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
Cement solubility in acid has proven to be a problem in the field and has recently been confirmed by laboratory testing. Traditional views have held that acid will react with cement for only a short period of time until it forms a protective skin which inhibits the acid attack. However, field experience has shown that a large percent (≈ 75%) of secondary squeeze cement jobs were breaking down after a HCI-HF acid stimulation. Further, 17% of primary cement jobs developed zonal isolation problems after a HCI-HF acid treatment of the producing perforations. This paper will document the field evidence from the Eastern Operating Area, (EOA), of the Prudhoe Bay Field, Alaska indicating that HCI-HF acid is damaging to squeezed cement. It will also show from a series of lab tests that cement is quite soluble in several of the various acids used at Prudhoe Bay. HCI-HF acid, the most destructive acid tested, showed a 96% dissolution of a 2×2×2 in. (5.1×5.1×5.1 cm) cement cube when immersed in a stirred solution of 12-3% HCI-HF acid at 190° F. Finally, the data indicates that the major factors that effect the reaction rate are HF acid volume to cement surface area and acid shear rate at the cement/acid interface.
Coiled-tubing drilling (CTD) is coming into the mainstream in a number of locations and applications, and the availability of improved bottomhole assembly (BHA) elements is a significant contributor in this regard. This paper describes BHA elements, including a hydraulic orienter and measurement-while-drilling (MWD) system, recently used by Arco in CTD operations; examines their capabilities; and provides examples in which these tools were used successfully.The BHA orienter is a newly engineered, simple, hydraulically actuated device. This paper examines its performance attributes affecting overall rate of penetration (ROP) and directional control of the BHA, including its actuation speed and its ability to change the BHA orientation while in a medium-radius curve. The MWD system represents a new application of an existing reliable system, with attributes that are especially important in the CTD environment. Modifications related to downsizing are examined, as well as relevant performance attributes including steering-data-update rate; real-time gamma log quality; mud-pulse-detection capability on coil in a range of drilling fluids; and reliability. The BHA is also looked at from an overall system perspective, including its integration with the coiled tubing (CT) and other equipment associated with the CTD operation.As of the writing of Ref. 1, Arco Alaska Inc. and THUMS Long Beach Co. (a wholly owned subsidiary of Arco) had completed 16 CTD re-entry sidetrack jobs in Alaska and Long Beach, California, with these BHA elements. This paper summarizes overall performance and reliability from these jobs. Additionally, specific cases are examined in greater detail, with these BHA elements contributing to successful CTD window milling, medium-radius curves, and laterals. (As of this publication, Arco companies have performed 47 such jobs with these BHA elements; however, this paper focuses on the first 16 jobs.)
Coiled tubing drilling (CTD) is coming into the mainstream in a number of locations and applications, and the availability of improved BHA elements is a significant contributor in this regard. This paper describes BHA elements including a hydraulic orienter and MWD system recently employed by ARCO in CTD operations, examines their capabilities, and provides a series of examples in which these tools were successfully employed. The BHA elements formerly utilized in coiled tubing drilling have often not performed as well or as reliably as the industry has become accustomed to in conventional drilling operations. 1 Principle concerns here include the overall ROP, attainment of timely survey and steering data, and directional control of the BHA. The BHA elements utilized in the recent operations represent an improvement in these critical functions. The BHA orienter is a newly engineered, simple, hydraulically actuated device. Its performance attributes affecting overall ROP and directional control of the BHA are examined, including its speed of actuation, and its ability to change the BHA orientation while in a medium radius curve. The MWD system really represents a new application of an existing reliable system, with attributes that are especially important in the CTD environment. Modifications related to downsizing are examined, as well as relevant performance attributes including: steering data update rate; real time gamma log quality; mud pulse detection capability on coil in a range of drilling fluids; and reliability. The BHA is also looked at from an overall system perspective, including its integration with the coiled tubing and other equipment associated with the CTD operation. As of this writing, ARCO Alaska, Inc. and THUMS Long Beach Co. (a wholly owned subsidiary of ARCO) have completed 16 CTD reentry sidetrack jobs utilizing these BHA elements in Alaska and Long Beach, California. Overall performance and reliability from these jobs are summarized. Additionally, specific cases are examined in greater detail, with these BHA elements contributing towards successful CTD window milling, medium radius curves, and laterals.
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