Lost Circulation Materials Research Articles

Investigation of Filter Cake Evolution in Carbonate Formation Using Polymer-Based Drilling Fluid.

Drilling fluid and filtrates invasion often alter the near-wellbore flow properties during overbalanced drilling. The utilization of polymeric gels to prevent drilling fluid loss reduces the risk of formation damage caused by this alteration. In this study, the internal and external filter cake evolution by polyacrylamide (PAM) cross-linked with polyethylenimine (PEI) was investigated. The analysis conducted in this study showed that the cross-linked polymer activates and forms a mature gel inside the formation’s pores. Gel also formed a dense uniform structure on the rock’s surface, preventing further fluid loss. A high sealing pressure of up to 1000 psi was achieved, allowing drilling to continue without the need for additional casing string to prevent lost circulation. Moreover, the PAM/PEI formula showed less invasion of filtrate and evolution of a thin shallow internal filter cake that penetrated less than half of the filter disk thickness. In comparison to the full invasion and particle depositions that occurred with the water-based mud (WBM), the PAM/PEI formula is expected to reduce the impact of lost circulation materials (LCMs) on formation damage.

Gelation kinetics of PAM/PEI based drilling mud for lost circulation applications

The problem of lost circulation is a direct consequence of the presence of fractures or high permeability. Lost circulation material (LCM) is used to treat the loss that occurs or proactively strengthen the wellbore of the potential loss zones. The polyacrylamide (PAM)/Polyethylenimine (PEI) cross-linkable polymer gel is proven to work best in zone-sealing and plugging applications. In addition to their thermal stability, the fact that these gels are flexible gives them the advantage of deforming and sealing-off multiple and complex near wellbore fractures.The primary objective of this study is to investigate the gelation kinetics of the PAM/PEI cross-linkable polymer to be used in drilling fluids formulations to enhance wellbore stability and strength. Different combinations of polymers and crosslinkers were evaluated; potential polymers were screened for gelation kinetics and gel strength in typical drilling mud formulations. The gelation time of the polymer-based drilling mud formulation is determined in salt-free and in saline systems. The results on rheological properties and gelation kinetics were used to prepare a gelling mud formulation using PAM/PEI cross-linkable polymer as LCM. The produced gel was thermally stable under high-temperature-high-pressure reservoir conditions, which will result in enhanced wellbore strengthening.

Optimizing selection method of continuous particle size distribution for lost circulation by dynamic fracture width evaluation device

The increase in the pressure bearing capacity is critical for mitigation of lost circulation, and to do so the strength of the plugging zone should be increased via picking an effective evaluation method and choosing the proper particle size distribution (PSD). Therefore, in order to successfully investigate this process, the traditional evaluation device of lost circulation should be improved for dynamic variation of fracture width to find the optimized plugging materials. This means, a new testing apparatus device capable of testing dynamic fracture width should be developed. Additionally, commonly used selection criteria of PSD are incapable of providing us with a continuous PSD curve since they only fulfill discrete PSDs. To overcome these two obstacles, a variety of continuous PSD models based on a new testing device and method are examined. Likewise, the impact of governing parameters and an optimized PSD selection criterion were studied with a specific lost circulation material (calcium carbonate) and concentration in a water-based drilling fluid. The experimental results demonstrate that the normal distribution model provides the best selection result while the sensitive analysis indicate that the factors influencing pressure bearing capacity in a sequential order are: minimum particle size, standard deviation, maximum particle size and average value. Besides, the optimized experiments illustrated that the maximum particle size should be 1.3 times of the maximum fracture width and the minimum particle size should be 0.8 times of the average fracture width. Moreover, the average value should be equal to the optimal particle average size and the standard deviation should be 0.3 times of the particle average size. It was found that the optimized selection method results in the highest plugging efficiency compared to conventional selection criteria. Collectively, the new device and optimal assessing approach would improve the prediction accuracy of pressure bearing capacity to select an appropriate PSD with varying of fracture width in field operations.

An Experimental Study on optimizing the particle size distribution of bridging agents in drilling fluids

Managing lost circulation has always been a serious challenge in the oil industry. This situation becomes further complicated when shale formation is involved. Particulate lost circulation materials (LCMs) have been used to prevent the drilling fluid from entering fractured, cavernous, or high-permeability formations for many years. However, existing LCMs are inefficient in terms of size and application methods in solving severe-to-total losses in shales due to the uncertainty of particle size distribution (PSD) in drilling fluids. This study focuses on an experimental method optimizing the PSD of solid particles in the drilling fluid at high temperature and pressure. An orthogonal experiment of L9 (3)4, including four factors and three levels, was designed to investigate the relationship between the PSD of solid particles and filtration loss. Results indicate that the orthogonal design method contributes to the optimization of the PSD of solid particles in the drilling fluid and reduce the test time and experimental workload during the drilling fluid process in the design stage. In addition, a polynomial relationship between the cumulative filtration loss and D90 or D50 is verified experimentally in this study. This study will remarkably benefit the drilling technologists in resolving the issue of lost circulation.

Ultra-High Temperature Drilling Fluid Technology for Drilling Well Jiantan-1

The well Jiantan-1 is a key exploratory well of PetroChina located in the Jianshan structure of the central rhinoplasty belt in the Chaidamu Basin. The main purpose of this 6343 m well is to explore the gas reserves in the base rocks of the Jianshan structure. The highest bottom hole temperature measured is 235℃, one of the highest bottom hole temperatures encountered in onshore drilling. The formations drilled in this area have fractures that are fully developed, long sections of gypsum and mudstones, high pressure saltwater zones, high CO2 concentrations and high geothermal gradient. Borehole wall collapse, mud losses, overflow, creeping and tight hole in the gypsum/mudstone formations are downhole troubles encountered and sometime intertwined with each other during drilling. To deal with these problems, laboratory study was conducted on several aspects of the drilling fluid used, which are high temperature stability, rheology and high temperature filtration control. An organic salt drilling fluid was formulated with additives specially selected, such as ultra-high-temperature filter loss reducers, anti-collapse additives with plugging capacity, lubricants and thermal stabilizers etc. This drilling fluid is able to resist high temperatures up to 240℃. After aging for 72 hours, the HTHP filtration rate was maintained with 10 mL, the coefficient of friction was less than 0.1, and the invasion depth on a sand-bed less than 12 cm. In addition, the fourth and fifth intervals of the well penetrated formations with micro-fissures and a basal rock weathered crust, and mud losses occurred many times into the fissures. A lost circulation material slurry for the well Jiantan-1 was then developed with choice rigid, flexible and gel-like plugging additives which are suitable for use at elevated temperatures. Using this mud loss control slurry, the pressure bearing capacity of the formations into which mud losses have occurred was finally enhanced. The organic salt drilling fluid and the mud loss control slurry played a major role in preventing and controlling borehole wall collapse and mud losses, ensuring the success of the drilling of the first ultra-high temperature Jiantan-1 well in Chaidamu Basin.

Evaluation of Aerated Drilling in K-01 Geothermal Well using Guo Ghalambor’s Gas-Liquid Rate Window

Common problem in geothermal drilling is loss circulation problem. One of the common methods to cure loss circulation problem is using Lost Circulation Materials (LCM), but in the production zone, using LCM can damage the production zone. Therefore, underbalanced drilling is method that can be used to prevent loss circulation problems in production zone in geothermal well. One of the most common underbalanced drilling methods is aerated drilling or foam drilling. Aerated drilling was used to overcome the loss of circulation problem in production zone in K-01 Geothermal Well. Even though, the aerated drilling was already used, but the loss circulation problem was still occurred. The purpose of this research is to evaluate aerated drilling operation in K-01 Well using Guo Ghalambor’s Gas-Liquid Rate Window and make recommended gas-liquid rate for the next drilling operation. Gas-Liquid Rate Window is constructed using characteristic of the formation, drilling parameters, daily drilling report and also fluid injection characteristics that was used for aerated drilling operation in K-01 geothermal well. Using the constructed Gas-Liquid Rate Window, an evaluation is carried out for the drilling operation in K-01 geothermal well. The gas-liquid rate parameters used in aerated drilling operations is evaluated while checking the loss circulation event from the mud logging data. After the evaluation of the aerated drilling is carried out in then a suggestion is made for the next drilling operation. Based on the evaluation, the combination of gas-liquid rates that was used on the 9.875"hole section in K-01 Well was in the outside of the constructed GLRW therefore loss circulation problem occurred. The recommended gas-liquid rate combination from this research can be used to determine the gas-liquid rate combination to prevent loss circulation problems, wellbore damage and cutting transport problems.

Study of effects of downhole conditions on the setting time and compressive strength of MOS settable system by orthogonal experimental design

Lost circulations of drilling fluids during drilling and completion can be an expensive and time-consuming problem. Commonly used lost circulation materials (LCMs) like nutshells or cross-linked polymer gels can’t be easily removed after plugging with a risk of formation damage. Magnesium oxysulfate cement (MOS cement) provides a reliable solution for solving the severe loss that occurs in production and non-production zones. In this paper, the effects of harsh downhole conditions including density, bentonite content, salt content, and temperature on the setting time and compressive strength of the MOS settable system are studied using the orthogonal experimental analysis method. At the same time, laboratory plugging experiments and field application are used to evaluate the ability of the MOS settable system to seal the severe loss. The results show that the downhole conditions have a significant effect on the setting time and compressive strength of the system. By adding calcium carbonate particles, the permeability of the MOS settable system can be reduced and the pressure-bearing capacity of the MOS solidified plug can be improved. Field application shows that this settable system can solve severe loss that conventional LCMs cannot solve and has a good application prospect.

Potentials of waste seashells as additives in drilling muds and in oil well cements

Abstract Additives to drilling fluids and oil well cements play a significant role during wellbore drilling operations. These additives (such as carboxymethyl cellulose and starch) are often expensive and most times degrade at high temperature conditions. Although seashells (usually considered waste materials) have been utilized for various purposes in drilling muds as weighting agents and lost circulation materials, however, to the best of our knowledge, there is no research conducted yet to test their feasibility and performance as retarders or accelerators in oil well cements. In addition, no study has compared the filter loss capabilities of these seashells as drilling mud additives. Filling this gap is what makes this work novel and is the major contribution of this work to the extant literature on this subject. The specific objective of this work is to bring to light how useful seashells could be as a drilling mud and oil well cement additive from an experimental standpoint. To this end, the capability of the pulverized and calcined shells of egg, snail, oyster and periwinkle as mud filter loss control agents and as oil well cement additives were evaluated. Thirteen water based mud samples and one oil well cement sample were formulated with the shells as additives. The basic tests run on the formulated mud and cement samples were the filter loss test (using the American Petroleum Institute filter press) and cement setting time tests (using the Vicat’s apparatus) for the mud and cement respectively. The conventional filter loss control material - carboxymethyl cellulose (CMC) was used as the control. Results from the filtration tests showed that the filter loss capability of the shells decreased from egg shell to snail shell to periwinkle shell. With respect to cement, incorporation of calcined oyster and periwinkle seashells resulted in extended final setting times of the cement. In quantitative terms, it was observed that the final setting time of the cement was extended by an average of 14 and 28 ​min as the amount of oyster and periwinkle shell respectively introduced to the cement was increased from 5 to 15%. Conclusions drawn are that both the oyster shell powder and the periwinkle shell powder act as retarders. It is concluded that the filtration property of the seashells had a direct relationship with the amount of the seashell added. Economically, the pulverized seashells have comparable cost performance with the conventional filter loss and cement retarder additives. By utilizing these waste shells in this way, the overall well drilling cost would be reduced and the environment would be protected.

Simulating Fracture Sealing by Granular LCM Particles in Geothermal Drilling

Lost circulation occurs when the returned fluid is less than what is pumped into the well due to loss of fluid to pores or fractures. A lost-circulation event is a common occurrence in a geothermal well. Typical geothermal reservoirs are often under-pressured and have larger fracture apertures. A severe lost-circulation event is costly and may lead to stuck pipe, well instability, and well abandonment. One typical treatment is adding lost-circulation materials (LCMs) to seal fractures. Conventional LCMs fail to properly seal fractures because their mechanical limit is exceeded at elevated temperatures. In this paper, parametric studies in numerical simulations are conducted to better understand different thermal effects on the sealing mechanisms of LCMs. The computational fluid dynamics (CFDs) and the discrete element method (DEM) are coupled to accurately capture the true physics of sealing by granular materials. Due to computational limits, the traditional Eulerian–Eulerian approach treats solid particles as a group of continuum matter. With the advance of modern computational power, particle bridging is achievable with DEM to track individual particles by modeling their interactive forces between each other. Particle–fluid interactions can be modeled by coupling CFD algorithms. Fracture sealing capability is investigated by studying the effect of four individual properties including fluid viscosity, particle size, friction coefficient, and Young’s modulus. It is found that thermally degraded properties lead to inefficient fracture sealing.

Utilizing a new eco-friendly drilling mud additive generated from wastes to minimize the use of the conventional chemical additives

The cost of the drilling operation is very high. Drilling fluid presents 15 to 30% of the entire expense of the drilling process. Ordinarily, the major drilling fluids additives are viscosity modifiers, filtration control agents, and partial loss treatments. In this experimental work, full-set measurements under fresh and aged conditions, as well as high-temperature and high-pressure (HTHP) API filtration, were conducted to study the impacts of adding 0.5%, 1.5%, 2.5%, and 3.5% of black sunflower seeds’ shell powder (BSSSP) to spud mud. BSSSP of various grain sizes showed their ability to be invested for viscosity modifying, seepage loss controlling, and partial loss remediation. In addition to BSSSP eminent efficiency to be used as a multifunctional additive, the BSSSP is cheap, locally obtainable in commercial quantities, environmentally friendly additive and easy to grind into various desired grain sizes. Besides its outstanding strength to behave under conditions up to 30 h aged time and under 50 °C (122 °F) temperature, the utilization of powdered waste black sunflower shells in the drilling process and other industrial applications can reduce the effects of food waste on the environment and the personnel safety. To sum it up, experimental findings revealed that BSSSP can be used for multiple applications as a novel fibrous and particulate additive. The results elucidated BSSSP suitability in substituting or at least minimizing some of the traditional chemical materials utilized in the petroleum industry such as salt clay, polymers, and lost circulation materials (LCM).

A method of sizing plugging nanoparticles to prevent water invasion for shale wellbore stability based on CFD-DEM simulation

Plugging shale pores and throats with nanoparticles is an efficient and economic method of preventing water invasion to sustain shale wellbore stability. The nanoparticles size distribution is the key factor affecting plugging performance. Currently, the selection of nanoparticle size is usually based on empirical plugging rules and qualitative evaluation experiments. Experimental evaluations only can reflect the macroscopic plugging performance, but cannot reveal the microscopic plugging mechanism. In addition, the experiment for shale pores plugging is complicated and costly. Thus, it is very meaningful to propose a new method for sizing plugging nanoparticles based on simulation.The essence of particles plugging process actually is the interactions of particle-particle, particle-rock and particle-fluid. In this paper, the thought of discrete element method is adopted and the plugging nanoparticles are treated as discrete elements. A coupled computational fluid dynamics-discrete element method (CFD-DEM) model is constructed to describe the motion behaviors of nanoparticles (discrete elements) in the process of plugging shale pores. In this model, taking into account the flow action of drilling fluid and the nanoscale effect of nanoparticles, the major considered forces include inter-particle forces (i.e. elastic contact force, friction force and Van der Waals force), hydrodynamic forces (i.e. drag force, Magnus force, buoyancy force and pressure gradient force) and gravitational force. Furthermore, a plugging criterion is established to check the particles plugging status through monitoring the microscopic movements of nanoparticles. Moreover, three evaluation indexes (i.e. porosity reduction of shale, permeability and porosity of external plugging layer, plugging ratio) are proposed to evaluate the macroscopic plugging performances of nanoparticles.Based on the constructed computational model, microscopic plugging criterion and macroscopic plugging performance evaluation indexes, a novel method of sizing plugging nanoparticles to prevent water invasion for shale wellbore stability is developed. In addition, the validation of the method is conducted and the results are similar to the previous results of plugging experiments. The proposed method can give support to the selection of plugging nanoparticles size through obtaining the microscopic plugging behaviors and macroscopic plugging performances of nanoparticles. And it can also be regarded as a fundamental method to study other kinds of particles plugging problems, such as formation damage and optimization of lost circulation materials.

Transient analysis of mud loss in fractured formations

Quantitative analysis of mud losses is a direct method for prediction of fracture width, which offers a criterion for designing lost preventive materials and guides early remedial actions. This study presents a mathematical model for prediction of mud loss in naturally fractured formations in two parts. First, a mathematical model is developed considering the fracture deformation and leak-off from the fracture faces to describe the trend of mud loss in fractured formations. The fracture deformation is described as a linear function of exerted differential pressure. An explicit approximate equation is also proposed to predict the fracture width from mud loss data for yield power-law drilling fluids. Second, a mathematical model is developed, which describes mud losses in fractured formations taking into account the pressure drops in mud bank and formation fluid bank. The governing equations are solved numerically using an implicit finite difference method for the first part and an iterative method with variable time-step for the second part.The comparison of approximate equation with the complete model indicates that the predicted fracture width of the approximate equation only differs 5% from that of complete numerical model. The substantial new finding is that depending on the magnitude of leak-off coefficient, three major type-curves of mud loss can be identified. It is found that formation fluid viscosity can affect mud loss volume in early times and thinner drilling fluids tend to increase the mud losses in fractured formations. However, the ultimate mud loss volume is governed by the mud-invasion factor, which is a direct function of drilling fluid yield stress. The validity of the model is examined using published data of mud loss measurement in the Gulf of Mexico. Automatic curve-fitting technique is adapted to find the fracture width by matching the model prediction with mud loss data. The results of this study is beneficial to predict the volume of mud loss for qualitative purposes during well planning and more importantly to quantify the conductive fractures for design of effective blend of lost circulation materials.

Observations and analyses of the first two hydraulic stimulations in the Pohang geothermal development site, South Korea

We present key observations and analyses of the first and second stimulations conducted at the Pohang enhanced geothermal system (EGS) site in Korea in 2016. The first hydraulic stimulation was conducted in the PX-2 well of 4.3 km depth, with the maximum wellhead pressure of 89.2 MPa, the maximum injection rate of 46.8 L/s, and a total injected volume of 1970 m3. The first stimulation showed non-linear and reversible fracture-opening behavior with injection pressure increase. The stimulation mechanism of PX-2 is interpreted as a combination of tensile fracture extension and hydraulic jacking. The second hydraulic stimulation was conducted in the PX-1 well of 4.2 km depth, with the maximum wellhead pressure of 27.7 MPa, the maximum injection rate of 18.0 L/s, and a total injected volume of 3907 m3. The fracture-opening pressure of PX-1 was evaluated from the clear pressure peaks and the pressure at differential injectivity increase, and was drastically lower than that of PX-2. The transmissivity of PX-1 permanently increased by 6.4 times during the second stimulation. The wellhead injectivity of PX-1 was 3.6 times as high as that of PX-2 at the same injection rate. The stimulation mechanism in PX-1 is interpreted as a combination of hydraulic shearing and hydraulic jacking of unmated or shear-dilated fracture. Both stimulations in the two wells showed consistently greater seismicity rate and magnitude during the shut-in phases than during the preceding injection phases. A close correlation between the injected fluid volume and seismic magnitude was observed in both wells, and the seismic events induced by the two stimulations were in general below the maximum magnitude envelopes expected by the previous studies. Despite the close distance of approximately 600 m in the same rock formation, the two wells showed distinctly different behavior in terms of the overall pressure ranges, pressure peaks, pressure for injectivity increases, transmissivity changes, and the stimulation mechanisms due possibly to the heavy mud and lost circulation material used during the drilling and completion of PX-2. The contrasting hydromechanical responses observed in the same reservoir at the two nearby wells emphasize the importance of proper drilling and completion with close consideration of stimulation strategy.

Investigation of LCM soaking process on fracture plugging for fluid loss remediation and formation damage control

When drilling through naturally fractured reservoirs, the remediation of drill-in fluid loss needs to be designed by taking both fracture plugging and formation damage into account. Lost circulation materials (LCMs) should be implemented efficiently to minimize the solid-laden fluid invasion into the fractures during the development of fracture plugs. This study investigated the effects of three LCM deployment factors: injection rate, soaking time, and soaking pressure, on the total fluid invasion volume and plug breaking pressure. Full factorial experimental design and statistical analysis were conducted to study the statistical significance of each factor. Scanning Electron Microscope (SEM) and Micro Computed Tomography (MicroCT) were applied to visualize the plug structures for the understanding of the soaking effects.Different combinations of LCMs were tested in a permeability plugging apparatus with a slotted disk. For granular LCM mixtures, the soaking pressure and time did not have a significant impact on total fluid invasion volume or ultimate plug breaking pressure. For mixtures blended with fibrous LCMs, longer soaking time contributed to reducing fluid loss and increasing plug breaking pressure. A lower LCM pill injection rate led to less fluid invasion upon the establishment of an effective seal. The SEM and MicroCT images indicated that a longer soaking time could lead to a less porous and strengthened plug structure.The results and conclusions from this study provide new insights into the identification of optimal LCM implementation scheme with better formation damage control.

Use of Low-ECD Synthetic-Based Mud During Topdrive Failure

While drilling in deepwater Gulf of Mexico, a topdrive failure forced the shutdown of all drilling operations for the rig operator and lasted for 114 hours (almost 5 days). At the time of the failure, the equivalent static density (ESD) was 11.36 ppg and the equivalent circulating density (ECD) was 11.5 ppg. The rate of penetration (ROP) was 100 ft/hr. Troubleshooting It was recommended that there be no utilization of the pumps while drilling was stopped, to reduce the chance of packing off the annulus with cuttings until the rotating and reciprocating could continue. During this time, a 100-psi backpressure was maintained using the managed-pressure drilling system. Kronos low-ECD synthetic-based mud (SBM) at a weight of 10.9 ppg was utilized during this event. During troubleshooting and repair work, a 1,500-bbl 11.4-ppg riser cap was spotted. Following the repair of the topdrive, a malfunctioning packer had to be replaced, causing additional delays. After the packer was set and the blowout preventers tested, the 11.4 ppg riser cap was displaced and subsequently replaced with 10.9-ppg low-ECD mud. The Kronos system was designed and maintained to have adequate suspension properties to maintain wellbore integrity without sag during unexpected downtime, even with cuttings left in the hole. The rig continued to trip into the hole to 9,800 ft. At that time, a bottom-up was circulated and a maximum of 575 units of gas was circulated out. Drilling then continued using a base oil and premix for dilution. The lost circulation material in the background continued to be blended into the dilution products being used at the time. During the drilling from 11,636 to 12,845 ft, the decision was made to increase the mud weight to 11.15 ppg. When the drilling continued from 12,845 to 13,234 ft, formation water was detected from the mud check entering the mud system. The system was treated to minimize the damage caused by formation water and maintain the properties sought by the operator. To assist in maintaining the desired weight and to prevent low-gravity solids from impacting the mud weight, two shakers were screened up to API Standard 120. At roughly the same time, the mud weight was increased to 11.2 ppg and then to 11.4 ppg. While continuing to drill, one of the engines that provided power to the mud pumps broke down, reducing the flow rate required to clean the hole. A cement unit was added to the pump to replace much of the volume lost with the broken mud pump. At 13,155 ft, the decision was made to call this current interval at total depth. The back-reaming process also started at that time. To assist with the hole cleaning, a high-viscosity sweep was blended and circulated. An additional high-viscosity sweep was blended and circulated, the second time at 9,058 ft. With continued back reaming through some tight spots to 7,272 ft, operators decided to wash back down 7,938 ft and pump one more high-viscosity sweep. Once the hole was clean, a flow check was performed and operators began to pull out of the hole.

A self-breaking supramolecular plugging system as lost circulation material in oilfield

AbstractLost circulation is a frequently encountered problem during workover operations of a low-pressure reservoir. Many lost circulation materials (LCM) have been used to solve the problem, but various disadvantages still exist, for example, oil-soluble materials are easy to get stuck in the pipe string as the temperature drops and gel time and gel strength of the cross-linking gel systems are difficult to control in the occasion of inadequate stirring. A self-breaking supramolecular plugging system used to control lost circulation of workover operations is developed, and it can encounter the aforementioned disadvantages of the traditional LCM. The system forms a space grid structure by non-covalent bonds between the molecules without adding a cross-linking agent. Sand beds plugged with the supramolecular plugging system had a pressure bearing capacity of 4.5 MPa. The self-breaking rate of the supramolecular system was 100% after 7 days at a temperature of 120°C. The core permeability recovery of the self-breaking liquid was 91.9%, indicated that the plugging system is compatible of the reservoir. The supramolecular plugging system has been used in four wells in the Jidong oilfield of East China. Field practice showed that the supramolecular LCM can effectively control lost circulation at different rates during workover operations. The increase in the daily fluid after operations indicated that the slurry has not damaged the reservoir.

Effects of permeable plugs on wellbore strengthening

Wellbore strengthening (WBS) treatment uses lost circulation materials (LCMs) to seal fractures emanated from a wellbore to enhance the pressure-bearing capability of a fractured rock formation during drilling a well. The plugging zone made of LCMs to prevent high wellbore pressure from being completely transferred to fracture tip is permeable and movable. The mechanical responses of wellbore-fracture-plug systems are thus important to WBS designs. A fully coupled hydraulic fracture model is presented to simulate the evolution of near-wellbore stress, fracture opening and internal pressure for finding the way of mitigating the risk of plug failure. Laboratory results were first reported on the ranges of plug permeability and strength in producing plug failure. A symmetric bi-wing fracture geometry is employed in the model, and the roles of in situ stresses, and plug location, dimension and permeability are studied in terms of dimensionless parameters. The numerical results show that in addition to increasing hoop stress, the less permeable plug can slow pressure enhancement rate at its downstream to postpone tensile plug failure caused by increasing fracture opening. The strengthening duration prolongs in rocks under an isotropic in situ stress condition, by plugging fractures at or near their fracture mouth, moderately increasing plug length and width, and decreasing plug permeability. Not only do the time-varying results recover most of the findings obtained by the fixed pressure fracture model, but they are more suitable to the plug failure analysis because the unbalanced pressure on both sides of the permeable plug, controlling shear plug failure, varies transiently.

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.

Feasibility Study of using Oil Palm Trunk Waste Fiber as Fluid Loss Control Agent and Lost Circulation Material in Drilling Mud

An investigation was conducted on Oil Palm Trunk (OPT) fiber that was used as fluid loss control (FLC) agent and lost circulation material (LCM) in drilling fluid. API filtration test was performed to evaluate the performance of OPT fiber as FLC while sand bed test was performed to evaluate the performance of OPT fiber as LCM. For the API filtration test, the addition of OPT fiber reduced the fluid loss while the filter cake thickness increased as higher amount of OPT fiber was added into the drilling mud. The use of fine size OPT fiber (<75?m) yielded the lowest fluid loss volume. For the sand bed test, mud invasion reduced as the concentration of OPT fiber increased. The findings also revealed that the OPT fiber with the size of 75-150?m performed best in reducing mud invasion with the value of merely 1 cm. Thus, it can be concluded that OPT fiber has a high potential to be used as FLC and LCM in drilling mud.

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.