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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.
The application of salt for primary cementing in the past has been restricted largely to salt formations. Recognition of its value in cementing through fresh-water-sensitive shales and bentonitic sands has recently brought about wide usage. Formations of this latter type from different areas have been sampled and tested for the applicability of salt cement with emphasis s on improved cement-formation bonding and minimization of formation deterioration by water contact. Field surveys indicate this economical additive has helped to reduce remedial work and to greatly improve the success of primary and squeeze cementing jobs. A study has also been made of the effects of various concentrations of salt in cement systems and how these concentrations modify their slurry properties.
The application and use of sodium chloride in oilwell cementing dates back over a decade. The initial recorded use of salt with cement appeared in the completion of wells through salt domes along the Gulf Coast in the 1940's. In the absence of bulk blending facilities, salt was added to the mixing water prior to mixing with cement. This practice was followed to help provide better bonding to salt formations, as illustrated in Fig. 1. Here it can be seen that the fresh-water slurry has dissolved a portion of the salt, resulting in no bonding between the two, while the salt-saturated slurry causes no solution problem and permits contact and bonding of cement and salt. The addition of sufficient sodium chloride to provide a saturated solution for mixing cement required considerable time and expense to the operator. Foaming, which was encountered during the mixing of salt water, necessitated development of anti-foam agents and it became fairly common to add 1 pt of tributylphosphate/10 bbl of salt water to minimize this nuisance. These operational difficulties and misunderstanding of the effect of salt in cement systems probably account for the long delay in widespread use of such slurries. Another application of brines for mixing cement occurred when early cementers found that certain shaly formations could be more effectively squeezed when using water from the producing zones. However, the addition of salt to the mixing water for this specific application was rarely considered, and only scattered uses are recorded. Perhaps the earliest significant use of salt cement appeared in the Williston basin area of North Dakota and Montana. The problem of collapsed casing and tubing in salt sections and investigation of the reasons for this made it a logical consideration. Here the application was to provide good bonding to salt sections. Previous developments in blending equipment made the dry blending of salt with the cement practical for the first time. Tests revealed that granulated salt added to the dry cement in sufficient quantity to saturate the mixing fluid was a practical approach to overcome previously objectionable features. Wellhead sampling showed that the mixing provided by pumping equipment resulted in solubilization of the salt before entering the wellhead. Today, practically all salt used in oilwell cementing is dry-blended with cement before delivery to the wellsite. In studying troublesome and often expensive squeeze jobs in shaly zones, salt cement was again given consideration. Success with squeeze cementing in shaly sections in Southern Oklahoma might be considered the initial application of salt slurries for shales and bentonitic sands.
The use of salt has many unique properties for oilwell cementing. Ludwig described the general effects of salt on cement and the basic chemistry involved when cement reacts with sodium chloride in concentrations ranging up to saturation of the mixing water. More recently, Beach recognized the benefits of small quantities of salt in gel cements. Salt produces two opposite effects on the setting of cement, depending on the concentration.
New lightweight cement technology, based on increased cement fineness, has been developed in response to problems associated with cementing primary oilfield casings in cold environments. When low density slurries are required at low temperature conditions, adequate compressive strengths may be difficult to obtain. The new technology presented in this paper, called ultrafine cement, offers lightweight cement slurries paper, called ultrafine cement, offers lightweight cement slurries (10.0 lb/gal-13.0 lb/gal) which exhibit much higher early compressive strengths than conventional lightweight systems when cured in cold environments (40 deg. F - 80 deg. F).
By means of laboratory investigation, this paper introduces ultrafine cement as a unique lightweight primary cementing material. Performance properties of ultrafine slurries are discussed and comparisons to commonly used lightweight cements are presented.
Cementing oilfield casings in cold environments has been a challenge to field operators for many years. The in capability of cement slurries to gain early compressive strengths in these operations has led to increased research efforts and development of various types of accelerated cement compositions. Low temperature cement designs and proper cementing techniques, from the permafrost of the Alaskan North Slope to the cold waters of offshore exploration, have been documented extensively. Few papers, however, have been written on the performance of lightweight cement slurries in cold environments because very few lightweight cement slurries are available which can perform under low temperature conditions.
Recent advancements in particle size technology, however, have yielded a new ultrafine cement that performs with exceptional results at low temperatures. Tested in the laboratory from 40 deg. F to 80 deg. F, ultrafine cement slurries exhibit early compressive strengths more rapidly than conventional lightweight slurries of equal density.
Ultrafine cement technology was introduced initially to oilfield operations for remedial cementing. While use in this area continues to grow, refinement of the cement in both its physical and chemical properties has resulted in particle sizes physical and chemical properties has resulted in particle sizes that are 10 times smaller than standard oilfield cements. Increased surface area, requiring significantly more water, results in a low density mix, ideal for use in lightweight primary cementing applications.
The advantages of ultrafine cement for primary uses include less waiting-on-cement (WOC) time, a stronger cement sheath during drillout, simple slurry design - (cement and water), and other secondary benefits.
A detailed discussion of the properties and applications of ultrafine cement for primary cementing are presented in this paper. Also presented is a review of lightweight cement paper. Also presented is a review of lightweight cement compositions currently used in cold environments, including performance comparisons to ultrafine cement. performance comparisons to ultrafine cement.
Lightweight cements play a vital role in primary cementing operations. A prime reason for the use of lightweight cement is the reduction of hydrostatic pressure when cementing across a weak formation. Other uses for lightweight slurries include preventing lost circulation during pumping operations, preventing lost circulation during pumping operations, eliminating "fallback" after the cement is placed, cementing long annular columns, or as a "filler" cement to lower costs.
This paper presents an investigation of several slurries using field and laboratory prepared drilling fluids solidified with Blast Furnace Slag. The data presented includes base mud properties, final slurry composition, and slurry properties. This investigation includes measurements of the common properties of thickening time, compressive strength, free water, etc. It also includes an evaluation of the bulk shrinkage ot the set material, shear bond, etc., as well as rheological compatibility studies of the finished slurries with the base muds. These additional tests are considered critical in the potential application of this process under field conditions. Results of large scale bond log tests are included.
One of the main benefits from any mud solidification process is the reduction in the environmental impact. The benefit is due solely to the reduction of the volume of mud disposal requirements. Due to the dilution requirements of the mud for the incorporation of the Blast Furnace Slag, the actual volume of mud that can be "saved" from disposal may be considerably less than that reported. This study evaluates the actual reductions in disposal volumes while accounting for the dilution volumes. Economic comparisons from field operations are included as well as a theoretical comparison for zero discharge areas like Mobile Bay. Operational considerations and the economics of required mud isolation and storage are reviewed.
From the laboratory data evaluated, environmental, and economic evaluations, it is apparent the use of Blast Furnace Slag slurries for oil field applications must be carefully evaluated on a per case basis. While the process may be a viable mud solidification process, the replacement of Portland cement by this material may compromise some properties considered essential in a cementing operation.
For this investigation, three typical field muds were chosen. The muds were taken from wells representing various parts of the drilling process. An 8.8 lb/gal lightweight spud mud typical of surface holes, a 12.6 lb/gal mud often seen at intermediate casing points and a 17.6 lb/gal mud representative of the final stages of a well were used as the base muds for this study. The mud properties for each mud are listed in Table 1.
Blast Furnace Slag (BFS) slurries were prepared for each of the muds. A temperature of l85°F BHST was chosen for the investigation as this is typical for a 10,000 ft well (with a 1.1 temperature gradient) in the Gulf of Mexico. The aim was to prepare a BFS slurry that would give three to five hours of thickening time at 150°F BHCT. The formulations used for preparing the BFS slurries are found in Table 2. Note that the BFS concentration is expressed in pounds of BFS per finished barrel of slurry. In earlier investigations1-3 it is not clear whether the BFS was added to a barrel of diluted mud or was expressed as pounds per finished barrel. If the concentration of BFS is expressed in pounds of BFS added to a barrel of mud, the apparent concentration is higher. This is due slmply to the additional volume taken up by the BFS. For example, in this study Mud 5 has 300 lb BFS per finished barrel or 428 lb added to a barrel of diluted mud.
ABSTRACT The use of bentonite in cements has increased rapidly during the past four years. Cements containing 8 to 12 percent bentonite have been used successfully in several thousand wells in which formation temperatures varied from about 100 to 250 F. Good results in the placement and performance of cements containing bentonite have been obtained by controlling the setting properties of regular portland cements through the use of calcium lignosulfonate in concentrations from 0 to 0.75 percent. Laboratory and field experience shows that the following properties of cements containing 8 to 12 percent bentonite are advantageous in the cementing of many wells: 1, low slurry density of 12.5 to 14 lb per gal; 2, low water loss of about 100 cc in a 30-min API filtration test; and 3, low set strength of about 200 to 250 psi tensile. A saving of approximately 15 to 25 percent in the cost of cementing materials for a given job is realized through the use of cements containing 8 to 12 percent bentonite in preference to cements containing no bentonite, An additional saving is realized in many areas because the time and expense of performing stage jobs are eliminated by the use of light-weight slurries prepared with cements containing 8 to 12 percent bentonite. INTRODUCTION Portland-type cements have always been used as the standard material for oil-well cementing. They are usually mixed with water alone and used as "neat slurries." Neat slurries weigh 15 to 16 1/2 Ib per gal, have high water loss by filtration, and develop set strengths of 500 to 700 psi tensile. Although these properties are suitable for many cementing operations, they often lead to serious trouble. For example, the high density often causes breakdown of weak formations and loss of slurry during primary cementing jobs: and it creates high pump pressures during the reversing out of excess cement slurry in squeeze operations. The high filtration rate allows deposition opposite exposed permeable formations of a thick, semi-dehydrated cement cake that may plug the annulus, stop circulation, and cause cement to be left in the casing on primary cement jobs. In squeeze cementing, the high filtration rate allows the formation of a stiff, high solids-content mass inside the casing, thereby reducing the pressure available to squeeze the slurry into all of the perforations. The high set strengths hinder penetration of the casing, cement sheath, and formation by perforating guns. For several years, the use of small amounts of bentonite with cements has been practiced primarily to increase the consistency of cement slurries. These low bentonite-content cements produced stable slurries weighing about 14 1/2 Ib per gal and developing set strength of about 400 psi tensile. Further reductions in density and set strength were needed for many jobs, but the addition of more then about 4 percent bentonite was generally considered to be unwise prior to 1948. Work done primarily to obtain improved perforating properties in set cements resulted in the development of a composition which is prepared by the addition of bentonite and calcium lignosulfonate to cement.The properties of a cementing composition containing 12 to 15 Ib bentonite and 0.5 Ib calcium lignosulfonate per sack of cement, and the results obtained on the first few experimental field jobs, were discussed in a paper presented in 1950.Since then cementing compositions containing bentonite and calcium lignosulfonate have been used extensively in operations of Humble Oil & Refining Co. and rather generally throughout the industry. These cementing compositions are commonly referred to as " modified cements." Many inquiries regarding the field appl