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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.
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.
A new fluid loss additive has been developed for a broad range of applications. A synthetic polymer has been used to control fluid loss values polymer has been used to control fluid loss values to meet specific requirements in the design of lightweight, standard-weight, and heavyweight slurries at circulating temperatures from 180 degrees F to 400 degrees F. Polymer-containing slurries mixed with fresh water, sea water, and water containing potassium chloride or sodium chloride (up to saturation) exhibit fluid loss values as low as 20 cc/30 min within these temperature ranges.
Various methods of determining fluid loss at temperatures above 190 degrees F are utilized within the industry. These methods are explained and compared. The new additive does not have a pronounced affect on cement slurry viscosity. Slurries may be formulated to have low fluid loss values while being neither overly-viscous nor overly-dispersed. Such properties permit displacement without undue properties permit displacement without undue friction pressure at reasonable annular velocities, thus helping improve displacement mechanics and ensure efficient drilling fluid removal.
Results of using this polymer have been to help improve the probability of obtaining a successful cement job by (1) minimizing the risk of bridging, (2) enhancing displacement mechanics, and (3) providing early casing protection. Field case providing early casing protection. Field case histories are provided.
Introduction of a recently developed cementing fluid loss additive has helped simplify the cement slurry design process under difficult design conditions. This new material, which has been used extensively in the Gulf Coast area, represents the introduction of a new chemical technology in cementing materials. Consisting of a polymeric material, the additive provides fluid loss control properties for a variety of cement compositions in properties for a variety of cement compositions in hostile well environments.
Formulation of cement compositions for many well environments can be difficult to achieve, particularly when a number of response parameters particularly when a number of response parameters (fluid loss, compressive strength, free water, etc.) must be met. High temperature well conditions and well intervals with large temperature differentials can further aggravate the situation. In general, it can be said that the more response conditions that the cement slurry must satisfy, the more complicated the slurry design process becomes.
In many cases the design process essentially becomes a balancing act of adjusting additive concentrations to obtain specific cement parameters, then readjusting certain other additive concentrations. Additive limitations concerning compatibility and durability can be intensified as the number of variables increases, and as well conditions become increasingly severe. The presence of high concentrations of salt, in particular, can have an adverse effect on the performance of some of the conventional, cellulose-based fluid loss additives.
An analysis of the laboratory development and field application of the synthetic polymer fluid loss additive illustrates its utility in a variety of cement job designs. A discussion of fluid loss measurement procedures is also presented, including a description of an instrument designed for high temperature (up to 400 degrees F) fluid loss testing.
With respect to the fluid loss behaviour of a cement slurry basically two stages need be considered: (i) a dynamic one corresponding to the placement and then (ii) a static one, the waiting on cement. During the first period the slurry flow is eroding the filter cake as it is growing thus rapidly a steady state is reached where the filtration occurs through a cake of constant thickness; at the same time, since the slurry is losing water but no solid particle, its density is increasing in line with the fluid loss rate. During the second period, referred to as "static", the cake grows due to the absence of flow. It may possibly grow up to a point where it locally but completely fills the annulus: bridging takes place and the hydrostatic pressure is no longer transmitted to the deeper zones. Dynamic: A maximum acceptable value of the total volume of fluid lost during the cement placement can be easily calculated from an upper density limit. Since basic slurry properties, like thickening time and rheology are greatly dependent on density (at least in the low water content domain), they may be used to define this limit. To convert the total amount of fluid loss into an API fluid loss value, three parameters are needed: the permeable formation area, the mudcake thickness and its permeability. Static: A maximum acceptable value of the thickness of the cake built-up during the waiting on cement period is deduced from the annular gap. The time taken by the filter cake to completely fill up the annulus, i.e., the bridging time, depends on the cement and mud cake properties (mudcake thickness and permeability and cement cake permeability). Therefore, by comparing thickening time and bridging time, a maximum value for the cement cake permeability can be deduced which is then expressed as an API fluid loss value.
From typical mudcake resistances it can be estimated that, both in dynamic and in static conditions, the fluid loss could, in some conditions, have to be reduced to an API value one order of magnitude lower than what is generally considered as a fair control of fluid loss. Some examples are given. However, still very little is known about the effect of spacers, washes, mechanical aids and cement itself on the mudcake state and more data have to be gathered on mudcake thickness and permeability under various conditions, before definite fluid loss limits can be asserted.
For more than 20 years, fluid loss control agents have been added to oil-well cement slurries and it is now recognized in the industry that the quality of cementing jobs has significantly improved. Indeed, it is generally clearly acknowledged that a lack of fluid loss control may be responsible for primary cementing failures, due to excessive density increase or annulus bridging and that formation invasion by cement filtrate may be deleterious to the production. With respect to squeeze-cementing, the problem is to adjust the level of fluid-loss to perforation size and formation nature. However, both for primary and remedial cementing very little has been written to justify the level of fluid loss control really required to achieve a good cement job. To address properly the quantitative evaluation of fluid loss limits compatible with successful cementing operations, two different stages have to be considered, first, placement or dynamic stage and then, waiting-on-cement or static stage. During the first stage, the slurry is flowing and eroding the cement cake which, after a short transient period, stops growing. In contrast, when the pumping is stopped the cake can grow freely.
Abstract In deepwater Gulf of Mexico, the use of synthetic-based drilling fluids (SBM) is common practice in all types of wells drilled by different operators. These fluids have been under constant development in past years. However, even with the latest in SBM technology, cementing operations can be adversely affected when this type of fluid is used, which can compromise the quality of the cement jobs. One of the challenges faced is the rheological incompatibility between the cement slurry and the SBM. This may lead to issues such as induced losses during primary cementing operations, due to higher friction pressures, or stuck pipe during plug placement, among others. The higher friction pressures during cement placement in primary jobs can also lead to an inaccurate or inconclusive post-job evaluation when attempting to match software-simulated pressures with actual pressures acquired during the jobs. Despite the use of mechanical separation and spacers, the post-job analysis of several recent cement jobs suggests that contact between cement slurry and drilling fluid is often still occurring. As expected, this contact is most frequently occurring in the annular space wherein there is typically an absence of mechanical separation. In these jobs, laboratory test results using mixtures of slurry and SBM with various ratios have shown levels of incompatibility, which have been correlated to evidence of higher-than-expected friction pressures in the same jobs. The solution proposed for this scenario is to add surfactant or surfactant-based chemical additives at low concentrations to the cement slurry. The addition of surfactant to cement slurries has been proven to reduce rheological incompatibility between the slurry and SBM and the impact of contamination on set cement properties. This paper presents the laboratory test results, operational concerns, mitigation, and a case study showing the application and effectiveness of this technique comparing similar strings that were cemented in different wells.