Oilfield Cement Research Articles

Repairing Casing Leaks Using Small-Particle-Size Cement

Summary Emerging small-particle-size cement technology has been applied successfully to one of the oil field's most frustrating problems, repairing tight casing leaks. Because of these leaks, a continuous injection rate cannot be established, yet test pressure leaks off 300 to 500 psi in 10 to 20 minutes. Confirmed through experimental data and more than 100 field operations, a new approach offers exceptionally high success rates for these troublesome leaks. This paper presents a detailed introduction to these new cement systems, along with comparative studies of small-particle-size cement vs. standard oilfield cements. This paper also discusses repair methods, several case histories, and field jobs performed to date.

Development and Application of a Knowledge-Based Expert System for Cement Slurry Design

Summary Traditionally, quality oilfield cement slurries have been based on rules of thumb and years of experience. The designs used in specific areas often were developed many years ago. Although these slurries may have used the best technology available when they were designed, these designs often can be improved with newer technology and products. A knowledge-based expert system can provide information regarding the latest products and technology, as well as the knowledge necessary to use them properly. This paper discusses the expert slurry-design system (ESDS) and the various considerations involved in developing a useful and usable expert system. This discussion covers establishing system goals and requirements, developing the knowledge base, creating the user interface, operating the system, and confirming ESDS results.

Low Strain Shear Behaviour of Cement Slurries at Quasi-static Rates.

AbstractThe response of a freshly mixed oil-field cement slurry to an imposed shear strain has been experimentally studied using a method based upon concentric cylinders rheometry. At various stages of the reaction of cement with water, the slurry was deformed at quasi-static rates and the resultant shear stresses calculated. In an attempt to reduce errors due to wall slip, porous coated bobs which allow some infiltration of the slurry have been used. Stress-strain curves for the cement have been produced and the variation of yield stress with time after mixing evaluated. The experimentally determined stress-strain curves exhibited three characteristic regions. Firstly, a region of negligible stress increase at very low strain. A region of rapid linear increase in stress with strain, and finally, a post-yield strain softening region. The low strain (<0.01) response of cement slurries has been compared to that of suspensions of polyethylene beads in water with similar solids volume fraction and mean particle size. A number of similarities in the stress versus strain curves of these two granular suspensions have been observed.

Effect of Perforations on Fracture Initiation

Summary. This paper describesperforation/fracture tests performed perforation/fracture tests performed in large sandstone blocks in a triaxialstress cell to determine perforatinggeometry and perforating-fractureprocedures for optimal fracture procedures for optimal fracture initiation. Four-shot, 1800 phasedperforating guns, steel casing, and perforating guns, steel casing, and oilfield cement were used. In oneseries of experiments, the casing wascemented and cured while undertriaxial stress. Most tests were madewith pore pressure, and vertical andhorizontal wells were simulated. Ingeneral, the tests showed that(1) fractures initiate either at the baseof perforations or at the intersectionof the plane normal to the minimumhorizontal stress that passes throughthe axis of the wellbore and thewellbore surface and (2) fracture initiationdepended on perforation orientationwith respect to the plane normal tothe minimum horizontal stress andthe properties and injection rate ofthe fracture fluid. All fracturesreoriented into the plane normal to theminimum horizontal stress within onewellbore diameter; although multiplefractures were initiated, only primarysingle fractures propagated beyondone wellbore diameter. Introduction Laboratory simulation of fracturing throughcased and perforated wellbores generally hasbeen performed with one or more of thefollowing limiting conditions:scaled tests(rate effects ignored),artificial rock(cement, plaster of Paris, or hydrostone),isolation of the perforations from thewellbore (no pressurization between thescaled casing and the rock),noporoelastic effects (impermeable rock or poroelastic effects (impermeable rock or high-viscosity fracturing fluid),artificially scaled perforations(no perforation damage), andno pore pressure (impermeable ordry rock). Extrapolation of laboratory results todownhole situations must proceed withcaution whenever any of these conditions arepart of the laboratory experiment. For part of the laboratory experiment. For example, Condition 3 is unrealistic downholeand Condition 5 may apply only in specialsituations of high underbalance perforating. Conditions 4 and 6, however, may simulatea well with extensive wellbore andperforation damage. To the best of our knowledge, perforation damage. To the best of our knowledge, Warpinski's mineback experiments, inwhich annulus fractures were observed, were the only experiments conducted in theabsence of these conditions. The experiments discussed in this paperwere planned to evaluate the effect ofperformations on fracture initiation. Because of performations on fracture initiation. Because of the limiting-condition issues discussedabove, fill-scale experiments wereconducted in sandstone rock in a large triaxial stressframe that simulated downhole conditions. Test Fixture The fracture-initiation experiments wereperformed in 27×27×32-in. sandstone performed in 27×27×32-in. sandstone blocks in Terra Tek's 8, psitriaxial stress frame. The circular frame andits top and bottom platens surround the rockand provide reactions for three independentpairs of flat jacks. Flat-jack efficiency was pairs of flat jacks. Flat-jack efficiency was measured at 91 %; all applied stress datause this correction. Access to the centralwellbore is provided through the top loadingplate. Pore pressure was applied by placing plate. Pore pressure was applied by placing the rock in a stainless-steel can with rubberseals on the top and bottom faces. The canallowed the placement of about 0.25 in. ofbauxite beads around the four faces. Theborehole was cored to 4% in., and 3-in. OD × 2 1/2 -in. ID) steel tubing was cementedin place. Through-tubing, 1 11/16-in. orin., 4-shot/ft (SPF), 180 degs phased guns withthree or four shots were used. Table 1 givesthe rock properties. Experimental Procedure Three sets of tests were conducted (Table2). Experimental techniques were enhancedwith each additional set. Rock saturation andpore pressure were added to Set 2. An in-situ pore pressure were added to Set 2. An in-situ pore pressure gauge, a large 2.38-gal pore pressure gauge, a large 2.38-gal intensifier, and casing cemented and cured understress were added to Set 3. The general test procedure was as follows.1. Vacuum saturate the rocks with 3%brine, flowing brine from the uncasedwellbore to the rock sides (Sets 2 and 3 only).2. Cement casing into rock and allow tocure (Sets 1 and 2 only).3. Place rock into test frame. For Set 3, casing was cemented in place before theframe was closed and cured while understress.4. Place perforating gun in wellbore.5. Close frame, apply desired flat-jackand pore pressures; the wellbore is ventedto atmosphere.6. Fire gun. For Set 3, the casing cementcured for a minimum of 24 hours before thegun was fired.7. Flow brine at 2,000 psi from the beadpack through perforations; measure flow pack through perforations; measure flow rate8. Perform various prefractureprocedures depending on test set. procedures depending on test set. 9. Fracture rock with red dye used infracture fluids.10. Remove, cut open, and examine rock. Experimental Results Set 1, Torrey Buff Sandstone. Equipmentfailure prevented on of the specimensin this set. After perforation, the wellborewas flushed with brine, and on Tests 2 and4, brine was injected at low pressure throughthe perforations to saturate the rock locallynear the wellbore. JPT P. 608

Chemical Shrinkage Properties of Oilfield Cements

Summary Chemical shrinkage was measured at elevated temperatures and pressures for 29 different cements with modified PVT equipment. Results showed that a "dilution" effect exists; i.e., the cements that had the highest yield had the lowest chemical shrinkage. For the 29 tests, the maximum chemical shrinkage measured was 4.6% and the minimum was 1.6%. Tube column tests showed that chemical shrinkage helped to reduce hydrostatic pressures.

Oscillating rheometer measurements on oilfield cement slurries

Cement slurries which have their application in the oilfield industry have been evaluated on an oscillating rheometer. Viscoelastic properties were observed in four slurries; a neat slurry, a polymer modified, a polymer/sodium silicate modified slurry and a sodium silicate modified slurry. The viscoelastic properties may originate from dissolved polymers, electrostatic attraction between solids or from matrix structures associated with cement curing. The viscoelasticity was not restricted to gelled slurries. The viscoelastic properties were still observed after the gel structure has been broken. The polymer/sodium silicate containing slurry also exhibited a yield stress as observed with the oscillating rheometer.

Mobile Laboratory Improves Oilfield Cementing Success

Summary A newly built mobile cement-testing laboratory assists in monitoring blended cement quality and design. The unit contains the equipment required to perform tests for oilwell cements described in API's Spec 10. Case histories are presented describing how use of the mobile cement laboratory improved cementing success on critical cement jobs. Introduction Cementing practices, equipment, and materials have changed as wells have become deeper and technology has advanced. Many cementing systems have been devised. The number of different systems that can be designed by varying the components is almost endless. The pumping time of a cementing system is controlled by the class of cement, the well temperature and pressure, and the type and amounts of additives. In 1939, the first pressure/temperature thickening-time tester was developed, enabling, the industry to forecast slurry performance accurately. Proper "aboveground" density is essential to the successful accomplishment of any well-cementing operation. Only a small amount of water (about 25%) is necessary for cement to set satisfactorily. More water must be added, however, for the cement system to be pumpable. Mixing and pumping equipment has evolved continuously over the past few years. Unfortunately, maintaining slurry density during, cement jobs is still a problem. Cement is usually handled in bulk form, In most cases, additives can be blended with bulk cement to suit any well condition. To achieve success. critical cement-jobs require accurate testing in the laboratory, proper blending of the cement and additives, and correct mixing of the slurry to designed density. A gamut of pilot tests must be run to ensure a good slurry design. After the system has been blended, samples also must be tested. Many oil companies depend solely on the cement service companies to meet these criteria. Chevron, however, has developed a cooperative testing program that works closely with the service company field and laboratory personnel. This program relies heavily on monitoring the quality and performance of cement and additives. Using this program, Chevron and the service company can verify the slurry performance. Over the past few years, a number of major oil companies have experienced situations in which pilot test and blend sample test results did not correlate. Some companies are willing, to accept a 40% variation in thickening time between the pilot and blend tests. In 1982, we implemented a field study to evaluate service companies' blending equipment and procedures. The following recommendations resulted from this study:layer the cement and additives,weigh additives on a close tolerance scale,move the cement a minimum of six times before samples are taken, andobtain accurate (representative) samples while the blend is going to the tank in which it will be transported to the rig. Very close correlation between pilot and blend sample tests was achieved with the implementation of these recommended procedures. While developing these recommendations. we observed a long period between blending, of the cement with additives and testing of the samples. Although the samples were "hot shotted" to both the Chevron and service company laboratories, in most cases, the cement was on location before testing of the blend samples began. In the past, other methods of analyzing cement blend samples. such as a chemical analysis, have been attempted, but none provided the accuracy of a thickening-time test on a high-pressure, high-temperature consistometer. Because of the time required to ship samples and inadequate alternative testing methods, a full-scale testing laboratory with the ability to operate at remote wellsites or service company blending facilities was needed. Capabilities We designed the mobile cement-testing laboratory with the following objectives:equip a self-contained vehicle with equipment that would reliably perform thickening-time. fluid-loss, free-water, and rheology tests on cement slurries for critical casing strings on any well:to provide living accommodations for two people on location:to provide reserve capabilities;to be within the weight limitation of the vehicle:to meet safety requirements;to provide access to laboratory equipment for maintenance and service:to provide for the calibration of the testing equipment; andto be cost-effective. JPT P. 1032^

Analytical Chemistry of Portland Cement and Its Oilfield Admixtures

Summary The successful cementing of boreholes in earthen formations requires careful control of the properties of cementing slurries and of the final set product. Accurate, dependable, time-efficient chemical analyses are prerequisites in determining quality of cement, additives, field blends, slurries, and set cement. A review highlighting chemical, spectrochemical, microscopic, and thermal methods of analysis of portland cement and oilfield cementing systems is presented.