Abstract

The production, transport, and refining of petroleum and gas are constantly affected by the deposition of unique phases such as wax, asphaltenes, diamondoids, hydrates and sulphur. The venue of the advanced acoustic resonance approach has been successfully tested to meet the challenges of solids problems in the new millennium. A mercury-free advanced acoustic resonance (AR) system, developed and tested at Hycal Energy Research Laboratories for reservoir applications, makes it possible to have fast, highly accurate, measurements during depressurization (9000 psia to 12000 psia) runs on dark live oil at reservoir temperature to identify the onset of asphaltenes precipitation and bubblepoint during the same run. Black oil, a problem for visual and optical methods, is no barrier for this technology. Effects of inhibitors on solid-liquid equilibrium have been investigated at various concentrations and temperatures. The results are presented and discussed. EOS modeling of the AR results have been presented with comparisons. There is good agreement between measured and modeled data.

Introduction

Asphaltene precipitation from reservoir fluids causes severe operational problems in the wellhead equipment, separators, tanks and surface equipment1. In offshore production, the clean-up costs have skyrocketed2. Asphaltenes are dark brown to black solid compounds with no definite melting point. They decompose while heating and leave a carbonaceous residue. They are non-crystalline substances or mixtures of relatively high molecular weight fractions of bitumen with characteristics of strong aromatic polar substances. They are classified by the particular solvent (nheptane, n-pentane) used to precipitate them3. Asphaltene precipitation can be measured experimentally. Light-scattering techniques4 and cross-polarization microscopy5 have been used to determine the onset of solids precipitation experimentally. Poor signal to noise ratio limits the dynamic range of operation in the case of N.I.R.

An advanced acoustic resonance system developed at Hycal has been successfully used in the determination of solids precipitation onset in live reservoir fluids. The system exploits the time evolution of the acoustic response in fluids under variable and well-controlled conditions of pressure, volume and temperature6,7. The system detects phase transitions on the basis of differences in orders of magnitude in sonic speeds of various fluid states (vapor/liquid/solid) when acoustic waves are propagated in a reservoir fluid going through phase changes.

EXPERIMENTAL DETAILS

Hycal's AR system, consisting of a cylindrical resonator (0.25 inches in diameter) is made of hastelloy with one transducer at the top to generate acoustic waves through the fluid in the resonator, and the other at the bottom to receive the signals that carry information about the fluids through the phase transitions. One can detect the onset of liquid-solid or liquidvapor transitions in fluids by analyzing the acoustic responses.

The assembly is housed in a well-insulated circulating airbath with precise temperature control from -40 °C to 150 °C. A digital pressure gauge is used to measure pressures up to 10,000 psia and a platinum resistance thermometer is used to measure temperatures precisely. A linear velocity displacement transducer arrangement is used to accurately measure the volume at any instant in time.

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