Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure of Granular Plugging Zone in Deep Fractured Reservoirs

Abstract

Fracture plugging zone with low strength is one of the key reasons for plugging failure in deep fractured reservoirs. Forming a high-strength plugging zone is a key engineering problem to be solved in wellbore strengthening. In this chapter, wellbore strengthening mechanisms of plugging zone for wellbore strengthening in deep fractured reservoirs are revealed from a relationship between mechanical structure and strength standpoint. Physical granular bridging materials dislocation and crushing under pressure fluctuation induce the strong force chains network failure, which leads to macroscale friction or shear failure of plugging zone. The main methods to improve microscale materials stability are to increase friction resistance, exert embedding effect, and strengthen bonding effect. Factors, which strengthen the meso-structure stability, include increasing shear strength and proportion of strong force chains. Key measures to strengthen the macrostructure stability of plugging zone are by improving its compactness, controlling its length, and ensuring the stability timeliness.

1. Introduction

Working fluid loss is a phenomenon that occurs frequently during the process of drilling and completion in fractured formations due to the low-pressure -bearing capacity of the formation [1, 2, 3]. Working fluid loss causes the most serious reservoir damage during drilling and completion in petroleum and geothermal resource. It is also one of the most complex engineering problems to solve in that it affects safe and efficient drilling and oil and gas well productivity for a long time [4]. The maximum liquid column pressure that the wellbore can bear without fluid loss is called formation pressure-bearing capacity [5, 6]. Technologies to improve the formation pressure-bearing capacity (also called wellbore strengthening technologies) are often used to control the loss of the working fluid. The pressure-bearing capacity of the formation can be improved by adding wellbore strengthening materials into the working fluid, which then expands the safety density window of the formation. The DEA-13 experiment carried out by the association of drilling engineers between the mid-1980s to the early 1990s confirmed that adding solid particles into the drilling fluid can greatly improve the pressure-bearing capacity of the formation [7]. Since then, a lot of research has been done on the loss control theory and on characterizing formations that experience high losses and have low - pressure-bearing capacities. The temporary plugging theory and its related method have solved the loss problem of pore type and fracture pore type reservoirs [8, 9].

In fractured reservoirs, the development of natural fractures greatly complicates efforts to improve the pressure-bearing capacity of the formation. The root cause of working fluid loss in fractured reservoirs is the imbalance between the wellbore pressure and the formation pressure/stress. At present, methods to improve pressure-bearing capacity of fractured reservoirs mainly include increasing fracture pressure, increasing fracture extension pressure, and plugging loss channel [10, 11]. Ways to improve the pressure-bearing capacity of the reservoir are closely related to the degree of fracture development within the reservoir. The reservoirs can be divided into the undeveloped fractured reservoir, non-leaky natural fracture reservoir, and leaky natural fracture reservoir based on the degree of fracture development. The corresponding working fluid loss types for these three reservoirs being induced fracture loss, fracture extension loss, and large and medium fracture loss [12]. For the induced fracture type loss reservoir, the mechanism that leads to fluid loss is the imbalance between the wellbore pressure field and the in situ stress field. The key to improving the pressure-bearing capacity is to form a plugging zone near the fracture opening, support the fracture opening, and improve the fracture pressure of the reservoir. This is called the stress cage and fracture closure stress method [13]. The root cause of fluid loss in the fracture extended loss reservoir is the imbalance between the wellbore pressure field and the stress field at the fracture tip. For this type of reservoir, a plugging zone is formed within the fracture, and the key to controlling its fluid loss is to isolate the fracture tip and improve the fracture extension pressure (called increasing the fracture extension pressure method) [14, 15, 16]. In large and medium fracture type reservoirs, the imbalance between the wellbore pressure field and the formation pressure field is the main fluid loss mechanism. To improve the pressure-bearing capacity of this type of reservoir, it is necessary to add lost circulation materials (LCMs) to the working fluid to plug the fractures. The plugging zone that is formed within the fracture can block the transmission between wellbore pressure and formation pressure (called the plugging the loss channel method) [17, 18, 19]. Therefore, the essence of improving the pressure-bearing capacity of the reservoir is to form an artificial high-pressure-bearing plugging zone around the well. For fractured reservoirs, the key to improving the pressure-bearing capacity of the plugging zone is by improving the pressure-bearing capacity of the reservoir.

The relationship between structure and performance is continuously researched [20, 21, 22]. The structural stability of the plugging zone is the key factor that controls fracture reservoir fluid loss. The structural analysis of the plugging zone has become important for the theory and technological development of lost circulation prevention and mitigation. Kang et al. introduced the particle material mechanics theory to study the plugging effect of fractures, pointing out that the plugging zone is a granular material system composed of LCMs. The structure of the plugging zone has multi-scale characteristics. The strength of the force chain in the mesoscale is closely related to the stability of the macroscale plugging zone. Xu et al. constructed the failure mode of the plugging zone under a high temperature, high pressure, and high-stress environment, established the strength model of the plugging zone and extracted the key performance parameters of LCMs [23]. Based on the theory of system science and mutation, She et al. proposed a mathematical model to describe the failure process of the fracture plugging system [24]. Considering the stress analysis of the microstructure of the plugging zone, Qiu et al. discussed the failure modes of the plugging zone such as crushing failure and seepage loss failure. Yan et al. defined the dominant force types among LCMs, extracted multi-scale structural parameters of plugging zone, and introduced a photoelastic experiment to characterize the force chain network [25].

Granular matter is called the fourth form of matter. Research on its multi-physical mechanisms and multi-scale structural stability has become a hot spot at the frontier of particle material research [26, 27, 28]. Although some progress has been made in the research on the structural characterization of the plugging zone, the multi-scale structural failure mechanism of the plugging zone is not yet clear. In this chapter, recent advances in wellbore strengthening mechanisms are presented from a relationship between mechanical structure and strength standpoint.

2. Multi-scale failure mechanisms of plugging zone

2.1 Macroscale structural failure of plugging zone

On the macro scale, the structural failure of plugging zone can be divided into friction failure and shear failure [23]. The reason why the plugging zone can exist within the fracture is that the friction force is formed by the contact between plugging zone and the fracture surface. After plugging zone has formed, it is affected by the combined action of the wellbore pressure and the fracture pressure along the direction of the fracture length (Figure 1a and b). When the friction between plugging zone and fracture surface is less than the resultant force of wellbore pressure and pressure in the fracture, plugging zone will slide to the fracture interior. The structure will gradually disintegrate, resulting in friction failure (Figure 1c). The plugging zone shear failure refers to the failure in which plugging zone is first destroyed at a weak point under the action of the pressure difference between wellbore pressure and fracture pressure (Figure 1d). The nonuniformity of the development of micro convex bodies on the fracture surface means that the friction between the plugging zone and the fracture surface is not equal everywhere [23]. The parts with low friction between the plugging zone and the fracture surface are more likely to be damaged. Most importantly, because the formation of plugging zone is a random process, the structure of the plugging zone has a certain heterogeneity, and therefore contains structural weaknesses in the plugging zone. When pressure is applied to the plugging zone, structural failure will first occur at these weak points and then induce shear failure.

Figure 1.

Schematic diagram of the multi-scale structural failure of plugging zone.

2.2 Mesoscale structural failure of plugging zone

Compared with an ordinary solid, the plugging zone is a discrete granular material system, which shows strong discontinuities and contact energy dissipation. This causes even more complex macro-mechanical behavior [29, 30]. Under an external load, the plugging zone will produce a certain mechanical response. The particles in the plugging zone will be squeezed and come into contact with each other, forming a quasi-linear chain network structure, called a force chain network (Figure 1e). The force chain network is the transmission path of the indirect contact force of LCM along with the particle contact network. It provides the physical pathway for the transmission of the external load and directly reflects the internal stress of the discrete particle material system. The force chain network can be divided into strong and weak areas. The number of force chains is small, but it carries the main external load of the system. The distribution of the weak force chain and the surrounding strong force chains help to maintain the stability of the system. The photoelastic experiment method can effectively characterize the evolution process of the meso force chain network of the plugging zone during a pressure-bearing process. As shown in Figure 1f, after the vertical load and shear load are applied to the plugging zone simultaneously, several bright force chains appear along the shear direction begin to appear and they connect to the force chains in the vertical direction to form a strong force chain network. When the shear load continues to increase to the bearing limit of the plugging zone, the strong force chain network inside the plugging zone breaks (Figure 1g). The results of the photoelastic experiment show that the pressure failure of the plugging zone results in a fracture of the strong force chain network at mesoscale.

2.3 Microscale structural failure of plugging zone

After the LCMs are retained in the fracture, it forms a plugging zone under the combined action of the wellbore pressure difference, the in situ stress, and the contact force of adjacent particles. The LCMs are squeezed and come into contact with each other to maintain a relatively stable state (Figure 1i). The bridging materials in the plugging zone bear the main external load. When the pressure fluctuates, some bridging materials in the plugging zone may lose their mechanical balance, and the particles may move together with each other. The dislocation modes of the micro LCMs include particle friction sliding (Figure 1j) and particle rotation (Figure 1k). In practice, the dislocations of LCMs include both frictional sliding and turning, such as particle climbing (Figure 1l). After plugging zone is formed, a strong force chain network is caused by the sliding or rotation friction of micro bridging particles, which leads to the friction and shear failure of the macro plugging zone.

3. Microstructure strengthening of plugging zone

3.1 Improving friction resistance of LCMs

Friction sliding of LCMs is one of the main reasons for the dislocation of LCMs on a microscale. An important means to strengthen the microstructure stability of LCMs is by improving friction resistance between LCMs. The geometry and elasticity of LCMs are the main controllable factors. The results of the friction coefficient test show that the lower the roundness of the LCM, the higher the friction coefficient. The friction coefficient between the LCMs can be improved by adding elastic particles to a single rigid particle (Figure 2). By optimizing the irregular rigid bridging materials, the meshing degree between the LCMs can be improved, and the friction coefficient between the LCMs can be improved. At the same time, the introduction of highly elastic and highly expansive filling materials can increase the contact area between the LCMs, and thereby increase the effective stress on the LCMs, to improve the friction resistance of the LCMs.

Figure 2.

Friction coefficient curve of single high sphericity rigid particles and a combination of rigid, elastic, and fiber LCMs (Modified from [27]).

3.2 Give full play to the embedding effect of the high-strength rigid particles

For deep fractured reservoirs, particle breakage can induce microstructure failure of the LCMs. Under conditions of high deep fracture closure stress, LCMs can easily be compressed and broken. As reported in [25], millimeter-grade calcium carbonate particles show significant particle size degradation after 30 minutes under a pressure of 25 MPa of fracture closure stress, while the D90 degradation rate of its particle size distribution reaches 17.7%. After the LCMs are broken, the space around the neighboring particles increases, which makes it easier for the particles to move. High fracture closure stresses can induce compression and breakage the LCMs. However, these stresses can also act as a favorable factor in cases, where they provide a power source for the proper embedding of LCMs. This is achieved by optimizing the strength and toughness of rigid bridging particles, which enhances the stability of the LCMs (Figure 3).

Figure 3.

Crushing rate under 30 MPa effective stress of rigid granular materials commonly used in drilling sites (Modified from [26]).

3.3 Introduction of bonding to strengthen the stability of LCMs

Two types of forces exist between the LCM particles: contact forces, and noncontact forces. Noncontact forces include electrostatic force, van der Waals force, and liquid bridge force. The electrostatic force between particles can be ignored because the particles are usually inert and electrically neutral. The particle size is generally between hundreds of microns and several millimeters, and therefore the van der Waals forces between the particles are often small. The liquid bridge force is mainly derived from the surface tension at the solid–liquid–gas interface. For the drilling fluid system or plugging slurry system, generally, there are only two phases: liquid phase and solid phase. While the solid–liquid–gas interface is negligible, so the inter-particle liquid bridge force effect in plugging zone can also be considered nonexistent. The contact force between particles is exerted by the fracture closure stress. The fracture closure stress that acts on the LCMs can often reach tens of MPa. The contact force between these particles, which plays a major role at this depth, is far greater than the noncontact forces between the particles [25]. To further enhance the structural stability of the LCMs, a soluble bonding material can be added into the granular plugging formula. This not only address the need for plugging removal before putting into operation but also play a bonding role that can further solidify the LCMs and improve the stability of the LCMs. The fracture plugging experiment shows that the plugging zone pressure-bearing capacity formed by a single type of spherical, sheet, and fiber LCMs is relatively low. The pressure-bearing capacity of a single fiber material is only 1.4 MPa, while the pressure-bearing capacity of the plugging zone formed by fiber combined with polypropylene binder can be increased to 6.9 MPa [31].

4. Meso-structure strengthening of plugging zone

4.1 Increase the shear strength of the strong force chain

The mesoscopic failure of the plugging zone can be attributed to failure of the strong force chains network in the plugging zone. The strong force chains network is formed by the connection of multiple strong force chains, but the shear strength of a single chain directly controls the pressure stability of the strong force chains network. According to the mechanics of the granular matter, the shear strength of a single force chain is equal to the product of the friction coefficient between the particles and the contact stress between the particles. The contact stress between the particles depends on the stiffness, deformation, and contact area of the particles. In a weak force chain, the contact degree between particles is low and the deformation of the particles is small, which makes it difficult to bear a higher shear load. Conversely, in strong force chains, the degree of contact between the particles is high, the external load borne by particles is large, and the deformation of particles is relatively large. When the action line of the particle contact force is within the range of particle friction angle, the particles in the strong force chains are in a relatively stable state. The higher the friction coefficient of the particle surface, the greater the stiffness and the greater the degree of extrusion deformation, and the higher shear strength of strong force chains. By optimizing the performance parameters of the LCMs, the strong force chain LCMs with a high shear strength can be selected to enhance the stability of the meso-structure of the plugging zone (Figure 4).

Figure 4.

The shear strength of the strong force chain of plugging zone formed several typical LCMs.

4.2 Increase the number of strong force chains

The bearing stability of the strong force chain network of the plugging zone is determined by both the shear strength of a single strong force chain and the number of strong force chains. The photoelastic experiment results show that strong force chains proportion can be increased by selecting rigid LCMs with low sphericity and by adding more elastic LCMs. The strong force chains exist in a dense granular material system. By optimizing the concentration and proportion of the bridging and filling particles, and by improving the coordination of the LCMs, the probability of forming strong force chains can be improved. The type, concentration, and ratio of LCMs can be optimized to increase the proportion of the strong force chains, which can increase the pressure-bearing stability of the meso-structure in the plugging zone. As shown in Figure 5, compared with the single round granule plugging zone, the development of the force chain network in the plugging zone is improved after adding triangular and square particles. The addition of angular particles to round particles makes the force chain network more diversified, increases the proportion of strong force chain, and improves the stability of the plugging zone.

Figure 5.

Photoelastic images of the plugging zone under an external load. (a) Single round particles, (b) combination of round and triangular particles, and (c) combination of round and square particles.

5. Macro-structure strengthening of plugging zone

5.1 Ensure the timeliness of plugging zone stability

As the exploration and development of oil and gas resources reach ever greater depths, repeated fluid loss occurs frequently [32]. For example, the Tarim Basin has a reservoir depth of 7700 m, a reservoir temperature of 168°C, and an average total salinity of 210,000 mg/L for the formation water. The minimum horizontal principal stress is 167 MPa, while the pore pressure is 120 MPa. In this example, the force exerted on the plugging zone was 47 MPa. During the drilling of the reservoir section in the Tarim basin, multiple fluid losses occurred, with the loss amount ranging from 24.5 to 573.0 m³, with an average loss amount as high as 164.7 m³. The long-term stability of plugging zone under a high-temperature, high-stress, high-salinity environment has become one of the key technical challenges in ensuring deep fractured reservoir loss control. Based on conventional particle size distributions, acid solubility, and other evaluation indexes of LCMs, it is necessary to consider the temperature resistance, salt resistance, and compression resistance of LCMs. Optimizing the temperature-resistant, salt-resistant, and high-stress resistant LCMs is important to ensure stable and timely plugging of the fracture zone.

5.2 Improve the compactness of plugging zone

Compactness is one of the key performance parameters to strengthen the macrostructure of the plugging zone. When the density of the plugging zone is poor, the particle system is loose and its structural stability poor. The density of the plugging zone is closely related to the concentration, gradation, and type of LCM. For a certain fracture width, particle interaction and the formation of a tight plugging zone are enhanced with an increase in LCM concentration. Compared with the single-particle size LCM, the LCM with a specific particle size distribution improves the plugging zone density. For the millimeter-sized loss fractures, which are difficult to control, the optimal combination of LCMs is a rigid granular material, elastic granular material, and fiber material. Rigid particles play a major role in bridging and form the main framework of the plugging zone. Elastic particles fill in the gaps between the rigid particles, which not only reduces the porosity of the plugging zone but also improves the toughness of the plugging zone. Fiber material with a high aspect ratio and good flexibility can further improve the fracture plugging compactness. By optimizing the performance matching of LCMs, the density of the plugging zone can be improved.

5.3 Control the plugging length of the plugging zone

The structural pressure-bearing stability of the plugging zone is not only controlled by LCMs performance parameters and the plugging slurry formula but is also closely related to the plugging technology. Bridging plugging is the most commonly used loss control technology in fractured reservoirs. The field practice of bridging plugging shows that controlling the squeezing pressure can gradually increase the length of the fracture plugging zone. The optional squeezing strategy is intermittent squeezing in small amounts to squeeze as much of the plugging slurry into the formation as possible. With the increase of the length of the fracture plugging zone, the stability of the fracture plugging zone is enhanced. By using long core fracture plugging simulation experiments, the bridging plugging technology can be optimized in terms of squeezing pressure, single squeezing volume, and squeezing times. And the favorable conditions for the formation of fracture plugging zone with high-pressure stability can be created.

6. Conclusion

The following conclusions are obtained:

• The dislocation and mechanical crushing of bridging materials under pressure fluctuation induce the failure of strong force chains network, which leads to macroscale friction or shear failure of plugging zone.

• The main methods to enhance the stability of LCMs include increasing the friction resistance, exerting the appropriate embedding effect of special-shaped particles, and strengthening the bonding effect between LCMs.

• The main methods to improve the meso-structure stability of the plugging zone are by increasing the shear strength of the strong force chains and increasing the number of the strong force chains.

• The main measures to strengthen the macrostructure stability of the plugging zone are by ensuring its stable and timely plugging, improving the compactness, and controlling the length of the plugging zone.

Acknowledgments

The authors gratefully acknowledge the Fourth Batch of Leading Innovative Talent’s Introduction and Cultivation Projects of Changzhou (No. CQ20220087), the Scientific Research Foundation for Innovative Talents Introduction of Changzhou University (No. ZMF23020041). The authors would also like to acknowledge the useful suggestions from Prof. Xiangyu Shang, Dr. Jingyi Zhang and Dr. Chong Lin.

Conflict of interest

The authors declare no conflict of interest.