HCl/HF Acid-Resistant Cement Blend: Model Study and Field Application
- Curtis G. Blount (Arco Alaska Inc.) | Jerry L. Brady (Arco Alaska Inc.) | Dustin M. Fife (Arco Alaska Inc.) | Lamar L. Gantt (Arco Alaska Inc.) | Julie M. Heusser (Arco Alaska Inc.) | Mel C. Hightower (Arco Alaska Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- February 1991
- Document Type
- Journal Paper
- 226 - 232
- 1991. Society of Petroleum Engineers
- 1.6 Drilling Operations, 2.2.2 Perforating, 2.4.3 Sand/Solids Control, 4.3.1 Hydrates, 5.2 Reservoir Fluid Dynamics, 3 Production and Well Operations, 4.1.3 Dehydration, 4.1.2 Separation and Treating, 4.3.4 Scale, 5.4.6 Thermal Methods, 2.2.3 Fluid Loss Control, 1.10 Drilling Equipment, 1.14.3 Cement Formulation (Chemistry, Properties), 1.14 Casing and Cementing, 3.2.4 Acidising, 4.2.3 Materials and Corrosion
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Summary. Analysis of Prudhoe Bay field stimulation data suggests that HF-acid treatments are harmful to wellbore cement. Laboratory tests confirmed the suspected solubility of conventional oilfield cements in hot-acid solutions. Test results indicate that a 2-in. cement cube is up to 96% soluble in a dynamic solution of 12% HCl/3% HF (mud) acid at 190 degrees F. A wellbore simulator was developed to correlate cube solubilities to down-hole conditions. This paper documents equipment and testing procedures used to confirm acid procedures used to confirm acid solubility of cement-squeezed perforations in a typical wellbore. perforations in a typical wellbore. It also documents additive screening that led to the development of an acidresistant cement (ARC) blend that uses liquid latex. This blend improved acid resistance by more than 700% over conventional formulations under simulated field conditions and configuration. The latex cement blend was also demonstrated to be compatible with coiled-tubing-unit cement-squeeze applications.
Traditional industry views hold that acid reacts with cement for a short period of time until a protective, acid-inhibiting skin forms. Field experience in the Eastern Operating Area of Prudhoe Bay, however, has shown that more than 37% of primary cement jobs developed zonal-isolation problems after HCl/HF acid treatment. Furthermore, 73% of squeeze cement jobs broke down after HCI/HF acid stimulation. Because of these problems, solubility tests were performed problems, solubility tests were performed with various acids on cement cubes. Contrary to popular belief, common industry cement blends were found to be very soluble in hot HCl/HF acid. Results from the cement-solubility study included the following. 1. Conventional cement cubes are up to 96% soluble in dynamic, hot HCl/HF acid. 2. Weight loss of cement cubes varied little when the cubes were cured and acidized under pressure compared with weight loss of cubes cured and acidized at atmospheric pressure. 3. Dehydrated cement filter cake reacted with HCl/HF acid at roughly the same dissolution rate as the slurry cement cubes. A simulator was constructed to model cement-squeezed perforations in a wellbore at downhole conditions. The cement-squeeze simulator (SCC) accommodated squeezing, curing, and fullbore acidizing of squeezed perforations. The CSS modeled Prudhoe perforations. The CSS modeled Prudhoe Bay bottomhole temperatures (BHT's) from 160 to 220 degrees F and common squeeze differential pressures of 1,500 psi. CSS test results showed that with conventional cement, hot HCl/HF acid dissolved cement nodes, dehydrated cement in perforations, and primary cement. Literature searches were performed to gain insight into what could be done to make cement more resistant to acid attack. The construction industry offered suggestions that complemented the sparse oil-industry information regarding cement degradation by corrosive fluid. Cement blends identified in the search were made with fly ash, latex, reduced water slurries, and proprietary polymers. The cement blends were screened in polymers. The cement blends were screened in a cement solubility study as documented here. If a product was identified as acidresistant, considerable testing was performed to ensure compatibility with oilfield performed to ensure compatibility with oilfield applications. Cement-blend test parameters included fluid-loss control and thickening time at BHT's and bottomhole pressures and the ability to pump through coiled tubing (see Appendix A). If the slurry met these compatibility requirements, solubility performance was verified in the CSS with performance was verified in the CSS with heated HCl/HF acid under dynamic and pressurized conditions. pressurized conditions. Liquid latex (styrene butadiene) is the only additive tested thus far that is highly resistant to HCl/HF acid and that meets the compatibility requirements. The latex blend's high resistance to acid attack under simulated downhole conditions was verified with the CSS.
ARC. In our literature search, we found little information on reducing cement solubility in acid. However, a few ideas had been used with some success. Reducing the permeability of cement was the first technique considered. Although oil industry cement blends typically have low permeability, the permeability can be permeability, the permeability can be reduced further with such additives as latex and low-water-ratio cement. When cement permeability is low, decomposition of permeability is low, decomposition of cementitious matter is limited to exposed surface areas. However, the rate of deterioration is accelerated when products of chemical decomposition are washed away by dynamic action, as in a typical acid stimulation at Prudhoe Bay. A second approach was the use of additives to protect cement from acid attack. One of the more common additives is fly ash, a silica fume used in pozzolan cement. The fly ash reacts with cement to reduce permeability and the amount of highly permeability and the amount of highly acid-reactive calcium hydroxide in the cement. Certain latex compounds and polymers also helped to reduce the solubility of cement in acid. The polymeric constituents inhibit acid attack by coating the cement particles and by reducing the permeability of the cement.
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