Lost Circulation Control Research Articles

Dynamically Evaluating the Performance of Naturally Occurring Additives to Control Lost Circulation: On the Effect of Lost Circulation Material Type, Particle-Size Distribution, and Fracture Width

Summary Lost circulation is one of the most challenging problems during drilling of oil and gas wells. This issue leads to significant loss of drilling fluid, increase of nonproductive time as well as dictating additional costs to drilling companies. Lost circulation may also lead to other consequences, including stuck pipe, poor hole cleaning, and well control issues. How to efficiently control lost circulation have been traditionally depending on the type of the used lost circulation material (LCM). Injection of commonly available materials (without any further process on their chemical properties) into the thief zone is a common method of lost circulation control. These nonmodified materials are named as conventional LCMs against the unconventional LCMs which are designed/produced just for fluid lost control. The objective of this paper is to comparatively investigate the performance of cane, oak shell, wheat, and mica as LCM of water-based drilling fluid exposed to fractured formations. These materials were chosen because of their low cost, easy access, and compatibility with the environment. The sealing efficiency of these materials was assessed at different particle-size distributions (PSDs) for proper treatment of loss circulation. To do so, an experimental setup containing a cell with adjustable fracture size was designed. Among the LCM formulations made of each of the materials, oak shell formulations are better than the others, followed by mica and cane blends, respectively. The results reveal that combining the materials together is a better treatment than the separate use of them. As it will be seen in detail later, high diversity in particle size (broad PSD) causes more efficient control of fluid loss. Also, to reduce the dependency of sealing ability of LCM formulation on fracture size, mixing of the materials with different particle sizes and shapes is recommended.

Low-Temperature Recrosslinkable Preformed Particle Gel as a Material for Lost Circulation Control

Summary One of the most prevalent, expensive, and time-consuming problems during drilling operations is the loss of circulation. Uncontrolled lost circulation of drilling fluids may lead to dangerous well control difficulties and, in some cases, complete loss of the well. In this paper, the ability of a low-temperature recrosslinkable preformed particle gel (LT-RPPG) has been evaluated to determine the extent to which it can be used to control drilling fluid losses during drilling operations. The RPPG consists of swellable gel particles that can self-crosslink to form a strong bulk gel in fractures to form strong plugging after being placed in the loss zones. We investigated the effect of the LT-RPPG swelling ratio and fracture width on its plugging efficiency to fractures through core flooding tests. Results showed that its sealing pressure can reach up to 1,381 psi/ft and permeability reduction more than 99.99% when the RPPG swelling ratio is five for the fracture with a width of 2.00 mm. LT-RPPG is a good candidate that can be used to control the severe or total loss during drilling operations.

Physical plugging of lost circulation fractures at microscopic level

Drilling fluid loss into formation fractures is one of the most common and costly problems encountered during the exploration and development of oil and gas resources. At present, the most extensive solution is to use physical lost circulation materials (LCMs) to form high-strength plugging zone in fractures. However, it is still unclear how the plugging is initiated and formed in the fracture. In this paper, a microscopic visualization experimental device for the formation of plugging zone is used to observe the dynamic plugging performance of spherical materials, flaky materials and fibers in fracture. Experimental results show that the formation of fracture plugging zone can be divided into retention stage and plugging stage. Spherical materials have three main retention modes: single-particle straining, dual-particle bridging and multi-particle bridging. Flaky materials assist the retention and plugging of spherical materials through three modes: embedded, intercepted and supported. Fibers take part in the retention of spherical materials through three ways: forming a net at the entrance of the fracture, forming a net after bridging the particle materials, and transverse filling in the fracture. The results achieved in this work provide a basis for the selection and design of LCMs for lost circulation control in deep fractured tight reservoir.

High temperature resistant polymer gel as lost circulation material for fractured formation during drilling

A cross-linked polymer gel system used for lost-circulation control during drilling with a high-temperature resistance of 160 °C was prepared from a formulation containing 1% of polymer ZB-1/ZB-2 (1:1 ratio), 0.2% of catechol, 0.4% of hexamethylenetetramine, and 0.3% of thiourea. The effects of time, temperature, and salinity on the gelation performance of polymer gel were analyzed. The results indicate that the gelation time of the polymer gel lost-circulation system at high temperature (160 ℃) is 6–20 h, with the maintenance of the storage modulus after gelation at greater than 60 Pa with the loss modulus retained at above 20 Pa. The polymer gel displayed excellent gel strength and viscoelasticity. The salinity has little influence on the viscoelasticity of the gel system, indicating that the gel system has excellent salt-resistance and is suitable for lost circulation control in the drilling process of high-temperature and high-salinity formations. The plugging performance of the polymer gel lost circulation material system on fractures with different widths and roughness was studied. The results show that the bearing resistance capacity of the gel gradually decreases as the fracture width increases, and the lost circulation control performance of the gel is significantly better on rough walls than on smooth walls. In addition, as the fracture width increases, the gel strength contributes to the bearing resistance capacity. This study indicates the excellent lost-circulation-control performance gained using the polymer gel LCM for fractures. To improve lost-circulation-control performance, the gel strength and the cementing force between the gel and the fractured wall must be improved.

A coupled CFD-DEM numerical simulation of formation and evolution of sealing zones

Lost circulation can be one of the most troublesome situations during drilling, especially in naturally fractured formations. This situation may lead to serious economic losses because of the loss of expensive drilling fluid into the formation and nonproductive time spent on regaining drilling ROP. Granular lost circulation materials (LCMs) are the most commonly used to prevent and cure lost circulations. However, poor understanding of how granular LCM works at a micro-scale has limited their effectivity. In this paper, we developed a coupled CFD-DEM model by combining computational fluid dynamics with discrete element methods to simulate the sealing process of LCM in a wedge-shaped fracture by tracking the motion of each individual particle. Formation and evolution of both the sealing zone and the resulting force chain network were investigated by combining micro process visualization and analysis of flow characteristic curves of LCM suspension injection. The results show that injected LCM particles eventually result in three situations after transport in the fracture, depending on their size and concentration: 1) no bridging no sealing, 2) bridging without sealing, and 3) bridging with sealing. The fracture sealing zone is formed by either single-particle bridging or dual/multi-particles bridging of LCMs. A successful sealing comprises four stages: 1) LCM suspension uniform flow, 2) unstable bridging, 3) sealing zone formation and growth, and 4) fluid flow through the porous sealing zone. These situations and evolutionary stages are clearly reflected in the morphological changes of the flow curve. The formation and evolution of the force chain network also include four stages: 1) discrete force chain dynamic initiation, 2) small force chain network unstable formation, 3) discrete force chain network aggregation, 4) force chain network stable propagation. Strong force chains are in the front section and weak force chains are in the middle and end section of the sealing zone. The formation and collapse of force chains within bridging particles determines if a bridging structure and a sealing zone can be formed and stable. Dual-particles bridging formed sealing zone is weaker than single-particle bridging one because of the weak point in the strong force chain in the fracture width's direction. This research provides a better insight into the process of fracture sealing by granular LCMs, which contributes to improve efficiency of lost circulation control jobs.

Optimization of an in-situ polymerized and crosslinked hydrogel formulation for lost circulation control

Lost circulation is a common problem during drilling and completion, and it contributes much to the nonproductive time of the operation. The Brazilian pre-salt is composed of highly heterogeneous carbonates and is highly susceptible to circulation loss. In this work, a removable, solid-free polymer gel formulation based on in-situ polymerization and gelation was proposed and optimized using design of experiments. The proposed gel contains acrylic acid (monomer), carboxymethylcellulose (thickener), ammonium persulfate (initiator), chromium (III) salt (crosslinking agent) and 240,000 NaCl brine as a solvent. The concentrations of these components, and the solution pH, were the variables considered in this study. The plateau modulus, maximum elastic modulus, gel stability, and gelation times were obtained and analyzed. The gelation time was obtained from oscillatory rheology time curves fitted by the Hill 5 model and from low-frequency nuclear magnetic resonance. Most of the studied gels presented elastic moduli from 1000 to 7000 Pa, close to and higher than the moduli of other gels for lost circulation. The gelation times ranged from 5 to 30 min, which could be improved by changing the initiator, protecting or inhibiting it. A pH of 9 led to faster gelation times, syneresis, and lower elastic moduli; therefore, pH values from 5 to 7 are ideal. Carboxymethylcellulose and the chromium salt increased slightly the gelation time, the elastic moduli, and the stability. Acrylic acid concentration controlled the overall elastic moduli, and higher concentrations led to shorter gelation times. Ammonium persulfate was able to both initiate the polymerization and also internally break the gel, depending on the concentration. Molecular explanations for these effects were provided. This proposed system is a promising candidate for lost circulation control for operations in the Brazilian pre-salt. Future steps involve studying the plugging and removable properties of the gels with optimized composition in high pressure and temperature settings. • A promising gel formulation for lost circulation control was proposed and optimized. • The pre-gel solution has low viscosity, but very high elasticity after gelation. • The gelation mechanism involves in-situ crosslinking and polymerization. • Rheological properties and gelation time can be modeled with R 2 ≥ 0.95. • Rheological properties match or surpass those from the literature.

Lost circulation prediction based on machine learning

Lost circulation, one of the most headache problems in drilling engineering, causes a significant increase in non-productive time and increases the uncertainty of well control risk. There is a tremendous challenge for lost circulation prediction/diagnosis using traditional methods in carbonate formation due to its complex and variable leakage zone. To solve this problem of the Mishrif reservoir in the H oil field, in this paper, we propose a whole practical workflow of lost circulation prediction based on the Mixture Density Network using deep learning. Firstly, we choose 16 comprehensive mudlogging parameters that are the most correlated with the mud loss rate. These parameters are selected from a total of 22 comprehensive mudlogging parameters using three different feature selection approach. Then, combining the parameters and the mud loss rate, we get the relationship between comprehensive mudlogging parameters and the mud loss rate based on the Mixture Density Network including five sub-Gaussian models. Finally, the mud loss rate distribution is obtained according to the relationship with input parameters in real-time. The reliability is also evaluated using the uncertainty information in the model. The results show that the parameters had the highest correlation with the mud loss rate, including Measure Depth, Vertical Depth, Rate of Penetration, Hook load, Pump Pressure, Stroke Per Minute, Flow In, Flow Line, Temperature In, Temperature Out, Mud weight In, Mud weight Out, Conductivity Out, Equivalent Circulating Density, Total Gas and Pit Volume Total. The Mixture Density Network has a solid ability to describe data suitable for mud loss prediction and diagnosis. Improving the quality of training data is very important to improve the prediction accuracy of the model. Due to uncertainty in the weights of the sub-Gaussian model obtained from training, the final probability density distribution curve may be single or multi-peaked. According to the prediction results, lost circulation can be controlled by the corresponding method. Thus, the workflow not only allows for real-time assessment of lost circulation risk during drilling but also provides a reference for optimizing lost circulation control.

Colmatating of productive gas formations while drilling wells with abnormally lower hydrostatic pressures

Today the pressures in productive gas collectors on the Dnieper-Donets rift are considerably lower than hydrostatic ones. Opening such formations while drilling is a challenging task in terms of preventing losses and differential pressures. In order to solve the problem, one needs to improve the methods of effective temporary colmation of productive formations with acid-soluble materials under the conditions of abnormally low formation pressures during drilling on fields with late terms of development. Traditional algorithms of lost circulation control in abnormally low reservoir pressure, developed by leading world services companies, often turn out to be unworkable. In order to conduct the lost circulation control, one often chooses colmatants, which totally align with the principles of ideal packaging, however do not meet the criterion of strength or resistance to significant differential pressure. As a result, in the course of the subsequent drilling there can be another circulation loss in already colmated layer. In Ukraine the lost circulation control sphere is being developed by means of selecting colmatants according to a new principle. The article has proposed the activities for the lost circulation curing and pollution reducing of productive collectors. For this purpose the generalized characteristic-criterion of colmatants has been developed and used. The criterion got a name “flexibility”, i.e. a deformation, compaction and penetration (into porous-fissured medium) capability under immense pressure. With the help of the criterion the set of particular colmatant have been chosen; also, principles of colmatant mixtures selection under the relevant geological and mining conditions have been identified. In many cases, there is no need to calculate the physical parameters of the formation and the circulation loss crack due to flexibility of colmatant; one only has to select the optimal concentrations of materials and the periodicity of input these materials to the mud. The developed technology confirms the need to improve the algorithms for circulation loss prevention and curing, primarily for conditions of abnormally low reservoir pressures.

Experimental study on an oil‐based polymer gel for lost circulation control in high‐temperature fractured formation

AbstractAiming at the lack of plugging agent for oil‐based drilling fluid leakage in fractured formation, an oil‐based gel‐plugging agent with high structural strength and strong thermal stability was synthesized based on monomer polymerization and crosslinking. According to the preparation experiment, when the total concentration of methyl methacrylate (MMA) and butyl acrylate (BA) is 30% and the ratio is 6:4, the oil‐based gel solution can be completely gelatinized from 80 to 200°C, which has high gel strength and controllable gelation time. The effects of total concentration of BA and MMA, the ratio of BA and MMA, synthesis temperature, oil‐based drilling fluid invasion, shear effect, and fiber concentration on gelling performance were systematically studied. Fourier transform infrared molecular structure analysis showed that the oil‐based gel contained benzene ring group and ester characteristic group. From the rheological properties of oil‐based gel with 30% gelling agent at 160°C, it could be seen that the elastic modulus G′ of 10 Hz is 232 kPa, which is much larger than the storage modulus G″ is 99 kPa, which reflects the solid properties and has good elasticity and high strength. The displacement experiment of sand filling pipe and dynamic and static plugging experiment of high temperature and high pressure shows that the oil‐based gel has high bearing capacity in fractures and sand filling pipe. The bearing pressure is 3.01 MPa in 6 mm fractures, and the bearing pressure gradient is 0.602 MPa/cm.

Dynamic fracture width prediction for lost circulation control and formation damage prevention in ultra-deep fractured tight reservoir

Dynamic fracture width prediction is of great importance to the control of lost circulation, which often leads to safety, environmental and economic issues and results in severe formation damage during the development of ultra-deep fractured tight reservoir. However, the connected fault boundary and nonlinear deformation behavior are rarely considered for the fracture width prediction. In the current paper, the fracture deformation equation is determined by fitting the stress sensitivity experiments data. The prediction model for dynamic fracture width is developed based on the non-Newtonian fluid loss dynamics theory, accounting for the power-law fluid, fracture index deformation equation and fault boundary. Parametrical analysis is conducted to investigate the effects of pressure difference, consistency coefficient, flow pattern index and fault boundary distance on the dynamic fracture width. Control strategy is proposed for drilling fluid loss in fractures with connected fault in ultra-deep fractured reservoir. The developed dynamic fracture width prediction model is validated by field data and successfully applied to the SHB ultra-deep fractured tight reservoir in Tarim Basin, China.

Impact of friction coefficient on the mesoscale structure evolution under shearing of granular plugging zone

Forming a high-strength granular plugging zone is important to lost circulation control success in fractured oil, gas, and geothermal resources. The stability of mesoscale force chain network, which is the way to transferring contact force between particles, determines granular plugging zone strength. However, the effect of granular performance parameters on mesoscale structure stability of plugging zone remains unclear. In this work, a photoelastic method is adopted to visually observe mesoscale structure evolution accounting for fracture occurrence, plugging zone stress condition and failure mode. Results show that force chains display better symmetry along the shear direction and distribute more uniformly in a plugging zone formed with higher friction coefficient particles. Contact force transmission causes lower friction coefficient particles siding and induces plugging zone instability. Symmetry and uniformity of the strong force chain network are the two key optimal distribution parameters to improve plugging zone stability and materials selection.

Lost Circulation Control for Pump-Injected Reverse Circulation Drilling: Experimental Optimization and Theoretical Analyses

The reverse circulation drilling method is now widely accepted as an effective way of exploration and grade control. However, the performance of reverse circulation drilling operations is greatly affected by the filtration effect (lost circulation) and no research has critically addressed this issue. In this study, a simulation experiment was conducted, the filtration effect was quantified as the mass of water loss measured at a flow rate of 4.00 m3/h to 8.00 m3/h for reverse and forward circulation drilling. Then the dynamic filtration effect was analyzed in three ways; first, a filtration versus circulation flow rate curve was obtained for both the forward and reverse circulation process. Second, the positive and negative effect of hydrodynamics on filtration was analyzed through a computational fluid dynamic approach, and finally, via qualitative analyses. The results showed that: (1) An optimal flow rate occurred when the filtration effect was minimum, (2) There is a point where the positive and negative effect of the increase in flow rate on filtration is the same, and (3) filtration monotonously increases with flow rate. This study provides a better understanding of the effect of filtration during the reverse circulation drilling process, a guide to field experts, and a theoretical framework for future research.

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.

A high-temperature resistant colloid gas aphron drilling fluid system prepared by using a novel graft copolymer xanthan gum-AA/AM/AMPS

Colloidal gas aphrons (CGA) drilling fluid has been worldwide used in drilling depleted reservoirs and other under-pressured zones for its special plugging mechanism and excellent ability of lost circulation control. However, the research temperature of CGA system reported in the previous literatures is within 140 °C, and it is still an urgent challenge to promote its application in high-temperature (≥150 °C) deep wells. In this study, a novel environmentally-friendly xanthan gum derivatives XG-AA/AM/AMPS was synthesized by grafting acrylic acid (AA), acrylamide (AM), 2-acrylamido-2-methylpropane sulfonic acid (AMPS) onto XG and CGA drilling fluids with a temperature resistance of 180 °C has been successfully generated by using XG-AA/AM/AMPS as a foam stabilizer. Results presented that the XG-AA/AM/AMPS-based CGA drilling fluids can maintain reasonable rheology properties within 180 °C, i.e., significant shear thinning behavior, higher low shear rate viscosity, and apparent viscosity, and reasonable rheological parameters, which was conducive to cuttings carrying. At 140–180 °C, the filtration volume of prepared CGA fluid can be controlled within 13.5 mL. Furthermore, the high-temperature resistance mechanism of XG-AA/AM/AMPS was studied by several techniques (PSD, XRD, SEM, and polarizing microscope), which can be attributed to the dual effects of XG-AA/AM/AMPS on enhancing the dispersion of clay particles and the stability of aphrons. • A novel high-temperature resistant stabilizer XG-AA/AM/AMPS was synthesized by grafting xanthan gum with AA, AM and AMPS. • CGA drilling fluid with a temperature resistance of 180 °C was successfully prepared. • The high temperature performance of XG-AA/AM/AMPS-based CGA drilling fluid totally meets the API standard. • The high temperature resistance mechanism of XG-AA/AM/AMPS was comprehensively discussed.

Mathematical Simulation of Lost Circulation in Fracture and Its Control

Lost circulation has been one of the major problems that impede efficient and cost-saving drilling operations. The nature of lost circulation and its control is not yet fully understood. A method to characterize the mud loss in fracture and the plugging process of lost circulation materials is highly desired to obtain a thorough understanding of mud losses in fracture and provide reference for lost circulation control. This paper presents an easy-to-use method to identify types of lost circulation in fracture and the corresponding control. Three analytical models are presented based on three loss mechanisms, namely, seepage/filtration in a fracture, pipe flow in a fracture, and gravity displacement in a fracture. A numerical model is developed to simulate the deposition of lost circulation materials in fractures and predict the time and the volume of drilling fluid needed for lost circulation control. Case studies with these analytical models provide a deeper insight of this subject. Sensitivity analyses with the numerical model identify the major factors responsible for lost circulation control. High viscosity of drilling fluid may prevent lost circulation, while low viscosity is desired for a fast control of lost circulation. Lowering the density of drilling fluid is another way to prevent the lost circulation and facilitate the deposition of lost circulation materials. Lost circulation materials with high density could deposit faster close to the wellbore and therefore accelerating the control process. High concentration of lost circulation materials is likely to shorten the plugging time, which should be determined referring to the severity of loss. This work provides drilling engineers a practical method for simulating the lost circulation and selecting lost circulation material.

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.

A novel material evaluation method for lost circulation control and formation damage prevention in deep fractured tight reservoir

Lost circulation induced by working fluid loss into formation fractures often leads to safety, environmental and economic issues, which has hindered the exploitation of oil, gas and geothermal resources for years. Fracture plugging with loss control materials is the most commonly used method for lost circulation control. However, traditional material selection is mainly guided by trial-and-error method. This paper proposes a novel material evaluation method for the systematic and scientific selection of loss control material. The new method consists of four steps: key material parameters determination, parameter weight calculation, parameter experimental evaluation, and material quantitative scoring. Fracture plugging strength model is analyzed to determine the key material parameters accounting for fracture plugging strength and the requirement of plugging removal before production. Analytic hierarchy process is adopted to calculate the material parameter weight. Experiments are designed to evaluate the material parameter levels. Quantitative scoring model is proposed to finally determine the material integrated score for the quantitative evaluation and scientific selection of loss control material. The reliability of the proposed evaluation method is validated by fracture plugging experiments. High-score materials are selected to form loss control formula based on the proposed evaluation method and successfully applied to gas field in Tarim Basin, China.

Improving the rheological properties of alkali-activated geopolymers using non-aqueous fluids for well cementing and lost circulation control purposes

Typical alkali-activated low-calcium fly ash (AAF) geopolymer formulations are yet to deliver acceptable rheologies for applications such as oil and gas well cementing, where the cement slurry needs to be pumped, and few effective superplasticizers have been identified. The work reported here centers on the discovery that the rheological properties (mixability and pumpability) of AAFs can be significantly improved by incorporating an organic-based non-aqueous fluid (NAF). NAFs such as oil-based muds and synthetic-based muds are water/brine-in-oil invert emulsions that are widely used in the oil and gas industry for drilling and completion operations. Adding NAF can significantly improve the rheological properties of AAF geopolymers, to the point where the rheology of the mixture approaches that of a Portland cement slurry. Overcoming the poor pumpability issue of AAF opens up a large variety of well cementing applications for AAF slurries including primary cementation and plug cementing in severe lost circulation zones.To modify the gelation and setting characteristics multiple fly ash sources and activators were considered in this study. The alkali activators tested in this study were sodium hydroxide and sodium hydroxide with addition of liquid or solid form sodium silicates. Sodium hydroxide activated AAFs showed improved rheological properties with the addition of NAFs. The liquid sodium silicate activated AAFs resulted in unfavorable rheology readings, gel strength measurements and accelerated the pumping time in comparison to use of pure sodium hydroxide activator. The addition of NAF further accelerated the pumping time, increased the viscosity of the AAF, leading to an unpumpable slurry with a rapid setting behavior. In contrast, activation with solid sodium silicate did not negatively affect the rheological behavior of AAF in the presence or absence of NAF. The use of a solid activator showed an initial high consistency peak that broke down with continued shearing. This early high consistency behavior may be exploited to combat lost circulation problems on troublesome oil and gas wells.

CFD–DEM simulation of mud cake formation in heterogeneous porous medium for lost circulation control

The lost circulation is a problem commonly observed in well drilling operations under overbalanced pressure conditions. Among the geological formation characteristics that can intensify the loss of drilling fluid, the presence of highly permeable regions is one of the most critical. In this case, corrective measures are necessary, like the use of lost circulation materials (LCM) to build a mud cake in the porous substrate, sealing the pores, and controlling the lost circulation. This work presents a numerical study to understand the influence of fluid–particle interaction under dynamic filtration on the process of particle packing in porous media, resembling the use of LCMs to grow a mud cake and reduce the fluid invasion. The flow of an incompressible Newtonian fluid in a porous medium modeled in pore scale is considered, being the voids structured as an anisotropic array of staggered solid cylinders. The LCM is considered as solid and discrete spherical particles (dp= 0.75 mm) immersed in a water–glycerin fluid. A Euler–Lagrange approach to model the liquid–solid two-phase flow is employed, with the simulation being resorted via the dense discrete phase model coupled to the discrete element method (DEM). The DEM computes collision and friction forces present in the particle–particle and particle–wall interactions. Variation effects for different values of the Reynolds number in the vertical channel ReCH,i (125, 250, 500) and the lost circulation intensity measured by the initial fluid loss ratio Qloss (5, 10, 20%) are considered. By releasing solid particles, the fluid loss ratio, relative permeability, particle layer dimensions, and also the vertical channel pressure are altered. For an initial fluid loss ratio of 20%, the decay in Qloss after 60 s for ReCH,i values of 125, 250 and 500 is, respectively, 17%, 13% and 12%. The variation of the initial fluid loss ratio Qloss dramatically influences the number of particles that forms the bed. After 60 s, no particle has entered the porous matrix for Qloss = 5%. When Qloss = 10%, the particles build a bed, and for 20%, the plugging of the substrate occurs.

Fracture plugging zone for lost circulation control in fractured reservoirs: Multiscale structure and structure characterization methods

The stability of plugging zone has a great impact on the lost circulation control effect in fractured reservoirs. In this work, the multiscale structure of plugging zone is described based on the analysis of its inter-particle forces. Structure characterization methods of plugging zone are developed accounting for geological characteristics and drilling operation features of fractured reservoirs. To conclude, fracture plugging zone is a dense granular matter system, and the contact forces are dominant in inter-particle forces. Plugging zone structure is multiscale, of which the microscale is single lost circulation materials (LCM), mesoscale refers to the particle cluster transferring force chains, and the macroscale refers to the plugging zone formed in the fracture. The correlation between performance parameters of microscale LCM and strength of macroscale plugging zone can be established by characterizing the mesoscale structures of plugging zone. This study provides the basis for LCM research and development in fractured reservoirs.