Real-Time Method for the Detection and Characterization of Scale

Abstract

A new technique is presented to detect scale deposition and to determine its composition and thickness in real-time in oilfield tubing and pipe. The method is based on continuous triple-energy gamma-ray attenuation measurements made at a dedicated, instrumented mandrel or spool piece. Detection and identification of extremely thin scale deposits are possible without disrupting the flow. Simulations of scale deposits in a three-phase pipe are given to demonstrate the method's practical application. The feasibility of the method has been verified with field measurements. The instrumentation required is based on existing technology with a proven track record in field conditions. Surface installations are safe to operating personnel and the environment. This new method enables a simple monitoring device to detect and characterize scale in its earliest stages of formation. The technique may be used to determine the appropriate type of inhibition or removal treatment according to the type of scale present, to evaluate the effectiveness of scale treatments in-situ in the pipe in real-time, and can help to optimize chemical consumption for continuous treatment. Introduction The problem of scaling in production tubulars (tubing and piping) is widespread. Scale deposits appear in several forms and are caused by several different phenomena. Scale may appear as the result of waterflooding, production commingling or simply the depressurization of the fluids as they flow to surface. What is common to all scale occurrences is that the mineral content of the fluid (usually water) has exceeded the fluid's saturation point in response to a change in conditions. The change in conditions may be a mixing of different waters, changes in temperature and pressure, water evaporation, or water chemistry and pH changes (such as due to CO2 out-gasing). As the mineral's saturation point is exceeded, unstable crystal nuclei evolve and devolve until they grow a critical radius, at which point the surface free energy of the nuclei decreases and crystal growth is spontaneous1. Crystal nucleation usually occurs against a substrate, such as scale growth on a pipe wall, because the crystal nuclei are more stable growing against a surface. Nucleation occurs more readily when the interfacial tension between the crystal and the substrate is very small. Crystal growth on a crystal substrate is highly favorable, which explains why scale can build up rapidly. The nature of scale formation in oilfield tubulars is unique by its environment. Oilfield scale forms in the presence of oil and gas, waxes and surfactants, metal corrosion, and in turbulent and high velocity flow. Oilfield scale often is composed of more than one mineral. It is not uncommon for several different compounds to be deposited together or in layers. Wax, oil and iron oxide can be trapped within the scale formation. Even the density of the scale can vary, depending on the depositional conditions. Treating oilfield scale is a complex challenge. In certain regions prone to scaling, such as the North Sea and Canada, treatments to prevent or minimize scale formation are well engineered and commonly practiced. Nevertheless, these treatments are not always completely effective. For instance, a inhibition treatment applied uniformly to several wells with different scaling tendencies may result in some wells being under-treated, allowing scale to form. When scale is multi-component, a treatment may be only partially effective. Over time, inhibition treatments lose effectiveness as the production environment changes, re-creating scaling conditions. Despite the best efforts to control it, scale deposits in production tubulars and surface facilities disrupt production and cause costly intervention work. The literature is replete with examples of the consequences of scale. In fact, there seems yet to be published a useful aspect of scale deposition in oil and gas production systems.

Calcite scale prediction at the near-well region: A radiotracer approach

“…In particular, a handled device was developed to detect the presence of scale in surface piping by measuring the nuclear attenuation across the pipe diameter and two field cases were presented in which a dual-energy-venturi multiphase flow meter was used to detect and characterize scale according to the attenuation of the nuclear spectrum [7]. A year later, Poyet and co-researchers presented a gammaray attenuation method, based on continuous triple-energy gamma-ray attenuation measurements, for the detection of scale deposition in real-time in oilfield production tubulars [8].…”