Formation damage is a common problem in petroleum reservoirs and happens in different stages of reservoir development from drilling to production. The causes of formation damage include particle invasion, formation fines migration, chemical precipitation, and pore deformation or collapse. Formation damage adversely affects productivity of wells by reducing the permeability of near wellbore region. Furthermore, formation damage also affects well logging results. Therefore, understanding the mechanism of formation damage is vital to predict the extent and severity of formation damage and to control it. This thesis is focused on the study of formation damage caused by external particle invasion. A simplified numerical method based on a commercial code PFC (Particle Flow Code) is proposed to simulate the particle invasion process. The fluid-particle interaction is simplified as hydrodynamic drag forces acted on particles by fluids; the particle-grain interaction is modeled as two rigid balls on contact. Furthermore, an pore network flow model is developed in this study to estimate permeability of damaged formations, which contain two well-separated particle sizes. The effects of the particle size and the initial formation porosity on formation damage are studied in detail. Our study shows that big particles tend to occupy the formation face, while small particles invade deep into the formation. Moreover, particles which are smaller than pore throats (entrances) impair permeability more than those bigger than pore throats. Our study also indicates that a higher initial formation porosity results in more particle invasion and permeability impairment. It is suggested that, in order to reduce formation damage, mud particle size distributions should be carefully selected according to given formation properties. Although our model has some limitations, it may serve as a tool to predict formation damage according to given parameters, and to understand the mechanism of formation damage from a micro-scopic point of view. / text

Asphalt mixture microstructure is formed by aggregates, bitumen binder and air voids. Aggregates make for up to 90% of the mixtures volume and the structure formed by them will depend mostly on their size distribution and shape. The study presented in this thesis has as main objective to develop a framework that allows the characterization of asphalt mixtures based on the aggregates gradation and its impact on pavement performance. Moreover, the study aims to identify the range of aggregate sizes which form the load carrying structure, called Primary Structure, and determine its quality. The method has been developed as a numerical procedure based on packing theory of spheres. Parameters like porosity, coordination number and disruption factor of the Primary Structure; and a binder distribution parameter for the different sub-structures have been used to evaluate the quality of the load carrying structure and predict the impact on several failure modes. The distribution of bitumen binder has been derived from a geometrical model which relates porosity of the mixture with film thickness of particles considering the overlapping reduction as the film grows. The model obtained is a closer approximation to a physical characteristic of the compacted mixture separated according to different elements of the structure. The framework has been evaluated on several field and laboratory mixtures and predictions have been made about their rutting performance and moisture resistance. The calculated parameters have compared favourably with the performances reported from the field and laboratory testing. The developed gradation analysis framework has proven to be a tool to identify those mixtures with a poor rutting performance based on the gradation of the aggregates. The Gradation - Based Framework has satisfactory distinguished between good and bad performance of asphalt mixtures when related to permanent deformation and moisture damage. The calculated parameters have allowed identifying and understanding the main mechanisms and variables involved in permanent deformation and moisture damage of asphalt mixtures. The developed model can be used as a tool to determine the optimal gradation to assure good performance for hot mix asphalt pavements. / QC 20120626

Recently, there has been growing interest on the behaviour of unbound granular material in road base layers. Researchers have studied that the design of a new pavement and prediction of service life need proper characterization of unbound granular materials, which is one of the requirements for a new mechanistic design method in flexible pavement. Adequate knowledge of the strength and deformation characteristics of unbound layer in pavements is a prerequisite for proper thickness design, residual life determination, and overall economic optimization of the pavement structure. The current knowledge concerning the granular materials employed in pavement structures is limited. In addition, to date, no general framework has been established to explain satisfactorily the behaviour of unbound granular materials under the complex repeated loading which they experience. In this study, a conceptual method, packing theory-based model is introduced; this framework evaluates the stability and performance of granular materials based on their packing arrangement. In the framework two basic aggregate structures named as Primary Structure (PS), and Secondary Structure (SS). The Primary Structure (PS) is a range of interactive grain sizes that forms the network of unbound granular materials. The Secondary Structure (SS) includes granular materials smaller than the primary structure. The Secondary Structures fill the gaps between the particles in the Primary Structure and larger particles essentially float in the skeleton. In this particular packing theory-based model; the Primary Structure porosity, the average contact points (coordination number) of Primary Structure, and a new parameter named Disruption Potential are the key parameters that determine whether or not a particular gradation results in a suitable aggregate structure. Parameters mentioned above play major role in the aggregate skeleton to perform well in terms of resistance to permanent deformation as well as load carrying capacity (resilient modulus). The skeleton of the materials must be composed of both coarse enough and a limited amount of fine granular materials to effectively resist deformation and carry traffic loads. / QC 20120601