Abstract

Several papers have reported the potential formation damage as a result of the adverse interactions among acids, formation fluids and minerals during acid stimulations operations. However, few studies have focused on the effect of the acid consumption capability of crude oils upon the efficiency of matrix acidification, particularly that capability arisen from the nitrogen compounds content. This paper ascertains:

  1. the release of basic nitrogen compounds from crude oils, due to reactions with HF (3%), HCl, citric and acetic acids (all of them 10%),

  2. the effect of these nitrogen compounds released into the aqueous phase upon the dissolution of silicate and alumina-silicate minerals.

Eight Venezuelans crude oils were brought into contact with acidic solutions for 10 days at 70 C in batch experiments. The resulting aqueous solutions were analyzed by coupled gas chromatography-mass spectroscopy (GC/MS) and used to dissolve quartz, kaolinite and K-feldspar at 70 C, during 5 days. The initial and final concentrations of Al, K and Si in solution were determined by inductively coupled plasma, atomic emission spectroscopy (ICP-AES).

GC/MS analyses showed that a complex mixture of cationic nitrogen compounds, such as R-Quinolines, R-Pyridines and Carbazoles, are released into solution when crude oils are treated with acid. The type of acid lead the amount and kind of nitrogen-organic compounds (N.O.C) extracted during crude oil - acid interactions.

The analysis by ICP-AES indicated that the presence of small amounts of the N.O.C. in the aqueous phase (0.002% to 0.04%) inhibits up to 72% the elements released by mineral dissolution. This is probably due to the shielding of the mineral surface active sites and should be the reason for efficiency decrease of matrix acidification. The presence of these compound reduced the oil recovery up to 20%, in laboratory displacement experiments.

Introduction

The main purpose in any acidification treatment is the dissolution of minerals from the formation or precipitated solid material in the near wellbore areas. All of these in order to increase permeability. This is usually achieved by employing HF and HCl or their mixtures, as well as acetic, formic and citric acids, among others.

The mechanisms and effects causing damages to the formation during stimulation processes have been well documented. Some of the inconvenients reported are secondary phase precipitation, formation of organic deposits, and fine migration. Nevertheless, the basic properties of crudes have been poorly considered when designing a stimulation process, just as acid properties (acid number) of crudes are considered in recovery processes with alkaline additives. Nitrogen compounds are well known constituents of crude oil, with an average content of 0.094% by weight and as high as 0.75% in some California oils. These compounds are believed to conform a major fraction of the basic compounds in crude oils. Pyridines, Quinolines and some substituted Pyrroles, such as 2,4-Dimethylpyrroles, particularly alkylated Benzoquinolines, have been recognized as major classes of basic compounds, whereas Indoles, some Pryrroles, and Carbazoles are regarded as non-basic compounds.

The significance of knowing the basic properties of a particular crude is due to the fact that basic compounds can interact with the acids injected in the formation, generating in-solution species that can affect mineral dissolution and precipitation processes in the porous medium. P. 137^

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