Optimum Gravel Sizing for Effective Sand Control

Abstract

ABSTRACT Efficient sand control is dependent on the design of the exclusion system and must take into account a number of formation characteristics and production factors which will affect the eventual well performance. For gravelpacking, the design requires the specification of the size of the commercial gravel to use with the final objective being the achievement of maximum productivity over the production life of the well with effective sand exclusion. To date, a number of formulae have been suggested for gravel size selection, the most popular being the Saucier formula. However in a large number of gravelpacks, operators have reported fines being produced in large quantities, suggesting ineffective bridging by the gravelpack. A review of the available gravel sizing formulae show that they may be too general and possibly too simplistic to take full cognisance of the depositional environment of the formation sand, the operational conditions to which the gravelpack is subjected, the reservoir fluid, the gravelpack structure, etc. A project has been initiated in the Department of Petroleum Engineering at Heriot-Watt University to address the problem of determining the optimum gravel size under a range of different operational conditions. The project has the objective of developing a theoretical approach and modelling technique to predict gravel size and also to evaluate particle-pore bridging phenomena, using a series of rigorously defined experiments. This paper presents the results of the comprehensive experimental investigations carried out to evaluate the bridging effectiveness of various commercial gravels as a function of different operational conditions including the sand sorting and shape. The experimental studies were carried out using a purposely-built packed column to evaluate the effects of production rate, fluid type, formation sand and gravel characteristics, absolute and differential pressure on the efficiency of the exclusion treatment. The bridging efficiency of selected gravels were measured. The experimental data generated formed the data base for developing a semi-empirical bridging efficiency equation which assists in the selection of gravel and the prediction of its bridging effectiveness under specified operational conditions. It also allows the design engineer to conduct a sensitivity analysis of the effectiveness of the chosen gravel to a range of variable conditions. The paper concludes with a description of the computer package being developed for optimum gravel size selection. This package is expected not only to select the optimum gravel size but also predict the eventual well performance.