Borehole Expansion Research Articles

Stability Analysis of Borehole Walls When Drilling with Normal-Temperature Drilling Fluids in Permafrost Strata

Permafrost is a temperature-sensitive geological formation characterized by low elasticity and high plasticity. Inappropriate engineering design during borehole drilling in permafrost can result in the collapse of surrounding strata. To evaluate the stability of borehole walls, a finite element model was developed based on the inherent physical properties of permafrost. This model was utilized to investigate the thermal, stress, and plastic yield zone evolution around the borehole during drilling with normal-temperature fluids. The borehole expansion rate was employed as a quantitative measure to assess wall stability. The analysis reveals that the strata adjacent to the borehole, when drilled with normal-temperature fluids, experience thawing and yielding, with secondary stress concentrations in unthawed strata driving the progressive expansion of the plastic zone. The degree of plastic deformation diminishes with increasing distance from the borehole. Consequently, the borehole expansion rate was utilized to evaluate collapse risk under varying conditions, including permafrost thickness, depth, plastic strain thresholds, and drilling fluid densities. The findings suggest that normal-temperature drilling fluids are appropriate for thin permafrost layers, whereas for thicker permafrost, adjustments in drilling fluid density are required to ensure the stability of borehole walls due to the elevated temperatures and geostress at greater depths.

Micromechanical investigation into the mechanism of slurry fracturing and the effect of covering depth concerning shield tunneling in a sandy stratum

With progressive advancements in slurry shield tunneling, increasingly larger tunnels are being constructed beneath shallow covering layers, leading to smaller cover-span ratios. If slurry pressure is not properly controlled, the covering layer is susceptible to fracturing or splitting. However, existing theories based on borehole expansion and filter cake forming fall short in interpreting the essence and mechanism of slurry fracturing in a discrete medium. To address this issue, this work employs a numerical approach grounded in laboratory experiments. This study offers a fresh perspective on the progression of slurry fracturing in sandy stratum, enabling direct observation of events within the stratum and analyses on dynamics and responses of the earth at various covering depths. Furthermore, particle scaling laws have been introduced to ensure the consistency in stress responses between real-world physical systems and simulations. The validity of this numerical approach in modeling such phenomena is demonstrated both theoretically and logically. New insights into the mechanism of slurry fracturing in a sandy stratum are provided.

Liquefaction assessment based on borehole expansion test with Ménard pressuremeter

Liquefaction assessment based on borehole expansion test with Ménard pressuremeter

Fracture Mechanics Induced by Drilling and Blasting in Underground Openings: Prevailing Practices and Studies

This article presents the prevailing practices and literature review on the study of fracture mechanics of rock mass, during the construction of underground openings, by drill and blast method (DBM). The paper makes an introductory discussion on basic fracture and failure mechanics on the rock, followed by a review due to the blast-induced failure mechanism. The damage zone in drill and blast openings is discussed which is followed by the conclusions of the literature. DBM is a widely acceptable and broadly applicable approach in the underground construction method. The blasting damaged zones are classified as the borehole expansion zone, crushed zone, nonlinear fracture zone, and radial crack propagation zone. Blasting activities create stress waves in construction work, which fundamentally influence the fracture and failure mechanics of rock with pre-existing discontinuities, in-situ stress, and groundwater conditions. On the other hand, the cumulative action of subsequent explosion gas and stress waves defines the final damage limit and crack pattern map. Thus, it is of utmost importance to investigate the blast-induced fracture and failure mechanism to mitigate the instability in the underground openings.

Creation of expanded diameter gravel wells in unconsolidated for-mations for High-Temperature Aquifer Thermal Energy Storage Sys-tems: Theoretical and numerical evaluation of borehole stability

High-Temperature Aquifer Thermal Energy Storage (HT-ATES) systems have the potential to cost-effectively store large volumes of thermal energy, bridging the supply-demand gap for variable renewable heat sources, such as solar thermal or power-2-heat conversion [3, 4]. These systems involve the injection and extraction of heated and cooled groundwater in aquifers via tube wells [11]. A HT-ATES system will be showcased at TU Delft, which involves the use of an Expanded Diameter Gravel Well (EDGW) to increase well capacity and reduce mechanical clogging compared to conventional wells [9]. This has the potential to reduce the number of wells needed and lower the costs of the HT-ATES system.
 An EDGW has previously been constructed at depth in unconsolidated formations using a jetting technique for borehole expansion [8]. This well (expanded 2.6 fold from 600 mm to 1570 mm diameter) was taken into routine operation for drinking water production. A second expanded borehole (expanded 4.1 fold from 600 to 2460 mm diameter) collapsed upon testing the enhanced removal of the filter cake before it could be completed as EDGW. The missing explanation for the collapse of the second well highlights a knowledge gap regarding the stability of an expanded diameter borehole in unconsolidated formations. To prevent collapse of future expanded boreholes and to better manage the drilling process, this study aims to investigate the effects of an enlarged diameter on well stability through a theoretical analysis.
 The stability of the EDGW borehole is evaluated in two ways, see Figure 1 for a schematic of the workflow. Firstly, the effects of an enlarged diameter on the stability of the well are evaluated analytically using a poroelastic framework [1, 2, 7]. Different conditions are taken into account regarding the stress state, mud pressure, and hydraulic conductivity of the aquifer. Secondly, field test conditions for the anticipated EDGW in the HT-ATES system are simulated numerically using the two and three-dimensional finite element software. The EDGW to be built in Delft, will be constructed within the Maassluis formation. The target aquifer is located at a depth of 120 – 182 m and is mainly composed of sandy units [5]. The simulation is divided into an initial, drilling, and open borehole (seepage) stage [6, 10]:
 
 During the initial stage, in-situ stresses and pore pressures are applied.
 The well bore volume will be removed during the drilling stage, resulting in a stress state that is no longer in equilibrium, and a mud pressure is applied to the borehole wall.
 For the open borehole prior to backfilling stage, the effect of time-dependent fluid flow due to mud losses is evaluated on well stability.
 
 During the open borehole stage, both situations with and without the presence of a filter cake (varying in thickness and hydraulic permeability) will be evaluated. Furthermore, the effect of the presence of thin clay/silt layers on the stability problem will also be taken into account, where special attention is paid to the roof of the well.
 The final results of this study are presented in the form of critical conditions regarding stress state, required mud pressure, and hydraulic conductivity for enlarged diameter boreholes in unconsolidated formations. Additionally, a design for the EDGW field test as part of the HT-ATES system in Delft is proposed, taking into account uncertainties such as the in-situ stress state and strength parameters of the formation.

Analysis of Rock Breaking Parameters and Simulation of Mechanical Characteristics of Multi-Nozzle Jet Impact

With the continuous development of high-pressure water jet technology, research on the optimization of structural parameters for multi-nozzle configurations, the perforation effect of jet rotational impact rock breaking, and the impact force during rock breaking has received increasing attention. Through the development of a self-designed high-pressure water jet rotational drilling test device, rock breaking experiments were conducted on sandstone of different strengths using jet streams with different inclination angles, various combinations of nozzles, different target distances, and different rotational speeds. The parameters and structure of the multi-nozzle jet drill bit were optimized, and the impact of water jet-rock breaking effects was studied. The rationality of different inclination-angle jet streams in rock breaking was verified using the ALE-FEM coupling method. The changes in the force on the target body and the fragmentation mode during rock breaking with different inclination-angle jet streams were analyzed. The results showed that under the condition of adjustable inclination angles, a smaller inclination angle resulted in greater depth and a smaller diameter of rock breaking, while a larger inclination angle resulted in greater width and a smaller depth of rock breaking. The optimal combination of multi-nozzle jet streams was determined to be 20°, 30°, and 60°, which achieved a balance between rock breaking and borehole expansion performance. The efficiency of multi-nozzle jet rotational rock breaking decreased with increasing target distance, with the optimal range being 2 to 4 mm. The rotational speed of the multi-nozzle jet stream had a significant impact on rock-breaking efficiency. Under the same target distance conditions, as the drilling speed increased, the volume of rock breaking initially increased and then decreased, and the rate of volume attenuation increased with increasing target distance. The forms of rock breaking in multi-nozzle jet streams were not identical. Jet streams with smaller inclination angles mainly caused tensile failure through axial impact, while those with larger inclination angles primarily caused shear failure through radial impact. This study provides valuable guidance for optimizing the structural parameters of multi-nozzle jet drill bits and researching rotational rock breaking.

Comparison between double caliper, imaging logs, and array sonic log for determining the in-situ stress direction: A case study from the ultra-deep fractured tight sandstone reservoirs, the Cretaceous Bashijiqike Formation in Keshen8 region of Kuqa depression, Tarim Basin, China

The tight sandstone in the Tarim Basin has the characteristics of large burial depth and development of nature fractures due to concentrated in-situ stress. Identifying the present-day in-situ stress orientation is important in hydrocarbon exploration and development, but also a key scientific question in understanding naturally fractured reservoirs. This paper presents a case study where we integrate various methods using wireline and image-log data, to identify present-day in-situ stress direction of ultra-deep fractured tight sandstone reservoirs, in the Kuqa depression. We discuss the formation mechanism of the elliptical borehole, compares the advantages and applicable conditions of the double caliper method, resistivity image logs and array sonic logs method. The well borehole diameter is measured orthogonally, then the ellipse is fitted, and the in-situ stress orientation is identified by the azimuth of the short-axis borehole, but it fails in the borehole expansion section, the fracture development section and the borehole collapse section. The micro-resistivity image logs method reveals the borehole breakouts azimuth, and also the strike of induced fractures, which are used to determine the orientation of in-situ stress. However, under water-based mud conditions, it's hard to distinguish natural fractures from induced fractures by image logs. Under oil-based mud conditions, the induced fractures are difficult to identify due to the compromised image quality. As for the sonic log, shear waves will split when passing through an anisotropic formation, shear waves will split during propagation, and the azimuth of fast shear waves is consistent with the orientation of in-situ stress. However, it is usually affected by the anisotropy caused by the excessively fast rotation of the well log tools, so that the azimuth of fast shear wave cannot effectively reflect the orientation of the in-situ stress. Based on comprehensive assessment and comparison, in this paper we propose a method integrating various logging data to identify the orientation of in-situ stress. Among various types of logging data, the breakouts azimuth identified by image logs is proved to be the most credible in identifying the orientation of in-situ stress, while using the direction of induced fractures under water-based mud conditions is also viable. However, the azimuth of the fast shear wave is consistent with the orientation of maximum in-situ stress only when the rotation speed of the logging tool is low. The caliper method can be used as a reference for verifying the other two methods. Using this integrated method to study the orientation of in-situ stress in the Keshen 8 trap, the results show that faults are an important factor affecting the direction of in-situ stress, while multi-level faults will produce superimposed effects that cause the current direction of in-situ stress to change.

Research on fast video mapping method of borehole in front view

Camera borehole technology is widely used in borehole engineering, which can obtain the structure of deep-buried rock mass. However, when people recognise these images, there will be problems such as heavy workload, low efficiency and easy recognition errors. In order to solve these problems, this paper proposes a fast unfolding mosaic and fusion method of looped image in the forward view video. This paper proposes a borehole image expansion method with autonomous positioning of the borehole centre and adaptive adjustment of the expansion radius to achieve rapid and automatic image expansion. On this basis, an adaptive adjustment algorithm of image stitching points based on grey difference registration is proposed, which solves the defect of determining stitching points by artificial experience. The practical results show that the image mosaic technology is simple, the processing is flexible and efficient and the forward view borehole expansion map obtained has an ideal effect.

Study on the hydration of hard and brittle mudstone and its influence on the wellbore stability

The instability of hard and brittle mudstone wellbore in the Xujiahe Formation in the Western Sichuan Depression is widespread, which has led to accidents such as sticking and collapse. In this paper, the Xujiahe Formation in the Western Sichuan Depression is taken as the research object, and the method of indoor experiment and theoretical simulation is used to study the instability phenomenon of the wellbore. The following points of knowledge have been obtained: The clay minerals in the mudstones of the Xujiahe Formation in the Western Sichuan Depression are mainly illite and chlorite (content>90%), with a small amount of illite/montmorillonite mixed layer, and the pore throat radius is between 14.56nm-24.47nm, belonging to the mesoporous type; After the mudstone hydrates, its compressive strength, cohesion, elastic modulus, and tensile strength will continue to decrease with the increase of water, while the Poisson’s ratio will continue to rise; The relationship between the dry core and the wet core can be used to correct the acoustic velocity of the collapsed mudstone section; the nature of the wellbore instability is that the liquid pressure in the wellbore is less than the formation pressure, and the formation pressure exceeds the rock strength, which leads to instability. We use the borehole expansion rate to realize the evaluation of wellbore stability. The research results have guiding significance for the evaluation of the stability of the hard and brittle mudstone wellbore of the Xujiahe Formation in the Western Sichuan Depression, and provide technical support for the next step of drilling safety.

Theoretical analysis of pressure-bearing performance on compression packer in low permeability reservoirs

Up to date, compression packers have been widely utilized to seal annular space between tubing and casing in the fracturing process of low permeability reservoirs. However, packer end damage under great annulus pressure difference and internal shear failure caused by excessive pressure in tubing are the main failure reasons. Current research works mainly focus on the theoretical model of sealing property with contact stress as evaluation index and less consideration was given to pressure-bearing property in rubber. This paper established a novel analytical model which took into account material properties, geometric dimension and borehole expansion ratio setting the maximum shear stress as evaluation criteria, which can investigate the distribution of shear stress along the axial diameters on compression packer under one end loaded and other end fixed. Also, the accuracy of the analytical model was verified by FEM. Main results illustrated that a) the analytical results can describe the mechanical behavior of the compression process well and the recommended setting pressure was less than 40 MPa; b) with the materials hardened, the compression distance reduced smoothly and the maximum shear stress almost kept unchanged; c) the compression deformation and maximum shear stress rose significantly as the inner diameter of the open hole well increased; d) the height of the packer played a negative role both in maximum compression distance and maximum shear stress of the compression packer.

Dependence Models of Borehole Expansion on Explosive Charge in Spherical Cavity Blasting

In geotechnical practice, it is often necessary to improve the properties of soil and rock in which different structures are built. For this purpose, spherical cavity blasting can be applied to expand the borehole. Such expansion may incorporate various constructive elements such as anchors and thus stabilize the slope. The paper presents the method for determining the increased volume, expansion, and deepening of the borehole as a result of spherical cavity blasting. In addition, mathematical models describing the dependency of the borehole expansion on the amount of explosive charge are presented. The models are mutually compared with the Akaike information criterion.

Experimental Investigation of Blast-Induced Fractures in Rock Cylinders

Fractures in rock cylinders with a central blasthole have been investigated, focusing on the borehole expansion, the crushed zone around the borehole, and the effects of the boundary conditions. Nine small-scale blasting tests were carried out on granite cylinders with diameters of either 228 or 240 mm and lengths of 300 mm, with fully coupled or decoupled explosive charges. The granite cylinders were confined by steel tubes with an inside diameter of 268 mm, where the gap (20 or 14 mm) between was left either empty, filled with gravel or filled with cement grout to simulate various lateral boundary conditions. The fractures around the blasthole were examined on cross-sections of the cylinders. The extent of the crushed zone and the expansion of the borehole were found to depend largely on the decoupling ratio of the charge, both decreasing with an increasing ratio. For small charges, the cylinders confined by gravel fill developed more and longer radial cracks than cylinders confined by the more rigid cement fill. For large charges, a cylinder with an empty gap fragmented into very small pieces, while a cylinder with cement fill broke into large fragments. Hoop strains measured on the steel tubes were smaller for a specimen with gravel fill than for similar specimens with cement fill. For the cylinder with an empty gap, fragment collisions with the steel tube caused significant hoop strains in the confining steel tube. The experimental findings of this investigation may contribute to a better understanding of rock blast fracturing, particularly in the region near the blasthole.

Volumetric Changes and Point of Saturation around a Pressuremeter Probe Used in Unsaturated Soils

The interpretation of pressuremeter test results in cohesive soils assumes that the soil is saturated and the rate of loading is such that undrained conditions apply. Pressuremeter tests in compacted soils or heavily overconsolidated clays may have a degree of saturation that is less than 100%. Consequently, the soil may undergo volume changes during the initial stages of loading. Loading of the soil will result in the compression of occluded air and consequently the initial circumferential strains may be a function of volumetric changes and not a result of borehole expansion alone. A method to determine the applied radial total stress that will result in saturation for the pressuremeter test is developed based on Hilf’s method of pore water pressure prediction during compressive loading of an unsaturated soil. A method to quantify the volume change that will occur within the plastic region around the pressuremeter probe prior to saturation is also developed.

Soil and Rock Classification from High Pressure Borehole Expansion Tests

Soil and Rock Classification from High Pressure Borehole Expansion Tests

25 MPa HyperPac Fills the Gap Between the Ménard Pressuremeter and the Flexible Dilatometer

The borehole expansion test can be used in any ground material, from the softer to the harder one, so as to obtain its stress–strain behaviour in situ. The authors submit their research work on equipment which permits to extend the use of the Menard pressuremeter up to 25 MPa test pressure. They also give the first test diagrams up to this pressure in slightly fractured rocks.

Soil and Rock Classification from High Pressure Borehole Expansion Tests

Physical and mechanical properties used to characterize soil and rock are different according to the various approaches and targets of the different activities involved, namely soil mechanics, rock mechanics or engineering geology. The Authors suggest that the data obtained during a borehole expansion test, which can be summarized by a Menard E-modulus and a limit pressure, be used in an overall classification ranging from loose soils to hard rock without any discontinuity based on the soil Pressiorama® as developed for soils these last 10 years.

Mine-by Experiment in a deep shaft in Neogene sedimentary rocks at Horonobe, Japan

The Japan Atomic Energy Agency (JAEA) has been implementing several research studies at the Horonobe Underground Research Laboratory (URL) in support of the Japanese R&D program for the geological disposal of High-Level Radioactive Waste (HLW). Understanding the development and properties of an EDZ (excavation disturbed or damaged zone), including the rock mechanical, hydrogeological and geochemical responses around excavated openings, is important to the design of plugs and sealing material and for performance assessment of geological disposal. The Mine-by Experiment described in this paper was carried out to investigate the development of an EDZ in one of the deep vertical shafts excavated in Neogene sedimentary rocks at the Horonobe URL site. The objective of this study is to obtain fundamental parameters of an EDZ in Neogene sedimentary rock, where case studies are few, in support of the geological disposal studies in Japan. Before and after shaft sinking, BTV (borehole television), borehole expansion tests, permeability tests and seismic velocity logging were carried out to determine initial conditions and changes in the mechanical and hydraulic properties. During shaft sinking, extensometers were installed in the rock mass to determine rock mass response to excavation. The variation of hydraulic properties was largest within one meter of the shaft wall and the variation in hydraulic conductivity was in the range of one order of magnitude.

Numerical modeling of ground borehole expansion induced by application of pulse discharge technology

Pulse discharge technology (PDT) is an innovative technique that can be used to enhance bearing capacity of piles and resisting capacity of anchors. It enlarges the section area and compacts the surrounding soil by high-powered shock wave pressure induced by an underwater electrical discharge. This study aims to establish a suitable numerical model for the simulation and prediction of ground borehole expansion induced by PDT. In order to examine the relationship between electricity and the characteristics of shockwaves generated by PDT, laboratory pulse discharge tests were performed using PDT equipment used in current practice. Then, based on the underwater explosion (UNDEX) model and a coupled acoustic–structural analysis scheme, the results of laboratory PDT tests were analyzed and numerically benchmarked to determine the equivalent UNDEX model parameters for providing shock loading input in a ground borehole expansion simulation. A series of expansion simulations for undrained clayey and sandy soils were performed, and the predicted borehole expansion behaviors were compared with the test results. Moreover, a parametric study was conducted to examine the effects of soil properties on the expansion behavior. The results of the numerical work in this study appeared to be consistent with field test results published in the literature and showed that the soil characteristics related with packing, state of stresses, and degree of saturation were important when analyzing borehole expansion behavior.

Simulation of blast induced crater in jointed rock mass by discontinuous deformation analysis method

Rock blasting is a dynamic process accompanied with the propagations of shock waves and the dispersion of the explosion gas. This paper adopts the discontinuous deformation analysis (DDA) method to simulate the rock blasting process. A dynamic parameter adjustment and the non-reflecting boundary condition are implemented in the DDA method. The sub-block DDA method to simulate fracture problems is used. The blasting process in jointed rock mass is simulated by application of the explosion gas pressure on the expanding borehole walls and induced connected fracture surfaces around the boreholes. The blast craters with different overburdens are derived. The whole process including the explosion gas dispersion, borehole expansion, rock mass failure and cast, and the formation of the final blasting piles in rock blasting are well reproduced numerically. Parametric study for different overburdens is carried out, and the results are analyzed and discussed.

Measurements of Soil Deformation by Means of Cone Penetrometer

Measurement of the settlement of a cone submitted to incremental loading allows the determination of a modulus correlated to those obtained using laboratory tests or derived from other in situ tests. This test can be carried out with standard static penetrometer equipment. After stopping the penetration of a static penetrometer cone at a selected depth, an incremental loading test is carried out until the failure of the ground under the cone is observed. Stress strain relationships can be drawn from cone resistance and sleeve friction. In this manner, a modulus and a friction angle can be obtained. Thus the classical CPT log is enriched offering new possibilities. In this paper the development of the first prototype and measuring devices and the proposed testing procedure are presented. A comparison of laboratory and field tests and geotechnical centrifuge models shows the validity of the testing procedure and the chosen interpretation method. The results of site investigations carried out on French national experimental sites are also discussed and compared with high quality borehole expansion test and field measurement of shear wave velocity. These investigations have been carried out using this new testing method in combination with a semi-heavy static Piezocone penetrometer in order to check the potential use for geotechnical design. An attempt is made in this paper to validate the interpretation method of the results, which can be used for routine geotechnical engineering.