Abstract

Stimulation of carbonate formations by acid dissolution of the rock has been an efficient and successful method of bimproving production in oil and gas wells. Hydrochloric acid is the normal fluid of choice. However, in high temperature applications corrosion issues limit usage, especially in chrome completions. Acetic acid has been used with some success and with adequate corrosion protection. But due to its low reactivity at higher temperatures, the efficiency with which a gallon of acid dissolves the formation is perceived as low. This perception comes from reaction efficiency of acetic acid reported in the literature ranging in values from 90% at 25 °C to 40% at 121 °C for 2 to 15 wt%, respectively. Acetic acid reaction on calcium carbonate is controlled by its small dissociation constant, 1.754E-05 at 25 °C (77 °F) and therefore is labeled a weak acid.

Abstract

Coagulation of asphaltenes in crude oil has been studied by the electronic spin resonance. Free radicals with the content of vanadium less than 0.1 mass% are found to recombine when asphaltenes precipitate. Polyaromatic fragments are shown to be localized in the inner part of the asphaltenes molecules. A possibility of the formation of diamagnetic compounds between vanadyl (VO2+) complexes and free radicals of asphaltenes is investigated.

Introduction

A study of asphaltenes from crude oils is a matter of the special interest and research in oil chemistry. The constantly increasing share of heavy crude oil and bitumen in the total oil production leads to the increasing importance of studies in this area. High concentrations of asphaltenes, resins, vanadium and sulfur are characteristic for most of the heavy crude oil and bitumen deposits. That makes oil production and refining more complicated.

Petroleum asphaltenes are the most high molecular species among other components of petroleum and just asphaltenes - the basic carriers of oil paramagnetysm. Basically paramagnetic components in oils are free stable radicals and vanadyl (VO2+) complexes. Behaviour of asphaltenes under reservoir conditions, preparation and transportation of crude oil is strongly related to the interactions between paramagnetic fragments in the structure of asphaltenes aggregates. There is a hypothesis [1] that asphaltenes molecules, especially the large ones, are not necessarily two-dimensional flat disks but they have the capacity, owing to the presence of long polymethylene bridges, to fold upon themselves into a complex three-dimensional globular conformer with internal structure. In other works [2, 3], however, a possibility is shown for the molecules containing alkyl fragments up to C24 (paraffins, ceresines, etc.) to co-precipitate together with asphaltenes.

Hypothetical structures of the asphaltenes molecules are presented [4, 5]. They have a different type: polyaromatic fragments are in the centre of molecules whereas naphtenealiphatic groups are located on the periphery.

From the point of view of explanation of the characteristics of crude oil and bitumen, the most significant model was proposed by Juan Murgich [6] to describe asphaltenes from different crude oils. According to the model, all asphaltenes are two types - A-type and S-type.

A-type asphaltenes have 15–20 condensed aromatic rings(in the centre), alkyl substitutes and heteroatoms on the periphery. Asphaltenes of this type are flat.

S-type asphaltenes have several aromatic parts (2–5 rings) which are connected with each other by alkyl and sulphide bridges. The molecules are almost spherical.

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