A Practical Solution to the Problem of Asphaltene Deposits-Hassi Messaoud Field, Algeria

Abstract

Abstract Deposits of asphaltenes in the tubing were a serious production problem at Hassi Messaoud, and required frequent tubing washing or scraping to maintain production. Profiles of the deposits, systematically taken, provided clues to the temperature and pressure conditions at the point of deposition, and the evolution of the shape of the deposit. Based on these observations, it was estimated that the problem would be largely controlled if the wells were produced at low wellhead pressures. Application of this idea has drastically reduced the number of tubing washes required. Limitations of the method are discussed. Introduction The Hassi Messaoud field has been reviewed previously, so a detailed description of the field is not required in this article. Briefly, the producing formation is the Cambrian sandstone at a depth of 11,000 ft, with a pay zone varying from 100 to 300 ft thick, covering an area of approximately 600 sq miles. The original reservoir pressure was 6,825 psi, and the bubble point of the crude varies from 2,880 psi to 2,130 psi on an east-west traverse of the field. The GOR (flash) varies from 1,390 to 1,030 cu ft/bbl depending on the geographical location within the field. The oil is an intermediate base crude by the standards of the Bureau of Mines, 42.3 degrees API, extremely rich in gasolines (40 per cent by weight) and a light-green color. An analysis of the average effluent is given in Table 1. The asphaltene content by the method of Louis is 500 mg/liter stock-tank oil. There are three tubing sizes for completion: 2 7/8-in. tubing for wells producing less than 2,400 BOPD, 3 1/2-in. tubing for 2,400 to 4,800 BOPD and 4 1/2-in. tubing for wells flowing over 4,800 BOPD. The tubing is set in a packer just above the formation, which is completed open-hole. From the first production, asphaltene deposits were observed in the tubings of the wells. These deposits consisted of 83.4 per cent asphaltenes, 3.3 per cent carbenes, and 13.3 per cent resins and heavy fractions of oil, and were a plastic, tacky consistency. Wells often lost 20 to 25 per cent of the wellhead pressure in 15 to 20 days, causing considerable loss in production. Cutting the deposits from the tubing by wire-line methods was too time-consuming and sometimes impractical, so a program of washing the tubings with a solvent was established. The profile and volume of the deposit were determined by successive gauge rings lowered by wire line, and the volume of chemical was calculated to assure the dissolution of the deposit with a factor of security. The chemical was displaced into the tubing opposite the deposit, and, after some agitation, flowed to the pit, and the well placed on production. This method, while satisfactory for cleaning the tubings, was expensive and cumbersome. Nevertheless, some 400 tubing washes were performed in this manner in 1960, 1961 and 1962.This article gives results of an attempt to attack this problem from a preventive rather than a remedial standpoint. Basic Theory of Asphaltene Deposition A number of researchers have investigated the properties of asphaltenes in crude oil, such as Nellenstyne, Katz, Sachenan and Witherspoon among others, and the complexity of the problem is indicated by the extensive literature on the subject. JPT P. 387ˆ