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Studies have been conducted on the properties of many deep well cementing compositions to determine their strength behavior over curing periods to 180 days at elevated temperatures and 3,000 psi pressure. This pressure results in essentially the same compressive strength as that obtained with much higher pressures in accordance with the findings of an API subcommittee. These different compositions include API Classes A, E, and F cements containing additives such as retarders, bentonite, heavy weight materials and pozzolans. All compositions except mixtures of pozzolans and hydrated lime show from mild to severe degrees of strength loss over a temperature range from 230 to 320°F after setting. Pozzolan-lime compositions actually gain in strength with time at these high temperatures.
Other chemical and physical properties evaluated to observe their interrelationship with strength loss show very little correlation except in permeability. As retrogression in strength Increases, the permeability of set cement increases. X-ray diffraction patterns on the set products indicate the formation of certain compounds having little or no strength value in those cementing compositions where strength loss was more severe.
As a result of the growing trend toward deep well completions, a study has been made on the properties of the various cementing slurries presently in use at elevated temperatures and pressure. The strength of these compositions is of prime importance in selecting the most suitable material for use at high temperatures, to determine WOC time and the proper time to perforate with a minimum of shattering. Of primary interest in this investigation was the strength behavior of various compositions currently in use, and the effect of additives on cement after long periods of curing under severe conditions.
Earlier investigators have pointed out that the strength of some cements will increase with increasing curing temperature to about 220 to 240°F, but at higher temperatures a loss in strength occurs at extended time intervals. Studies by the API Mid-Continent District Study Committee on Oil Well Cements outlined a testing procedure whereby field conditions of temperature and pressure could be simulated in the laboratory. It was observed that retarded cements undergo changes at elevated temperatures and some lose as much as 50 per cent of their early strength when cured at high temperatures. The scope of these tests was limited to curing periods of 1 to 28 days; additives were not covered in this study.
An increase in the number of deep wells being drilled where extreme bottom-hole temperatures are en countered, and the anticipated drilling of wells where temperatures in the range of 500°F or higher may occur, has brought about a comprehensive investigation of cementing materials and of the techniques involved in their proper usage at these elevated temperatures.
Included are developments in cements, retarders, weighting materials and other cement additives which make it possible to formulate a variety of compositions to help resolve the cementing problems of these extreme well conditions. The problems associated with the selection and testing of cements are discussed, and a resume of field results is included.
Previous studies on strength retrogression indicate that caution should be exercised in the selection of a cementing composition for use in high-temperature wells. It now appears that, by the addition of silica flour as a stabilizing additive to certain cements, compositions covering a wide range of slurry densities can be designed to meet extreme well conditions without strength retrogression.
Improvements in cement retarders make it possible to produce a four-hour thickening time at static temperatures up to 500°F. This temperature is considerably higher than conditions presently being encountered in drilling and completion work. Inert weighting materials, used to produce 20-lb/gal or heavier cement slurries, are reviewed.
As the search for oil continues, the number of wells being drilled to depths of 10,000 ft or below has continued to increase over the past decade. During 1959, there were 50,893 wells drilled in the United States for a total of 209 million ft of hole. Of this total, approximately 4.45 per cent were drilled below 10,000 ft and 0.30 per cent below 15,000 ft, with a record depth of 25,340 ft recorded (Table 1).
With the drilling of this record-shattering hole comes the question, "How deep can wells be drilled?" Limitations with respect to equipment (surface and subsurface), cementing materials and techniques would cause some concern; however, a panel of experts have agreed that the U.S. oil industry has the equipment and know-how to drill a 50,000-ft hole. The greatest deterrent would be the 700°F temperature they would anticipate encountering at this depth.
A number of cementitious materials used for cementing wells do not fall into any specific API or ASTM classification.These materials include: Pozzolanic materials include any natural or industrial siliceous or silico-aluminous material, which will combine with lime in the presence of water at ordinary temperatures to produce strength-developing insoluble compounds similar to those formed from hydration of Portland cement. Typically, pozzolanic material is categorized as natural or artificial, and can be either processed or unprocessed. The most common sources of natural pozzolanic materials are volcanic materials and diatomaceous earth (DE). Artificial pozzolanic materials are produced by partially calcining natural materials such as clays, shales, and certain siliceous rocks, or are more usually obtained as an industrial byproduct. Pozzolanic oilwell cements are typically used to produce lightweight slurries.
Remedial cementing requires as much technical, engineering, and operational experience, as primary cementing but is often done when wellbore conditions are unknown or out of control, and when wasted rig time and escalating costs force poor decisions and high risk. Squeeze cementing is a "correction" process that is usually only necessary to correct a problem in the wellbore. Before using a squeeze application, a series of decisions must be made to determine (1) if a problem exists, (2) the magnitude of the problem, (3) if squeeze cementing will correct it, (4) the risk factors present, and (5) if economics will support it. Most squeeze applications are unnecessary because they result from poor primary-cement-job evaluations or job diagnostics. Squeeze cementing is a dehydration process.
This paper presents a review of experience for successful cementing in oil and gas wells. Recommendations and considerations for use of various cements, flushes, control additives and placement techniques are given for best results in placement techniques are given for best results in zone isolation and support of pipe in wells. Recommendations for control of difficulties with gas migration, mud contamination, high temperature retrogression, small annular clearances, and displacement of drilling fluids are described.
The purpose of this paper is to describe the latest advancements in oil and gas well cementing.
As drilling operations become deeper and more costly, the significance of zonal isolation in cemented wells cannot be overemphasized. High temperature strength retrogression of cements and inefficient displacement of contaminants from small annular clearances has become more critical for economical exploration. Use of specially formulate cements, special control additives, and well planned programs for cementing have largely alleviated the programs for cementing have largely alleviated the basic problem of cementing failures, i. e., channeling.
With basic cements, additives are used that control filtrate loss, control retardation of hardening, provide desired hydrostatic column weight, and assist placement of permanent bonding materials to help prevent fluid migration and communication. Other additives stabilize hardened cement strength and maintain low permeability in cement even under extreme conditions of temperature and pressure associated with deep drilling. The problems of cementing in complex geometries and narrow annuli have been combated by perfecting techniques of slurry placement through rheological design, use of effective washes and flushes, and improving the mechanical aspects by using pipe movement with centralizers and scratchers. The delayed set technique for cementing has also been introduced to overcome some of these complications.
This paper describes these cements, additives and techniques as they are currently being used for successful cementing.
All factors responsible for fluid migration after cementing are not yet clearly understood.