Lost Circulation Materials Research Articles

Screening of Lost Circulation Materials for Geothermal Applications: Experimental Study at High Temperature

Abstract This study presents a laboratory experimental research to determine the characteristics of lost circulation materials (LCMs) capable of addressing thermal degradation, providing bridging and sealing in geothermal conditions. Eleven different materials were tested: Walnut Fine, Walnut Medium, Sawdust, Altavert, Graphite Blend, Bentonite Chips, Micronized Cellulose (MICRO-C), Magma Fiber Fine, diatomaceous earth/amorphous silica powder (DEASP), Cotton Seed Hulls, and a Calcium Carbonate Blend. The filtration and sealing pressure of the LCMs were measured with HPHT equipment up to 149 °C (300 °F). Besides, the particle size distribution (PSD) of fine granular materials was measured. The results show that the performance of some LCM materials commonly used in geothermal operations is affected by high temperature. Characteristics such as shape and size made some materials more prone to thermal degradation. Also, it was found that the PSD of LCMs is a key factor in the effectiveness of bridging and sealing fractures. The results suggest that granular materials with a wide particle size distribution PSD are suitable for geothermal applications.

Tensile Failure and Fracture Width of Partially Permeable Wellbores with Applications in Lost Circulation Material Design

SummaryEstimation of near-wellbore fracture widths remains central to designing the particle size distribution (PSD) and composition of lost circulation material (LCM) blends. Although elastic rock models are often used for this purpose, they fall short in capturing the substantial effect of pore fluid pressure on the fracture width. The problem is addressed in this paper by incorporating the poroelastic back stress in width estimation of axial fractures nearby an inclined wellbore. The poroelastic back stress is caused by a nonideal drilling fluid filter cake allowing for fluid pressure communication between the wellbore and pore space of the rock surrounding the wellbore. In this aspect, a proper definition of the filter-cake efficiency is made in terms of the wellbore pressure, far-field pore fluid pressure, and pore fluid pressure of the rock surrounding the wellbore. The value of this parameter is estimated from the standard drilling fluid filtration test results, as well as the formation rock permeability. The filter-cake efficiency is next used to develop the long-time, asymptotic analytical solution for the poroelastic stress of an inclined wellbore. By accounting for the obtained poroelastic back stress, an improved estimation of the wellbore tensile limit that depends on the filter-cake efficiency parameter is developed. For wellbore pressures beyond the wellbore tensile limit, the width of the near-wellbore fractures is estimated. The fracture width estimation is made through an analytical, dislocation-based fracture mechanics solution to the integral equation describing the nonlocal stress equilibrium along the fracture faces. The commonly practiced scheme for geometric design of LCM blends is enhanced by using the presented improvement in estimation of the near-wellbore fracture width. A case study is used to demonstrate the substantial effect of drilling fluid filtration properties and the resulting poroelastic back stress on the wellbore tensile limit, estimated fracture width, and consequently, composition of the recommended LCM blend.

Sealing Mechanism Based on Force Chain and Experimental Investigation of Sealing Fractures

Abstract Lost circulation often occurs in fractured formations, which was a main technological problem during drilling. Conventional lost circulation material (LCM) was often used to form a plugging zone to prevent fluid loss during drilling. The formed seal was a granular material system composed of LCMs. This paper presented the physical mechanism of the force chain within the plugging zone. The seal performance is related to the properties of LCMs. A device for testing seal performance of LCMs with long fracture was developed. The effects of LCM performance on seal integrity were investigated using a plugging device with long fracture. The results showed that the wide particle size distribution (PSD) of LCMs tended to form a strong force chain network structure within the sealing zone. Increasing the stiffness and roughness of LCMs resulted in higher breaking pressure. The addition of fiber with high length-diameter ratio could improve the shear strength of the sealing zone and form a strong force chain network structure, and it can reduce fluid loss.

Shape memory polyurethane as a wellbore strengthening material

Wellbore strengthening is a technique for increasing formation fracture pressure and widening the mud window. In this study, smart wellbore strengthening materials (WSMs) built on shape memory polyurethane (SMPU) are introduced. SMPU WSMs are programmed to release pre-determined recovery stress once unfold at predetermined temperature. The molecular structure of the SMPU WSM synthesized here is characterized and its shape memory, recovery stress and mechanical properties are determined. Compatibility of SMPU with xanthan gum-based drilling fluid is confirmed through mud rheology and filtration tests. SMPU WSMs mixed with SMPU lost circulation materials (LCMs) smoothly flow through the bit, pack against and seal fractures and release recovery stress once activated. Wellbore strengthening and fracture sealing of the SMPU WSM and LCM mixture show a recovery stress of 3.52 MPa in a constrained environment. The seal showed high plugging pressure of 110 bar with only 150 cm3 total fluid loss.

Technological solutions for drilling a trap intrusion section on the Srednebotuobinskoe oil gas condensate field, East Siberia

One of the largest trap intrusion provinces, Siberian traps, is located in the East-Siberian Platform in Russia. A trap intrusion zone usually has abnormally low reservoir pressures and natural fractures. Consequently, trap drilling is associated with fluid losses that can be catastrophic. The section of trap intrusion is a part of a Ø 174 mm production casing section of the Srednebotuobinskoe field. The basic well design of this section also includes the Osinsky horizon, which features an abnormally high reservoir pressure zone. The latter creates incompatible with drilling conditions environment due to the presence of catastrophic loss zones in traps. Time spent on drilling a trap intrusion zone accounts for up to 30 % of the total well drilling time. The abovementioned geological issues in directional wells become the key topic in solving it. The method to resolve this issue is an integrated approach employing all the technologies and technical facilities aimed at finding a technological solution. First of all, in order to optimize the well construction cycles and reduce the complications, all the wells were classified in three categories as per the type of behavior in trap intrusion. This allowed to work out multi-level activities, that depended on severity of losses and non-operational time spent on drilling through this section. An alternative well design was developed and trialed on several wells, which showed positive results on decreasing non-productive timing. The main concept of this design was significantly different from the basic well design that was used in the past on the Srednebotuobinskoe field. The main difference was isolation of traps from the high pressure Osinsky horizon lower section with a Ø 245 mm casing string, which allowed safe splitting of two incompatible drilling zones. The economic effect of this solution allowed saving of up to 10.4 days or 15 % of the construction time in the wells of first category. These results were reviewed at the Technical Committee of the Company and agreed to implement the alternative well design on first category wells on the Srednebotuobinskoe field. In addition, for the rest of well categories the Drilling Team has produced and successfully implemented the preventative measures that allowed drilling through traps with lost circulation material. It is worth to mention that this method assumed a by-passing mud-cleaning system on the rig to allow building up a solid phase thus stemming the losses while drilling. For all the categories of the wells the Drilling team has selected and trialed different types and designs of drilling bits that would allow drilling hard rock such as dolerite section in traps with minimal number of runs. For the last five years the Drilling team together with the bit producing companies have designed a new type of PDC cutters that would allow to enhance durability as well as improve drilling speed both in dolerite formations and in overlying formations in the Production casing section. The Drilling team has also managed to reduce the number of trips associated with early bit wear decreasing them from five to two runs and create an experimental basis for drilling the entire section of the production casing in one run. From 2019 team continue looking for further solutions that would allow to improve bottomhole assembly elements balancing between durability and drilling rate in different types of formations above and below traps. A range of proposed technological solutions significantly reduced the impact on the well construction performance by preventing and reducing the downhole losses events in the section of trap intrusion on the Srednebotuobinskoe field.

Study on the loss control performance of smart adhesive lost circulation materials with Ethylene Vinyl Acetate Copolymer (EVA)

The conventional lost circulation materials (LCMs) particles of seal in the formation are in linear contact. Therefore, there is no cohesive force interaction, which is susceptible to pressure fluctuations, resulting in lost circulation and control failure. In response to these problems, this paper tested the high-temperature adhesion properties of EVA in the viscous flow transition zone through high-temperature adhesion experiments; then a smart adhesive LCMs is prepared using thermoplastic EVA resin with conventional LCMs; finally, through the high-temperature and high-pressure plugging experiment, the system's performance for sealing fractures was tested. The experiment results show that the smart adhesive LCMs have good plugging performance and can bond LCMs in the fracture to form a high-strength sealing with a pressure-bearing capacity of up to 9.3MPa.

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

Loss of drilling fluids in large porous and fractured zones inevitably up-regulates the overall cost of drilling. As a type of acid-soluble cement, magnesium oxysulfate (MOS) cement is arousing huge attention for the less hygroscopic nature and less damaging to steel casings compared with magnesium oxychloride (MOC) cement. The present study developed MOS cement as a fast setting, high strength and acid-soluble lost circulation material to reduce the problem of losses. As suggested in this study, a higher strength of MOS cement at 70 °C could be achieved by elevating MgO/MgSO4·7H2O molar ratio or down-regulating H2O/MgSO4·7H2O molar ratio. Boric acid and borax could act as effective retarders. Plugging slurry based on MOS cement could effectively block the simulated porous loss zones exhibiting a diameter from 1.24 mm to 1.55 mm, as well as the fractured loss zones with a width from 2 mm to 5 mm and bearing a pressure difference up to 8 MPa. Permeability recovery test demonstrated that it facilitated future oil and gas production. The successful field application in the Junggar Basin, Xinjiang, China verified the significant plugging effect of MOS cement for severe loss problems.

An Integrated Method for Evaluating Lost Circulation Risk for Shale Gas Drilling in Sichuan Basin

Accurate prediction of lost circulation fracture width and lost circulation pressure is the basis for preventing lost circulation and curing the losses. They are controlled by many factors, such as in-situ stress, wellbore fluctuating pressure, rock properties, natural fractures, and so on. Researchers have put forward a variety of calculation models of lost circulation pressure for fractured formations; however, in the field practice, it is still mainly based on the minimum horizontal principal stress. The lost circulation of fractured formation seriously jeopardizes the safety and progress of drilling and production. At present, the success rate of curing the losses is low, one of the important reasons is that the fracture width cannot be accurately predicted. It is difficult to determine the wellbore strengthening method and particle size of the lost circulation materials due to unknown fracture width. Various prediction models of fracture width are used to calculate the fracture width of the loss layer, and the influence of the factors such as the loss time, drilling fluid properties (viscosity and yield value), and the angle between the fracture and the wellbore on the calculated fracture width is studied. For the lost circulation pressure, the acceptable loss rate in the drilling process is given. On the premise of the calculated fracture width, the lost circulation pressure under the acceptable loss rate is determined. The results show that the values of fracture width and lost circulation pressure predicted by the three methods are close to each other, and the prediction results of the three methods can be averaged in the field application to provide reference for the following work. The fracture width is sensitive to drilling fluid viscosity, bottom hole pressure, and other parameters, so the value of these parameters should be accurately measured in the field to reduce the uncertainty. The results also show that the larger the angle between fracture and wellbore, the more serious the lost circulation will be.

Experimental Investigation of Particle Size Degradation and Plugging Efficiency of Three Granular Lost Circulation Materials

This work delineates a comprehensive study of one of the main problems that contributes towards nonproductive time (NPT) in a drilling operation, which is lost circulation. The focus of this study was to investigate the performance of walnut, graphite, and marble, which are three widely used and industry-available granular lost circulation materials (LCMs). Additionally, the study aimed to establish a particle size selection guideline for better operational performance and plugging efficiency. Four water-based carrier fluid systems (water–bentonite mix, water–polymer mix, and two polymer–salt systems) were tested with the LCMs in this study. Dry and wet particle size degradation studies were conducted on all the LCMs with the different carrier fluid systems to study their compatibility and efficiency. The effect of the carrier fluid type was proven to be significant only on marble particles size degradation; walnut and graphite were not affected by the carrier fluids and showed consistent size degradation performance with all fluids. The results of this work led to newly developed particle size selection guidelines to enhance plugging efficiency—guidelines that are custom-made for each material by taking into consideration the rate of the degradation and type of material and by correlating the findings with fracture width. Applying this method of investigation to the current lost circulation management practice can help resolve many lost circulation incidents by effectively and efficiently selecting the appropriate LCM.

Rheological investigation of effect of high temperature on geothermal drilling fluids additives and lost circulation materials

High temperatures (HT) and the corrosive environment make the design of a geothermal drilling fluid a highly complex task. The presence of HT exacerbates the problems due to thermal degradation. Geothermal aquifers usually combine high-temperature and complex chemistry, making them highly corrosive. Although geothermal drilling fluids are exposed to thermal degradation, research of the effect on mud rheological properties in temperatures near or above 149°C (300°F) is limited.In this study, a High Pressure-High Temperature (HPHT) rheology setup was used to measure drilling fluids' properties up to 204.4°C (400°F). Rheological behavior of viscosity, density, lost circulation, and alkalinity control additives were analyzed at HT. It was found experimentally that Bentonite alone is thermally stable at HT. In contrast, Caustic Soda showed a tendency to gelate at temperatures above 93.3°C (200°F), adversely affecting mud rheology. Alkalinity control materials to replace Caustic Soda were evaluated. Among them, Lime presented the best thermal stability at 204.4°C with a range of variation of 0.0024 Pa.s (2.4 cP) on apparent viscosity after three consecutive tests. Besides, it was tested 11 different lost circulation materials (LCM) for understanding their rheological behavior at HT. It was found that HT has less impact on fine granular materials rheological behavior with an average increase of 17.7% compared with the baseline. In contrast, flaky, fibrous, and coarse granular materials presented a more apparent impact in WBM rheology with a deviation of 166% compared with the baseline.

Shape Memory Polyurethane as a Drilling Fluid Lost Circulation Material

AbstractDrilling fluid loss is a major problem with serious economic and environmental consequences. The use of traditional lost circulation materials (LCMs) to seal wide fractures increases the risk of bit nozzle plugging. In this work, smart LCMs based on shape memory polyurethane (SMPU) are proposed for the first time. SMPU can be programmed to recover at temperatures suited to a given well. As such, SMPU smoothly passes through the bit nozzles, while effectively seal wide fractures once activated. The SMPU is prepared by two step pre‐polymerization and characterized by Fourier transform infrared spectra, X‐ray diffraction, and differential scanning calorimeter. The SMPU is programmed by changing and fixing the original shape to a temporary shape through a thermo‐mechanical process. The shape memory behavior of SMPU is analyzed by tensile apparatus. Compatibility of SMPU with WBMs is determined from mud rheology and filtration tests. Fracture sealing efficiency and shape recovery of SMPU are evaluated by a modified particle permeability apparatus fitted with a model fracture. The results confirm high sealing and shape recovery attributes of SMPU. The plug formed at 114 kg m–3 SMPU and 80 °C experiences a sealing pressure of 100 bar with 71.5 cm3 cumulative fluid loss.

Shape Memory Polymers as Lost Circulation Materials for Sealing Wide-Opened Natural Fractures

Summary While there have been various lost circulation materials (LCMs) available in the market for treating fractures during the drilling of oil and gas wells, there is still a demand for a technology to seal large fractures. Considering limitations on the size of the particles that can be circulated through the drilling equipment, especially the bottomhole assembly, simply enlarging conventional LCM particles becomes ineffective for sealing large vugs and fractures. In this study, we use shape memory polymers (SMPs) to prepare programmed LCMs with various temporary shapes, which can transform to their permanent shapes with much larger dimensions as compared to their temporary shapes. A series of steps for thermomechanical programming of SMP is designed to trigger their expansion at the reservoir temperature. The dimensions of the programmed shapes can be an order of magnitude smaller than the ones for the original shapes, making their transport through the flowlines feasible, and bridging wide-opened fractures possible. The basic idea is that, after recovery, the SMP-based LCMs form an entangled network across a large width of fracture, and SMP particles recovered within the network, filling in the pores to form an effective sealing. We seek the capability of entangled ladders and interwoven fibers in forming a network across the fracture. A permeability plugging apparatus (PPA) is used to examine the efficiency of developed LCMs. The technique of 3D X-ray computed tomography (CT) is used to visualize the internal structure of formed plugs, enabling us to understand the mechanisms of bridging, plugging, and sealing.

Application of wellbore strengthening drilling fluid technology in Lingshui gas field

Fractured reservoir is developed in Lingshui gas field and the problem of lost circulation is prominent. To improve the pressure bearing capacity of fractured formation, the wellbore strengthening technology was applied in this block. The fracture width of Lingshui gas field was predicted with finite element method, which considered the effect of wellbore pressure, in-situ stress, seepage fluid and pore pressure. The simulation results showed that the fracture width of Lingshui block ranged from 0 to 464 μm. Based on the prediction results of fracture width, the particle size distribution of lost circulation material (LCM) was selected with dynamic fracture width testing device. The results indicated that the D50 rule has the best sealing efficiency. The optimized LCM formula had better compatibility performance with water-based drilling fluid. The sealing results illustrated that invasion depth of seepage loss was smaller than 0.3 cm and the pressure bearing capacity of fracture reached to 17 MPa.

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 novel temperature-sensitive expandable lost circulation material based on shape memory epoxy foams to prevent losses in geothermal drilling

Lost circulation will inevitably increase non-productive time (NPT) and well cost. The application of high-efficiency lost circulation materials (LCMs) plays an indispensable role in preventing and mitigating loss. The development of new LCMs based on stimuli-responsive materials is an important research direction and it is expected to realize the revolutionary development of LCMs. In recent years, the temperature-responsive plugging agent based on shape memory polymers has caused a lot of attention. In this paper, based on the shape memory epoxy polymer (SMEP) and hollow glass beads (HGBs), a shape memory epoxy foam (SMEF) was successfully synthesized. The results show that the shape memory temperature (51.6 °C - 97.5 °C) of the SMEPs has a good linear relationship with the content of curing agent 4, 4′-diaminodiphenyl methane (DDM) (12% - 20%), and the expansion ratio (0% - 120.2%) of the SMEFs has a good linear relationship with the content of HGBs (0% - 50%). The mechanism of the SMEFs was proposed. A novel temperature-sensitive and expandable plugging agent (SMP-LCM) has been developed. The plugging experiment shows that SMP-LCM has a good plugging effect on high permeability loss zone simulated by sand beds. Compared with conventional LCMs (e.g., nutshells or fibers), SMP-LCMs can eliminate the risk of plugging of downhole drilling tools and enhance the plugging effect.

Comprehensive approach to torque and lost circulation problems in geothermal wells in terms of drilling fluid

Drilling in a lost circulation zone has long been a challenge in geothermal wells due to its strong potential for high torque and wellbore instability. Particularly in deviated, extended reach and horizontal wells, frictional forces between drill string and wellbore or casing may be high enough to interfere with running the pipe in and out of the hole. These forces may also increase the torque and the stress on drill string which may end up with severe problems like stuck pipe and lost in hole. High-temperature high-pressure (HTHP) drilling fluids design, which minimize loss rates and friction values, is critical for the success in the face of these challenging drilling practices.The goal of this research is to provide a solution-oriented approach to torque and lost circulation problems in potential seepage and partial loss zones in geothermal wells; by developing efficient water-based drilling fluid compositions.Non-aqueous fluids (NAFs) are known to have the highest lubricity performance. However, application of these drilling fluid systems is limited because of high cost and environmental constraints. Hence, water-based drilling fluids are good candidates for replacing conventional NAFs. This paper investigates water-based drilling fluid compositions with high lubricity performance, HTHP resistance and high loss zone plugging performance.The design and selection of HTHP drilling fluid system have been the focus of experimental studies and worldwide field experience. Several chemical commercial lubricants are added in this fluid system to find the compositions for the highest lubricity performance closest to NAF systems. Also, considering the formation characteristics, environmental constraints, reservoir contamination concern and drilling limitations; proper fluid compositions are formulated to plug the potential seepage and partial loss zones and reduce the differential-sticking tendency by using proper lost circulation materials and lubricants.This study aims to encourage the geothermal drilling industry worldwide, to utilize the developed drilling fluid compositions with enhanced lubricity and loss zone plugging performance together to overcome related major drilling problems.Results reveal that lubricity and pore plugging characteristics of the selected compositions are highly innovative and noteworthy to be used in field applications for geothermal drilling industry.

Effect of LCM and polymeric additives on mudcake and filtrate rheology parameters

In oil and gas well drilling operations, lost circulation is characterized by the invasion of drilling fluid into permeable formations. Besides generating additional costs to the process, the loss of circulation can damage producer formations and, if uncorrected, reduce pressure inside the well, inducing kicks. One of the alternatives for combating this problem is the addition of lost circulation materials (LCM) to the drilling fluid. The added LCMs will accumulate and form a filter cake on the interface between the well and the porous formation, thus reducing the volume of fluid lost. In the present work, the cake formation process resulting from the deposition of particulate material on a porous substrate is studied experimentally. The objective is evaluating the influence of type and concentration of two polymeric additives and four types of LCM on the formation of the filter cake. The experiments were carried out in an API filter press at pressures of 100 psi, using solutions of carboxymethylcellulose (CMC) and xanthan gum (XG) in water at 1.05 and 2.10 lbm/bbl. The particle size used as loss of circulation material comprises two values (2 – 44 and 44 – 106 μm) of calcium carbonate (CaCO3), glass microspheres (180 – 300 μm), and plastic particles (250 – 420 μm). The efficiency of the fluid-LCM combination in combating lost circulation is evaluated based on the filtrate volume, thickness, dry and wet cake masses, and filtrate flow curves. Results show that the increase of the LCM concentration and the use of the calcium carbonate particulate with smaller particles favor the filtrate volume reduction. Also, the filtrates exhibit non-Newtonian behavior with reduced values of viscosity due to the retention of polymeric chains by the formed filter cake. Such phenomenon is most significantly detected when calcium carbonate is used.

Research on the rheological and flow characteristics of a supramolecular gel in fractures

AbstractLoss circulation is one of the major concerns in drilling and well construction. An effective way to control drilling fluid loss and strengthen the wellbore is to plug fractures and holes with loss circulation materials. In this work, a hydrophobic association supramolecular hydrogel GP‐A developed by n‐dodecylacrylamide, methacrylamide and 2‐acrylamide‐2‐methylpropylsulfonic acid was proposed as a potential lost circulation material for malignant drilling fluid loss. The results show that the initial decomposition temperature of GP‐A was 172°C, and the flow characteristics conform to the Herschel‐Bulkley model with yield stress. Microstructural analysis shows that GP‐A gel with a 3D spatial network, and hydrogen bonds between branched chains form a dynamically recoverable structure. Based on POLYFLOW, the Phan Thien–Tanner model with viscoelastic parameters was used to simulate the viscoelastic flow characteristics of the fluid in fractures. The higher the concentration is, the greaterK, and the higher the inlet driving pressure, while the nonlinear relationship between the driving pressure and the gel slug length is evident. The introduction of supramolecular gel polymer as a loss circulation material is an innovative research topic, which provides a new method to simulate the flow of polymer fluid in fractures.

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.

Magnesium oxysulfate cement as a fast curing agent in drilling fluids to solve the severe loss

Severe loss of drilling fluids that occurs in Xinjiang, China, increases the Non-Production Time (NPT) and the overall cost of drilling. Magnesium oxysulfate (MOS) cement as lost circulation material has the advantages of short curing time, acid-soluble property and low cost, showing great potentials to solve severe loss. In this paper, the loss treatment of adding MOS cement directly into the drilling fluids was purposed and practiced. The effects of the amount of MgO, MgSO4·7H2O and borax on the compressive strength and curing time of the drilling fluids at 50 °C and 70 °C were studied. The plugging performance of the curable plugging slurry in minimizing fractured loss was evaluated. The results indicate that with the increase of MgO and MgSO4·7H2O, the compressive strength increases while the curing time would be shortened. The addition of borax retarder could extend the curing time while it would also decrease the compressive strength. The curable plugging slurry could effectively reduce the loss rate of fractured loss bearing a pressure difference above 8.0 MPa. The successful field applications in Xinjiang, China were introduced. This paper demonstrates that using MOS cement could provide a convenient, fast and efficient way to solve severe loss challenges.