Wellbore Simulator Research Articles

HCl/HF Acid-Resistant Cement Blend: Model Study and Field Application

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

Effects of Bit Hydraulics on Full-Scale Laboratory Drilled Shale

Summary The effects of bit hydraulics while drilling shale with a standard three-cone bit are examined in this paper. Tests were conducted by drilling into large-diameter, intact shale samples at simulated downhole conditions in Drilling Research Laboratory's wellbore simulator. The shale samples were recovered from massive surface outcroppings and preserved for laboratory use. The effects of hydraulic horsepower from 20 to 400 hhp and bit weights from 20,000 to 50,000 lbm on rate of penetration are presented. Introduction Most deep shales and some intermediate shales found in the U.S. are typically "slow" drilling formations. Efforts to improve rate of penetration in shale have been performed in both field tests and small-scale laboratory investigations. Until recently, laboratory drilling studies on shale have been confined to microbit1 or single-cutter2 studies because the ability to obtain and preserve large, intact shale samples had not been developed. In addition, laboratory facilities where full-scale drilling tests could be conducted at simulated deep-well conditions did not exist. Massive surface shale formations have been located and techniques have been developed to extract and preserve large-diameter, intact samples. With these samples and the ability to simulate full-scale drilling conditions, a systematic, technical approach was taken where the effects of drill bit hydraulics were examined while drilling shale at simulated downhole conditions. Background and Definitions The in-situ conditions of a typical deep wellbore and surrounding rock formation are illustrated in Fig. 1. The formation is subjected to overburden stress, confining stress, wellbore pressure, and formation pressure. As previously demonstrated,3–5 rate of penetration is influenced strongly by the bottomhole conditions and appears to be most sensitive to the differential pressure between the wellbore and formation. These effects, however, can vary widely depending on rock properties such as rock type, strength, density, permeability, and mud properties such as composition, filtration rate, viscosity, solids content, and particle size.6 Bit hydraulics - i.e., the means of removing cuttings from the hole bottom and cleaning the bit with the drilling fluid - are a key factor in improved bit performance. Bottomhole cleaning theories,7 microbit studies,8 and full-scale laboratory drilling experiments in hard, impermeable rock9 have shown the need for adequate bit hydraulics to maximize rate of penetration and avoid bit balling. Field tests with extended nozzles10 also have demonstrated great potential for increasing rate of penetration in certain formations with improved bit hydraulics. Some nondrilling laboratory studies have shown the effects of nozzle size on pressure distribution at the hole bottom.11

Evaluation of Control Techniques for Unconsolidated Silty Sands

In a laboratory study of sand-control techniques for unconsolidated silty sands in high-capacity wells, wellbore simulations were used to model a typical completion in the offshore Teak field. Among other results, it was found that the optimum control system was an open-hole gravel pack or a triple-wrap screen when used in open hole. Introduction Amoco Trinidad produces from three offshore fields near Trinidad in the West Indies. As shown in Fig. 1, these are the Samoan, Teak, and Poui fields. These fields produce from Miocene-age sands and contain both oil and gas wells. Although sand production exists, the Samoan and Poui fields do not have the same degree of sand-control problems as does the Teak field. After discovery of the Teak field, it became apparent that some type of sand control would be necessary; the two types of control chosen were triple-wrap screens and gravel packing. Initially, wells completed with the triple-wrap screen were capable of high producing rates; therefore, emphasis was placed on this type of installation. At the same time, wells that were completed with gravel packs were performing poorly. Although attempts were being made to stop sand production down hole and the triple-wrap screens appeared to be working, trace amounts of sand were continually being produced from all wells. These traces of sand were sufficient to erode choke manifolds and flowlines and to fill up onshore and offshore separators. A triple-wrap screen was pulled after only 21 days on production. This screen, as pulled after only 21 days on production. This screen, as shown by Fig. 2, had multiple eroded holes. Shortly thereafter, a gravel-pack screen that was pulled after less than 2 weeks on production had similar eroded holes. Subsequently, most wells pulled had holes eroded in the screens. At this time, a project was initiated to determine the optimum sand-control technique for these operations. Using specially designed wellbore simulators, tests were run to evaluate conventional gravel packs, to determine the effectiveness of prepacking, and to compare the triple-wrap screen with prepacking, and to compare the triple-wrap screen with the gravel-pack system in either cased or open-hole completions. Project Objectives Project Objectives Our basic approach to this sand-control problem had been to stop the migration of sand down hole by installation of either the conventional gravel pack or triple-wrap screen. Neither system was completely successful because of erosion of the screens, chokes, flowlines, and valves by produced sand. Disposal of the produced sand was also a problem. The primary purpose of the project was to find a way to stop sand production and project was to find a way to stop sand production and maintain high productive capacity. The objectives of this project were to (1) evaluate a conventional gravel pack, where gravel is placed only in the annulus between the screen and casing; (2) evaluate the effectiveness of prepacking or placing gravel outside the casing in conjunction with a gravel pack; (3) compare the triple-wrap screen with gravel pack; (3) compare the triple-wrap screen with gravel packing to determine which would better retain sand packing to determine which would better retain sand and allow maximum productivity; and (4) compare open-hole and perforated-casing completions with gravel packs and triple-wrap screens. Experimental Procedures Apparatus - Wellbore Simulators JPT P. 979