Despite decades of numerical, analytical and experimental researches, sand production remains a significant operational challenge in petroleum industry. Amongst all techniques, analytical solutions have gained more popularity in industry applications because the numerical analysis is time consuming; computationally demanding and solutions are unstable in many instances. Analytical solutions on the other hand are yet to evolve to represent the rock behaviour more accurately.
We therefore developed a new set of closed-form solutions for poro-elastoplasticity with strain softening behaviour to predict stress-strain distributions around the borehole. A set of hollow cylinder experiments was then conducted under different compression scenarios and 3D X-Ray Computed Tomography was performed to analyse the internal structural damage. The results of the proposed analytical solutions were compared with the experimental results and good agreement between the model prediction and experimental data was observed. The model performance was then tested by analysing the onset of sand production in a well drilled in Bohai Bay in Northeast of China. Acoustic and density log along with core data were used to provide the input parameters for the proposed analytical model in order to predict the potential sanding in this well. The proposed solution predicted the development of a significant plastic zone thus confirming sand production observed by today sanding issue in this well.
ABSTRACT: Analysis of stresses and deformations around circular tunnels and shafts are critical for evaluation of the interaction between the support system and the ground and therefore the tunnel support design and stability. The damage induced in the material by the excavation method (i.e., blasting) can significantly influence the ground response as the excavation alters the rock mass properties over the damaged zone. When the provided support pressure inside the tunnel falls below a critical value, a zone of plastic (broken) material develops around the tunnel. The self-weight of the broken material is significant at the crown of the tunnel and may subject the support to larger stresses. This study presents a new analytical-numerical solution for the determination of stresses, strains, and deformations around a tunnel with the consideration of the gravity effect and the blast induced damaged zone. A modified equilibrium equation for the ground is used and the elastic and plastic zones of the tunnel are analyzed. The results indicate that gravity and the damaged zone have significant effect on the tunnel convergence. The presented method in this paper is novel and allows the tunnel designers to assess the combined effect of blasting quality and gravity in the tunnel convergence.
In order to characterize the stresses and displacements around an excavation, it is necessary to understand the behavior of the rock mass around the tunnel. As excavation in an initially elastic rock mass occurs, the original properties of the rock mass are changed. Based on the method of excavation, the properties of the surrounding material can be influenced by the development of an excavation damaged or disturbed zone. The degree and extent of this damaged zone varies significantly based on the selected method of excavation. In tunnels excavated mechanically by TBM drilling, the effect of the damage in the surrounding rock mass is negligible. In the drill and blast method, the influence of excavation disturbance in the rock mass near the tunnel radius is far more significant (Martino and Chandler, 2004; Kwon et al. 2009; Bastante et al., 2012; Zhang et al., 2017). Therefore, it is of interest to consider the effects of a blast-induced damaged zone (BIDZ) when analyzing the stresses and deformations around an excavation. The development of a BIDZ has a considerable effect on the strength and stiffness of the rock mass. This damage to the material is assumed to form a cylindrical zone of influence at a constant extent. The zone beyond the BIDZ is characterized by undamaged material properties. Recently, geophysical techniques such as seismic wave propagation (Hedayat 2013; Hedayat et al., 2012; 2013; 2014a-d; 2018; Hedayat and Walton, 2017; Gheibi and Hedayat, 2018a-b) have been used to estimate the extent and quality of BIDZ (Day and Murthy, 2013). Hoek et al. (2002) introduced a factor accounting for degree of disturbance due to blast damage and stress redistribution, D. This disturbance factor depends directly on the quality of the blasting applied to the excavation, and is estimated based on the appearance of the rock mass. Based on D, associated properties of the damaged zone can be calculated using empirical equations.
ABSTRACT: Previous studies show that the effective fracture toughness will increase with the rise of rock ductility during the hydraulic fracture propagation. Higher pump horsepower is required for fracture propagation and stimulation volume will decrease extensively. However, such conclusion was mainly validated by numerical modeling and field observation with much fewer experimental investigations. Three different sample types of silica-rich, calcite-rich, and clay-rich with distinct ductility was made. Mineralogy and clay content have a profound effect on ductility. Mechanical properties and ductility were measured and compared using UCS, Brazilian, semicircular bending toughness, and hydraulic fracturing tests. Hydraulic fracturing tests were conducted on cube of 10cm on true triaxial stress condition. Also, scaling laws were applied to simulate the field-like viscosity dominated propagation condition. A direct relation between sample ductility and lag time (between the fracture initiation and breakdown) was observed (i.e. longer lag time for samples with higher ductility). In addition, it was shown that ductile samples have slower fracture propagation.
The role of ductility on hydraulic fracture has been extensively investigated based on theoretical studies and numerical modelling. The theory of plasticity is applied for modelling ductile behaviour (Fjar et al., 2008), it was later concluded that the plasticity increases the effective fracture toughness, which leads to more energy consumption during fracture propagation in ductile rock (Dean and Schmidt, 2009; Jiang et al, 2003; Martin, 2000; Papanastasiou, 1999, 2000; Yao, 2012).
Numerical simulation based on linear elastic fracture mechanics (LEFM) contradicts the prediction of fracture propagation in soft rock where the crack-tip effect plays a vital role where the plastic deformation is likely to occur. Therefore, different fracture models were developed to investigate the effect of plastic deformation on initiation and propagation of hydraulic fracture. Papanastasiou (1999) performed numerical simulation based on elastoplastic model and mentioned that the elasto-plastic fracture is propagated under higher net pressure compare to elastic fracture. Martin (2000) modeled the tip plasticity in soft formations and confirmed that fracture tip propagation requires more energy from fracturing fluid due to plastic deformation generated. Van Dam et al. (2000) performed laboratory experiments and numerical simulation, he demonstrated that the plasticity increases the fracture width, but significantly decreases the closure pressure. Recently, Yao (2012) considered a 3D pore pressure cohesive zone model and showed that the predicted geometry of a hydraulic fracture in ductile rock with this model is more accurate than LEFM method.. However, only a few of the conclusions are validated through experimental studies, which is technically difficult but virtually more reliable. The role of ductility on hydraulic fracture initiation and propagation based on experimental analysis is still poorly understood.
ABSTRACT: A modified version of conventional Hoek cell (QRT Cell) has been designed and developed for advanced triaxial te sting. QRT cell is capable of capturing radial deformation of the core samples through a new set of Linear Variable Differential Transducer (LVDT) designed to attach to core sample diametrically. The cell has significant advantage of minimal setup time, measuring permeability and controlling the temperature during the experiment with high accuracy. A number of triaxial experiments were performed on Gosford sandstone using the new cell to demonstrate its intrinsic advantages.
Conducting triaxial experiment is vital for detailed characterization of the mechanical properties of rock under confinement in many engineering projects. In addition, with emerge of Multiphysics studies and its importance in investigating reactive rocks behavior (Roshan and Oeser 2012, Roshan and Aghighi 2012, Roshan and Fahad 2012) the use of triaxial cells capable of measuring thermo-chemo-hydro-mechanical interactions are highlighted in recent years. Currently two devices are available for such test: Hoek cell (Hoek and Franklin 1968) and Triaxial cell (Kovari et al. 1983). The former is fast and easy to use but is incapable of providing the radial deformation of the tested sample. The latter can measure on sample deformation but has a long setup time especially due to its size and weight and associated portability issues (Masoumi et al. 2015).
In this paper a newly designed triaxial cell is presented. The design of new cell is inspired by the initial Hoek cell. The cell has all the benefits of the conventional Hoek cell plus the capability of measuring the radial deformation of the core samples from the commencement of loading up to the residual stress using the miniature Linear Variable Differential Transduces (LVDT) i.e. axial deformation can be measured using displacement of loading frame or on platen LVDT. A number of triaxial tests were performed on Gosford sandstone samples at three different confining pressures including 10, 30 and 50 to demonstrate the characteristics of this triaxial cell.
2. CONVENTIONAL TECHNIQUES
2.1. Triaxial cell
International Society for Rock Mechanics (Kovari et al. 1983, ISRM 2007) and American Systems for Testing and Materials (ASTM 2000) have suggested the application of the triaxial cell for rock testing under triaxial loading. The axial and radial deformations of the samples from the commencement of loading up to large shear strains beyond the post-peak can be measured and recorded with this cell. LVDTs are used for recording both axial and radial deformations. However, there are two drawbacks associated with this methodology including the setup time and the accuracy of the radial deformation measurement. Due to the unique design of the circumferential extensometers which are used in triaxial cells, the measured radial deformations need some additional modifications as suggested by Masoumi et al. (2015). In addition, considering the high cost of triaxial systems, they are not economically justified in many instances.
Underbalanced drilling (UBD) has been used in the oil industry to avoid formation damage, reduce lost circulation and increase the rate of penetration. Underbalanced drilling in incompetent rocks, however, poses severe borehole stability problems. Numerous models have been developed to predict the time dependent wellbore collapse during drilling, in particular underbalanced drilling. Experience shows that these models require data that are hard to obtain and without these data results become unreliable. A three dimensional wellbore model has been developed at university of New South Wales to help drilling engineers determine the optimal mud composition and a range of bottom hole pressure for underbalanced drilling. The concept of thermodynamics of irreversible processes, consolidation and elastoplastic constitutive law are employed to predict the temperature and ion distribution, pore pressure and effective stresses around a borehole. These data sets can be used as input to borehole stability analysis. From the results of this study it was revealed that the effect of plasticity on stress relaxation around the wellbore is a major concern during underbalanced drilling. This relaxation especially in hoop stresses can lead the wellbore to collapse. The results also showed that the size of plastic zone depends not only on the horizontal in-situ stresses but also on the vertical in-situ stress.
Keywords: Underbalanced drilling, Finite element, Three dimensional, Stress distribution
Most shale formations are naturally fractured and wellbore instability is a common problem in these formations. In previous wellbore stability analysis shale formations have been treated as a homogeneous rock which underestimates drilling mud parameters. The aim of this paper is to study the transient change in pore pressure and stress state around a wellbore drilled in a chemically active fractured shale formation. For this purpose a finite element model for coupled chemo-thermo-poro-plasticity is developed. A discrete fracture network is generated based on an object-based hybrid neuro-stochastic simulation. Permeability tensors of the fracture network were calculated by using boundary element method which is based on periodic boundary conditions around the grid block and interface boundary conditions around the fracture edges.
In order to solve the plasticity problem a single step backward Euler algorithm including a yield surface correction scheme is used to integrate the plastic stress-strain relation and an initial stress method is employed to solve the non-linearity of the plastic equation. Super convergent patch recovery is used to accurately evaluate the time dependent stress tensor. The solute advection and thermal convection are also considered due to presence of the natural fracture system.
From the results of this study it was revealed that the pore pressure decreases around the wellbore due to the backflow of chemical and thermal osmosis. When drilling through fractured shale formations, however, the decrease in pore pressure is lower than that of intact shale formations which is mainly due to solute advection through the fractures. Also it was found that the effective radial and tangential stresses can reach yield strength of the rock in presence of fracture system thus casusing serious instability problems. It is more likely to form a plastic zone near the wellbore wall in the presence of natural fracture than otherwise. In this study the formation of plastic zone leads to wellbore failure by exfoliation.
Key words: chemically active rock, naturally fractured shale, osmotic flow, plasticity
The effects of carboxymethylcellulose (CMC) polymer with some other additives on Class G cement were investigated to design a slurry for a field in the south of Iran subject to tectonically induced horizontal in-situ stress. For a number of alternative slurries, characteristics such as compressive strength, permeability, free water, and fluid loss were examined. Fundamental theoretical studies and laboratory experiments were conducted using a system-based approach, endeavoring to minimize undesired properties progressively. The result was a cement slurry with specifications of early and high compressive strength, low permeability, suitable rheological properties, and a wide range of thickening time and suitable slurry for this case. Accordingly, the major contributions of this research lie in two aspects. First, a new package of additives is introduced to be used in similar cases in the oilwell-drilling industry. Second, a systematic experimental/theoretical approach for optimization of cement slurry in accordance with functional requirements is documented.