Lost Circulation Materials Research Articles

Development and performance evaluation of a mechanically triggered release microcapsule based on in-situ polymerization for lost circulation control

Lost circulation is a critical technical challenge in the drilling and production process of underground oil and gas energy. In this study, a mechanically triggered release microcapsule for lost circulation control was designed and prepared with an in-situ polymerization using melamine, urea, formaldehyde, and diglycidyl ether of bisphenol A (healing agent) as raw materials. The structure and properties of the microcapsule were characterized by different techniques. The results show that the healing agent was successfully encapsulated by the resin shell, the median particle size (D50) of the microcapsules is 120.3 μm, and the thermal stability is excellent. The compatibility experiments show that the surface wettability and shear stability of the microcapsules are good in water-based drilling fluid. The compressive strength test reveals that the healing agent in the microcapsule is released and filled in the gap of the plugging zone under the contact pressure. The concentration of microcapsules and the size distribution of rigid particles are critical for the in-situ reinforcement effect. In addition, the fracture plugging performance of the microcapsule was studied. The results show that the pressure-bearing capacity of the plugging zone increases as the irregularity of the fracture surface increases. These findings indicate the microencapsulated healing agent is an effective lost circulation material (LCM), and provide a new technical idea for lost circulation control.

Shape memory resin with high temperature resistance for plugging fracture formations drilled with oil-based drilling fluid

Lost circulation usually occurs when drilling fracture formations using oil-based drilling fluids (OBDFs). This study synthesized the shape memory epoxy resins (EHN) with a glass transition temperature range of 94–122 °C. They exhibited good thermal stability and dynamical mechanism performance. The EHN did not swell in oil at different temperatures; however, its shape changed with temperature. Optimized by an orthogonal experiment, the thickness swelling rate prepared under the optimal conditions was more than 74%. The particle size reduction rate did not exceed 5% after aging at 150 °C in oil, which was less than 1/4 of that of the nutshell. The EHN1 with a glass transition temperature of 122 °C, prepared under the optimal conditions (OEHN1), was utilized to control lost circulation in OBDF at high temperatures. At a heating rate of 1–10 °C/min, the shape memory rate of OEHN1 required 14–107 min to reach 100% in the temperature range of 30–150 °C, respectively. The shape memory recovery rate of OEHN1 reached to 10, 28, 57, and 97% at 80, 100, 120, and 125 °C within 60 min, respectively. In addition, it required 4–10 min to recover to 100% at 135–150 °C, respectively. OEHN1 did not recover below 80 °C, partially recovered between 80 and 120 °C, and completely recovered above 125 °C (>glass transition temperature). When plugging the wedge fracture with inlet/outlet fracture widths of 3/1 mm at 80–150 °C, formula 2 (F2) containing OEHN1 exhibited lower fluid loss and stabler pressure-bearing capacity under 15 MPa. When plugging the wedge fracture with inlet/outlet fracture widths of 4/2 mm, F2 withstood a pressure of 5 MPa at 80 °C and 15 MPa at 120 °C and 150 °C. Further, OEHN1 exhibited excellent temperature responsiveness and plugging performance. They transformed from the glassy to rubber state following full activation at 125–150 °C. They became elastic particles, expanded from flakes to cubes, and could adapt to different crack sizes. In addition, they bridged, accumulated, filled, and formed compact cracks with other lost circulation materials. Consequently, they formed a force-chain network that improved the stability of the plugging layer.

Visualization experiment on dynamic migration and sealing mechanism of irregular materials in rough fractures

Lost circulation is one of the main problems causing low drilling efficiency and high drilling costs in the oil and gas industry. Due to an insufficient understanding of the migration and sealing mechanism of lost circulation materials (LCMs) in rough fractures, the success of fracture sealing is highly unpredictable. This study conducted visualization experiments using four types of LCMs to observe the entire dynamic sealing process of rough fractures. Adaptive fracture sealing experiments were designed to improve the sealing performance of LCMs at low concentrations. The results indicated that the key to sealing rough fractures is forming an effective sealing zone inside the fracture space rather than in the inlet buffer zone. The dynamic sealing process has experienced four typical stages: sporadic bridging, bridging structure expansion, sealing front formation, and sealing layer thickening. The first three typical stages correspond to flow from plate flow to trench flow and seepage. The formation of an effective sealing zone within fractures depends on forming a practical sealing front and subsequent particle accumulation of LCMs that thicken the sealing layer. Compared with regular particles, the sealing front of irregular particles is an area with a specific aperture range. The increase in irregular particle concentration makes the sealing front migrate to the fracture entrance, which is more likely to lead to inlet blockage. The distribution of a sealing zone depends upon the surface morphology of rough fracture. Increasing the particle size distribution improves the favorable bridging area and enhances the sealing performance. The adaptive sealing strategy uses low-concentration granular materials to form multi-point bridging within rough fractures and low-concentration flaky materials to adaptively and quickly extend and connect dispersed bridging structures. Mica flakes are more conducive to extending along the contour of fracture aperture than calcium carbonate particles. Adding mica flakes promotes the expansion of the bridging structures, forming a sealing front and a robust sealing layer in advance. This study delves into irregular materials' migration and sealing mechanisms in rough fractures, enriching theoretical understanding and providing guidance for on-site sealing operations.

Experimental study on the design method of lost circulation materials for induced fractures

Owing to the influence of fluctuating wellbore pressure, the fracture width in formations with induced fractures changes dynamically. However, the traditional design method for fracture plugging materials is based on a static fracture width, which leads to the easy destruction of the plugging layer established underground and a high risk of secondary loss. Until now, the treatment of lost circulation in formations with induced fractures still faces significant challenges. Therefore, a dynamic fracture plugging experimental device was developed in this study to investigate the bridging–plugging process of lost circulation materials (LCMs) within induced fractures, as well as the design criteria for LCMs for induced fractures. Experimental results show that through adjusting the mass ratio and total concentration of the three-level bridging material, the plugging position of the bridging material can be changed, and the fracture reopen pressure can be increased. When the mass ratio of the three-level bridging material satisfies primary bridge particle: secondary bridge particle: tertiary bridge particle = 1:1:2 and the total concentration is greater than 5%, the plugging material compounded with the three-level bridging material is prone to retention in the induced fractures, exhibiting short bridging time, low permeability of the formed plugging layer, and high fracture initiation pressure. It can effectively prevent the opening of fractures or the generation of new induced fractures, thereby reducing the risk of secondary loss. The research results can provide a scientific theoretical basis for the loss control of induced fractures.

Investigation of fracture-plugging wellbore strengthening: Large scale true tri-axial experiments and field tests

Lost circulation caused by developed natural fracture occurs frequently in tight sandstone formations located in Sichuan Basin, China. Fracture-plugging wellbore strengthening by lost circulation materials (FPWSLCM) is a widely applied fluid loss control technique globally. The upper limit of the pressure-bearing capacity treated using FPWSLCM and the relevant engineering influencing factors needs to be investigated further. In this paper, a self-designed large-scale true tri-axial cell was developed to simulate the fracturing and sealing processes in a cubic sandstone sample (30 cm × 30 cm×30 cm) under anisotropic stress to investigate the effect of lost circulation materials (LCM) and the experimental processes on the formation pressure-bearing capacity. Three homogeneous cubic tight sandstone samples taken from Xujiahe Formation in Sichuan Basin with a central hole were used for the wellbore strengthening experiments with FPWSLCM, which was used to eliminate the heterogeneity effect of the rock. SRIPE (SINOPEC Research Institute of Petroleum Engineering) bridge plugging materials were used as LCM. The results show that the formation pressure-bearing capacity after treatment by FPWSLCM was affected by the initial injection pressure, the intrusion amount of LCM, the pressure holding time, and the injection rate. The formation pressure-bearing capacity did not decrease consistently with the increase of plugging zone instability times, but showed an obvious characteristic of fluctuations; the formation pressure-bearing capacity exceeded the fracturing pressure in some cases. The experimental results could be explained by the stress cage theory. Finally, the modified FPWSLCM was applied as a lost circulation control approach by drilling into the tight sandstone formation in the Shunbei oil field, which has a history of severe loss of fluid circulations. The result of the field test indicated that the modified approaches were more successful than the previous approaches used in other wells in this block. The research results are of great significance for improving the success rate of lost circulation control and a reduction in drilling costs.

Experimental Study of the Swelling and Rheological Properties of a High-Strength Preformed Particle Gel Lost Circulation Material

Summary Preformed particulate gel (PPG) has emerged as a widely utilized lost circulation material in deep oil and gas drilling operations. The objective of our study was to devise a high-strength preformed particle gel (HSPPG) specifically designed to address drilling fluid loss in high-temperature fractured formations. To achieve this, a comprehensive set of laboratory experiments was conducted to assess the swelling and rheological properties of HSPPG under various conditions, and these investigations aimed to provide deeper insights into the pressure-bearing mechanism exhibited by HSPPG. The synthesis of HSPPG involved the copolymerization of acrylamide (AM) and N-hydroxymethacrylamide (NMA) molecular chains, catalyzed by organic peroxides, to form the primary network. Additionally, to enhance its temperature resistance, urea-formaldehyde (UF) resin, known for its superior thermal stability, was incorporated into the secondary network. This unique combination of primary and secondary networks imparted remarkable thermal endurance and structural stability to the resulting HSPPG. The swelling and rheological experiments revealed that HSPPG, with a particle size of 1000 µm, exhibited an equilibrium swelling rate (SR) value of 30.55 and a storage modulus (G’) of 1050 Pa at 120℃. These findings attested to its excellent temperature resistance and structural stability. Furthermore, when subjected to a sodium chloride solution at a temperature of 120℃ and a concentration of 25.0%, HSPPG achieved equilibrium swelling with an SR value of 24.93 and a G’ of approximately 7000 Pa. This significant increase in structural strength was attributed to charge shielding within the highly concentrated brine environment. In the plugging experiments, a wedge-shaped slit with an inlet of 3 mm and an outlet of 1 mm was successfully blocked using a concentration of 4% of HSPPG with a particle size of 1000 μm. The blocking strength achieved was 8.06 MPa. The results of these experiments, as well as the observed filling and plugging state of HSPPG in steel fractured cores, indicated that HSPPG possesses the properties of water absorption, swelling, and extrusion filling. These attributes facilitate the effective formation of a dense blocking layer within the fracture space, exhibiting excellent pressure-bearing capacity. In conclusion, the HSPPG developed in this study represents an advanced swellable granular plugging agent with excellent swelling capacity and structural strength at high temperatures. It offers an ideal solution to mitigate drilling fluid loss from fractured formations under high-temperature and high-salinity conditions.

Experimental study on the updated optimization method for particle size distribution of lost circulation materials

The use of laboratory methods to simulate the process of plugging slurry sealing the fracture is an efficient and convenient method for Lost Circulation Materials (LCMs) selection and formula design. However, commonly used plugging evaluation devices have defects such as small-scale fractures and low reproducibility. To address these shortcomings, we applied a wellbore-fracture coupling plugging experimental device to simulate the process of plugging slurry extrusion. We processed a large-scale visualized fracture using Plexiglas based on the geometric parameters of the formation fracture, reproduced the undulation characteristics of the fracture surface using laser etching technology, and characterized the three-dimensional morphology characteristics of commonly used calcite, walnut shells, and flake polybutyl ester at the wellsite to carry out experiments on the transportation of LCMs under wellbore-fracture coupling conditions. The results indicate that the discrepancy between laboratory evaluation outcomes and field application practices, as well as the low success rate of single-attempt on-site plugging, can be primarily attributed to mouth sealing. The migration behavior of the LCMs is dictated by its morphological characteristics. A larger disparity among the three-axis diameters leads to increased irregularity in the LCM. Characterization of irregularity yielded the following results: walnut shell exhibits the highest irregularity, followed by flaky polybutene and calcite. Increased irregularity in the LCM corresponds to a heightened risk of mouth sealing. Entry into the fracture can only be guaranteed when the D90/Wf ratios for calcite, flaky polybutene, and walnut shell are less than or equal to 0.56, 0.42, and 0.39, respectively. The methods and findings presented in this study provide a scientific basis for selecting LCMs in fractured formations and explain the reasons for poor loss control and significant repetitive loss in wellsite practice.

Experimental Study on the Effect of Rock Mechanical Properties and Fracture Morphology Features on Lost Circulation

Summary At present, lost circulation remains a complicated drilling problem in fractured formations that needs to be addressed urgently. However, the influence of actual rock mechanical properties (RMP) and fracture morphological features (FMF) on lost circulation is easily ignored in the current research on leakage mechanism and evaluation, which may lead to deviation from the analysis results, thus affecting the success rate of plugging treatments. Therefore, the complicated effects have been investigated using the improved plugging experimental instruments in this paper. The results indicate that both RMP and FMF have a prominent influence on the plugging and sealing effects of plugging slurries. This research suggests that the bridging and plugging capabilities of the slurry can be improved by increasing the type and amount of lost circulation materials (LCM). Moreover, depending on the fracture morphology difference, the same plugging slurry will have different plugging effects on the same width-size opening fracture channel. In addition, a novel evaluation method is developed to assess the effective sealing ability of plugging slurry against formation fractures, which has been successfully applied in the field. To the best of our knowledge, this is the first evaluation method that investigates simultaneously the mechanical properties of rocks and fracture characteristics of formations. The novel evaluation method incorporates the critical parameters of the lost circulation effect into the design of the plugging evaluation model. Thus, the proposed method can be used to quantitatively evaluate the plugging capability of the LCM and slurries and the loss capacity of the loss channels. However, the higher plugging coefficient (λ) of the slurry does not necessarily mean that the plugging slurry has a stronger plugging capacity (SP). Adopting the suitable fracture channel model can avoid overestimating or underestimating the plugging capability of the LCM slurries. Therefore, it is necessary to improve the formula design of the LCM slurry in combination with the geological engineering background. This perception has significant implications for the analysis of the lost circulation mechanisms and the optimization formula design of the plugging slurries.

Rapid self-healing nanocomposite gel crosslinked by LDH for lost circulation control

Lost circulation presents a complex challenge in drilling operations. Ascribed to the favorable remaining in fracture, strong resistance to efflux, and entire gel block rather than individual particles, the self-healing gels have been broadly applied as lost circulation material (LCM). In this study, we utilized layered bimetallic hydroxide (LDH) as a dynamic crosslinking agent to synthesize a novel nanocomposite (NC) gel through in-situ polymerization following the stripping of LDH in acrylic acid (AA) solution. LDH/PAA NC gels exhibit excellent self-healing properties, rheological strength, mechanical properties and adhesion properties, with these attributes being influenced by the LDH content. At 110 °C, the optimal LDH/PAA NC gel rapidly self-healed in bentonite mud within 1 h, revealing storage modulus (G′) of 4041 Pa, tensile strength of 8.04 kPa, and adhesion strength of 2.1 kPa. Visual plugging tests revealed that the LDH/PAA NC gel could withstand pressures up to 4 MPa, surpassing the capabilities of MA gel, which lacked self-healing properties. These findings highlight the potential of LDH/PAA NC gels as efficient LCM, offering a promising solution for controlling lost circulation during drilling operations.

Time-dependent thermal degradation of lost circulation materials in geothermal systems

Treatment of lost circulation can represent anywhere from 5 to 25 % of the cost in drilling geothermal wells. The cost of the materials used for lost circulation treatment is less important than their effectiveness at reducing fluid losses. In geothermal systems, the high temperatures (>90 °C) are expected to degrade many commonly used lost circulation materials over time. This degradation could compromise different materials ability to mitigate fluid loss, creating more non-productive time as multiple treatments are needed, but may result in recovering desired permeability zones within the reservoir section over time. This research aimed to study how thermal degradation of eight different lost circulation materials affected their properties relevant to sealing loss zones in geothermal wells. Mass loss experiments were conducted with each material at temperatures of 90–250 °C for 1–42 days to measure the breakdown of the material at geothermal conditions, collecting gases during several experiments to determine the waste produced during degradation. Compaction experiments were conducted with the degraded materials to show how temperatures reduced the rigidity and increased packing of the materials. Viscosity tests were conducted to show the impact of different materials on drilling fluid rheology. Microscope observations were conducted to characterize the alterations to each material due to thermal degradation. Organic materials tend to degrade more than inorganic materials, with organics like microcellulose, cotton seed hulls and sawdust losing 30–50 % of their mass after 1 day of heating at 200 °C, while inorganics like magma fiber only lose ∼5–10 % of its mass after one day of heating at 200 °C. Granular materials are the strongest when compacted despite any mass loss, while fibrous and flaky materials are fairly weak and breakdown easily under stress. The materials do not generally affect fluid rheology unless they have a viscosifying agent as part of the mixture. Microscopic analysis showed that more rigid materials like microcellulose and cedar fiber degrade in brittle manners with splitting and fracturing, while others like cotton seed hulls degrade in more ductile manners forming meshes or clumps of material. The thermal breakdown of lost circulation materials tested suggests that each material should also be classified by its degree of thermal degradability, as at certain temperatures the materials can lose the capability to bridge loss zones around the wellbore.

Geomechanical analysis of lost circulation control in tight formations

Unconventional oil and gas reservoirs, especially that in tight formations, contribute great parts to the global energy. During drilling in tight formations, lost circulation was one of the major problems, which can cause large amount of non operation time and millions of losses. In order to migrate the problem, lost circulation materials (LCMs) were used to prevent reopening of the fracture by isolating the fracture tip while the calculation of stress intensity factor (SIF) and fracture width is the key to LCMs design. In this paper, a dual porosity medium flow model suitable for tight formation is established to calculate the pressure distribution in fracture, and the fracture width and fracture reopening pressure (FROP) is then calculated by using the semi-analytical fracture mechanics model. Sensitivity analysis of critical parameters, for example, fracture length, wellbore radius, LCMs permeability, viscosity, wellbore pressure, and two rock-mechanics-related properties are implemented. The fracture width is larger in the formation with large horizontal principal stress anisotropy, low Young’s modulus and Poisson’s ratio. The increase in fracture length, wellbore radius and wellbore pressure also contributes to fracture opening. Meanwhile, we compared the situation before and after fracture plugging and the results emphasize that the fracture reopening is less likely to occur under the conditions of high viscosity and low permeability LCMs. The method proposed in this study can be used to calculate fracture width and FROP, which has potential significant application for lost circulation control in tight formation.

Integrated Geophysical Approaches for Assessing Lost Circulation While Drilling in the Shiranish Formation, Saddle Area of the Rumaila Oilfield, Southern Iraq

Loss of circulation while oil well drilling is a major problem and is regarded as the most challenging difficulty during drilling and well construction. A significant circulation loss can result in drilling risks such as sticking, blowout and well collapse. This research focused on Shiranish Formation downhole mud losses while drilling and stuck pipe in the saddle area of the Rumaila super giant oilfield, in southern Iraq. Over 1100 wells were drilled in the Rumaila oilfield and only a few wells within the saddle area experienced downhole formation losses/problems and stuck pipes. Seismic data, petrophysical data and drilling reports were used to investigate Shiranish Formation loss and stuck pipe issues. The seismic interpretation revealed small-scale faults and fractures within the examined area, most likely influenced by the strike-slip deep-seated Al-Hammar fault.Correlation within three wells was performed to understand the effect of these faults within the Shiranish Formation, which demonstrated that the Shiranish Formation top is roughly 40 m different from the neighboring wells within 1.5 km and other formation tops are not affected within these faults. Identifying the location of faults and fractures will aid in avoiding drilling in these particular areas and choosing locations where there are no faults, or at the very least in mitigating and assisting the risk of drilling in such areas, such as controlling the rate of Penetration (ROP) while drilling and preparing extra mud and lost circulation materials (LCM).

A Self-Healing Gel with an Organic-Inorganic Network Structure for Mitigating Circulation Loss.

Lost circulation control remains a challenge in drilling operations. Self-healing gels, capable of self-healing in fractures and forming entire gel block, exhibit excellent resilience and erosion resistance, thus finding extensive studies in lost circulation control. In this study, layered double hydroxide, Acrylic acid, 2-Acrylamido-2-methylpropane sulfonic acid, and CaCl2 were employed to synthesize organic-inorganic nanocomposite gel with self-healing properties. The chemical properties of nanocomposite gels were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope, X-ray photoelectron spectroscopy and thermogravimetric analysis. layered double hydroxide could be dispersed and exfoliated in the mixed solution of Acrylic acid and 2-Acrylamido-2-methylpropane sulfonic acid, and the swelling behavior, self-healing time, rheological properties, and mechanical performance of the nanocomposite gels were influenced by the addition of layered double hydroxide and Ca2+. Optimized nanocomposite gel AC6L3, at 90 °C, exhibits only a self-healing time of 3.5 h in bentonite mud, with a storage modulus of 4176 Pa, tensile strength of 6.02 kPa, and adhesive strength of 1.94 kPa. In comparison to conventional gel, the nanocomposite gel with self-healing capabilities demonstrated superior pressure-bearing capacity. Based on these characteristics, the nanocomposite gel proposed in this work hold promise as a candidate lost circulation material.

Evaluation of Kaolin Clay as a Lost Circulation Material in Water Based Mud

Evaluation of Kaolin Clay as a Lost Circulation Material in Water Based Mud

Challenges of formation damage control technology for ultra-deep tight gas reservoirs: A case study from Tarim Basin

Formation damage mechanisms and the corresponding control technology for the tight gas reservoirs have been reported, whereas few studies have discussed ultra-deep fractured tight gas reservoirs. Ultra-deep fractured tight gas reservoirs were susceptible to be damaged owing to geological conditions and engineering status. Taking the ultra-deep fractured tight gas reservoirs located in the Tarim Basin as an example, ultra-tight, high-pressure, high temperature (HPHT), and high-salinity formation water, ultra-low water saturation and fracture networks were identified as special geological characteristics. High-density oil-based drill-in fluids and serious lost circulation were the special engineering status. Challenges in laboratory experiments to evaluate formation damage include rigorous experimental conditions and unsuitable experimental methods. In addition, improving the formation protection ability of working fluids and minimizing the formation damage induced by the sequential use of different types of working fluids were the main challenges associated with using working fluids. Challenges in lost circulation control include the failure of plugging zone due to the degradation of lost circulation materials and repeated lost circulation due to the strength reduction of the plugging zone soaked in diesel oil. Recommendations for key technologies to improve targeted formation damage control technology have been proposed. The comprehensive analysis of these issues provides a road-map for researching formation damage control technologies.

High Performance Shape Memory Epoxy Syntactic Foam Composites as Lost Circulation Material in Deep Drilling

AbstractLost circulation is a critical issue in drilling. This work reports the investigations on shape memory epoxy polymers (SMEPs) with high switching temperatures, and good thermal, mechanical and shape memory properties for plugging in deep drilling. The SMEPs are produced by curing diglycidyl ether of bisphenol A (DGEBA) and castor oil glycidyl ether (COGE) with a flexible poly(oxypropylene) diamine (D230) and a rigid 4, 4′ ‐diaminodiphenyl methane (DDM). Increasing D230 contents or using COGE, glass transition temperature (Tg, 93–163 °C, DMA analyses) and crosslink density of SMEPs are reduced, shape recovery rate is increased, while tensile strength, elongation at break and impact strength are improved. Further, shape memory epoxy syntactic foams (SMEFs) with high‐temperature adhesion are developed by introducing polymer microspheres into SMEPs. The SMEFs have compression ratio of 52–58%, and expansion ratio of 100% by adding 2% wt. microspheres. They retain the Tg and thermal stability of SMEPs, and show a delayed, slower recovery than SMEPs. Long fracture plugging experiments exhibit that a pressure‐bearing capability of 8.99–9.81 MPa is achieved with SMEFs in sealing fracture slots at a high temperature.

Clarify the effect of fracture propagation on force chains evolution of plugging zone in deep fractured tight gas reservoir based on photoelastic experiment

The mesoscale structural stability of plugging zone formed by lost circulation materials (LCMs) frequently dictates the effectiveness of wellbore strengthening operations in deep fractured reservoirs. To address the stability of these mesoscale structures, researchers have conducted extensive investigations encompassing the establishment of photoelastic experimental and coupled computational fluid dynamics with discrete element simulation methods, as well as the influence of LCMs' geometric shape and frictional properties. However, contemporary studies primarily focus on the overall structural evolution of plugging zone under static fracture tip boundaries, lacking a detailed characterization of the pressure-bearing evolution process of force chain structures within plugging zone fracture tip regions under dynamic fracture tip conditions. This study employs the photoelastic experimental technique to explore the development of force chains under shear in a photoelastic granular system with a dynamic fracture confining boundary. Additionally, it utilizes the average intensity gradient method to accurately describe the evolution of average contact force, the proportion of strong force chains, and the strong force chains geometric structure in each segment under a dynamic fracture confining boundary. The test results showed that the presence of a dynamic fracture boundary leads to both a reduction in the abundance of strong force chains proximal to the fracture tip and a simplification of their geometric architecture, subsequently impacting the stability of the plugging zone. Within the low-propagation fracture plugging zone, an escalation in the shear displacement is observed uniformly progressing with an increase in shear pressure, whereas the high-propagation fracture plugging zone demonstrates climactic fluctuations in shear pressure as shear displacement intensifies. The peak shear strength of force chains within the low-propagation fracture reaches 20 MPa, a value 2.2 times higher than that of the high-propagation fracture boundary plugging zone. The ratio of strong force chains within low-propagation fracture exhibits a consistent increment from 10.5% to 13.3% in response to the ascension of shear displacement. On achieving the maximum shear displacement, this ratio in the front-middle-rear segmentations of the plugging zone reaches 14.6%, 14.7%, and 10.5%, respectively. Notably, within the high-propagation fracture, the proportion displays a fluctuating decline from 11.2% to 7.7%. At the pinnacle of shear displacement, the proportion of strong force chains in the front, middle, and rear divisions of the plugging zone are 7.6%, 7.8% and 7.7% respectively. Adjacent to the low-propagation fracture, strong force chains intermittently gather around the shear zone within the plugging area. Here, they gradually configure into staggered arrangements that eventually fuse into a densely packed, high-luminosity network resembling the structure of a fishbone. Conversely, within the high-propagation fracture regions, strong force chains transition from isolated to offset formations, leading to a persistently disrupted sparsely chain-like structure. This study provides theoretical basis for the wellbore strengthening in fracture propagation deep fractured tight gas reservoir.

The role of physical lost circulation materials type on the evolution of strong force chains of plugging zone in deep fractured tight reservoir

The selection of physical lost circulation materials (LCMs) type plays a crucial role in formulating effective plugging slurry. The structural stability of the plugging zone, formed by LCMs, is a decisive factor in determining their effectiveness in deep fractured reservoirs. In this study, combined with the Hertz contact model and calibration of contact forces of photoelastic particles, the color gradient mean square method was employed to quantitatively characterize the strong force chains evolution of the three typical types of plugging zones. Results showed that the incorporation of flexible particles into the LCMs enhanced the average shear strength, increased the proportion, and promoted the development of circular structures within the strong force chains, thereby facilitating stronger shear stability. This research provides new insight to understand the role of physical LCMs type on the mesoscale failure mechanism of plugging zone in deep fractured tight reservoir.

Fiber Stiffness: An Essential Parameter of the Effectiveness of Fiber-Based Lost Circulation Materials—A CFD-DEM Numerical Investigation

Summary Fiber materials have become an attractive choice for lost circulation material (LCM) applications recently. While there has been significant attention on the size, aspect ratio, and size distribution of fibers, the stiffness or basically the effect of their deformability on the sealing capability has not been studied rigorously. Experimental evaluations of fibers with different material properties could be a cumbersome, time-consuming, and expensive process. Most laboratory studies are limited to one or just a few different types of materials. Hence, a novel two-way-coupled computational fluid dynamics and discrete element method (CFD-DEM)-based numerical model is used to overcome this limitation and to simulate motion, collision, deformation, and finally entanglement of individual LCM fibers moving with the fluid along a fracture. Fiber stiffness is determined by the Young’s modulus, the fiber diameter, and the fiber length. Therefore, we investigate this effect in a parametric study with a focus on the impact of the length, diameter, and Young’s modulus of the fiber on their sealing capability. An in-depth analysis reveals that the bridging mechanism for fiber LCM changes with the stiffness of the fiber. Two distinct bridging mechanisms dependent on the fiber stiffness for fiber LCMs are identified. Based on the simulation results, we developed a conceptual model for the different mechanisms that fibers use for bridge initiation. It is also observed that in determining LCM effectiveness, both the fiber stiffness and the fiber dimensions go hand in hand. Stiff fibers were associated with greater maximum plugging pressures (MPPs). The effect of using a mix of soft and stiff fibers on fracture plugging effectiveness has been evaluated. The fiber LCM effectiveness as a consequence of the bending stiffness on bridging larger fractures is also investigated. Key Terms and Phrases lost circulation materials, fibers, fracture sealing, bridging mechanism

A New Methodology to Evaluate the Sealing Process Based on Pressure Drop and Fluid Loss in Fractures during the Drilling Operation

Summary Natural or artificial fractures are common in the wellbore during drilling operations. These fractures allow the flow of drilling fluid into the rock formation. The loss of circulation increases the operation’s cost and nonproductive time, which may threaten the well’s structural integrity. To overcome this problem, it is necessary to understand the flow of fluids through fractures and develop methods to mitigate the loss of circulation. This work’s main contributions are expanding the knowledge on the flow of drilling fluids through fractured channels, conducting an experimental study on the flow of suspensions of lost circulation materials (LCM) in fractures, and performing a theoretical analysis to obtain mathematical models describing fractured channels’ sealing. This work proposes a correlation between the pressure drop and the volumetric flow rate of fluid through fractures. To validate the model, a physical simulator collected fluid flow data and pressure drop in fractures with 2-mm, 5-mm, and 10-mm apertures and 1.02-m length. A blend of polymers and calcium borate was used in suspension in water viscosified with xanthan gum (XG). Density and rheological behavior tests were performed to characterize the studied fluids. The LCM had a bimodal particle-size distribution, and the formulated fluids had a Herschel-Bulkley rheological behavior. Pressure drop, flow rate, and rheological data were used to propose a correlation between pressure drop and volumetric flow rate through the fracture. The proposed correlation was used to monitor the sealing of fractures by calculating their hydraulic diameter throughout the sealing process. The LCM suspensions underwent filtration tests to observe the effects of sealing particles on the mudcake and filtrate volume. The proposed correlation fitted the experimental data with less than 10% deviation. The fracture hydraulic diameter was estimated using experimental data of volumetric flow rate and pressure drop, which made it possible to monitor the sealing process of fractures through time. The sealing and filtration tests showed that the borate and polymer blends are effective as filtration control agents and LCM.