A three-dimensional numerical model was developed to simulate the stability of wellbores and perforations in poorly consolidated sandstone formations. The model integrates the post-yield plastic behavior of granular materials in order to investigate the mechanical instabilities associated with wellbores completed in such formations. Fluid flow and poroelastic stresses are computationally coupled with mechanical calculations to generate pore pressure and stress distribution in the sand. The sand erosion model developed by Kim (2010) is adopted to predict the rate of sand production based on the proposed erosion criterion. It has been widely reported in the literature that sanding can be greatly influenced by in-situ stress anisotropy, completion geometry, wellbore placement, and perforation orientation. Through advanced modeling and meshing techniques, the model developed in this thesis is capable of simulating complex completion configurations and operational conditions for the purpose of researching the impact of these factors on the wellbore and perforation stability. Accordingly, the model can be utilized to design a completion that minimizes sand production and optimizes the mechanical stability of the wellbore for a specific in-situ state of stress. Results obtained from the model show that vertical wellbores produce less sand compared to horizontal wellbores in the case where the overburden stress is the maximum in-situ stress. In addition, orienting the perforation in the direction with the least plastic strain development results in a more stable perforation tunnel with less sand production. Therefore, in a horizontal wellbore, vertically oriented perforations are more stable than horizontally oriented perforations and can withstand higher drawdown pressure before sand is produced. The model was extended to simulate the impact of mechanical and hydraulic interference from adjacent perforations on the evolution of plastic strain. Results from simulation runs show that the perforation spacing has an influence on both the magnitude and the spatial spread of the plastic strain. The model combines the effect of the wellbore diameter, shot density, and the phasing angle to determine the completion configuration with the least sanding risk. / text

[pt] Freqüentemente, durante os estágios de produção de um poço de óleo, ocorre a produção simultânea de partículas sólidas que originam da massa rochosa ao redor, um processo chamado de produção de areia. Concentrações de tensões nas vizinhanças do poço podem causar perda de coesão entre os grãos, criando assim uma região de matéria granular solta suscetível a ser arrastada pela força de infiltrações. Este trabalho se baseia no estudo da formação de danos nas vizinhanças do poço. Este programa experimental é conduzido ao simular, do interior de uma célula especialmente construída, as pressões crescentes durante esses estágios de escavação numa amostra de rocha, geralmente arenito, pouco consolidada ou um material montado artificialmente simulando arenito. Este processo é conduzido em tempo real dentro de tomógrafo computadorizado de Raio X. Os ensaios tiveram por objetivo identificar a pressão de início e o modo de propagação da ruptura da parede da cavidade interna da amostra ensaiada. Estes são estágios iniciais dos processos de produção de areia em rochas. As análises das imagens tomográficas adquiridas durante os ensaios permitiram a visualização de breakouts e arrombamentos dos poços. Estudos mais detalhados possibilitaram estimar a quantidade de areia produzida e reconstruir tridimensionalmente o processo de propagação de ruptura. / [en] Frequently, during the production stages of an oil well, simultaneous production of solid particles originating from the surrounding rock mass occur, a process called sand production. Stress concentrations around of the well may cause loss of cohesion between grains, thus creating a region of loose granular matter susceptible for drag bye seepage forces. The present work reports on the study of the formation of breakouts/damage around of the well. This experimental program is carried out by simulating, inside an especially constructed cell, the stresses arising during these excavation stages on a sample of a rock, generally a poorly consolidated sandstone or an artificially assembled material simulating a sandstone. This process is carried out in real time inside an X ray computerized tomography. The tests investigated the initial and the evolution of failure at the cavity wall of samples. These are initial stages of the sand production process. The analysis of the CT-scans obtained during tests allowed the visualization of breakouts and collapses of the wells. From studies more details were possible estimate the sand production and produce 3-D images of the propagation of the failure.

[pt] A produção de areia é um dos mais freqüentes e graves problemas observados durante a produção de um poço de petróleo, completado em arenitos mal consolidados. Este fenômeno pode causar obstrução do poço, abrasão dos equipamentos de elevação e de superfície, colapso do revestimento e problemas ambientais derivados da disposição em superfície das areias contaminadas. Por outro lado, em reservatórios de baixa produtividade que produzem óleo de alta viscosidade, uma limitada produção de areia incrementa a produtividade do poço. Nos estudos para previsão da produção de areia é de fundamental importância definir-se um modelo constitutivo capaz de reproduzir o comportamento tensão-deformação do arenito e os mecanismos de ruptura que provocam instabilidade do poço. Este é o tema central desta tese. Em especial, o trabalho procurou explorar modelos constitutivos baseados em meios contínuos de Cosserat. Uma justificativa para isto está relacionada ao fato de experimentos demonstrarem a influência da microestrutura nos processos de ruptura que ocorrem na vizinhança do poço. O modelo utilizado foi o elastoplástico de Bogdanova-Bontcheva & Lippmann (1975) incorporando as leis de fluxo associada e não associada. Inicialmente são definidos alguns conceitos básicos relevantes ao entendimento do fenômeno de produção de sólidos. Uma revisão da teoria dos meios contínuos generalizados de Cosserat é apresentada e em seguida é discutido o modelo elastoplástico de Bogdanova- Bontcheva e Lippmann com detalhes das implementações computacionais necessárias. Finalmente, são feitas análises de geometrias de poços, procurando-se identificar os mecanismos de ruptura que provocam instabilidade e que são uma potencial fonte de produção de areia. / [en] Sand production is one of the most frequent and serious problems observed during the production of an oil well completed in weak sandstones. This phenomenon can cause wellbore plugging, surface and rise equipment abrasion, casing collapse and environmental problems derived from the disposal in surface of contaminated sands. On the other hand, for reservoirs of low productivity, that produce oil of high viscosity, a limited sand production increase the productivity of the well. In the studies for sand production prediction, it is of basic importance to define a consitutive model capable of reproducing the stress-strain behaviour of the sandstones and the failure mechanisms that causes wellbore instability. This is the central focus of the present work. In order to represent the behaviour of the rock masses, models based in Cosserat continuum were used. Elastoplastic models (associated/not associated) under that theory (Bogdanova-Bontcheva and Lippmann) were implemented. Initially, basic concepts related to the understanding of the phenomenon of solid production are presented. A review of the theory of generalized Cosserat continuum is presented, the elastoplastic model of Bogdanova-Bontcheva and Lippmann is described, together with details of the computational implementations. Finally, analyses of well geometries with the implemented Cosserat based elastoplastic models are shown,identifying the failure mechanisms.

[pt] Durante a fase produtiva de um poço de petróleo ou gás, muitas vezes há a produção simultânea de partículas sólidas arrancadas da matriz da rocha reservatório. Este fenômeno recebe o nome de produção de areia. Neste caso as tensões e as condições de fluxo nas vizinhanças do poço são fatores fundamentais para a deflagração do processo. As tensões que se concentram na parede do poço com a perfuração do mesmo, pode ser de tal magnitude que pode causar a perda da coesão entre os grãos e criando , assim , uma região de material granular susceptível ao arraste pelas forças de percolação derivadas do fluxo. Este trabalho visou a realização de ensaios em amostras de arenito Rio Bonito e arenito sintético utilizando a técnica da Tomografia Computadorizada de Raios-X para acompanhamento em tempo real dos ensaios. Os ensaios tiveram por objetivo identificar a pressão de início e o modo de propagação da ruptura da parede da cavidade interna da amostra ensaiada. Estes são estágios iniciais dos processos de produção de areia em rochas. As análises das imagens tomográficas adquiridas durante os ensaios permitiram a visualização de breakouts e arrombamentos dos poços. Estudos mais detalhados possibilitaram estimar a quantidade de areia produzida e reconstruir tridimensionalmente o processo de propagação da ruptura. / [en] During productive phase of the well, manytimes there is simultaneous production of the solid particles detached from matrix of the reservoir rock. This phenomenon receive the name of the sand production. In this case stress and flow conditions around of the well are fundamental factors for deflagration of the process. Stress concentration in the wall of the well lead to the loss of cohesion between grains arising, consequently, a granular material region susceptible for dragging by seepage forces derived from fluid flow. The objective of this work was to perform sand production tests in Rio Bonito and synthetic sandstone samples using real-time X-Ray Computerized Tomography. The tests investigated the initial and the evolution of failure at the cavity wall of samples. These are initial stages of the sand production process. The analysis of the CT-scans obtained during tests allowed the visualization of breakouts and collapses of the wells. From studies more details were possible estimate the sand production and produce 3-D images of the propagation of the failure.

The possibility of commercial natural gas production from gas hydrates has been tested by researchers and industry for more than ten years. Depressurisation of gas hydrates in porous and permeable sandstones has successfully produced water and natural gas. However long term sustainable production is still elusive. Catastrophic sand production into the wellbore has terminated at least three of the significant depressurisation trials including the 2013 trial at the Daini-Atsumi knoll, Nankai Trough, offshore Japan. Sand production is generally thought to be the result of mechanical and hydrodynamic instability, however it appears the failure mechanism is not the same for all reservoirs and the location of reservoir porosity and pressure on the normal compression line for sands could be a controlling factor. Sand production in reservoirs at shallow depths and low confining stresses (less than 10 MPa) are likely to be influenced by fluid flow effects like those described by the Shields (1936) diagram. The relative density of the formation may also affect the nature of the sand production in these reservoirs. The Daini-Atsumi knoll is a structural high on the outer ridge of the Kumano forearc basin, offshore Japan. Hydrate saturations of 50 to 80 % occur within three geological units of the Middle Pleistocene Ogasa group. This group is made up of deep water sediments including sediment gravity flow deposits distinguished by alternating silt and sand layers. The presence of these alternating layers could have influenced the sand production seen during the trial. This reservoir heterogeneity at the 2013 Daini-Atsumi knoll gas hydrate production trial site was characterised using the descriptions of geological units, analogues and statistical techniques. Scenarios of this heterogeneity were tested in a high pressure plane-strain sand production apparatus. The results of these tests suggest the boundary shear stress of the fluid on the grains is a significant control on sand production for the Daini-Atsumi Knoll reservoir and the layering and grainsize structure of the sediments encourages sand production. Relative density of the sediments appears to impact the nature of the sand production where denser sediments show more localised movement. These results indicate that even minor weaknesses in sand control devices will result in uncontrollable sand production rates from the Daini-Atsumi Knoll gas hydrate reservoir. Managing the fluid flow rate in the reservoir and selectively completing coarser grained zones at the base of sand layers could help limit sand production in future trials.

This thesis presents a model for the process of sand production that allows us to predict the stability of wellbores and perforation tunnels as well as mass of sand produced. Past analytical, numerical, and empirical models on material failure and erosion mechanisms were analyzed. The sand production model incorporates shear and tensile failure mechanisms. A criterion for sand erosion in failed sand was proposed based on a force balance calculation on the sand face. It is shown that failure, post failure sand mechanics and flow-dominated erosion mechanisms are important in the sand production process. The model has a small number of required input parameters that can be directly measured in the lab and does not require the use of empirical correlations for determining sand erosion. The model was implemented in a numerical simulator. Three different experiments using different materials were simulated and the results were compared to test the model. The model-generated results successfully matched the sand production profiles in experiments. When the post-failure behavior of materials was well-known, the match between the simulation and experiment was excellent. Sensitivity studies on the effect of mechanical stresses, flow rates, cohesion, and permeability show qualitative agreement with experimental observations. In addition, the effect of two-phase flow was presented to emphasize the importance of the water-weakening of the sand. These results show that catastrophic sand production can occur following water breakthrough. Finally the impact of increasing sand cohesion by the use of sand consolidation chemicals was shown to be an effective strategy for preventing sand production. / text

To provide technical support for sand control decision-making, it is necessary to predict the production condition at which sand production occurs. Sanding onset prediction involves simulating the stress state on the surface of an oil/gas producing cavity (e.g. borehole, perforation tunnel) and applying appropriate sand production criterion to predict the fluid pressure or pressure gradient at which sand production occurs. In this work, we present numerical and analytical poroelastoplastic stress models describing stress around producing cavity and verify those models against each other. Using those models, we evaluate the stress state on the cavity surface and derive sanding onset prediction models in terms of fluid pressure or pressure gradient based on the given sand production criterion. We then run field case studies and validate the sanding onset prediction models. Rock strength criterion plays important roles in sanding onset prediction. We investigate how the sanding onset prediction results vary with the selection of one or another rock strength criterion. In this work, we present four commonly used rock strength criteria in sanding onset prediction and wellbore stability studies: Mohr-Coulomb, Hoek-Brown, Drucker-Prager, and Modified Lade criteria. In each of the criterion, there are two or more parameters involved. In the literature, a two-step procedure is applied to determine the parameters in the rock strength criterion. First, the Mohr-Coulomb parameters like cohesion So and internal friction angle ff are regressed from the laboratory test data. Then, the parameters in other criteria are calculated using the regressed Mohr-Coulomb parameters. We propose that the best way to evaluate the parameters in a specific rock strength criterion is to perform direct regression of the laboratory test data using that criterion. Using this methodology, we demonstrate that the effect of various rock strength criteria on sanding onset prediction is less dramatic than using the commonly used method. With this methodology, the uncertainties of the effect of rock strength criterion on sanding onset prediction are also reduced.

This work shows that toughness (resistance) to fracture propagation is an inherent characteristic of cohesionless particulate materials, which is significant for understanding hydraulic fracturing in geotechnical, geological, and petroleum applications. We have developed experimental techniques to quantify the initiation and propagation of fluid-driven fractures in saturated particulate materials. The fracturing liquid is injected into particulate materials, where the fluid flow is localized in thin crack-like conduits. By analogy, we call them 'cracks' or 'hydraulic fractures'. Based on the laboratory observations and scale analysis, this work offers physical concepts to explain the observed phenomena. When a fracture propagates in a solid, new surfaces are created by breaking material bonds. Consequently, the material is in tension at the fracture tip. In contrast, all parts of the cohesionless particulate material (including the tip zone of hydraulic fracture) are likely to be in compression. In solid materials, the fluid front lags behind the front of the propagating fracture, while the lag zone is absent for fluid-driven fractures in cohesionless materials. The compressive stress state and the absence of the fluid lag are important characteristics of hydraulic fracturing in particulate materials with low, or no, cohesion. Our experimental results show that the primary factor affecting peak (initiation) pressure is the magnitude of the remote stresses. The morphology of fracture and fluid leak-off zone, however, changes significantly not only with stresses, but also with other parameters such as flow rate, fluid rheology, and permeability. Typical features of the observed fractures are multiple off-shots and the bluntness of the fracture tip. This suggests the importance of inelastic deformation in the process of fracture propagation in cohesionless materials. Similar to solid materials, fractures propagated perpendicular to the least compressive stress. However, peak injection pressures are significantly greater than the maximum principle stresses in the experiments. Further, by incorporating the dominate experimental parameters into dimensionless form; a reasonable power-law fit is achieved between a dimensionless peak injection pressure and dimensionless stress. Scaling indicates that there is a high pressure gradient in the leak-off zone in the direction normal to the fracture. Fluid pressure does not decrease considerably along the fracture, however, due to the relatively wide fracture aperture. This suggests that hydraulic fractures in unconsolidated materials propagate within the toughness-dominated regime. Furthermore, the theoretical model of toughness-dominated hydraulic fracturing can be matched to the experimental pressure-time dependences with only one fitting parameter. Scale analysis shows that large apertures at the fracture tip correspond to relatively large 'effective' fracture (surface) energy, which can be orders of magnitude greater than typical for hard rocks.

A gas hydrate production test using the depressurization method was conducted in early April 2007 as part of the JOGMEC/NRCan/Aurora Mallik production research program. The results of the production test were analyzed using a numerical simulator (MH21-HYDRES) coded especially for gas hydrate reservoirs. This paper evaluates the test results based on analyses of production test data, numerical modeling and a series of history matching simulations. Methane gas and water was produced from a 12 m perforation interval within one of the major methane hydrate (MH) reservoirs at the Mallik MH field, by reducing the bottomhole pressure down to about 7 MPa. The measured gas production rate was far higher than that expected for a comparatively small pressure drawdown. However, irregular (on-off) pumping operations, probably related to excessive sand production, resulted in unstable fluid flow within the wellbore, which made the analysis of test performance extremely complicated. A numerical reservoir model was constructed as a series of grid blocks, including those mimicking the wellbore, to enable rigorous simulation of fluid flow patterns in the vicinity of the wellbore. The model was then tuned through history matching, not by simply adjusting reservoir parameters, but by introducing the concept that sand production might have dramatically increased the near-wellbore permeability. The good agreement between observed and simulated performances suggests the mechanism of MH dissociation/production during the test. The history matched reservoir model was employed to predict the second-year production test performance, in order to examine the gas production potential of the Mallik MH reservoir, and to provide insight into future exploration and development planning for MH reservoirs.

[pt] A produção de areia em poços de petróleo é um problema bem conhecido devido às conqüências catastróficas, que muitas vezes chegam a afetar a vida produtiva de um reservatório inteiro. Estudos sobre o assunto vêm sendo desenvolvidos, e uma importante linha de pesquisa tem se formado. No entanto, estes estudos focam principalmente na predição ou começo da produção de areia, dando uma importância menor à sua quantificação. Há mais de vinte anos, este problema tem sido relatado em um Campo da Bacia de Maracaibo na Venezuela, levando ao fechamento de um grande numero de poços. O problema é ainda maior devido às condições do reservatorio: profundidade (maior a 5000 (m)), alta pressão e temperatura assim como a resistência mecânica da rocha (maior a 50 (Mpa)) fazem que estudos mais avançados sejam precisados para além de determinar que fatores influenciam este fenômeno, observar se os modelos de quantificação utilizados neste trabalho representam o que ocorre no campo. Para isto, utilizo-se o software comercial de elementos finitos Abaqus junto com a sub-rotina de erosão assim como o modelo analitico de Willson, baseada na resistência equivalente da formação obtida a partir dos ensaios de cilindro oco ou TWC (thick Walled Cylinder) desenvolvidos também neste trabalho. Em ambos métodos, foram imposta condições reais do campo: estado de tensão, pressão de fundo do poço, poro-pressão, assim como o comportamento tensão-deformação da rocha, obtidos a partir de ensaios de compressão uniaxial. Para o caso da modelagem numerica, forem analisados dois casos, poço aberto (modelo de duas dimensoes) e tunel canhoneado (modelos de tres dimensoes). No caso do modelo de Willson foi implementado o túnel canhoneado. / [en] Sand production at oil wells is a well-known problem, due to the disastrous consequences that in some cases affect the productivity of the entire reservoir. Studies about the subject have been development and a very important research line has been created. However, those progresses are more focus on sand production onset, giving less importance to the quantification issue. Whit more than twenty years, there is a field located on Maracaibo Basin, Venezuela that has been affected with this problem occasionating the closure of a big amount of wells. The problem is even bigger due to the reservoir conditions: depth (more than 5000 (m)), high pressure and temperature conditions and also rock mechanics strength (more than 50 (Mpa)) make that more advanced studies will be need for seeking the factors that affect in this phenomenon and observe if the quantifications models used in this work represent the field observations. For this, was used the commercial software of finite elements Abaqus whit the erosion sub-routine as well as the analytical Willson model based on the equivalent strength formation obtain from thick walled cylinder test development in this work also. In both methods were used real field conditions: state of stress, bottom hole pressure, pore pressure and the stress-strain rock behavior obtain form uniaxial compression test. For the numerical model were analyzed two cases, open hole (two dimension model) and perforation tunnel (three dimension model). For the analytical Willson model was just implemented the perforation tunnel case.