Selection of Screen Slot Width to Prevent Plugging and Sand Production

Abstract

Abstract A numerical model has been developed that addresses both plugging of, and sand production through single wrapped screens. The model was developed on the basis of a fractal model for the particle size distribution of reservoir sands. A database of sand types from the North Sea and Haltenbanken areas was established. Principal component analysis was used to reduce the number of significant variables in the database, and to provide a basis for a prediction model for critical slot widths. A series of laboratory experiments were performed, and four critical slot widths were identified for each sand type, defining a safe design interval for screen slot width. A mathematical model was developed that can be used to predict the critical slot widths for other sand types from the area. Introduction Single, wire wrapped screens with keystone shaped wire have been used to control sand production in oil and gas wells since the 1930 s. They have the advantage over prepacked screens in that they do not become plugged as easily by drilling mud. Furthermore they function as a surface filter, where the plugging material is easily removed, whereas prepacked screens are depth filters where plugging material tends to get trapped inside the prepack. Single wrapped screens do, however, have a reputation for being susceptible to plugging and/or sand production when designed according to the various traditional criteria (Refs. 1 and 2). This indicates that the design criteria for single wrapped screen completions should be revised. Sand control with screens is basically a function of the relationship between particle size and screen slot width. The pioneering work was published by Coverly (Ref. 3) in 1937. Coberly concluded that spherical particles could generally be retained when the slot width was 2.5 times the particle diameter or smaller. He also stated that in a mixture of particles of different size, the sand control properties of a screen depends on the largest particles in the mixture. He suggested that screen completions should be designed with screen slots that were 2 times wider than the d10 of the formation sand. He did not address the problem of screens becoming plugged by fines from the formation sand. This criterion has been used in California, while slot widths equal to d10 has been used on the U.S. Gulf Coast area (Ref. 4). In this paper it is shown that the design criterion suggested by Coverly, or any other criteria based on a single point on the particle size distribution curve, can not adequately describe either sand production or plugging of single wrapped screen. Instead a method is developed where a more complete description of the particle size distribution is used to predict the plugging and sand control properties of single wrapped screens. The study described in this paper has been limited to one screen type, single wrapped screens, and erosion of the screens have not been considered. An extension of the study is currently being planned that will include alternative screen designs, and also compare the susceptibility of the various screen types to erosion. Description of the particle size distribution In a traditional presentation of the results from a sieve analysis, the accumulated mass percentage of particles larger than a certain diameter is plotted on a semi-logarithmic scale. Since the particle distribution is plotted as a function of particle mass, the distribution function will emphasise the largest particles. When the purpose is to describe plugging of screen slots, it is more relevant to concentrate on the smaller particles. It is obvious that a particle matrix with zero porosity will be able to plug a screen slot completely as long as it contains particles large enough to be retained by the slot.