ABSTRACT

Increasing interest in utilizing hydrofluoric acid (HF) for acid stimulation of offshore wells to improve hydrocarbon productivity has raised acid compatibility concerns with titanium alloy riser system components such as Titanium Stress Joints (TSJs). Recent published lab test findings by the authors1,2 confirmed that UNS R564041 Titanium exposure to warm, naturally-aerated, dilute, HF-containing, fresh acid mixtures does result in unacceptable, elevated etch rates, which dramatically increase with temperature and/or acid concentration. These lab studies, however, also revealed that several common inorganic salts such as aluminum, molybdate, and/or borate salts, can effectively inhibit this attack if added in sufficient molar ratios to free fluoride in these fluids.

Corrosion of titanium in these well stimulation acids may also induce significant absorption and/or penetration of byproduct hydrogen, raising additional concern for hydrogen damage to a stressed component if not properly inhibited. Continued lab acid testing of UNS R56404(1) and UNS R56407 titanium coupons confirms that substantial hydrogen uptake and microstructure-dependent penetration can occur during severe HF-containing acid etching. Primary influencing factors are identified from assessment of the effects of CO2 gas saturation; acid formulation, pH, and temperature; fluid flow; and fresh versus spent acid condition; and are considered with respect to inorganic salt inhibitor effectiveness.

BACKGROUND

Matrix acidizing (i.e., acid stimulation) of wells is often used to enhance hydrocarbon flow. In sandstone or silica-rich geologies, HF-containing acid mixtures are often considered to dissolve migrant sands and siliceous minerals to improve hydrocarbon permeability and well productivity1. In offshore wells, injection of fresh acids can be achieved using drilling or workover risers, or utilizing floating intervention vessels via coiled tubing injection; which all avoid direct acid fluid exposure to the production riser bore. However, significant cost savings are possible utilizing intervention vessels if the spent acid return flow could be returned through the production riser system.

Spent HF-containing acid return flow through the riser raises the serious concern and risk of severe acid attack of all offshore riser component bore surfaces, especially within the titanium stress joint (TSJ)1,3. Titanium alloys are known to exhibit substantial, elevated rates of uniform surface corrosion (i.e., etching), and possible hydrogen absorption, when exposed to cold or warm, dilute HF acid media; such that historic guidelines3,4 strongly recommended total avoidance of contact with all HF containing fluids.

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