Abstract

A producer in Alaska had been experiencing asphaltene deposition problems in one well of a dual string completion, producing from two zones. Laboratory tests performed on both dead and live oil samples identified inhibitor A as the most effective chemical for this well. The field performance of inhibitor A was monitored by a wellhead side stream filter plugging loop. Results from laboratory tests and a field trial are presented. In this paper the correlation between lab data and field performance shows that predictive lab tests can successfully select an asphaltene inhibitor for field applications.

Introduction

Problems associated with asphaltene deposition have been well documented in the petroleum industry1–2. Asphaltenes are heterocyclic unsaturated macromolecules consisting primarily of carbon, hydrogen, and a minor proportion of heteroelements such as oxygen, sulfur and nitrogen. Asphaltenes are typically defined by solubility as benzene soluble and pentane or heptane insoluble3.

The asphaltenes are believed to exist in the oil as a colloidal suspension, and are stabilized by resins adsorbed on their surface. These higher molecular weight components of crude oil are normally in equilibrium at reservoir conditions. As crude oil is produced this equilibrium may be disrupted by a number of factors including pressure reductions, crude oil chemical composition changes, introduction of miscible gases4 and liquids, mixing with diluents and other oils, and during acid stimulation, hot oiling and other oilfield operations. The upset of the colloidal system may result in irreversible flocculation of asphaltenes. The deposition and precipitation of flocculated asphaltenes can severely reduce the permeability of the reservoir, cause formation damage and can also plug-up the wellbore and tubing.

An offshore Alaskan producer experienced a deposition problem in one of their wells. This well was a gas lift well with dual string completion producing from two zones. The crude was a black oil of 32.5 API gravity. The average production of the well was 109 BOPD. In December of 1998, a 2–1/4" gauge ring was run in hole and encountered a large deposit from 7057' and down.

Subsequent analysis of the deposit revealed that it consisted primarily of asphaltenes. The stability of asphaltenes in the crude was assessed using SARA analysis, a heptane titration precipitation test and Oliensis spot test. Results from all three tests indicated the asphaltenes in the crude were indeed unstable.

Laboratory chemical screening tests on both live and dead crude oil identified Inhibitor A as the most effective chemical for this oil. To monitor the field performance of the asphaltene inhibitor, a side stream filter loop was installed at the wellhead. The objective was to measure and compare the time needed to plug a filter by untreated and treated produced fluids.

Laboratory Tests
Stability and Field Deposit Analysis.

Three tests were used together to access the stability of a crude oil and propensity of asphaltenes to deposit or sludge in production systems were used. These tests include the SARA (weight fraction of Saturates, Aromatics, Resins, Asphaltenes) screen, Oliensis spot test5, and Asphaltene Precipitation Detection Apparatus (APDU).

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