Generalization of the API RP 14E Guideline for Erosive Services

Abstract

The commonly used practice for controlling sand erosion in gas and oil producing wells is to limit production velocities following the API RP 14E. This guideline contains a procedure to calculate a threshold velocity, the flow velocity below which an allowable amount of erosion occurs. While providing advantages of computational ease, the approach has some disadvantages. One is that, while many factors influence the erosion rate, RP 14E includes only one factor, the density of the medium. Thus, factors such as flow geometry, type of metal, sand size, and Reynolds number are not accounted for. Another disadvantage is that the tolerable amount of erosion, in terms of loss of wall thickness, is not specified in RP 14E. A method is presented to overcome these disadvantages by (1) accounting for many of the physical variables in the flow and erosion processes and (2) including a way to predict the maximum penetration rate for sand erosion. The capabilities of the method are evaluated by comparing predicted penetration rates with experimental data found in the literature.

In this paper, the method is applied to calculate threshold velocities. The computational procedure allows an allowable amount of erosion to be specified in mils per year for elbows, tees and direct impingement geometries. Threshold flowstream velocities are calculated for carrier fluids of crude oil, water, and methane at elevated pressure. Resulting threshold velocities are presented for a range of sand sizes, pipe diameters, sand production rates, and methane pressures. Results show that threshold velocities for single phase gas flows are much lower than threshold velocities for single phase liquid flows.

Introduction

Sand erosion is a major problem in many production situations because small amounts of sand entrained in the produced fluid can result in significant erosion and erosion/corrosion damage. Even in "sand free" or clean service situations where sand production rate is as low as a few pounds per day, erosion damage could be very severe at high production velocities. Sand erosion can also cause localized erosion damage to protective corrosion scales on pipe walls and result in accelerated erosion/corrosion damage.

The objective of this work is to develop guidelines for predicting a threshold velocity and/or particle size below which only a small, allowable amount of erosion occurs. Threshold velocity guidelines help operators to estimate safe production velocities in erosive services, and serve as a tool to engineers for the cost effective design of piping systems, the selection of piping materials, and the selection of appropriate screen sizes for new wells.