Evaluation of API RP 14E Erosional Velocity Limitations for Offshore Gas Wells

Abstract

Abstract In order to avoid erosion damage and associated problems in two-phase flow systems, API RP14E recommends limiting the maximum production velocity to a value defined by the following empirical equation:(Equation Available In Full Paper) Evaluation of the above equation has shown that in cases where the form of the equation can be rationalized, the value of C, as recommended by API, is extremely conservative. These cases include erosion due to liquid particle Impingement and corrosion-assisted erosion due to the stripping of corrosion inhibitor films. For the case of erosion due to sand particles entrained in the fluid, the form of the equation appears to be incorrect. The API RP14E recommendations to reduce the value of C to account for sand in the produced fluid is, therefore, improper. A method for calculating erosion damage as a function of fluid and flow characteristics is proposed. This approach can be used to calculate a limiting flow velocity for any specified allowable erosion rate. Introduction The production of hydrocarbons from underground reservoirs is associated mainly with the flow of a liquid (oil and water), gas (natural gas), and/or solid (sand). This flow situation is essentially one of a liquid-gas, two-phase flow with entrained solid particles. When the fluid flow in a pipe is disturbed due to a local change in direction, a velocity component normal to the pipe wall will be introduced, resulting in repeated impacts on the pipe wall. Erosion damage of the pipe is caused by the repeated bombardment of liquid and solid particles. The erosion damage is enhanced by increasing the production- capacity of a given flow system (Le., increasing flow velocity). In order to avoid potential erosion problems, most oil companies have been limiting their production rate by reducing the flow velocity to a level below which it is believed that erosion does not occur. This limiting flow velocity is calculated using the API RP14Erecommended empirical equation1: (Equation Available In Full Paper) For a sand-free, two-phase flow situation, the C factor is limited to 100 for continuous flow and 125 for intermittent flow. The API RP14E recommends the use of a lower unspecified C factor for fluids containing sand. Previously, production rates were generally restricted to lower values than those specified by Equation (1) due to fear of formation damage or excessive sand production. However, the recent advances in well completion procedures, sand control techniques, and our understanding of reservoirs' flow behavior permit higher production rates. These developments, in addition to the current economic incentives, are motivating the oil industry to increase production rates, particularly for high-capacity gas wells. Under this condition, the API RP14E erosional velocity equation represents a major obstacle. Although the API equation has been widely accepted, the authors were unsuccessful in all attempts to determine the basis of this equation.