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Abstract Gas invasion of sediments is one mechanism by which methane hydrates are believed to form. As the capillary pressure exerted by an accumulated gas phase below the hydrate stability zone increases, it can exceed the entry pressure of the sediment within the hydrate stability zone, leading to a drainage displacement. Alternatively it can exceed the parting pressure of the sediment, leading to a gas-filled fracture propagating into the sediment. In unconsolidated ocean sediments, the capillary pressure may also be large enough to move grains apart during drainage. This motion alters the pore throat sizes which control subsequent drainage of the sediment. A model for the dynamics of this process is useful for assessing the competition between drainage (controlled by capillary forces) and fracturing (controlled by pore pressure and earth stresses). This in turn provides insight into the possible growth habits within the hydrate stability zone. To model this process we consider immiscible displacements when fluid/fluid interfaces are controlled by capillary forces. The progressive quasistatic (PQS) algorithm based on the level set method readily determines the pore level geometry of these interfaces. Capillary pressure generally exerts a net force on grains supporting an interface. We extend PQS algorithm to implement a kinematic model of grain displacement in response to that force. We examine the changes in the drainage curve caused by this coupling. We compute the interfacial area associated with the bulk water phase, anticipating preferential growth of methane hydrate there. When grains can move in response to net force exerted by the gas phase, small variations in an otherwise uniform distribution of pore throat sizes lead to self-reinforcing, focused channels of gas phase. In contrast to behavior in stationary grains, the drainage curve exhibits no clear percolation threshold. Displacements in materials with broad throat size distributions also exhibit self-reinforcing channels. Behind the leading edge of the displacement front, the net force exerted on the grains tends to push them together. This effectively seals off these regions from subsequent invasion. Thus hydrate growth tends to be localized along the channel of displaced grains. 1. Introduction The Earth's crust contains a very large amount of carbon held as methane hydrates in relatively shallow sediments (Collett et al. 1999). Unfortunately the uncertainty regarding the amount is also large, and this hinders the task of assessing the potential resource. One way to reduce the uncertainty is to understand better the growth habit of hydrates, e.g., whether they occur as cement at grain contacts, as material filling pores between sediment grains, as a solid comprising part of the load-bearing framework, as veins or fractures, etc. Knowledge of the growth habit at the grain scale would enable improved inversion of standard logging measurements for hydrate saturation SH, This knowledge would also inform methods and models of gas production from hydrate accumulations. The growth habit of methane hydrates depends on the mechanism by which methane is brought into the hydrate stability zone (HSZ). (The HSZ refers to a range of depths, either below permafrost or below the ocean floor, within which the pressure and temperature are such that methane hydrate is thermodynamically stable.) Paradoxically, gas is observed to co-exist with hydrates in some hydrate provinces. Moreover, the solubility of methane in brine is two orders of magnitude smaller than the concentration of methane in hydrate. These observations have led us to examine the mechanisms by which a gas phase could enter the HSZ (Behseresht et al. 2007 and 2008b). There are two limiting cases (Behseresht et al. 2008a): drainage, in which gas displaces brine from the sediment after building up enough pressure to exceed the capillary entry pressure, and fracture propagation, in which the gas phase pressure builds up enough to exceed the least confining stress. In the latter case, the gas-water meniscus is assumed to form an "elastic membrane" that exerts a net force on the grains supporting the meniscus, but the gas phase capillary pressure does not lead to drainage into the walls of the fracture.
- North America > United States > Texas (0.28)
- North America > Canada > British Columbia (0.28)
Evaluation of Discrete Element Method for Realistic Modeling of Reservoir Rock
Nabipour, A.. (Curtin University of Technology) | Sarmadivaleh, M.. (Curtin University of Technology) | Asadi, M. S. (Curtin University of Technology) | Sabogal Polania, J. M. (Curtin University of Technology) | Evans, B.. (Curtin University of Technology) | Rasouli, V.. (Curtin University of Technology)
Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Asia Pacific Oil & Gas Conference and Exhibition held in Brisbane, Queensland, Australia, 18-20 October 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract One of the most important concerns in perforation operation is to reduce the extent and severity of damage caused by perforating the reservoir formation in order to maximise production. In contrast to several experimental studies on modeling perforation, not many numerical attempts have been made for modeling this process. In this study a Discrete Element Method (DEM) based code was used for perforation modeling. After generating the rock sample using an assembly of bonded particles, the advantages and shortcomings of the method are discussed. By pointing out the differences between real perforation conditions and modeling assumptions, a number of practical solutions for improvement of the numerical model are explained.
- North America > United States (1.00)
- Asia (0.66)
- Oceania > Australia > Queensland > Brisbane (0.24)
Experimental Study of MSE of a Single PDC Cutter Interacting With Rock Under Simulated Pressurized Conditions
Rafatian, Navid (University of Tulsa) | Miska, Stefan (University of Tulsa) | Ledgerwood, L.W.. W. (Baker Hughes) | Ahmed, Ramadan (University of Tulsa) | Yu, Mengjiao (University of Tulsa) | Takach, Nicholas (University of Tulsa)
Summary The downhole pressure environment is one of the most important factors affecting the rate of penetration (ROP). It is believed that impermeable rocks experience high differential pressures because of shear dilatancy and become stronger and, thus, more difficult to drill. However, recently performed atmospheric and pressurized single-cutter experiments show that contrary to this belief, even at low pressures (100-200 psig) and even with permeable rocks, significant increase in mechanical specific energy (MSE) is observed compared to atmospheric tests. The experiments were carried out in a single-cutter high-pressure testing facility refurbished with high-precision sensors and a data acquisition system. In the experiments, a 13-mm polycrystalline-diamond-compact (PDC) cutter was used to cut Carthage marble and Indiana limestone samples with depths of cut ranging from 0.025 to 0.050 in. More than 70 high-precision tests were performed on these two rock types under confining pressures ranging from 0 to 1000 psig. The confining fluids were either water or mineral oil. Unexpectedly, analysis of the MSE consistently showed that increases in the confining pressure as small as 150 psig can increase the MSE of the cutting process significantly and reduce the cutting efficiency by half. These reductions in the cutting efficiency that were even more dramatic in the permeable and saturated Indiana limestone could not be explained by the strengthening of the rock under confining pressures. Upon analysis of the results of experiments (cutting forces, volume of cuts, and visual inspections of cuttings), a new theory was proposed to explain this unexpected behavior. This new theory, based on the frictional forces and the cutting mechanism under pressure, gives useful insights into the physics of cutter-rock interaction. Such insights are invaluable to the improvements of drilling practices selection [weight on bit (WOB), type of drilling fluid, and its properties] and the ROPs.
- Research Report > Experimental Study (0.82)
- Research Report > New Finding (0.64)
- Well Drilling > Drilling Operations (1.00)
- Well Drilling > Drill Bits > Bit design (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
Abstract We rigorously model immiscible displacements in unconsolidated sediments subject to confining stress. Fluid-fluid interfaces are assumed controlled by capillary forces, and the progressive quasi-static (PQS) algorithm based on the level set method determines the pore level geometry of those interfaces. From the pore-level fluid configuration we compute the net force exerted on each sediment grain by capillary pressure, including cohesion at grain contacts supporting pendular rings. We combine those forces with mechanical stress and elastic properties of grains to determine the resultant movement of grains using a discrete element method code (Itasca's PFC3D). To our knowledge this is the first rigorous coupling of capillarity and grain solid mechanics in 3D. When grains can move in response to net force exerted by the nonwetting phase, small variations in the distribution of pore throat sizes lead to self-reinforcing, focused channels of nonwetting phase during drainage. When forces exerted by capillary pressure are the same magnitude as the force required to displace grains, this channeling prevents the emergence of a recognizable fracture. 1. Introduction The migration of gas through water-saturated soft sediment is an important aspect of fluid dynamics under the seafloor in several parts of the world (Collett et al 1999). A model for this process would be useful for assessing the competition between drainage (controlled by capillary forces) and fracturing (controlled by pore pressure and earth stresses). This competition governs the evolution of natural gas seeps, the formation of methane hydrates and many schemes to produce gas from hydrates, and the viability of carbon sequestration in sub-seafloor sediments. When gas invades an unconsolidated, weakly confined, water-saturated sediment, the large-scale behavior ranges between two limiting cases. One is fracturing of the sediment with emplacement of gas limited to the volume of the fracture. The other is drainage of the pore space and broadly distributed emplacement of gas at large saturation. The large-scale behavior emerges from grain-scale competition between capillarity-controlled movement of the gas/water meniscus and pore-pressure controlled displacement of the grains. The competition is primarily influenced by the grain size: Capillary invasion happens in coarse grained sediments while fracturing is dominant in fine grained (Jain and Juanes 2009, Behseresht et al. 2008). The cited work provides a consistent, semi-quantitative picture of behavior, but relies on several restrictions or simplifications of the physics, such as 2D grain mechanics or a purely kinematic grain displacement model (see also Prodanovic and Bryant, 2008b). This paper builds on this work with the goal of verifying our previous insights. We have particular interest in the behavior at intermediate grain sizes, when the pressures required for drainage and for fracturing are comparable. Previous investigation with kinematic models indicates that if sediment grains are randomly arranged and movable, gas invasion forms channels (Prodanovic and Bryant, 2008b). If sediment grains are (locally) ordered and movable, gas invasion form fracture-like patterns oriented by the original ordering. If the grains in the sediment are randomly arranged and fixed, gas invasion forms a highly ramified structure corresponding to classical drainage.
- North America > Canada (0.68)
- North America > United States > Texas (0.46)
Uniaxial stress cycling experiments were conducted on dry, brine saturated and hexadecane saturated Berea sandstone samples to observe in detail the hysteresis in stressstrain diagrams and to understand the influence of different fluids on the strain amplitude dependence of elastic moduli and attenuation. Cycling experiments were also conducted with sandstone samples saturated with CTAB, a cationic surfactant that renders the mineral surfaces hydrophobic. Hexadecane and CTAB were selected so as to investigate the relative contributions of adhesion hysteresis and stickslip sliding on attenuation in sedimentary granular rocks. Youngs moduli and Poissons ratios obtained from the cycling tests show a significant dependence on strain amplitude on dry as well as water and hexadecane saturated samples. Bowtieshaped diagrams are obtained when loading and unloading tangent moduli are plotted against strain. The type of fluid in the pore space and at the grain contacts has a large influence on the hysteresis observed in the stressstrain diagrams.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.49)
- Well Drilling (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.69)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.64)