Publication Details (including relevant citation information):
Nasr-El-Din, Hisham A., Al-Driweesh, Saad M., Sierra, Leopoldo, van Domelen, Mary, Welton, Thomas -
Abstract: Abstract The high levels of carbon dioxide and low levels of hydrogen sulfide content of some deep and high temperature gas producers contributed in the requirement to complete these wells using super Cr-13 tubings. Due to the low permeability of the formation and the associated formation damage issues, acid fracturing treatments were required to optimize the productivity of these wells. This paper describes the selection, optimization and long term comparative evaluation of the gelled and in-situ cross-linked HCl/formic acid systems used this type of wells. The high temperatures encountered in deep wells and the susceptibility of super Cr-13 to severe corrosion in high concentration HCl systems used for stimulation purposes added one additional difficulty to the acid stimulation process. To overcome these problems, extensive experimental and field studies were performed to select an acid system to enhance the productivity of these wells. Core flood tests performed with HCl/formic acid systems showed their ability to create deep wormholes in tight carbonate cores; however the corrosiveness of these systems at downhole conditions could be severe if the correct type and concentration of corrosion inhibitor is not used. In general, for the HCl/formic acid systems at downhole conditions (275 F) it was found that high concentrations of corrosion inhibitors are required to protect the super Cr-13 completions. Based on lab tests study acid stimulations were performed, the flow back fluid was recovered and analyzed to observe the corrosion problems and to optimize the corrosion inhibitor. In all the cases the wells responded very well to the acid stimulation and the completion integrity was not compromised in a short or long term. The paper also shows for the first time a comparative long term well response to the acid stimulation of the two acid systems used in the area, showing the better performance of the in-situ crosslinked HCl/formic system over the gelled HCl/formic system. Introduction Non-associated gas is being produced from the Khuff formation (carbonate) in Saudi Arabia. This formation belongs to the late Permian age and is encountered at an average depth of 11,500 ft. The Khuff formation is subdivided into four main zones, denoted A, B, C, and D. Zones B and C are the two main intervals producing non-associated gas. Lithological studies show that the reservoir is composed of dolomite intermingled with limestone and intermittent anhydrite stringers within the tighter section of the reservoir. The two reservoirs have varying pay thickness, on average, from 120 ft in the Khuff B to 200 ft in the Khuff C. The average initial reservoir pressure is 7,500 psi and the average bottom hole static temperature is 275 F. The reservoir has a permeability of 0.1 to 5 mD and tends to have significant porosity. The non-associated gas is sour with hydrogen sulfide content that varies from 0 to 10 mol%. Wells that produce sour gas with high levels of hydrogen sulfide are completed with low-carbon steel tubulars (L-80 and C-95). To enhance the productivity of these wells both matrix acidizing and acid fracturing treatments were used.5 The acid systems used were based on 28 wt% HCl and included: regular, emulsified, in situ gelled acid,[3,7] and recently, acids based on viscoelastic surfactants. The southern part of this reservoir has very low hydrogen sulfide content, less than 100 ppm, and more than 2 mol% CO2. Low-carbon steel tubulars cannot be utilized under these conditions because of excessive corrosion to the tubing. Corrosion resistant alloys (CRA) are recommended under these conditions.[9,10] To address corrosion problems in the sweat environments, these wells were completed with super Cr-13. Unlike regular Cr-13 tubulars, super Cr-13 contains nickel and molybdenum, which increase corrosion resistant of the tubulars. Table 1 gives the elemental composition of super Cr-13 that was used in these wells.