Thomas Welton - First Field Application of In-Situ Gelled HCl-Formic Acid System

Document created by Thomas Welton on Feb 10, 2017
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  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.[1] 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.[2] The reservoir has a   permeability of 0.1 to 5 mD and tends to have significant   porosity.[3] The non-associated gas is sour with hydrogen sulfide   content that varies from 0 to 10 mol%.[4] 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,[5] emulsified,[6] in   situ gelled acid,[3,7] and recently, acids based on viscoelastic   surfactants.[8] 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.[11] Table 1 gives the elemental composition of super   Cr-13 that was used in these wells.

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