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Formation damage due to chemical reactions or mechanical failure of the rock is a well-known issue in the oil and gas industry. In this paper, we look at injectivity changes of water injectors targeting unconsolidated formations as a result of sand channelization. In this situation, sand particles migrate from the formation matrix toward the wellbore and consequently affect permeability of the formation and completion components like frac-packs or gravel-packs. To simulate this phenomenon, we propose a model that decomposes the sand matrix into two different phases by means of their volumetric fractions. The first phase consists of the immobile granular phase while the other is the fraction of grains that are detached from the matrix and carried away by the fluid. The ratio of these two phases evolves over time by means of erosion and deposition. A finite element code for this model is developed and implemented for different reservoir conditions. The results show how local porosity evolves over time around the wellbore by the formation of channels that enhance the flow; however, upon well shut-in, flowback induced by the water hammer effect brings large amounts of sand into the well and fills it up. A study of the effect of frac-packs which are typical completions for wells targeting soft formations is performed to demonstrate potential damage of repetitive shut-ins on injectivity in these wells. The presented model appears as a robust tool to predict sand production for different completion systems to achieve more sustainable injection operations.

Fluid injection into unconsolidated formations serves many different purposes in the oil and gas industry. EOR projects require fluid injection at sustained high flow rates into the reservoir for years. Depending on the matrix properties and injection conditions, individual grains may be detached from the sand matrix and carried away by the fluid. During well shut-ins, pressure transients are generated and the direction of the flow is reversed allowing the grains to be transported and deposited near and even inside the wellbore. The deposited grains may plug the pores around the wellbore and decrease the injectivity, a phenomenon often observed at injection wells targeting unconsolidated formations. In order to address this issue, we implement a model consisting of partial differential equations solved through the numerical finite element methods that decomposes the porosity into mobile and immobile solid phases plus the liquid phase. The mass exchange from mobile to immobile solid phases is dependent on the deposition and erosion rates that are a function of the pressure gradient and stress concentrations around the wellbore. The system uses appropriate scales in size and time as well as appropriate field parameters. The results show how the porosity evolves over time around a hypothetical wellbore; the erosion reduces the pressure gradient until the erosion is negligible. The injection rate, the initial porosity heterogeneity and inter-grain forces (degree of consolidation) proved to have a significant impact on the matrix erosion. Simulations that emulate the effect having gravel and frac-packs were also performed evidencing that the different completion systems help to reduce the formation of high porosity channels around the wellbore during fluid injection.

Fluid injection is a common practice in the Oil and Gas industry found in many applications such as waterflooding and disposal of produced fluids. Maintaining high injection rates is crucial to guarantee the economic success of these projects; however, there are geomechanical risks and difficulties involved in this process that may threat the viability of fluid injection projects. Near wellbore reduction of permeability due to pore plugging, formation failure, out of zone injection, sand production, and local compaction are challenging the effectiveness of the injection process. Due to these complications, modeling and simulation has been used as an effective tool to assess injectors’ performance, however, different problems have yet be addressed. In this paper, we review some of these challenges and the solutions that have been proposed as a primary step to understand mechanisms affecting well performance.Copyright © 2016 by ASME

Formation damage due to chemical reactions or physical failure of rock is a well-known issue in the oil and gas industry. In this paper, we look at the changes of injectivity at water injectors located in unconsolidated formations as a result of sand channelization. In these situations, sand particles migrate from the formation matrix toward the wellbore and consequently affect permeability of the formation and completion components like frac-packs or gravel-packs. To model this phenomenon, we propose a model that decomposes the sand matrix into two different phases by means of their volumetric fractions. The first phase consists of the immobile granular phase while the other is the fraction of grains that are detached from the matrix and carried away by the fluid. The ratio of these two phases evolves over time by means of erosion and deposition. A finite element code is developed for numerical implementation of this model for different reservoir properties. The results show how the porosity evolves over time around the wellbore by the formation of channels that enhance the flow, however, upon well shut-in, Flowback induced by the water hammer effect brings a large amount sand into the well and fill it up. A study of the effect of frac-packs which are typical completions for wells targeting soft formations is performed to demonstrate potential damage of repetitive shut-ins on injectivity in these wells. The presented model appears as a robust tool to predict sand production in different completion systems to achieve more sustainable injection treatments.

The selection of terminology, though seemingly unimportant, can have profound effects on our thinking. A seizure classification system based exclusively on seizure semiology emphasizes the wide diversity and richness of seizure expression. In the field of epilepsy surgery, the use of a seizure classification system that emphasizes ictal semiology in an invaluable first step in the precise localization of ictal onset. This proves to be particularly true in pediatric epilepsy, because seizure semiology is more subtle and difficult to characterize. The following article describes a semiological seizure classification and its relevance to both pediatric epilepsy surgery and status epilepticus.