ABSTRACT: This study presents the feasibility of performing permeability measurement by interpretation of consolidation-induced time-dependent deformation of sample when an instant pore pressure pulse is applied. For the interpretation of measured transient behavior, a procedure to back-calculate permeability from measured consolidation strain is developed. The estimation shows promising results that gave similar order of magnitude of permeability compared to that measured from steady state method. However, this study shows that creep behavior affects the interpretation of permeability significantly as the effective stress increases. This study indicates that proper calibration of creep effect is important to get reliable values. This study also present some recommendations to optimize the results.
Permeability measurement using the steady-state method requires much time for low permeable lithologies (e.g. shale or mud stone), leading to very long experimental time if the aim is several permeability measurements per test. For example, it may take more than one week for one permeability measurement for low permeable rock. However, if an instant pressure pulse loading can capture the transient/consolidation behavior within some hours and the measurement can be converted to the permeability, it can provide a cost and time effective alternative test method.
Change in pore pressure of porous material change the stress conditions acting on its grain structure, which is known as the effective stress. Consequently, the pore volume changes due to expulsion/influx of the pore fluid that occupies the void spaces, a process known as consolidation (or primary consolidation) (Biot, 1941; Terzhaghi, 1943). Thus, hydraulic parameters, especially the permeability, can be deduced from the consolidation-induced deformation, which is strongly coupled with hydraulic behaviour.
As schematically illustrated in Figure 1a, we can assume two-way 1-D consolidation condition when the pore pressure at the top and bottom boundaries change rapidly with a step function of ΔΡ, which is labeled ‘pulse-loading’ hereafter. After the pulse loading, the pore pressure will be gradually dissipated from the top and bottom to the middle of the specimen over time (i.e. red curves in left Figure 1a). Consequently, as shown in Figure 1b, associated volume changes may continue until the pore pressure becomes fully dissipated or reaches steady-state condition.
ABSTRACT: This study reports on strength measurements performed on sandstone samples from a shallow offshore field and is part of a larger petrophysical evaluation including static and dynamic elastic properties under various stress conditions. The samples tested were retrieved from three one-foot-long preserved core sections spanning a depth range of about 20 meters. For each depth, three types of mechanical tests were performed: brazilian tensile test, unconfined compression test (UCS) and confined compression test with ultrasonic wave velocity monitoring along and perpendicular to the plug axis. The mechanical data are augmented with X-ray CT image volumes acquired pre-test and post-test in order to elucidate some of the observations as well as to outline possible elements of a laboratory workflow that would comprise image-based heterogeneity assessment and prediction. We emphasize in particular the fact that if such workflow is to be standardized, care should be taken in acquiring properly calibrated plug scale X-ray images in order to allow heterogeneity quantification and its use as an input into future predictive models.
Geomechanical testing of rock samples in the laboratory is typically limited in that every measurement accounts for the sample behavior as a whole with seldom acknowledgement of its internal degree of preexisting or induced heterogeneity. Preexisting heterogeneities such as mechanically weak layers in strongly laminated rocks may cause early failure. This effect, which causes a rock to behave in a strongly anisotropic fashion since the weaker surfaces need to be presented at an appropriate angle to the maximum stress to fail, is exemplified with the data of Figure 1 borrowed from Paterson and Wong . In order for such an effect to be predicted, weak layers need to be identified, which involves imaging since no bulk measurement such as porosity, density or composition is able to capture that information. On the other hand, induced heterogeneity such as localized shear bands formed during brittle failure is known to strongly impact fluid flow, and cannot be accurately predicted in terms of e.g. number, thickness and orientation. Nondestructive 3D imaging greatly complements laboratory measurements by providing access to the internal make-up of a rock at various stages of a test. Moreover, it does seem sensible to expect that a core plug will often exhibit some degree of heterogeneity, as plug dimensions are not determined based on an assessment of the representative elementary volume (REV) for the material of interest.
Goteti, R. (Aramco Services Company: Aramco Research Center - Houston) | Agar, S. M. (Aramco Services Company: Aramco Research Center - Houston) | Brown, J. P. (Red Sea Exploration Division) | Sibon, H. J. (Red Sea Exploration Division) | Zuhlke, R. (EXPEC Advanced Research Center)
ABSTRACT: Layered evaporite sequences have been documented from various rifted margins, including the South Atlantic and the Red Sea. The intervening sedimentary layers in such sequences can undergo large deformation and present drilling hazards associated with high pore fluid pressures and/or rubble zones. Although physical and numerical models provide insights to the deformation of such layers, the former are limited in terms of scalability of material failure parameters to natural examples, while the latter predominantly focus on massive salt and adjacent frictional-plastic sediments. In this paper we present a 2D evolutionary large strain finite element model of a salt diapir in an idealized layered evaporite sequence (LES). Gravitational loading and sedimentation provide the driving force for halokinesis. Salt is assigned a temperature-dependent non-Newtonian rheology, whereas the sediments are assigned a non-associative cap-plasticity model that supports both compaction and shear localization. The model results suggest that mechanical stratification plays an prominent role in the evolution of a LES. Stresses and strains in the sediment layers evolve in a complex manner and are predominantly controlled by their structural position. The presence of multiple salt layers in a LES decouples the deformation at different depths such that poly-harmonic folds can develop near the salt diapir. Structural dip and position, in addition to curvature, impact the deformation within the sedimentary layers. Geomechanical forward models also provide directional guidance on the likely variations in in-situ stresses and in well planning in LES settings.
A Layered Evaporite Sequence (LES) is a compositionally stratified heterolithic sequence comprising salt(s) and sediments. Some examples of salt basins containing layered evaporites are the Brazilian salt basins (Fiduk and Rowan, 2012), Levant Basin (Cartwright et al., 2012), North Sea (Strozyk et al., 2012), South Oman salt basin (Li et al., 2012) and Lower Permian Basin (Raith et al., 2016). The stratification in a LES could be due to spatial variations in salt composition (e.g., halite, anhydrite, tachyhydrite, carnallite) or inter-layering of salt with sedimentary inclusions or ‘stringers’. During geological loading, evaporites in a LES could undergo viscous deformation while the inter-layered sediments deform by frictional-plastic processes. The bulk mechanical response of a LES to geological loading can be significantly different, than when the salt is compositionally homogeneous, such as in the massive allochthonous halite structures in the Gulf of Mexico.
ABSTRACT: Investigations on fracturing (cracking) processes in shale, specifically crack initiation, -propagation, and -coalescence contribute to the understanding of shale behavior. This is important for stability and deformability problems encountered in civil and mining engineering. It is also very important in the context of hydraulic fracturing for hydrocarbon extraction.
Vaca Muerta shale is a petroleum bearing formation in the Neuquén Basin in Argentina for which the application of hydraulic fracturing is considered. To provide a basis for understanding the hydraulic fracturing mechanisms in this material, it is necessary to conduct experiments on specimens without hydraulic pressure first, and this is what is presented in this paper.
The paper provides some background on mineralogic and mechanical properties of the material. Specimen preparation will be discussed in detail given its importance and challenges posed by the Vaca Muerta shale in this regard, but also given its importance for experiments with other shales. The fracturing process will be investigated on prismatic specimens with two different flaw- (pre-existing cracks) geometries subjected to uniaxial compression. The use of high-speed and high-resolution video makes it possible to interpret the fracturing process in detail. The interpretation shows that flaw geometry has a strong effect on the resulting crack pattern and controls crack coalescence.
Investigations on fracturing (cracking) processes in shale, specifically crack initiation, -propagation, and -coalescence contribute to the understanding of shale behavior. This is important for stability and deformability problems encountered in civil and mining engineering. It is also very important in the context of hydraulic fracturing for hydrocarbon extraction.
Vaca Muerta shale is a petroleum bearing formation in the Neuquén Basin for which the application of hydraulic fracturing is considered. To provide a basis for understanding the hydraulic fracturing mechanisms in this material, it is necessary to conduct experiments on specimens without hydraulic pressure first, and this is what is presented in this paper.
ABSTRACT: Cutter, the fundamental cutting element of PDC (Polycrystalline Diamond Compact) bit, directly determines the overall performance of PDC bit and thus influences Rate of Penetration (ROP), drilling efficiency and drilling cost. Bit scrapped caused by cutter failure accounts for about 90% of all the failure in field operations. A majority of researches have been conducted on PDC cutter design, which were mainly concentrated on structure itself, such as stress state, self-sharpening and interface bonding strength, while neglecting the closely related factor of the rock-breaking method. This paper presents a novel PDC cutter which is designed based on the bionics theory to illustrate the layered spalling concept of rock-breaking. This paper has paid much attention to studying the numerical simulation of dynamic rock-breaking process under new bionic PDC cutter to illustrate the rock-breaking mechanism. Meanwhile, the corresponding experiments have been performed with a single cutter to validate the theory. The results indicate that: (a) the process of rock-breaking under the new bionic PDC cutter is characterized as “layered spalling” with slight oscillation comparing to the property-“Block Dropping” of conventional cutter. (b) the cutting process of bionic cutter is more stable with minor impact vibration, which contributes to improving the cutter’s life to some extent. The amplitude of cutting force reduces by 42% and the average value of the new bionic PDC cutter is 460N while the conventional PDC cutter is 790N.
PDC bit is the most important tool for oil drilling. A large number of field tests and laboratory studies (Kaitkay and Lei 2005, Gouda et al. 2011) have shown that the cutting teeth are easy to fail, such as cutter chipped and broken, diamond layer peel off and wear (Lin et al. 1992). So the quality of the cutter directly affects the performance and life of the drill bit (Yahiaoui et al. 2013). There were many studies focused on the comprehensive performance of PDC cutter (Martinez et al. 2013). Most of their researches prone to ameliorate the stress condition of PDC cutter, improve cutters’ self-sharpening performance and enhance the interface bonding strength, etc.(Tammineni et al. 2013, Azar et al. 2013, Raghav et al. 2013). Meanwhile, through many new design methods have been proposed and some good effects were obtained (Zhang et al. 2013), but the problem still exists. The designs merely pay attention to the design of the cutter itself, ignoring the impact of rock breaking modes on the cutters’ performance (Gerbaud 2011).
Yang, Xiao (China University of Petroleum-Beijing) | Zhang, Guangqing (China University of Petroleum-Beijing) | Du, Xianfei (Oil and Gas Technology Institute PetroChina Changqing Oilfield) | Liu, Zhibin (State Key Laboratory of Petroleum Resources and Engineering) | Dong, Haoran (State Key Laboratory of Petroleum Resources and Engineering) | Wang, Yuanyuan (State Key Laboratory of Petroleum Resources and Engineering) | Nie, Yuanxun (State Key Laboratory of Petroleum Resources and Engineering)
ABSTRACT: Fracture width and its variation are of significance to hydraulic fracture model validation. And it is also important to determine how the fracture extends as well as the properties of fracturing fluid and proppant in hydraulic fracturing. Hydraulic fracture width is 3-4 orders of magnitude less than the length and the height. Because fracture width is very tiny and the reservoir is at great depth, fracture width can only be determined through model estimation or numerical simulations. At present, even in the laboratory conditions, no direct measurement of fracture width is possible. This article is based on Fiber Bragg Grating (FBG) strain sensor and strain transfer theory. The hydraulic fracturing is controlled by great stress difference and prefabricated fracture direction in the true-triaxial rock test equipment. FBG strain sensors are distributed in the fracture propagation direction to monitor fracture behavior. So dynamic monitoring hydraulic fracture width in concrete materials can be realized in laboratory. The entire process of hydraulic fracture initiation and propagation is recorded by FBG strain sensors during hydraulic fracturing, with various stages of micro-fractures, macro-fractures and width changes. By multiple FBG strain sensors, displacements of fracture process zone before hydraulic fracture reaches FBG and fracture widths when the hydraulic fracture is crossing FBG are obtained. Then we found the speed and the shape of fracture in the process of hydraulic fracture propagation. Data show that fracture width does not increase monotonously with hydraulic fracture extension in the process of hydraulic fracturing. It fluctuates within a certain range along with the fracture extension.
In the 1950s, with the emergence of hydraulic fracturing technology, oil and gas development has entered a new stage (Hassebroek and Waters, 1964). Hydraulic fracturing as an unconventional oil and gas extraction technique, high pressure water-sand slurries are pumped into reservoirs to fracture rock with low permeability to extract oil and gas(Nolte, 2000). This technology has led to a significant increase in oil and gas production (T.g. Fan and G.q. Zhang, 2014). In this process, the hydraulic fracture width has a great effect on the fracturing effect. Monitoring the accurate value of the fracture width is of great significance to study the fracturing process and fracture models (Perkins and Kern, 1961).
ABSTRACT: Increasing formation pressure during fluid or gas injection might cause reactivation or instability of existing faults. Effective in-situ stresses evolve with changes in pore pressure which is called coupling effect. Understanding this effect is important during hydrocarbon field development to assess the fault reactivation risks during reservoir depletion and injection processes. Utilizing the poroelasticity theory and applying analytical solution of Biot’s diffusion equation is helpful when investigating the coupling effects on various states of in-situ stresses and formation pore pressures. This paper focuses on the horizontal stress and pore pressure coupling effects on a fault stability study during cushion gas injection in a gas storage field in the Netherlands. To do so, initial field stresses were estimated using field data. Thereafter the poroelasticity theory and diffusion equation were implemented to estimate the injection-induced stresses at different time-steps caused by pore pressure changes. In this paper, the sensitivity analysis was performed to explore the influences of injection time and fault distance on the induced instabilities. The results of this study showed relatively good agreements of injection-induced stresses obtained from the analytical method with corresponding values from complex finite element simulations. Moreover, the results indicated that the injection point could be located closer to the central fault.
Pore pressure and temperature variation in a reservoir and its vicinity occur during gas injection or withdrawal which could cause sudden slip on the pre-existing faults. Coupling between pore pressure and stress is one of the factors causing deformation and changes in the total stress field in the reservoir and its surroundings. Changing pore pressure directly influences the effective stresses as coupling effects mainly depend on the elastic and diffusion properties of the formation rocks as well as on geometrical characteristics such as positions and/or distances from wells. Rock properties can be simplified via elastic or constant permeability assumptions based on rock structure or compositions (Josh et al., 2012). However, coupling effects are still tedious to study due to the complexities existing in operational regimes and geometrical configurations. However, they are essential for geo-mechanical predictions related to caprock integrity, well bore stability, subsidence, fault reactivation and micro seismicity hazards (Ruistuen, Teufel, & Rhett, 1999). Terzaghi (1943) described the coupling effects via poroelastic theory for the first time; he noted that total stress is equal to effective stress plus pore pressure; and this theory has been the basic concept of rock engineering for many years.
Oldenburg, C. M. (Lawrence Berkeley National Laboratory) | Dobson, P. F. (Lawrence Berkeley National Laboratory) | Wu, Y. (Lawrence Berkeley National Laboratory) | Cook, P. J. (Lawrence Berkeley National Laboratory) | Kneafsey, T. J. (Lawrence Berkeley National Laboratory) | Nakagawa, S. (Lawrence Berkeley National Laboratory) | Ulrich, C. (Lawrence Berkeley National Laboratory) | Siler, D. L. (Lawrence Berkeley National Laboratory) | Guglielmi, Y. (Lawrence Berkeley National Laboratory) | Ajo-Franklin, J. (Lawrence Berkeley National Laboratory) | Rutqvist, J. (Lawrence Berkeley National Laboratory) | Daley, T. M. (Lawrence Berkeley National Laboratory) | Birkholzer, J. T. (Lawrence Berkeley National Laboratory) | Wang, H. F. (University of Wisconsin-Madison) | Lord, N. E. (University of Wisconsin-Madison) | Haimson, B. C. (University of Wisconsin-Madison) | Sone, H. (University of Wisconsin-Madison) | Vigilante, P. (University of Wisconsin-Madison) | Roggenthen, W. M. (South Dakota School of Mines and Technology) | Doe, T. W. (Golder Associates) | Lee, M. Y. (Sandia National Laboratories) | Ingraham, M. (Sandia National Laboratories) | Huang, H. (Idaho National Laboratory) | Mattson, E. D. (Idaho National Laboratory) | Zhou, J. (Idaho National Laboratory) | Johnson, T. J. (Pacific Northwest National Laboratory) | Zoback , M. D. (Stanford University) | Morris, J. P. (Lawrence Livermore National Laboratory) | White, J. A. (Lawrence Livermore National Laboratory) | Johnson, P. A. (Los Alamos National Laboratory) | Coblentz, D. D. (Los Alamos National Laboratory) | Heise, J. (Sanford Underground Research Facility)
ABSTRACT: In support of the U.S. DOE SubTER Crosscut initiative, a team comprising national laboratory and university researchers has established a field test facility in a deep mine. Initial activities were aimed at in situ hydraulic fracturing experiments to characterize the stress field, understanding the effects of crystalline rock fabric on fracturing, and gaining experience in stimulation monitoring using geophysical methods. The kISMET (permeability (k) and Induced Seismicity Management for Energy Technologies) project test site was established in the West Access Drift of the Sanford Underground Research Facility (SURF) nominally 4850 ft (1478 m) below ground in phyllite of the Precambrian Poorman Formation. The kISMET team drilled and cored five near-vertical boreholes in a line on 3 m (10 ft) spacing, deviating the two outermost boreholes slightly to create a five-spot pattern around the test borehole centered in the test volume 40 m below the drift invert (floor). We present an overview of activities and results in the areas of site selection, laboratory studies, pre-test modeling, and hydraulic fracturing and monitoring.
In order to address the challenges of subsurface energy-related processes involving fractures, fracturing, and permeability enhancement, earth scientists from several national laboratories and three universities have carried out a project to develop a new underground facility called kISMET (permeability (k) and Induced Seismicity Management for Energy Technologies) located on the 4850 level (4850L) nominally 4850 ft (1478 m) below ground (actually 4757 ft (1450 m) below ground) at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The purpose of the new facility is to provide a test site for research on stress characterization, permeability enhancement, and induced seismicity in crystalline rock. Rocks at SURF comprise a sequence of intensely folded Precambrian metamorphic schists, phyllites, and amphibolites cut by a number of Tertiary rhyolite dikes. The kISMET experiments meet objectives in the U.S. Department of Energy SubTER initiative under the “stress” pillar and in the “new subsurface signals” pillar through the monitoring of fracturing by active seismic, electrical resistivity tomography (ERT), and passive microseismic approaches accompanied by numerical modeling. The rocks at SURF are representative of deep crystalline rocks with low natural permeability targeted in some regions of the U.S. and around the world for enhanced geothermal system (EGS) development.
ABSTRACT: Fluid injection into an unconsolidated medium is modeled numerically using the DEM code PFC2D®. The injection process is modeled with the simplification that fluid leakoff is negligible. The fluid can penetrate in between grains and fluid front advances only if the gap size between two neighboring grains exceed a threshold value. Such a simplification can be considered as similar to taking into account of the effect of surface tension. Dependence of the fluid-grain displacement patterns on the threshold gap size is analyzed for the cases of constant rate injection. A bifurcation analysis for cavity expansion of a thick-walled cylinder is performed to predict the borehole pressure versus volume relationship. The critical pressure corresponding to the onset of bifurcation is compared with the peak injection pressure obtained from the numerical simulations.
Failure mechanisms due to fluid injection in a nearly unconsolidated formation are not yet well understood due to the highly nonlinear and coupled nature of the problem. It is generally assumed that onset of failure may occur in a tensile or a shear mode, depending on whether cohesion is present or not. In a cohesive medium, fluid pressurization in a borehole is assumed to lead to tensile crack initiation and propagation. On the other hand, for a cohesionless material, a constitutive model of Coulomb type predicts that the material must fail in shear and failure may be manifested in the formation of spiral-shaped shear bands.
Evidences of shear band development were observed in the cavity expansion experiments in dry sands in Alsiny et al. (1992), where borehole pressurization is realized by inflating a membrane. In these experiments, the membrane does not penetrate into the sands. Meanwhile, the experiments in Chang (2004), performed by injecting a highly viscous fluid into dry sands or silica flour, demonstrate that planar features resembling an opening mode crack can be created in a purely frictional granular material. A major difference between these two experiments is the pressurization mechanism. We therefore conjecture that the action of fluid penetration to cause grain displacements is a critical element in producing the opening mode crack- or finger-like features.
In this study, the effect of fluid penetration on the fluid-grain displacement patterns in a cohesionless medium is investigated using the Discrete Element Method (DEM) code PFC2D® (Itasca, 2015). A numerical scheme is devised to consider a particular case, where the injected fluid is highly viscous and can penetrate into open spaces in between grains if the gap size between two neighboring grains exceeds a threshold value. Such an implementation could be considered analogous to taking into account the effect of surface tension without leakoff; the larger the critical gap size, the higher the surface tension or the fluid viscosity.
Pramanik, R. (Massachusetts Institute of Technology) | Pan, K. (Massachusetts Institute of Technology) | Jones, B. D. (Massachusetts Institute of Technology) | Albaiz, A. (Massachusetts Institute of Technology) | Williams, J. R. (Massachusetts Institute of Technology) | Douillet-Grellier, T. (Total E&P) | Pourpak, H. (Total E&P)
ABSTRACT: We have developed a numerical simulation methodology where different materials can be analyzed in the same framework. At the interface between different phases, the necessary velocity and stress continuity conditions are maintained, allowing interaction between particles from different materials when solving the momentum equation. The SPH approximation to the continuity equation is corrected in order to handle the density discontinuity at the interface between different materials. For the different types of layered rock, each rock type is presently assumed to be homogeneous and isotropic, with perfect bonding at the interface. Failure of the rock or de-bonding of the layers occurs due to elasto-plastic damage model which includes the Drucker-Prager plasticity model and the Grady-Kipp damage model. We begin with a description of the theory governing this SPH framework. Following this, validation results are presented for the simulation of layered materials. Results using this SPH framework are compared to an analytical solution for geostatic stress, where good agreement is observed. Following this, the three point bending of a layered material as reported by Lee et al. (2015) is simulated. Simulation of the three-point bending experiment shows that inclined layers in the path of a propagating tensile fracture may have a significant influence in the ultimate fracture propagation direction.
Prediction of hydraulic fracture propagation in a layered reservoir with natural discontinuities remains a big challenge to unconventional reservoir developments. The interaction of a Hydraulic fracture and natural discontinuities can alter the fracture path and lead to a complex fracture network which has significance consequences on the design of fracturing treatments and productivity. It is believed that the complexity of the resulting fracture network primarily depends on in-situ stresses, mechanical properties of the host rock, natural discontinuities, including layering and interfaces, and fluid properties and injection rate. Experimental observations show that in heterogeneous and anisotropic rock, hydraulic fractures may be become arrested, divert into or cross at contacts (or other discontinuities) between layers.