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Abstract
Existing geometrical gravel-pack design criteria do not consider the effect of sand migration through and clogging of the gravel-pack matrix. Therefore, a visual and qualitative laboratory investigation of moving particulates through gravel-pack confined within a parallel configuration (linear) and a convergent configuration (radial) visual cells, has been carried out. Castings of the clogged gravel-pack have been studied to obtain information about the geometrical alteration and deposition morphology. The structural changes due to the clogging process have been characterized by injecting a NaCl-tracer and by analyzing the breakthrough curves (BTCs) before and after clogging.
Results indicate that the migration of particulates into the gravel-pack is a time dependent process which is completed when a self-filtration zone of intruded fine particulates is formed next to the interface of the gravel-pack-particulates at a given flow rate. The particulates migration process is characterized by a discontinuity of particulates concentration. The concentration of particulates is high up to a well-defined line located at the interface. Ahead of the front, there is very little or no particulates at all. It is further observed that an empty pore just ahead of the moving front of particulates remained almost entirely empty until the concentration of moving particulates within the pore immediately behind it is sufficiently high. In this way, the well-defined front of particulates moves through the gravel-pack, conquering pore after pore. It is shown that dispersion measurement characterizes the changes in the structure and therefore in the flow pattern accurately. Tracer analysis indicated that the clogging process created multiple flow paths manifested in highly tailed breakthrough curves.
Besides providing the parameter values for models, these observations indicate that it is necessary to consider a collective motion of particulates instead of the classical particle-by-particle approach used in phenomenological models.
Introduction
The nature and function of a well in a poorly consolidated formation are inevitably influenced with fine byproducts that cause erosion in tubular materials and that are responsible for plugging of wells as well as the surface equipment. Moreover, when large amount of load bearing solid particles migrate into a well, cavities form around the well, and consequently the settlement occurs, diminishing the performance of the well. A popular method of combating particulates production in oil/gas fields, based on limits set by successful and unsuccessful experiments, is to define maximum ratios of grain size diameter for sand and gravel-pack at a certain percentage by weight, for which a stable behavior (no sand migration) has been observed.
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