Rocks extracted from deep drilling are brittle due to high pore pressure and stress relief upon retrieval
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Retrieved literature discusses high pore pressure in deep formations, stress unloading effects, and brittle rock failure characteristics separately, but does not fully establish the combined claim about extracted rocks being brittle exclusively due to pore pressure and stress relief.
Controlling surrounding rock stability in deep soft rock tunnels solely by increasing the support strength is challenging. The key areas where the surrounding rock of a roadway is prone to “instability” should be treated with comprehensive measures, such as drilling to relieve pressure, grouting of the surrounding rock, floor trenching technology, and secondary support, to achieve overall stability of the roadway surrounding rock. In this paper, based on a typical straight wall semi-circular arch roadway project in the Huaibei mining area of Anhui Province, mechanical parameters, such as cohesion, internal friction angle, Poisson’s ratio, and elastic modulus of the surrounding rock, were measured. Displacement stress distribution in surrounding roadway rock were simulated and analyzed using FLAC 3D, and the key parts of the surrounding rock prone to instability under the original roadway support conditions were determined. A suitable arrangement and parameters of the borehole pressure relief in the side roadway, floor grouting, floor trenching, and floor bolt secondary support were selected. Engineering measurements indicated that the engineering techniques were successful. It provided a significant reference for the comprehensive management and treatment of the surrounding rock stability in deep soft rock roadway on a global scale.
Frontiers | Pressure relief for drilling (trenching) and support technology in deep soft rock tunnels ORIGINAL RESEARCH article Front. Earth Sci. , 18 December 2024 Sec.
View in article ORIGINAL RESEARCH article Front. Earth Sci. , 18 December 2024 Sec. Solid Earth Geophysics Volume 12 - 2024 | https://doi.org/10.3389/feart.2024.1501420 Pressure relief for drilling (trenching) and support technology in deep soft rock tunnels Y L Yang Ling 1 * X Y Xu
Displacement stress distribution in surrounding roadway rock were simulated and analyzed using FLAC 3D, and the key parts of the surrounding rock prone to instability under the original roadway support conditions were determined. A suitable arrangement and parameters of the borehole pressure relief in the side roadway, floor grouting, floor trenching, and floor bolt secondary support were selected. Engineering measurements indicated that the engineering techniques were successful. It provided a significant reference for the comprehensive management and treatment of the surrounding rock stability in deep soft rock roadway on a global scale.
In the case of deep high-stress tunnels with poor rock stability, selecting suitable drilling pressure relief arrangements and parameters can reduce the rock maximum principal stress within a certain range around the tunnel and transfer the maximum principal stress to further areas, which reduces the looseness and fragmentation of the surrounding rock, controls the stability of the surrounding rock, and extends roadway service period ( Shang et al., 2022 ; Chen et al., 2022 ; Yue et al., 2022 ; Mogi, 1967 ; Xie S. R. et al., 2023 ; Shi et al., 2023 ; Xie J. et al., 2023 ; Wang X. Q. et al., 2023 ).
The influence of maximum principal stress size and distribution of the middle cC part of the surrounding rock is shown in Figure 8 . The maximum principal stress peak value of the undrilled pressure relief is in the r = 13.2 m position. As shown in Figures 6A–D , the maximum principal stress peaks are located at the distances r = 15.8 m, r =16.2 m, r = 17.1 m, and r = 17.1 m from the surface of the roadway, respectively; compared with the pressure relief without drilling, the maximum principal stress peak shifts are Δr = 2.6 m, Δr = 3.0 m, Δr = 3.94 m, and Δr = 3.94 m, respectively.
FIGURE 6 Numerical calculation model of borehole pressure relief with different number and row spacing of sidewalls (A) Single row (B) a × b = 0.5 m × 0.5 m (C) a × b = 1.0 m × 1.0 m (D) a × b = 2.0 m × 2.0 m. FIGURE 7 Maximum principal stress cloud diagram of surrounding rock with different row spacing a × b between pressure relief boreholes (A) Single row, (B) a × b = 0.5 m × 0.5 m, (C) a × b = 1.0 m × 1.0 m, (D) a × b = 2.0 m × 2.0 m (E) No pressure relief drilling hole. FIGURE 8 Distribution of the maximum principal stress of pressure relief in different numbers and spacing of boreholes in the side.
Despite the specific roadway feature (including a burial depth of 700 m, mudstone lithology, a straight wall semi-circular arch cross-section, and an approximate cross-sectional area of 15 square meters etc.), the research methods in this paper have extensive applicability in analyzing the stability of diverse tunnels. The key measures such as drilling pressure relief, floor grouting, floor trenching, and secondary bolt support can be extended to deep tunnels globally, especially those in soft rock under high stress. (2) A FLAC 3D model tailored to specific engineering conditions is developed to analyze the loosening and fracturing of surrounding rock under diverse scenarios.
10.1111/j.1747-1567.2011.00706.x CrossRef Google Scholar 28 Wang P P. Jiang Y. Li P. Zhou J. Zhou Z. ( 2023 ). Experimental analysis of pressure relief effect of surrounding rock in high-stress roadways under different drilling parameters . Appl. Sci. 13 , 2511 . 10.3390/app13042511 CrossRef Google Scholar 29 Wang X. H. Zhang H. H. Wu Z. Li X. L. Sui Y. Gao R. Q. ( 2022 ). Selection and optimization mechanism of the lower return roadway layout in the near residual coal pillar area . Processes 10 , 2471 . 10.3390/PR10122471 CrossRef Google Scholar 30 Wang X. Q. Wu S. J. Tang H. P. Chen B. ( 2023 ).
10.19606/j.cnki.jmst.2021.02.002 CrossRef Google Scholar View reference in article Summary Keywords deep soft rock roadway , numerical simulation , displacement stress distribution , drilling pressure relief , floor grouting , floor trenching , secondary support of floor bolts Citation Ling Y and Ying X (2024) Pressure relief for drilling (trenching) and support technology in deep soft rock tunnels . Front. Earth Sci. 12:1501420.
Abstract
The main goal was to successfully drill a re-entry well in the Rahiyah Field, West Kuwait, overcoming the challenge of commingle drilling from the Najmah to Middle Marrat formations characterized by High-pressure High-Temperature (HPHT) conditions and significant pressure uncertainty.
A high-level Managed Pressure Drilling (MPD) strategy was developed, involving the use of a 15.2 ppg mud weight with an Equivalent Circulating Density (ECD) of 16.9 ppg. The plan included performing Dynamic Pore Pressure Tests and MPD Build Up Pressure Tests at the bottom of the Dharuma Formation to accurately determine the pore pressure across transitional zones. Adjustments to mud weight and ECD were made based on real-time data, facilitating adaptation to varying well conditions.
The application of MPD techniques allowed for efficient and safe drilling through a highly uncertain pressure window, reducing the drilling time of commingled sections to 10 days, compared to over 30 days typically required for re-entry wells. The conventional approach would have necessitated a higher mud weight of 16.9 ppg, potentially causing a cycle of fluid losses and differential sticking. The oil-based mud (OBM) for the well’s final total depth (TD) was modified to a lighter density of 12.6 ppg with an ECD of 14.0 ppg, mitigating common issues in HPHT environments. This resulted in significant time savings and reduced non-productive time (NPT), demonstrating substantial improvements over traditional methods.
This case study demonstrates the effectiveness of combining MPD with real-time pressure assessments to manage significant pressure variations in deep well environments. The learnings from this pilot project are crucial for refining cost-effective field development strategies and could significantly influence well trajectory planning, completion designs, and recovery optimization in similarly challenging geological settings.
To reveal the microscopic damage evolution law of rocks under the effect of unloading disturbances with different amplitudes, electron microscope scanning, nuclear magnetic resonance (NMR), and triaxial compression tests were carried out. The evolution patterns of surface and internal pore types and mechanical properties of rock specimens after unloading perturbation were analyzed. In this paper, a classification of the ratio of dmax/dmin (dmax and dmin refer to the maximum and minimum pore size of each pore, respectively) is proposed to examine the pore and crack evolution extension developme
Mechanical properties of rocks under different stresses were studied. The crack evolution laws of rocks in different environments were also analyzed. Guo et al. [ 11 ] characterized the deformation and fracture patterns of shale samples under different stress paths by triaxial unloading tests under different stress paths. Lei et al. [ 12 ] conducted a series of loading and unloading tests on the rock mass around the roadway using high-precision acoustic emission technology. The results of the study provide guidance for deep shaft tunnel support work and disaster prevention and control.
Fig 3 shows that there are three peaks in the T 2 energy spectrum, the first and second peaks are larger in amplitude, and the third peak is almost invisible, and the geometrical pattern of the T 2 energy spectra of each group of samples is similar, the amplitude of each peak is close to that, which indicates that the internal pore development of the samples is the same, and they belong to the same stratum of the same rock in the same block. 2.3 Experimental design of rock unloading disturbance In deep geological environments, rocks are typically considered to be under hydrostatic
During drilling operations, the vertical confining pressure on the rock (initially in a hydrostatic state at the excavation face) remains constant, while the horizontal stress progressively decreases due to unloading effects. The tests were conducted using stress control to simulate the unloading disturbance of the well wall perimeter rock. A shale specimen with a stratigraphic pinch angle of 90º was used for the study.
Figs 5 bc also shows essentially no obvious signs of nascent cracking, again due to the small magnitude of unloading; Fig 5d – f local large solution hole wall or near the region to produce signs of nascent cracks, and with the unloading amplitude increases nascent cracks have increased, there is a tendency to expand, the cause of which is caused by shear extension cracks, As the horizontal principal stresses are unloaded, but the overlying formation pressure remains constant, in this case, the shear force q = σ 1 -σ 3 , where σ 1 is unchanged, σ 3 reduced, q enlarged, As σ 3 decreases the amplitude value the greater the increase in q , After considering the stress concentration at the pore wall again, shear damage occurs at the pore wall when the shear force q is greater than the shear strength of the local pore wall rock.
Such pores with d max / d min between [1, 2] have a mega decrease condition, indicating that almost no new pores are sprouting and there is an expansion of this type of pores. During stress unloading in surrounding rock, macroscopic fractures demonstrate the highest susceptibility to crack initiation, propagation, interconnection, and localized damage evolution. This mechanism facilitates the full development of damage along potential slip surfaces, with shale specimens exhibiting pronounced brittle failure characteristics. Furthermore, immediately after borehole excavation (before mudcake formation), the wellbore lacks effective wall support.
Only limited stabilization is provided by near-wellbore seepage resistance – a mechanism contingent on well-developed formation permeability, yet typically insufficient due to minimal seepage constraints. Consequently, stress redistribution occurs radially from the wellbore into the surrounding rock during this pre-mud cake phase, inducing stress relief perturbations in the near-wellbore zone. Fig 12 Area percentage of pores with different d max / d min ratios. 3.2 Analysis of NMR test results The distribution patterns of T 2 energy spectra of rock samples at different unloading amplitudes were measured by NMR tests, as shown in Fig 13 .
Prior to the drilling of the borehole, the raw geopathic stresses are in a state of static equilibrium. The surrounding rock is under pressure while the borehole is being drilled. Meanwhile, before the mud cake is formed, the rock is permeable, and the drilling fluid does not act as a support. Changes in the stress state at various points in the well wall envelope cause the stress state to readjust. When the mud cake is formed, it can be used as an isolation layer due to its very low permeability and isolation of the drilling fluid from sex well wall percolation.
(31) and (33) into perspective, one can define the random damage variable as the ratio of N i and N and further assume that each disturbance produces some cells of damage of N ( η i ) when the volume expansion rate reaches a certain level η i . (34) D u = N ( η i ) N = ∫ 0 η i N P ( t ) d t N Defects such as rock pores and cracks deep in the formation are generally tightly closed under the original state of ground stress, and there is some elastic deformation of the bedrock skeleton.
Mechanical properties of rocks under different stresses were studied. The crack evolution laws of rocks in different environments were also analyzed. Guo et al. [11] characterized the deformation and fracture patterns of shale samples under different stress paths by triaxial unloading tests under different stress paths. Lei et al. [12] conducted a series of loading and unloading tests on the rock mass around the roadway using high-precision acoustic emission technology. The results of the study provide guidance for deep shaft tunnel support work and disaster prevention and control.
It is generally difficult to predict fractures of low-permeability reservoirs under high confining pressures by data statistical method and simplified strain energy density method. In order to establish a series of geomechanical models for the prediction of multi-scale fractures in brittle tight sandstones, firstly, through a series of rock mechanics experiments and CT scanning, we determined 0.85 σc as the key thresholds for mass release of elastic strain energy and bursting of micro-fractures. A correlation between fracture volume density and strain energy density under uniaxial stress state
In order to establish a series of geomechanical models for the prediction of multi-scale fractures in brittle tight sandstones, firstly, through a series of rock mechanics experiments and CT scanning, we determined 0.85 σ c as the key thresholds for mass release of elastic strain energy and bursting of micro-fractures. A correlation between fracture volume density and strain energy density under uniaxial stress state was developed based on the Theory of Geomechanics.
Therefore, one efficient geomechanical modeling strategy used in recent exploration and development of brittle reservoirs is studying concentrations and changes in paleotectonic stress field so as to determine critical process involved in fracture development and combine various rupture criterions to predict favorable zones of fractures [ 20 ], [ 28 ], [ 29 ], [ 23 ], [ 30 ], [ 31 ]. Since the 1960’s, many studies have been published on the mechanisms of fracture-generating structural movement, including rock failure criterion, indicator of comprehensive rupture rate, and strain energy density.
The tensional fracture has a dendritic structure, a relatively shorter distance, and frequently bypasses rock grains, wherever they are observable by drill core and FMI. Observations from cores and analysis from FMI show that dip of fractures mainly ranges from 75°– 90° (i.e. vertical fractures), followed by 45°–75° (i.e. high-angle fractures) and 15°–45° (i.e. low-angle fractures) ( Fig 2b ). Due to the effect of stress unloading after exposure to ground, the physical parameters of fracture network will change slightly relative to underground conditions, such as fracture aperture and porosity.
Over the course of loading, the confining pressure was manually set to seven stages at an interval of 5 MPa (0, 5, 10, 15, 20, 25 and 30 MPa) as the axial pressure increased up to peak stress. At all times, axial loads exceeded confining pressure by no more than one-tenth of the rock uniaxial compression strength (UCS). Before the CT scanning tests, uniaxial compression tests of several spare samples were conducted to make estimation of uniaxial compression strength or peak strength (σ c ).
Seven scans were performed in the loading process, i.e., at the initial loading, 25%, 50%, 65%, 85%, 100%, and 110% of stress peak, and post-peak, respectively, with a CT slice thickness of 2 mm, 25 slices per scan ( S2 Appendix ). For a grey-level CT image, the bright color denotes the low-density part such as fractured zone or pores, and the dark color denotes the high-density rock matrix, as shown in Fig 3b . The scanned 2-D images were reconstructed into 3-D geometry and then imported to Avizo (ThermoFisher Scientific, USA) for further image analysis ( Fig 3c ). In this study, it is important to distinguish induced different-sized fractures and natural vugs from CT images.
According to brittle fracture mechanics theory and maximum tensile stress theory, brittle rock will break when elastic strain energy accumulated in the brittle material equals to the energy demanded for generating fractures per unit volume of the element [ 47 ], [ 48 ]. Generally, brittle macro-fractures occur with strain energy releasing, especially when the surrounding three-dimensional stress state reaches the rock’s strength [ 49 ]. At this time, part of the strain energy will be released as the surface energy of the new fractures while the rest will be released in form of elastic waves.
Geomechanical modeling of fracture parameters under paleostress field Fracture geometric parameters As Huang [ 51 ] and Song [ 33 ] demonstrated previously, the relationship between fracture volume density and strain energy density under triaxial compression experiments is no longer presented as a regular linear relationship. In other words, the energy accumulated in the brittle rocks must not only overcome the molecular internal cohesive forces, but also overcome the energy expenditure that naturally occurs to resist the confining pressure [ 51 ], [ 33 ].
For the fractures under confining pressure conditions, if there existed only compressive stresses ( σ 3 ≥ 0), the Mohr-Coulomb criterion would be selected, which was σ 1 - σ 3 2 ≥ C 0 c o s ϕ + σ 1 + σ 3 2 s i n ϕ (5) The Mohr-Coulomb Criterion suggested that a shear fracture only formed if the rock cohesion ( C 0 ) was exceeded, which was depended on the magnitude of normal stress along a fracture plane.
[ 57 ] derived an equation to calculate the aperture of a fracture due to the current in situ stress field as follows: b m = b 0 1 + 9 σ n ′ σ n r e f + b r e s (17) where b 0 and b m were the original and current aperture of the fracture (m), respectively; σ ’ n was the effective normal stress (MPa), namely was the result of the normal stress acting perpendicular to fracture plane minus the fluid pressure acting inside fracture [ 8 ], [ 9 ]; b res was the residual aperture of the fracture (m); and σ nref was the
From a microcosmic point of view, there existed only three failure modes in brittle rocks: shear fracturing, tensional fracturing, and composite fracturing, which depended mainly on confining pressure, principal stress intensity, and the pre-existing plane of weakness. After many empirical experiments, a relationship between the fracture volume density and stress-strain of tight sandstone reservoirs was finally established.
Cyclic pore pressure is closely related to the stability and safety of
deep underground engineering. A micro-macro mechanical model of creep
fracture under cyclic pore pressure, external confining pressure, and
axial stress was established to reveal the mechanism of cyclic pore
pressure of brittle rock. The model combines the improved microcrack
model, the function of cyclic pore pressure, the subcritical crack law,
the Hooke-Kelvin model, and the crack-strain model. The complete
elastic, viscoelastic and plastic deformation of rock is described. The
model solution before crack extension determines the elastic and
viscoelastic deformation, and that of after crack extension determines
the plastic deformation. Time-dependent elastic modulus evolution of
rock under cyclic pore pressure is obtained, which affects viscoelastic
strain under cyclic pore pressure. The effects of cyclic pore pressure
parameters
Δ
P
Pa
and
Δ
P
Pb
and cycle period on rock viscoelastic
rebound value and creep fracture time are discussed. Rationality of the
proposed model is verified by experimental data. The analytical results
provide help for evaluation in safety and stability of rock engineering.
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