The Discovery of High-Molecular-Weight Naphthenic Acids (ARN Acid) Responsible for Calcium Naphthenate Deposits

Abstract

Calcium naphthenate deposition is among the most challenging obstacles to high production regularity for oilfields where acidic crude oils are produced. Until now it has generally been acknowledged that the deposit is made up of calcium soaps of the naphthenic acids in the crude oil, though with a slight overrepresentation of the lighter acids. In this paper, however, we demonstrate that this is not the case. Through a combination of several analytical techniques - the most important being Potentiometric Titration, LC/MS, NMR, and VPO - the ARN acid has been identified as the dominating constituent of these deposits. The ARN acid is a family of 4-protic carboxylic acids containing 4 - 8 unsaturated sites (rings) in the hydrocarbon skeleton with mole weights in the range 1227–1235 g/mol. The mole weight of the homologous ARN acids series are 1227, 1229, 1231, 1233, 1235 (basic structures) + n×14 (n = number of additional CH[2]-groups in hydrocarbon skeleton).The ARN acid with mole weight 1231 has C[80]H[142]O[8] as empirical formula. The present paper describes the different analytical methods leading to the ARN acid discovery. Furthermore it discusses possible ARN structures and methods for quantitative ARN detection in crude oils. The ARN acid has proved to be the main component in naphthenate deposit from oilfields offshore Norway, Great Britain, China and West Africa.The implications of the discovery to current calcium naphthenate treating strategies will be briefly discussed. Introduction An increasing share of the oilfields found and developed around the world falls in the category "high-TAN crudes", i.e. contains significant amounts of carboxylic (mainly naphthenic) acids. Producing and refining high-TAN crude oils introduces a number of problems, among which calcium naphthenate deposition in process facilities is the most serious production issue.[1–4] The mechanistic understanding of calcium naphthenate deposition is still very limited, though. It is generally acknowledged that a reaction takes place between naphthenic acids in the oil and calcium ions in the water. The reaction product, calcium naphthenate, is basically insoluble in either of the phases and, hence, precipitates out and accumulates at the oil/water interface. Although this simple model describes the naphthenate deposition phenomenon nicely, it doesn't give any clue as to why the acids in the deposit do not resemble the acids in the crude oil (Mediaas et al.[5] have titrated acids isolated from a calcium naphthenate deposit sample and from the corresponding crude oil to show that the average mole weight of the former is significantly lower than that of the acids in the crude oil; 330 and 430 g/mol, respectively). Furthermore, in some cases, it takes only a few parts per million of a naphthenate inhibitor to suppress naphthenate deposition from oil and water containing 2 wt% naphthenic acids and 0.1 wt% calcium, respectively.[1] Together, these observations indicate that some rigid selection criteria direct which naphthenic acids are active in the naphthenate deposition process. These are nothing but field observations confirming laboratory experiments showing that high pH (~10 or higher) and high reactant concentrations are needed for detectable amounts of ordinary organic (including naphthenic) acids to deposit as calcium salts.[2] ConocoPhillips and Statoil have cooperated for several years to unravel the fundamental secrets of calcium naphthenate deposition. Our working hypothesis has been that the reason for the above described "discrepancies" between the model and field- and laboratory observations is that one or more specific structural elements need to be present in a naphthenic acid in order to render it receptive to deposition upon contact with calcium-containing water at pH ~6. The objective has been to identify structural keys that enable naphthenic acids to deposit as calcium naphthenate under production conditions. The first fruit of this cooperation - the identification of the ARN acid as a prerequisite for calcium naphthenate deposition - was presented at the ACS National Meeting in August 2004.[6] In this paper we will elaborate somewhat more on the ARN acid discovery before we present our present ideas regarding the structure of the ARN acid.

Equilibrium Partitioning of Naphthenic Acid Mixture Part 2: Crude Oil-Extracted Naphthenic Acids

“…In the presence of calcium, oil-soluble calcium naphthenate can form 39 which can cause problems in downstream processing 40 . In order to combat the naphthenate problems 17 , Dyer, et al 41 made a huge matrix of experimental results on the topic by screening dozens of carboxylic acids in systems with various pH values, concentrations and water phase compositions, before it was discovered that a specific family of naphthenic acids named ARN was the main component behind the calcium naphthenate problems 42 . This article will study the equilibrium partitioning of an extracted crude oil acid mixture, determine acid partition ratios and present insight into at which pH the phase change occurs.…”

“…Other factors like the presence of calcium has been shown to affect naphthenic acid behavior in oil–water systems. ,− In the presence of calcium, oil-soluble calcium naphthenate can form, which can cause problems in downstream processing . To combat the naphthenate problems, Dyer et al made a huge matrix of experimental results on the topic by screening dozens of carboxylic acids in systems with various pH values, concentrations, and water phase compositions before it was discovered that a specific family of naphthenic acids named ARN was the main component behind the calcium naphthenate problems …”

Equilibrium partitioning of naphthenic acids and bases and their consequences on interfacial properties

“…The n represents the number of carbon atoms while Z is a negative, even integer specifying the hydrogen deficiency [20,21]. Oxy-naphthenic acids with the general formula C n H 2n+Z O x have also been introduced to describe naphthenic acids with added hydroxyl groups or multiple carboxylic groups ( x ≥ 3 ) [22] like the ARN acid responsible for calcium naphthenate deposition [23,24]. Naphthenic acids can stabilize water in oil (W/O) and oil in water (O/W) emulsions depending on several conditions, especially pH [17,25], cause corrosion [26] or naphthenate deposits [27].…”

Interfacial Films of Calcium Naphthenate: Influence of Polycardanol Structures

“…The group of acids that originates naphthenate deposition was discovered after an extensive characterization research. , These acids, known as ARN acids, are relative high molar masses molecules with a principal structure based on four aliphatic ramifications with a carboxylic acid as ending groups. − Charged naphthenic acids tend to migrate to the oil/water interface, owing to its structure with carboxylic acids groups attached to aliphatic backbones. Through fluid transportation, from reservoir to surface, there is a pressure drop that originates a pH increase in production water due to a modification in CO 2 equilibrium.…”