Effect of water saturation and loading rate on the mechanical properties of Red and Buff Sandstones (original) (raw)
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Fracture initiation pressures in permeable poorly consolidated sands
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1982
This paper studies the stress distribution around an il~jection well drilled through a layer of poorly consolidated sand. As a plastically strained zone may form around a hole drilled through this type of rock, both elastic and plastic theories must be applied. Solutions have been worked out in order to Jollow the development of stresses as the injection pressure increases fi'om zero up to the point where a fracture is initiated. How these stresses develop is dependent on the rock and ,fluid properties, mainly Poisson's ratio and the ratio between the horizontal and vertical stress. Several different stress distributions will appear as the injection pressure is increased. Each stress distribution is composed of one or more different stress solutions linked together. This is solved by numerical iteration. The injection pressure is increased until a .fracture is induced. Fracture initiation pressures for several different parameter sets have been examined. The results have then been contpared with fi'acture initiation pressures calculated by classical elastic theory. This comparison shows that the fi'acture initiation pressures can be lower than results calculated by elastic theory. Typically, with a Poisson's ratio of 0.20 and a horizontal to vertical effective stress ratio of 0.40, the d!fference is in the order of ntagnitude of I0%. NOMENCLATURE Ai, Bi, Dl, O2 = C O = Cma C b = D,, Db = E= f G= h= K= k= p= Pi = Pc = Po : Pf. el = ef, pl : O= r= Ri= Ro: RI, R2, R3 = R~ Sco Integration constants. 2S¢otan~ = uniaxial compressive strength. Rock matrix compressibility. Rock bulk compressibility. Constants defined in Appendix B. Young's modulus. Plastic flow function. Shear modulus. Height of permeable layer. Horizontal to vertical effective stress ratio. Permeability, Fluid pressure. Fluid pressure at R~. Fluid pressure at plastic-elastic boundary. Fluid pressure at Ro. Fracture initiation pressure calculated by elastic theory. Fracture initiation pressure calculated by plastic theory. Fluid flowrate (negative when injecting). Radical distance from center of wellbore. Wellbore radius. Outer boundary radius. Radii of different plastic zones. = Radius of entire plastic zone. = Cohesive strength. *Continental Shelf Institute, Hhkon Magnussons gt. IB, 7001 Trondhe]m, Norway. 5" Norsk Agip, Sandnes. Norway. 255 t, V = Constants defined in Appendix B. u = Radial displacement. z = Depth. = Failure angle.
Stress-Dependent Perforation in Carbonate Rocks: An Experimental Study
SPE drilling & completion, 2018
Perforation with shaped charges as a conventional well-completion technique is widely used in the oil industry. Different phenomena influence perforation performance and depth of penetration (DOP). The authors examined the effect of in-situ stresses and shot density on DOP and created fracture patterns in concrete and limestone samples with surface and polyaxial/triaxial-stress-loading conditions. To achieve this aim, we designed and developed a polyaxial-perforation test machine. We optimized the number of experimental tests using the Taguchi-design test method. The Taguchi orthogonal scheme is well-known and is a highly recommended method to optimize the number of required experiments (Taguchi 1990; Ross 1996; Jeyapaul et al. 2005; Gupta et al. 2014). Our experimental setup resembles vertical wells in the strike/slip-faulting regions and horizontal wells in the reverse-faulting regions. The results show that DOP is more controlled by stresses normal to the shooting direction in polyaxial tests than by the stress in the direction of penetration. DOP and the maximum hole diameter from the second charge had a direct relation with shot density. The DOP observed in polyaxial-loading conditions was a little lower than in the triaxial-loading mode, where the mean value of stresses normal to the shooting direction in the polyaxial tests was the same as the horizontal stresses in the triaxial tests. In both surface and triaxial-loading conditions, the patterns of perforation fractures were radial and regular, whereas the cracks created were oriented along the direction of maximum horizontal stress in the polyaxial tests. Polyaxial-Compressive-Test Machine The authors designed and constructed the blocky-scale polyaxial-compressive-perforation apparatus and conducted perforation tests on different rock types with shaped charges and spacing. Fig. 1 shows a schematic of the polyaxial-perforation machine. The main
Evolution of permeability in a natural fracture: Significant role of pressure solution
Journal of Geophysical Research, 2004
1] A mechanistic model is presented to describe closure of a fracture mediated by pressure solution; closure controls permeability reduction and incorporates the serial processes of dissolution at contacting asperities, interfacial diffusion, and precipitation at the free face of fractures. These processes progress over a representative contacting asperity and define compaction at the macroscopic level, together with evolving changes in solute concentration for arbitrarily open or closed systems for prescribed ranges of driving effective stresses, equilibrium fluid and rock temperatures, and fluid flow rates. Measured fracture surface profiles are applied to define simple relations between fracture wall contact area ratio and fracture aperture that represents the irreversible alteration of the fracture surface geometry as compaction proceeds. Comparisons with experimental measurements of aperture reduction conducted on a natural fracture in novaculite show good agreement if the unknown magnitude of microscopic asperity contact area is increased over the nominal fracture contact area. Predictions of silica concentration slightly underestimate the experimental results even for elevated microscopic contact areas and may result from the unaccounted contribution of free face dissolution. For the modest temperatures (20-150°C) and short duration (900 hours) of the test, pressure solution is demonstrated to be the dominant mechanism contributing to both compaction and permeability reduction, despite net dissolution and removal of mineral mass. Pressure solution results in an 80% reduction in fracture aperture from 12 mm, in contrast to a $10 nm contribution by precipitation, even for the case of a closed system. For the considered dissolution-dominated system, fracture closure rates are shown to scale roughly linearly with stress increase and exponentially with temperature increase, taking between days and decades for closure to reach completion.
INTRODUCTION Fractured rock hydraulics
Fractured rock hydraulics, 2010
Acid igneous rocks, like granites, basalts and gabbros, as well as metamorphic rocks, like gneisses, schist and slates, form more than 95 % of the Earth's crust and make up the rigid basements of the continents, attaining more than 70 km depth at the root of high mountain chains. Basic igneous rocks, like petrified seafloor basalt extrusions, pile up to 7 km thickness under the oceans. However, igneous and metamorphic rocks emerge as outcrops only over 15 % of the Earth surface and are frequently covered by layers of sedimentary rocks, like sandstones, siltstones and limestone, up to hundreds meters thick, and also by less resistant thinner veneers, as weathered rocks and residual soils or unconsolidated sediments like gravels, sand and clays. Yet, rock masses near the Earth surface, up to tens or hundreds meters depth, are not massive. Indeed, distinct groups of almost planar discontinuities split all igneous and metamorphic rock masses into contiguous and practically impervious blocks of quite resistant intact rock. From a strict mechanical point of view, even if genetically dissimilar, these rock masses, as well as some types of crystalline limestone and dolomites, can be collectively classified as "hard-rocks" or, simply, as "fractured rocks". By custom, the generic term fracture stands for all kinds of restricted partitions or discontinuities within rock masses. Irrelevant and small amounts of groundwater, almost clogged, accumulate inside the minute voids and micro cracks of intact rock blocks and hardly drop under the action of gravity. That kind of water, remain practically unavailable for exploitation. On the other hand, significant quantities of groundwater may saturate the fractures of rock masses and also other sort of open discontinuities. That kind of water, called free water, may drain and percolate under its self-weight. Only this type of "gravity-driven" groundwater is the focus of the present book. In the last 50 years, many technical publications had dealt with the theoretical principles and practical field and laboratory tests related to the appraisal of the groundwater flow through sedimentary rocks and unconsolidated sediments. In the same period, as "hard-rocks" are not good aquifers by themselves, few publications had approached their hydraulics. In fact, groundwater storage volume is proportional to the porosity of the rock reservoir, including interconnected interstitial voids, fractures and dissolution discontinuities. For hard-rocks, porosities seldom attain 2 % of their total volume while may range from 5 to 15 % for consolidated clastic rocks and up to 25 % for unconsolidated sands and gravels. Yet, during the last decades, the growing need of groundwater supply in "hard-rock" land, mainly in poor countries, coupled with a better valuation of the seasonal groundwater recharge and corresponding transient storage at the top of deep weathered "hard rock" land in wet inter-tropical climate, had altered that point of view. Moreover, during the same time, new investigation methods and new technologies based on sounder theoretical concepts were devised to improve "hard-rock" hydraulics models, commonly motivated by attempts to enhance the structural and environmental safety factors of increasingly larger engineering
A critical stress analyses indicate that plastic yield, fracture initiation near or between packers or from a perforated hole are all possible during hydraulic fracturing. Further a fracture can be created from one of perforation holes where a corresponding fracturing pressure can be determined based on the orientation of the perforated hole and a deviated wellbore subject to three in-situ stresses regime. While a plastic yielding process on a wellbore or perforation hole surfaces may prevent a fracture from initiating, a fracture or fractures may initiate from a near packer region or at an oriented perforation when different completion strategies utilized.
Journal of Petroleum Science and Engineering, 2000
Considering the influence of casing, analytical solutions for stress distribution around a cased wellbore are derived, based on which a prediction model for hydraulic fracture initiation with the oriented perforation technique (OPT) is established. Taking well J2 of Z5 oilfield for an example, the predicted initiation pressure with the OPT of our model is about 4.2 MPa higher than the existing model, which neglects the influence of casing. In comparison with the results of laboratory fracturing experiments with OPT on a 400 9 400 9 400 mm 3 rock sample for a cased well with the deviation of 45°, the fracture initiation pressure of our model has an error of 3.2 %, while the error of the existing model is 6.6 %; when the well azimuth angle is 0°a nd the perforation angle is 45°, the prediction error of the fracture initiation pressure of the existing model and our model are 3.4 and 7.7 %, respectively. The study verifies that our model is more applicable for hydraulic fracturing prediction of wells with OPT completion; while the existing model is more suitable for hydraulic fracturing with conventional perforation completion.
Oil & Gas Science and Technology, 2001
-L'effet de la perméabilité initiale sur l'importance de la réduction d'injectivité due à l'injection d'eau salée au travers de formations fracturées-La réduction d'injectivité au cours du temps est un facteur important dans la définition des projets d'injection d'eau. Un des facteurs principaux affectant l'injectivité, en raison de l'invasion de particules, est la perméabilité initiale du milieu. Ce facteur a été étudié expérimentalement, et évalué par de nombreux chercheurs, mais tous ces travaux expérimentaux reposaient sur des essais d'écoulement linéaire. Cependant, dans certains projets, la réduction d'injectivité au cours du temps est bien moindre que celle prévue par les modèles expérimentaux. Cette incompatibilité est liée à l'apparition de fractures causées par l'injection à une pression supérieure à la pression de fracturation de la formation. Cette étude a été réalisée pour évaluer expérimentalement l'effet de la perméabilité initiale de la matrice sur la réduction de l'injectivité causée par l'injection d'eau salée dans une matrice rocheuse avec une seule fracture. L'eau salée injectée contient soit des particules solides jusqu'à 6 µm de diamètre, soit des particules solides jusqu'à 20 µm, à une concentration de 9 mg/l. Les premiers résultats montrent expérimentalement une différence considérable de l'étendue de la réduction d'injectivité selon que les essais d'injection sont réalisés à fracture ouverte ou à fracture fermée. Dans la suite, on présente l'indice d'injectivité à volume de pore injecté donné en fonction de la perméabilité initiale. La pente de cette courbe est appelée taux de réduction. Ce taux a été évalué pour différents cas. Dans le cas des suspensions salées contenant les petites particules, le taux de réduction de l'indice d'injectivité dû à la variation de la perméabilité initiale a été nettement moindre dans les essais à fracture ouverte que celui obtenu dans les essais à fracture fermée. Dans le cas de l'injection de suspensions contenant les grosses particules, le taux de réduction a été trois fois plus grand que celui obtenu avec les suspensions de petites particules. Cela prouve que la taille de particule est un paramètre qui joue un rôle important dans la détermination de l'étendue de l'effet de la perméabilité initiale de la roche sur l'indice d'injectivité lors d'injection dans des fractures. Mots-clés : injection d'eau, dommage de formation, perméabilité modifiée, fracture induite.
Strategies for Effective Stimulation of Multiple Perforation Clusters in Horizontal Wells
SPE production & operations, 2017
Increasing the efficiency of completions in horizontal wells is an important concern in the oil and gas industry. To decrease the number of fracturing stages per well, it is common practice to use multiple clusters per stage. This is done with the hope that most of the clusters in the stage will be effectively stimulated. Diagnostic evidence, however, suggests that in many cases, only one or two out of four or five clusters in a stage are effectively stimulated. In this paper, strategies to maximize the number of effectively stimulated perforation clusters are discussed. A fully 3D poroelastic model that simulates the propagation of nonplanar fractures in heterogeneous media is developed and used to model the propagation of multiple competing fractures. A parametric study is first conducted to demonstrate how important fracture-design variables, such as limited-entry perforations and cluster spacing, and formation parameters, such as permeability and lateral and vertical heterogeneity, affect the growth of competing fractures. The effect of stress shadowing caused by both mechanical and poroelastic effects is accounted for. 3D numerical simulations have been performed to show the effect of some operational and reservoir parameters on simultaneouscompetitive-fracture propagation. It was found that an increase in stage spacing decreases the stress interference between propagating fractures and increases the number of propagating fractures in a stage. It was also found that an increase in reservoir permeability can decrease the stress interference between propagating fractures because of poroelastic-stress changes. A modest (approximately 25%) variability in reservoir mechanical properties along the wellbore is shown to be enough to alter the number of fractures created in a hydraulic-fracturing stage and mask the effects of stress shadowing. Interstage fracture simulations show post-shut-in fracture extension induced by stress interference from adjacent propagating fractures. The effect of poroelasticity is highlighted for infill-well-fracture design, and preferential fracture propagation toward depleted regions is clearly observed in multiwell-pad-fracture simulations. The results in this paper attempt to provide practitioners with a better understanding of multicluster-fracturing dynamics. On the basis of these findings, recommendations are made on how best to design fracture treatments that will lead to the successful placement of fluid and proppant in a single fracture, and result in a set of fractures that are competing for growth. The ability to successfully stimulate all perforation clusters is shown to be a function of key fracture-design parameters. Prior experimental work has also clearly shown that the perforation-cluster spacing influences the fracture-growth pattern. When closely spaced multiple fractures were propagated simultaneously, some fractures were much larger than others (El-Rabaa 1982; Abass et al. 1996). It was shown that in some cases, one fracture could become the dominant fracture propagating among the clusters. Bunger et al. (2012) used an analytical model and performed a dimensional analysis to understand the most-important parameters that need to be addressed when optimizing multiple-fracture-growth problems. They considered the deflection patterns that are generated because of interaction of the fractures with existing fractures. They applied their model to a 2D fracture-growth simulator. In a later paper, the Bunger et al. (2012) model was used to understand the effect of viscosity and toughness-dominated regimes on multiple-fracture propagation (Ames and Bunger 2015). The latter used a mathematical model to couple the contributions of fluid flow, rock breakage, and perforation pressure drop to the total power requirement for the growth of multiple hydraulic fractures. Their model predicts that when the stage spacing is less than the created fracture height, the probability of multiple-fracture growth is small. The fundamental understanding their model provides can be very useful in explaining observations from numerical models. Many researchers have used the displacement-discontinuity method to model the stress interference created by hydraulic fractures. Using this method, researchers have attempted to analyze the effect of simultaneous-multiple-fracture growth (