Abstract

Lost circulation is a common and relatively difficult to control underground complex accident in the current drilling process, which has become one of the main factors affecting drilling progress and may even cause safety accidents of varying degrees.This article comprehensively analyzes the types and mechanisms of leakage causes, the performance of new leak stopping materials in different environments and conditions at home and abroad, and their interactions with factors such as wellbore and formation. It elaborates on the characteristics and leak stopping mechanisms of cement leak stopping materials, cross-linked system leak stopping materials, metal leak stopping materials, particle LCMs leak stopping materials, fiber leak stopping materials, and curable and LCMs mixed leak stopping materials. It summarizes the application effects and advantages and disadvantages of different types of leak stopping materials in different formations.The results showed that, taking into account the success rate of plugging and economic practicality factors, the selection of cement plugging materials in the treatment of high permeability porous formations resulted in a plugging success rate of 91%; Choosing a mixture of solidifiable and LCMs for leak sealing materials in dealing with karst cave formations resulted in a success rate of 89% in leak sealing; Choosing fiber based plugging materials in dealing with natural fracture type formation leakage has a success rate of 75%; The selection of particle LCMs plugging materials in the treatment of induced fracture type formation leakage has achieved a success rate of 92%.The research results have important theoretical and technical reference significance for improving the plugging effect of drilling fluid and promoting the development of plugging technology.

Leakage is one of the important engineering issues that constrain safe and efficient drilling. The leakage of expensive drilling fluid into the formation and the significant amount of non production time spent on re establishing circulation can significantly increase the cost of drilling.Leakage is one of the important engineering issues that constrain safe and efficient drilling. The leakage of expensive drilling fluid into the formation and the significant amount of non production time spent on re establishing circulation can significantly increase the cost of drilling.Scholars at home and abroad have developed a series of plugging materials to address different types of drilling fluid leakage problems. According to the plugging mechanism, it can be generally divided into three categories: curable plugging materials (such as cement, polymer gel, etc.), conventional plugging materials (LCMs for short) (such as particles, mineral fibers, etc.), and curable and LCMs mixture plugging materials (such as gel and LCMs complex, etc.).Although the development of the above-mentioned plugging materials has provided effective measures to deal with drilling fluid leakage, there is still a certain degree of blindness and unpredictability in dealing with complex formation leakage. The current plugging methods lack scientific and targeted approaches, resulting in significant differences in plugging effects, unstable temperature resistance, insufficient pressure bearing and retention capabilities of the same plugging material in different formations. These problems have led to a low success rate of wellbore plugging and difficulty in promoting plugging technology. Therefore, well leakage accidents have not been effectively controlled and eliminated.

In response to the current difficulty of plugging leaks in complex formations, including low success rates of one-time plugging and difficulty in replicating plugging techniques, this article comprehensively analyzes domestic and foreign literature in the past decade, and elaborates on the characteristics and plugging mechanisms of curable plugging materials, conventional plugging materials (LCMs), and a mixture of curable and LCMs plugging materials. It deeply explores the application effects of different plugging materials in the face of different formation leaks, which has important theoretical and technical implications for improving the plugging effect of drilling fluids and promoting the further development of plugging technology.

1. Mechanism and Diagnosis of Drilling Fluid Leakage

Studying the mechanism of leakage can help to gain a deeper understanding of the causes of well leakage and provide guidance for selecting appropriate plugging agents. Different types of plugging agents have differences in composition, particle size, flowability, etc. Therefore, analyzing the mechanism of drilling fluid leakage can help to accurately select suitable plugging agents and improve the plugging effect.Nelson et al. classified drilling fluid leakage into the following types based on the degree of leakage: leakage (leakage rate less than 1.6m³/h), partial leakage (leakage rate between 1.6-16m³/h), severe leakage (leakage rate exceeding 16m³/h), and irreversible leakage (no drilling fluid returning to the wellhead).Classifying based solely on the degree of drilling fluid leakage is relatively simple, without considering the leakage mechanism or occurrence conditions. Different leakage mechanisms may play different roles in different formations. Therefore, according to the type of leakage channel, leakage can be classified into the following types: high permeability matrix leakage, karst cave leakage, natural fracture leakage, and drilling induced fracture leakage. The diagnostic methods are shown in Table 1.

1.1 High Permeability Matrix Leakage

High permeability matrix leakage refers to the presence of highly permeable matrix in the downhole formation during drilling operations, which allows drilling fluid to penetrate and leak into these matrices, ultimately leading to the loss of drilling fluid into the formation.This type of leakage usually occurs in highly permeable rocks or soils in the formation, where drilling fluid is relatively easy to enter the pores and cracks of the rocks or soils, leading to the occurrence of highly permeable matrix type leakage.The leakage characteristic is that the leakage is initially very slow, but as the subsequent drilling progresses, more highly permeable rocks come into contact with the drilling fluid, and the rate of drilling fluid leakage gradually increases.As time goes by, the drilling fluid forms a filter cake on the wellbore, causing the leakage rate to gradually decrease, but the leakage will not completely stop until drilling into a highly permeable formation, as shown in Figure 1.In fractured formations, drilling fluid leakage is also considered as high permeability matrix type leakage, such as when drilling into salt gypsum layers.

1.2 Cave Type Leakage

Cave type leakage refers to the leakage of drilling fluid into the formation during the drilling process when it passes through cave or karst tunnel structures in the formation.When the drill bit encounters a karst cave, leakage will immediately occur, especially when several caves are connected. The volume of lost drilling fluid may be very large, and even cause irreversible leakage (no drilling fluid returns to the wellhead), leading to wellbore jamming, collapse, blowout and other accidents. The characteristics of drilling fluid leakage are shown in Figure 2.

1.3 Natural Crack Type Leakage

The leakage of drilling fluid caused by natural fractures refers to the leakage of drilling fluid from the wellbore into the formation during the drilling process due to the presence of natural fractures or fissures in the formation.This type of leakage usually occurs in areas where natural cracks, fissures, or faults exist in underground rock formations, with approximately 76% of leakage caused by natural cracks.Whether natural fractures can cause drilling fluid leakage depends on three factors: wellbore fluid column pressure, fracture hydraulic width, and drilling fluid rheology.For natural fracture type leakage, the two main controlling factors are fracture hydraulic width W_h and bottomhole pressure (BHP), with bottomhole pressure being a key factor. Based on these two factors, further explanation of the natural crack type leakage situation is shown in Table 2.

Natural fractures need to be wide enough and have sufficient permeability to cause natural fracture type leakage, but not all natural fractures can cause drilling fluid leakage because most natural fractures are closed or filled with minerals, making them narrow or impermeable.If natural cracks are interconnected, the resulting leakage is severe leakage or irreversible leakage. Sanfilippo et al. analyzed the on-site data and concluded that the later stage of drilling fluid leakage events can be approximated as

Where

V_cum - the cumulative volume of drilling fluid leaked into the fracture from the beginning of leakage until time t, L；

V₀- instantaneous leakage, L/min；

t - duration of leakage, min；

C - empirical coefficient.

The leakage rate and time curve of drilling fluid show a sudden increase and gradual decrease in leakage rate, as shown in Figure 3.