A New Method To Detect Blockage in Gas Pipelines
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 174751, “A New Method To Detect Blockage in Gas Pipelines,” by Kegang Ling, SPE, University of North Dakota; Xingru Wu, SPE, University of Oklahoma; and Zheng Shen, SPE, Weatherford, prepared for the 2015 SPE Annual Technical Conference and Exhibition, Houston, 28–30 September. The paper has not been peer reviewed.

Introduction

The consumption of natural gas is next to that of oil and coal according to Energy Information Administration statistics. New gas pipelines are constructed and existing pipelines are expanded to transport more natural gas to plants or end users. A blockage, which is a common problem in gas-pipeline operations, is a result of chemical or physical deposition caused by changes in composition, pressure, or temperature. The reliable and timely detection of blockages along pipelines is one of the critical topics in flow assurance.

The detection of pipeline blockage falls into two categories: physical inspections and mathematical models. Conventionally, physical-inspection methods include acoustic reflectometry, gamma ray transmission scanning, radio-isotope-tracer injection, tomography measurements, radiographic detection, and pipeline-diameter measurements. Physical inspections usually are accurate but come with the expense of production shutdown, high costs, and long downtimes, which may be infeasible with long pipelines or complex pipeline networks. Mathematical models use data such as flow rate, pressure, temperature, and pressure-wave reflection to locate a blockage. The models use mass conservation, momentum conservation, energy-balance equations, pressure-pulse decay, and phase shift to estimate blockage size and location. Mathematical models have the advantage of quick evaluation at lower costs and can monitor the pipeline continuously without interrupting pipeline operations. Usually, mathematical models require flow parameters, which are not always available; therefore, operators are more confident with physical inspection than mathematical modeling.

Gas Flow Through Blockage in Pipeline

Gas flow through blockage in pipelines is similar to the flow through a restriction, a throated pipe, or a choke, such as a nozzle or an orifice. It can be evaluated using choke-performance relationships. The flow regime can be classified into subsonic and sonic flows on the basis of gas velocity. Sonic flow occurs when gas velocity through a restriction reaches the sonic velocity in the fluid under the in-situ condition. Under sonic-flow conditions, the upstream cannot sense the pressure wave propagated from downstream upward because the gas is traveling in the opposite direction with the same velocity. Subsonic flow happens when gas velocity is lower than the sonic velocity in the gas at in-situ conditions. Sonic flow takes place when the downstream- to upstream-pressure ratio is less than the critical-pressure ratio. Please see the complete paper for equations to calculate a single-phase gas-flow rate at sonic-flow conditions using oilfield units.