HCl/HF Acid-Resistant Cement Blend: Model Study and Field Application
Authors: Blount Curtis G. , Brady Jerry L. , Fife Dustin M. , Gantt Lamar L. , Heusser Julie M. , Hightower Mel C. 
Published: December 2007

Summary.
Analysis of PrudhoeBay field stimulation data suggeststhat HF-acid treatments are harmfulto wellbore cement. Laboratory testsconfirmed the suspected solubility ofconventional oilfield cements inhot-acid solutions. Test results indicatethat 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 tocorrelate cube solubilities todown-hole conditions. This paperdocuments equipment and testingprocedures used to confirm acid procedures used to confirm acid solubility of cement-squeezedperforations in a typical wellbore. perforations in a typical wellbore. It also documents additive screeningthat led to the development of anacidresistant cement (ARC) blend thatuses liquid latex. This blendimproved acid resistance by more than700% over conventional formulationsunder simulated field conditions andconfiguration. The latex cementblend was also demonstrated to becompatible with coiled-tubing-unitcement-squeeze applications.

Introduction
Traditional industry views hold that acidreacts with cement for a short period of timeuntil a protective, acid-inhibiting skin forms. Field experience in the Eastern Operating Area of Prudhoe Bay, however, has shownthat more than 37% of primary cement jobsdeveloped zonal-isolation problems after HCl/HF acid treatment. Furthermore, 73%of squeeze cement jobs broke down after HCI/HF acid stimulation. Because of theseproblems, solubility tests were performed problems, solubility tests were performed with various acids on cement cubes. Contrary to popular belief, common industrycement blends were found to be very solublein hot HCl/HF acid. Results from thecement-solubility study included the following.1. Conventional cement cubes are up to96% soluble in dynamic, hot HCl/HF acid.2. Weight loss of cement cubes variedlittle when the cubes were cured andacidized under pressure compared with weightloss of cubes cured and acidized atatmospheric pressure.3. Dehydrated cement filter cake reactedwith HCl/HF acid at roughly the samedissolution rate as the slurry cement cubes. A simulator was constructed to modelcement-squeezed perforations in a wellboreat downhole conditions. The cement-squeezesimulator (SCC) accommodated squeezing, curing, and fullbore acidizing of squeezedperforations. The CSS modeled Prudhoe perforations. The CSS modeled Prudhoe Bay bottomhole temperatures (BHT's) from160 to 220 degrees F and common squeezedifferential pressures of 1,500 psi. CSStest results showed that with conventionalcement, hot HCl/HF acid dissolved cement nodes, dehydrated cement in perforations, and primarycement. Literature searches were performed togain insight into what could be done to makecement more resistant to acid attack. Theconstruction industry offered suggestionsthat complemented the sparse oil-industryinformation regarding cement degradationby corrosive fluid. Cement blends identifiedin the search were made with fly ash, latex, reduced water slurries, and proprietarypolymers. The cement blends were screened in polymers. The cement blends were screened in a cement solubility study as documentedhere. If a product was identified asacidresistant, considerable testing wasperformed to ensure compatibility with oilfield performed to ensure compatibility with oilfield applications. Cement-blend test parametersincluded fluid-loss control and thickeningtime at BHT's and bottomhole pressures andthe ability to pump through coiled tubing(see Appendix A). If the slurry met thesecompatibility requirements, solubilityperformance was verified in the CSS with performance was verified in the CSS with heated HCl/HF acid under dynamic andpressurized conditions. pressurized conditions. Liquid latex (styrene butadiene) is the onlyadditive tested thus far that is highly resistantto HCl/HF acid and that meets the compatibilityrequirements. The latex blend's high resistanceto acid attack under simulated downhole conditionswas verified with the CSS.

Discussion

ARC.
In our literature search, we foundlittle information on reducing cementsolubility in acid. However, a few ideas hadbeen used with some success. Reducing the permeability of cement wasthe first technique considered. Although oilindustry cement blends typically have lowpermeability, the permeability can be permeability, the permeability can be reduced further with such additives as latexand low-water-ratio cement. When cementpermeability is low, decomposition of permeability is low, decomposition of cementitious matter is limited to exposedsurface areas. However, the rate ofdeterioration is accelerated when products ofchemical decomposition are washed awayby dynamic action, as in a typical acidstimulation at Prudhoe Bay. A second approach was the use of additivesto protect cement from acid attack. Oneof the more common additives is fly ash, asilica fume used in pozzolan cement. Thefly ash reacts with cement to reducepermeability and the amount of highly permeability and the amount of highly acid-reactive calcium hydroxide in the cement. Certain latex compounds and polymersalso helped to reduce the solubility of cementin acid. The polymeric constituents inhibitacid attack by coating the cement particlesand by reducing the permeability of thecement.
JPT
P. 226