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It is observed that wells in unconventional reservoirs can experience sharp rate decline in the early stage of production, especially with aggressive drawdown. One key factor affecting rate decline is rock sensitivity to increasing compressive stress. Based on comprehensive rock tests of a fractured tight reservoir, this paper describes and quantifies the stress-dependence of compaction and permeability for anisotropic rock matrix, natural fractures, and hydraulic fractures.
A series of core tests were designed to test low-porosity low-permeability rocks with abundant natural laminations. The effect of drawdown was simulated with increases of effective confining and loading stresses. The tests included rock compaction, anisotropic permeabilities of laminated rock, permeabilities of both tensile (i.e. hydraulic) and shear (i.e. bedding slippage) fractures, and acoustic anisotropy.
The laboratory tests reveal that
Despite rock strength in excess of 10,000 psi, the tight rock samples tested in the laboratory showed significant compaction with increasing confining stresses. Acoustic monitoring, rock mechanical behavior, and permeability measurements confirmed that pore collapse occurs as low as a few 1000s psi of effective confining stress. High friction angle, low modulus of rock components (e. g. kerogen, clay, or carbonate minerals), and the presence of weak grains and unstable grain-to-grain contacts, contribute to the low compaction strength.
High values of permeability anisotropy in the rock matrix correlate with abundant laminations. The anisotropy largely vanished as the confining stress increased.
In samples with proppant-free hydraulic fractures, the fracture permeability was reduced 70% with a moderate increase in effective confining stress.
In comparison, natural fractures or shear fractures stimulated by hydraulic fracturing are more resistant to closure stresses from pressure drawdown. The effective stimulated volume will be proportional to the presence of these fractures.
These findings quantify the effectiveness and longevity of the stimulated reservoir volume as an important variable in unconventional reservoir development.
Han, Songcai (School of Petroleum Engineering / China University of Petroleum (East China)) | Cheng, Yuanfang (School of Petroleum Engineering / China University of Petroleum (East China)) | Gao, Qi (School of Petroleum Engineering / China University of Petroleum (East China)) | Ding, Jiping (School of Petroleum Engineering / China University of Petroleum (East China)) | Li, Yang (School of Petroleum Engineering / China University of Petroleum (East China)) | Yan, Chuanliang (School of Petroleum Engineering / China University of Petroleum (East China)) | Zhang, Jincheng (Research Institute of Petroleum Engineering)
ABSTRACT: A fluid-driven fracture propagation model was presented to investigate the mechanical stability of natural fracture (NF) when hydraulic fracture (HF) approaching. The induced stresses generated by the approaching HF were semi-analytically calculated based on the complex variable method in theory of elasticity. The pore pressure effect caused by fracturing fluid leak-off was simulated by adopting the dual porosity medium theory. The critical failure conditions of NF were determined by the maximum tensile criterion and the Barton-Bandis criterion. The sensitivity of the stability of NF to approaching angle, approaching distance, in-situ stress anisotropy, injection rate, fracturing fluid viscosity, and surface roughness was analyzed in detail. Simulation results reveal that the shearing-mode failure of NF is obviously easier than the opening-mode failure, and shear slip zone is much larger than tensile failure zone. As HF approaches NF non-orthogonally, the induced debonding process is unstable, and the debonding zone is asymmetric with respect to the center of NF. The tensile-induced expansion zone is primarily located in the portion of NF ahead of the HF tip, however the shear-induced slip zone can even occur on NF behind of the HF tip. Induced stresses alone have a negligible effect on the stability of NF that is unconnected to HF. The instability of NF dominates the propagation trajectory of subsequent HF. A more suitable arrested/crossing condition by extending Gu-Weng criterion was established to predict the path of HF propagation.
Large-scale hydraulic fracturing has become the essential technology to efficiently extract shale gas resources from tight shale reservoirs. This is mainly because hydraulic fracturing can break up shale reservoirs to generate complex fracture networks that can improve the conductivity between the rock matrix and the wellbore. Given widely pre-existing natural fractures (NFs) in shale reservoirs, the morphology of hydraulic fracture (HF) propagation is evidently different from the single bi-wing planar fracture in conventional sand reservoirs. Fracturing simulation experiments under tri-axial stresses (Chen 2008; Zhou et al., 2008, 2010; Hou et al., 2014; Han et al., 2018) and mine micro-seismic monitoring events (Maxwell et al., 2002; Fisher et al.,2002, 2005) have verified that some non-planar, non-single and asymmetric fractures are usually generated in shale reservoirs during hydraulic fracturing. This mainly attributes to pre-existing NFs in shale reservoirs that can provide the weak zones for HF deviations from straight growth. When the propagating HF is arrested by activated NFs, a complex fracture network system consisting of HF and NFs will be formed. Therefore, investigating on the mechanical stability of NFs when HF approaching is especially important to predict the propagation trajectory of HF in shale reservoirs. On the other hand, when the undisturbed fault or bedding is activated by the approaching HF, this will be capable of inducing formation instability, surface subsidence, or even earthquakes. Recently, injection-induced earthquakes have become a focus of discussion as the application of hydraulic fracturing to tight shale formations (Wang et al., 2011; Ellsworth,2013; Hough, 2014, Catalli et al., 2016). At present, studies on the activation mechanisms of geological discontinuities induced by hydraulic fracturing are still insufficient.
ABSTRACT: During loss of control situations, fracture initiation occurring during the capping stages after uncontrolled fluid discharge, can lead to reservoir fluids broaching to the sea floor. A classic example is Union Oil’s 1969 oil spill in Santa Barbara channel, whose impacts on California’s oil industry are still strongly felt today. Reservoir fluids spewed in the channel at rates of 5,000 bbl/day in the first days. Fracture initiation at five different locations caused thousands of gallons per hour to broach to the ocean floor over a period of a month before it was controlled (Mullineaux, 1970; Easton, 1972). Disasters like this can be prevented if the effects of the uncontrolled discharge and capping stages of loss of well control are incorporated into the wellbore architecture. A critical capping pressure and subsequently a critical worst case discharge rate, below which fracture initiation would occur, during capping can be calculated analytically for a “hard” shut-in. For a “soft” shut-in, two numerical models are built; a geomechanics model simulating the stress loads on the wellbore in the two stages of loss of control and a reservoir model simulating capping pressure build-up. Propagation of this fracture up towards the seafloor can ultimately lead to broaching of reservoir fluids to the sea.
If a kick is not detected and circulated properly out of the well, it can lead to blowouts. In this case, loss of control leads to reservoir fluids gushing out of the wellbore uncontrollably without restriction. Well capping attempts generating pressure build-up in the wellbore, can lead to fractures initiating just below the casing shoe and propagating upwards creating channels for reservoir fluids to flow to the seafloor. The ability to predict these fracture failures will enhance wellbore integrity understanding during loss of control, predict and therefore prevent broaching occurrences (Zaki et al., 2015; Michael and Gupta, 2018; 2019a).
ABSTRACT: The Gulf of Mexico (GoM) is home to more than 50,000 oil and gas wells with approximately 30,000 wells that are plugged and abandoned leading to concerns of oil and gas leakage where currently, little to no monitoring is performed. The cement used when completing and eventually plugging wells is subjected to harsh conditions leading to failure of the cement due to debonding of the cement to the formation and/or casing, shrinkage of the cement, and chemical degradation in the cement. The goal of this study is to identify and rank the contributing factors of stress development that influence the potential of debonding along the cement interfaces for wells in the Eugene Island OPD in the GoM using staged poro-elastic Finite Element Models (FEM). The results show that the setting stress and the pore pressure in the cement that develop during hydration cause the most potential for debonding whereas the geographic in-situ stress magnitudes and cement mechanical properties have minimal effect on the stress development.
Drilling in the Gulf of Mexico (GoM) started in the early 1900's with primitive rigs connected to land by piers. Today there are over 50,000 oil and gas wells with approximately 30,000 that are plugged and abandoned (P&A) (data.bsee.gov 2018). Many of the P&A'ed wells have been that way for decades leading to concerns of oil and gas leakage. Current P&A practices in the GoM dictate that the well casing must be cut and buried beneath the sea floor (30 C.F.R. §250.1715). If a P&A'ed well is leaking, the leakage would have to travel from the cut casing to the sea floor where it would be diluted and swept away by currents and tides. This combined with the fact that monitoring of P&A'ed wells is not required by the Bureau of Ocean Energy Management (BOEM) and Bureau of Safety and Environmental Enforcement (BSEE) making it difficult to determine which wellbores are leaking. Thus alternative methods, such as numerical modeling, can be used to evaluate wellbore integrity. However, one major challenge with such alternative methods is creating realistic models with accurately determined input parameters.
ABSTRACT: We developed a coupled THMC model that can describe the long-term evolution in hydraulic and mechanical properties of the rock masses such as permeability and stiffness due to geochemical reactions within rock fractures induced by cavity excavation. Using the developed model, long-term prediction analysis by assuming the subsurface environments near the radioactive waste repository was conducted. Prediction results show that although many fractures are generated near the disposal cavity, which induces the permeability increase and the elastic modulus decrease in the cracked zone during the excavation, after the excavation, the permeability and the elastic modulus of the damaged zone decreased to that of the intact zone and increased to 30 % of the initial permeability, respectively. This evolution in rock permeability and stiffness after excavation was caused by pressure solution at contacting asperities within fractures. Therefore, it is concluded that pressure solution within the fractures has significant impact on the damage of rock masses in EDZ area by cavity excavation.
When evaluating the performance of high-level radioactive geological disposal system, it is essential to examine the evolution of the hydraulic and mechanical properties of rocks near the disposal facility. The surrounding bedrocks should be influenced by the phenomena such as the heat transfer from waste package, the transport of groundwater, cracking due to stress concentration during cavity excavation, and geochemical reactions of rock minerals and pore water. Therefore, these multi-physics phenomena should be evaluated comprehensively with coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) analysis in order to predict the change of the hydraulic and mechanical properties of rocks. It is well known that cracking in Excavation Damaged Zone (EDZ) exerts critical influence on the change of hydraulic and mechanical properties of the rock masses during cavity excavation. In addition, geochemical reactions such as pressure solution within rock fractures induced by cavity excavation should be considered to examine long-term evolution in permeability and stiffness of the rock masses in EDZ. For example, previous works [1,2] have confirmed reduction in the rock permeability of rock fractures by several orders of magnitude and increase of contact area in rock fractures due to fracture sealing with pressure solution at the contacting asperities within the fractures. Such increase of contact area in rock fractures should may result in the recovery of fractured rock stiffness and strength. To date, although many coupled models have been developed [3-8], the existing models cannot describe above-mentioned long-term hydro-mechanical evolution induced by geochemical reactions. Bond et al.  presented the coupled THMC simulators that examine permeability change within rock fractures due to geochemical reactions. The coupled THMC model developed by Ogata et al.  calculated the processes from the fracture generation to the evolution of rock permeability due to geochemical reactions within the generated fractures. These coupled models considered only permeability change due to geochemical reactions within rock fractures and evolution of fracture stiffness was not be taken into account.
Experimental uniaxial compression loading tests and scanning electron microscope (SEM) tests are carried out on rock-like specimens containing single pre-existing cracks to study the mechanical properties and microscopic damage evolution. The present study has distinguished tensile or shear cracks based on different SEM observations on a micro scale. Specifically, six typical micro patterns are defined according to their geometry shapes, namely, flocculent, flaw, circle, flow, layered, and broken circle pattern. These micro patterns display distinct characteristics on structure surfaces, boundary lines, and the distribution of grain debris. Moreover, the microscopic damage of both tensile and shear cracks is quantitatively studied using the image post-processing technique. The damage evolution, which associates the macroscopic cracking processes, has been investigated. It is indicated that the microcracks develop from the pre-existing cracks prior to the initiation of any macroscopic observable cracks, and the damage is not rapidly accumulated after the initiation of both tensile and shear cracks.
Natural rock contains discontinuities, including fractures, pores, and other defects, which govern the fracturing behaviors of the rock masses under loading. Numerous theoretical, experimental, and numerical studies have been carried out to study mechanical properties of jointed rocks or other rock-like materials (Griffith, 1921; Brace and Bombolakis, 1963; Horii and Nematnasser, 1985; Bobet and Einstein, 1998; Wong and Einstein, 2009a; 2009b; 2009c; Park and Bobet, 2010; Zhang and Wong, 2012; 2013; Gonçalves da Silva and Einstein, 2013; Haeri et al., 2014; Yang et al., 2017; Zhao et al., 2018). In these researches, tensile and shear cracks are always be regarded as two basic crack types and fundamental of the rock mechanic. (Cheng and Wong, 2018). Bombolakis (1963) firstly observed the propagation of tensile wing cracks from straight cracks under uniaxial compression, which consists well with the Griffith theory. Lajtai (1974) carried out uniaxial compression loading tests on plaster of Paris, the results consist of five crack types, including both tensile and shear cracks. Petit and Barquins (1988) observed that shear zones develop extensively in addition to the occurrence of tensile wing cracks. Using scanning electron microscope (SEM), Sagong and Bobet (2003) investigated tensile and shear cracks in gypsum specimens on a micro scale. Li et al. (2005) conducted experimental tests on marble specimens, and they discovered two cracking phenomena: wing cracks and secondary quasi-coplanar cracks. Although the mechanical properties of these two cracks were not clearly identified by the authors, it is accepted that wing cracks are tensile cracks and secondary cracks are shear cracks. Wong and Einstein (2009a) systematically characterized the tensile/shear cracks which emanate from a single pre-existing crack. Seven different crack types (including three tensile types, three shear types, and one mixed type) were identified based on geometry and propagation mechanism. Subsequently, they studied the orientation of microcracking zones of the wing cracks (Wong and Einstein, 2009c). As a summary, the previous studies focused on the differences between tensile cracks and shear cracks in three main aspects (Cheng and Wong, 2018): First, the tensile/compressive stress concentration phenomenon around the pre-crack tips; Second, the initiation direction and propagation trajectories of observable cracks; Third, the microscopic observation of the crack surfaces.
ABSTRACT: A highly altered rock mass can have adverse impacts on the stability of stopes and drives in underground mines. This work presents a case study investigating the rock characterization and engineering behavior of altered granites of Fengliushan Sn-Cu polymetallic deposit in Gejiu mineral district of China. The altered granites are formed by hydrothermal metasomatism of the host rock, typified by epidote and chloritization. Tests of mineral compositions, mechanical parameters, nuclear magnetic resonance, porosity and slaking characteristics were conducted on granite specimens on three different types of alteration. The results show that the amounts of montmorillonite and chlorite present in the altered granite are the main controlling factors forporosity, water absorption and dilatancy, and weaken the mechanical parameters of rocks. Under wet conditions, the granites which are intensely altered, slake into fragments rapidly nearly losing their entire bearing capacity. The porosity and slake-durability index of granites are also found to strongly correlate with point load strengths. Based on the experimental results, suggestions and recommendations are made concerning the construction and mining around altered granites in drives and stopes of altered granites. The results of this work have a significant bearing on the safe implementation of mining activities in highly geologically-altered rock masses.
The problems of granite erosion alteration in geotechnical construction are very common, and are widely distributed around the world (Shi, 1991; Shoji and Hidekazu, 2009; Giménez et al., 2013). The altered granites are formed by hydrothermal metasomatism of the late intruded granite. Due to geological processes such as magmatic infiltration and tectonic movement, this type of rock mass has the characteristics of fracture development, structural failure, poor stability and low intensity (Abad et al., 2015; Zhao et al., 2013). In particular, the altered rock mass rich in montmorillonite and other components will disintegrate due to water swelling of the montmorillonite (Brindley, 1980), resulting in dramatic changes in its physical and mechanical properties, and serious impacts on the stability of the engineering structures (Brekke and Selmer-Olsen, 1965; Chai et al., 2015; Lin, 2012). The 34 # orebody at Fengliushan in Gejiu, Yunnan province, China, is found in altered granites. After excavation of the drives, the granite slaked rapidly in a short period of time, resulting in varying degrees of deformation of the drives, as shown in Fig. 1.
ABSTRACT: Grain-scale heterogeneity is an inherent feature of rock, which is mainly caused by different minerals with various grain shapes, sizes, and strengths. Grain-scale strength heterogeneity is an important factor that should be considered in analyzing macroscopic mechanical behaviors of rock. This study investigates the effect of grain contact strength heterogeneity on brittle rock failure. Based on a two-scale tessellation technique in an open source code Neper, a 3DEC grain-based modeling (GBM) approach considering inter- and intra-grain contact failures is used to study the brittle rock failure process of rock under uniaxial compression. The modeling results indicate that the strength heterogeneity of inter- and intra-grain contacts affects the microcrack spatial location and hence the fracturing path and fracture pattern. Moreover, the tensile strength of grain contact controls the crack initiation stress, tensile stress distribution, and macro tensile strength. In addition, tensile cracks occur mostly along inter-grain boundaries and inter-grain contact failure dominates at the early deformation stage. Using the proposed 3DEC-GBM approach, inter- and intra-grain contact strengths can be considered and this approach can be a supplementary to traditional grain-based models for modeling brittle rock failure.
Material heterogeneity is an important factor controlling rock strength. For intact rock, material heterogeneity is closely related to rock structures. Grain-scale microstructures control micro-mechanical behaviors of the grain itself and hence control the overall macroscopic behavior of rock (i.e., Kazerani and Zhao, 2010; Lan et al., 2010; Farahmand and Diederichs, 2015; Gao et al., 2016; Liu et al., 2018). The mean grain size has a large influence on the yield stress of marble (Olsson, 1974), and grain shape induced heterogeneity can affect the macroscopic behavior due to local stress perturbations (Blair and Cook, 1998; Lan et al., 2013). Lan et al., 2010 summarized three main sources of grain-scale heterogeneity: (1) grain-geometry heterogeneity due to the variation of size and shape of microstructures, (2) grain-deformability heterogeneity due to the variation of mineral phases, and (3) grain-grain contact heterogeneity due to the variation of contact distribution and stiffness anisotropy. They stated that any successful modeling should incorporate these heterogeneities in a model. However, it is not easy to consider all grain heterogeneities in a numerical model. As mentioned by Bewick et al., 2012, the understanding on the influence of rock heterogeneity on brittle rock failure remains incomplete and additional research is needed.
Residual stresses are introduced into engineering components during welding and other manufacturing processes. Knowledge of these stresses can help to determine the structural integrity of the component and prevent stress corrosion cracking, therefore understanding the distribution and magnitude of residual stresses is very important. This paper presents the work carried out on a stainless steel pipe manufactured by cold rolling 17mm thick, 304 stainless steel plates to form three tubes attached with circumferential welds to create a full cylinder of roughly 3660mm axial length, 1710mm outer diameter and 17mm thickness.
In order to quantify the residual stresses in the cylinder the Contour, iDHD, ICHD and XRD measurements were carried out at different locations, including the circumferential weld, the seam weld and the repaired location. Additionally, ultrasonic residual stress measurements were carried out to detect weld repair locations and were particularly effective at identifying the hotspot locations.
The goal of the work described in this paper is to determine the stress states that exists at various locations within a stainless steel pipe by evaluating the properties of a full-diameter cylindrical mockup. This paper describes the design and procurement of the mockup and the characterization of the stress state associated with various portions of the container.
In order for SCC to be a viable degradation mode, three criteria must be met - there must be a sufficiently large tensile stress in the material to support crack growth, the material itself must be susceptible to SCC, and the environment must be sufficiently aggressive to support crack initiation and propagation. The work described in this paper is aimed at evaluating the first of these criteria for in-service pipe by characterizing the material properties of the base metal and weld zones on the canister mockup. Assessment of residual stresses associated with forming and welding was performed using a combination of five techniques. These include deep-hole drilling, the contour method, the centre-hole drilling x-ray diffraction, and ultrasonic testing. The deep-hole drilling technique allows measurement of residual stresses along a one-dimensional hole drilled through the wall of the cylinder; it allows the residual stresses within the container to be assessed while it is intact and hence, captures the effects of the cylindrical constraint on the stresses. The contour method provides a two-dimensional map of stresses along a cross section through a region of interest; however, the mockup must be cut into pieces to measure the face of the cross section, and stresses due to the constraint of the intact cylinder are lost. The centre-hole drilling method provides measurement at the surface while the X-ray diffraction allows assessment of very shallow near-surface stresses associated with shaping and grinding the mockup. It is also used to map stress components that are in-plane with the cross sectional surface, when using the contour method. Ultrasonic testing evaluates the change in sound velocity due to a change in local stress, and is able to evaluate stresses at a depth of 2.5mm within the stainless steel (as implemented here). The ultrasonic technique is non-destructive, and relatively new as a stress analysis method. It has been included here for comparison to the other techniques discussed above.
Conductivity after fracture closure results from uneven etching, because asperities created by the acid hold as pillars to keep a fracture open [...] (DANESHY et al, 1998) (SMITH and SHLYAPOBERSKY, 2000) (LIU, 2005) This surface mismatch generates acid conductivity. There is not a well defined trend of linear roughness after acid reaction on tensile fracture faces. Linear roughness can increase, stay equal or even decrease. As soon as the rocks had nipped off her tail feathers, and recoiled again, the Argonauts rowed through with all speed, aided by Athene and by Orpheus's lyre, and lost only their stern ornament. Thereafter, in accordance with a prophecy, the rocks remained rooted, one on either side of the straits […].