potential. Finally, reservoirs numerical simulation was used to further evaluate reservoirs energy and material basis of the optimum refracturing candidates. The hybrid method has been successfully applied to a tight oil reservoir in China. The average steady production was 15.8 t/d after refracturing treatment, increasing significantly compared with previous status. The research results can guide the development of tight oil and gas reservoirs effectively.

Numerical simulation of fracture propagation in Russia carbonate reservoirs during refracturing

Refracturing treatment is often performed on Russian carbonate reservoirs because of the quick production decline of reservoirs.The traditional refracturing model assumes that a refracture initiates in the normal direction relative to the initial hydro-fracture.This assumption is inconsistent with oilfield measurements of refracture propagation trajectories.Indeed,the existing model is not based on an indepth understanding of initiation and propagation mechanisms of the second hydraulic fractures during refracturing.In this study,we use the extended finite element method to investigate refracture propagation paths at different initiation angles.Both the enriched function approach and phantom mode technique are incorporated into the refracturing model,thereby ensuring that the refracture can freely extend on the structured mesh without any refinement near the crack tips.Key factors including production time,stress anisotropy and initiation angle,and the propped mechanical effect are analyzed in detail.This study provides new insight into the mechanism of refracture propagation in unconventional reservoirs.

Optimization of refracturing timing for horizontal wells in tight oil reservoirs: A case study of Cretaceous Qingshankou Formation, Songliao Basin, NE China

Tight oil reservoirs in Songliao Basin were taken as subjects and a novel idealized refracturing well concept was proposed by considering the special parameters of volume fracturing horizontal wells, the refracturing potential of candidate wells were graded and prioritized, and a production prediction model of refracturing considering the stress sensitivity was established using numerical simulation method to sort out the optimal refracturing method and timing. The simulations show that: with the same perforation clusters, the order of fracturing technologies with contribution to productivity from big to small is refracturing between existent fractured sections, orientation diversion inside fractures, extended refracturing, refracturing of existent fractures; and the later the refracturing timing, the shorter the effective time. Based on this, the prediction model of breakdown pressure considering the variation of formation pressure was used to find out the variation pattern of breakdown pressure of different positions at different production time. Through the classification of the breakdown pressure, the times of temporary plugging and diverting and the amount of temporary plugging agent were determined under the optimal refracturing timing. Daily oil production per well increased from 2.3 t/d to 16.5 t/d in the field test. The research results provide important reference for refracturing optimization design of similar tight oil reservoirs.

Cemented vertebra and adjacent vertebra refractured in a chronic kidney disease-mineral and bone disorder patient: A case report

BACKGROUND Although percutaneous vertebral augmentation(PVA)is a commonly used procedure for treating vertebral compression fracture(VCF),the risk of vertebral refracture should be considered.Chronic kidney disease-mineral and bone disorder(CKD-MBD)is a systemic disease of mineral and bone metabolism.It is associated with an increased risk of fracture.Few studies have reported the use of PVA in patients with CKD-MBD.We herein report a rare case wherein the cemented vertebra and the adjacent vertebra refractured simultaneously in a CKD-MBD patient after PVA.CASE SUMMARY A 74-year-old man suffered from low back pain after taking a fall about 3 wk ago.According to physical examination,imaging and laboratory findings,diagnoses of T12 VCF,CKD-MBD,and chronic kidney disease stage 5 were established.He then received percutaneous vertebroplasty at T12 vertebra.Fourteen weeks later,he presented with T12 and L1 vertebral refractures caused by lumbar sprain.Once again,he was given PVA which was optimized for the refractured vertebrae.Although the short-term postoperative effect was satisfactory,he reported chronic low back pain again at the 3-month follow-up.CONCLUSION It is necessary that patients with CKD-MBD who have received PVA are aware of the adverse effects of CKD-MBD.It may increase the risk of vertebral refracture.Furthermore,the PVA surgical technique needs to be optimized according to the condition of the patient.The medium-and long-term effects of PVA remain uncertain in patients with CKD-MBD.

Selection of candidate wells for re-fracturing in tight gas sand reservoirs using fuzzy inference

An artificial-intelligence based decision-making protocol is developed for tight gas sands to identify re-fracturing wells and used in case studies. The methodology is based on fuzzy logic to deal with imprecision and subjectivity through mathematical representations of linguistic vagueness, and is a computing system based on the concepts of fuzzy set theory, fuzzy if-then rules, and fuzzy reasoning. Five indexes are used to characterize hydraulic fracture quality, reservoir characteristics, operational parameters, initial conditions, and production related to the selection of re-fracturing well, and each index includes 3 related parameters. The value of each index/parameter is grouped into three categories that are low, medium, and high. For each category, a trapezoidal membership function all related rules are defined. The related parameters of an index are input into the rule-based fuzzy-inference system to output value of the index. Another fuzzy-inference system is built with the reservoir index, operational index, initial condition index and production index as input parameters and re-fracturing potential index as output parameter to screen out re-fracturing wells. This approach was successfully validated using published data.

Performance-based fractal fracture model for complex fracture network simulation

The paper presents a novel hydraulic fracturing model for the characterization and simulation of the complex fracture network in shale gas reservoirs. We go beyond the existing method that uses planar or orthogonal conjugate fractures for representing the ＇＇complexity＇＇ of the network. Bifurcation of fractures is performed utilizing the Lindenmayer system based on fractal geometry to describe the fracture propagation pattern, density and network connectivity. Four controlling parameters are proposed to describe the details of complex fractures and stimulated reservoir volume（SRV）. The results show that due to the multilevel feature of fractal fractures, the model could provide a simple method for contributing reservoir volume calibration. The primary-and second-stage fracture networks across the overall SRV are the main contributions to the production, while the induced fracture network just contributes another 20% in the late producing period. We also conduct simulation with respect to different refracturing cases and find that increasing the complexity of the fracture network provides better performance than only enhancing the fracture conductivity.

Intervertebral bridging ossification after kyphoplasty in a Parkinson’s patient with Kummell’s disease:A case report

BACKGROUND The short-term therapeutic efficacy of kyphoplasty on Kummell’s disease is obvious.However,postoperative refracture and adjacent vertebral fracture occur occasionally and are difficult to treat.Parkinson's disease(PD)is a pathological disorder associated with heterotopic ossification.In a patient with PD,an intervertebral bridge was formed in a short period of time after postoperative refracture and adjacent vertebral fracture,providing new stability.CASE SUMMARY A 78-year-old woman had been suffering from PD for more than 10 years.Three months before operation,she developed lower back pain and discomfort.The visual analog scale(VAS)score was 9 points.Preoperative magnetic resonance imaging indicated collapse of the L2 vertebra.Kyphoplasty was performed and significantly decreased the severity of intractable pain.The patient’s VAS score for pain improved from 9 to 2.Fifty days postoperatively,the patient suddenly developed severe back pain,and the VAS score was 9 points.X-ray showed L2 vertebral body collapse,slight forward bone cement displacement,L1 vertebral compression fracture,and severe L1 collapse.The patient was given calcium acetate capsules 0.6 g po qd and alfacalcidol 0.5ug po qd,and bed rest and brace protection were ordered.After conservative treatment for 2 mo,the patient's back pain was alleviated,and the VAS score improved from 9 to 2.Computed tomography at the 7-mo follow-up indicated extensive callus formation around the T12-L2 vertebrae and intervertebral bridging ossification,providing new stability.CONCLUSION Kyphoplasty is currently a conventional treatment for Kummell's disease,with definite short-term effects.However,complications still occur in the long term,and these complications are difficult to address;thus,the treatment needs to be selected carefully.To avoid refracture,an interlaced structure of bone cement with trabeculae should be created to the greatest extent possible during the injection of bone cement.Surgical intervention may not be urgently needed when a patient with PD experiences refracture and adjacent vertebral fracture,as a strong bridge may help stabilize the vertebrae and relieve pain.

Diverting agents in the oil and gas industry:A comprehensive analysis of their origins,types,and applications

During the life of a well,treatments are carried out to boost productivity by stimulating initially unproduced zones.These treatments include hydraulic fracturing,matrix acidization,and acid fracturing,among others.Hydraulic fracturing treatment is generally applied to deeper reservoirs of oil or natural gas for enhanced recovery.By infusing proppant,water,and chemicals under extreme pressure during the fracturing procedure,fissures in and beneath the reservoir layer can be accessed and expanded.Another stimulating procedure,matrix acidization,involves injecting acid down the drilling hole to permeate the rock fissures at stresses lower than the fracture stress.In addition,carbonate reservoir acid fracturing stimulation is commonly used as an acid treatment technique whereby a pressure greater than the formation disintegration pressure or spontaneous fracture closure pressure is used to compress acid into the reservoir.These treatments allow existing wells to sustain hydrocarbon production without new wells being drilled.Diverters,when employed efficiently,can prevent the need to use a rig to provide momentary physical barriers,thus lowering the cost of the workover.Recent improvements in diversion technology make use of a variety of degradable particles that act as momentary bridges,either at the perforation entries or inside the existing fractures.The aim of this study is to introduce different types of mechanical and chemical diverters used to enhance the productivity of wells.This study explains the concepts of different types of diverters and their applications in several formations,it will also help readers to understand the selection procedures based on the suitability and requirements of diverter use by case studies from around the world.

Outcomes and satisfaction of patients with orthopaedic implants left in-situ for more than three years： a retrospective study

Background There are no clear guidelines on implant removal. Few have assessed the long-term outcomes of patients with implants left in-situ, or removed. Therefore, removal of implants after fracture fixation remains controversial. Methods In this retrospective study, we reviewed 53 patients with implant for fracture fixation in-situ for more than 3 years. All patients were younger than 60 years. Quality of life of each patient was assessed with the Chinese （Hong Kong） validated Short Form-36 and the pain was assessed with visual analogue scale （VAS）. All patients were clinically examined and plain radiographs were taken. Results The total SF-36 score of the patients was not statistically different from the Hong Kong norm （P 〉0.05）. Mean score of VAS was 2.08. Thirty-three patients （62.3%） reported limited range of movement, 9 patients （17%） complained of cosmetic problems, and 10 patients （18.9%） complained of weakness. Clinically, 82.6% of patients had no scarring, 84.7% of patients had full range of movement and all had no tenderness on assessment. Radiologically, no abnormality was detected except for one patient with known avascular necrosis of the femoral head after screw fixation. Conclusion As most patients were clinically and radiologically normal with quality of life scores comparable to the norm, removal of implants is not advisable as a routine practice.