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Abstract This paper investigates the performance of different kinds of polycarboxylate (PCE) dispersants in combination with cellulose ethers as fluid loss additives and presents their influence on the fluid loss. The objective was to find out whether PCEs can better enhance the fluid loss performance of HECs than the currently applied acetone-based dispersant (AFS). Two different polycarboxylates were synthesized via free radical copolymerization using either methacrylate ester or isoprenyl ether as macromonomers and tested. The AFS polycondensate and the hydroxyethyl cellulose (HEC) were commercial samples. The impact of the PCE and AFS dispersants on the fluid loss of cement slurries achieved from hydroxyethyl cellulose was investigated via static fluid loss and rheology measurements. All PCE polymers improved the fluid loss performance of HEC significantly more than AFS; i.e. they required considerably lower dosages than AFS to achieve the same volume of fluid loss. For example, at 27 °C the addition of 0.1 % bwoc PCE to 0.4 % HEC improved the fluid loss from 250 mL/30 min to 50 mL/30 min. For the same effect, 0.4 % of AFS were needed. Most important, such low fluid loss values were achieved at very thin filter cakes, indicating that although the PCEs reduced slurry rheology considerably, no sedimentation had occurred which can also lead to low fluid loss, but is highly undesirable. Furthermore, a linear correlation between cement slurry rheology and fluid loss was observed: The stronger the dispersing effect of the PCE, the better is the fluid loss control. Among the PCEs tested, the isoprenol ether-based copolymer stood out as performing best. Dynamic light-scattering measurements revealed that the presence of PCEs strongly enhances the association of the HEC molecules, thus improving their ability to plug cement filter cake pores.
Dispersants, also known as friction reducers, are used extensively in cement slurries to improve the rheological properties that relate to the flow behavior of the slurry. Dispersants are used primarily to lower the frictional pressures of cement slurries while they are being pumped into the well. Converting frictional pressure of a slurry, during pumping, reduces the pumping rate necessary to obtain turbulent flow for specific well conditions, reduces surface pumping pressures and horsepower required to pump the cement into the well, and reduces pressures exerted on weak formations, possibly preventing circulation losses. Another advantage of dispersants is that they provide slurries with high solids-to-water ratios that have good rheological properties. This factor has been used in designing high-density slurries up to approximately 17 lbm/gal without the need for a weighting additive.
The cementation of tophole sections in deepwater operations is very challenging due to the conditions in which those jobs take place. Tophole sections in deepwater operations often have a narrow margin between the pore and fracture gradients and shallow hazards, such as gas or water flow, associated with them. The low temperature found at seabed results in high fluid viscosities, slow gel strength development, and delayed early compressive strength development of the slurry. To optimize the low-temperature slurries currently used, a novel low-temperature dispersant was developed that enables designing the slurry to meet the required properties. The new dispersant has several main benefits in slurry design and properties. The dispersant enables the slurry to develop faster gel strength and has no retarding effect compared to conventional dispersants. The new dispersant will provide a flat rheology over time, allowing better control of the circulating pressures during placement. Laboratory tests show less gelation and an improvement in static gel strength development as compared to current dispersants. The dispersant also provides better rheological properties, often at lower concentrations, thereby facilitating a simplified slurry design. Following laboratory testing, the low-temperature dispersant was introduced in Gulf of Mexico operations with very good results. Operationally, the dispersant is easy to handle. It is compatible with all typical cementing additives and can be used with conventional cementing equipment and liquid additive systems. The low-temperature dispersant has been used in various types of deepwater slurries such as lightweight, conventional, and foamed slurry system, using drill water or seawater as the base fluid, often requiring less concentration and improving key slurry properties to achieve successful cementing operations in challenging low-temperature conditions.
Summary The dispersant's synthesis and its effect on slurry rheology are the basis of mud to cement (MTC) technique application. In this article are two methods to synthesize the dispersant by using fractions of C9 which are byproducts of the Daqing ethene production unit. The dispersant's production cost is reduced. Experimental work verifies that material produced by the new process can be used in a MTC design. The effect of the amounts of the dispersant and mud used on the rheology of MTC slurry are studied and discussed. The results show that the thickening time of a MTC slurry is clearly extended by mixing in the dispersant, thus improving the mobility of the slurry. We find that there is a low mobility or high consistency region when a MTC slurry is initially prepared. The appropriate measures for improving the quality of the MTC slurry are provided in this article. Moreover, the rheology of a MTC slurry is studied. We find that the Herschel-Bulkley model is the most accurate for describing the rheological properties of a MTC slurry. Last, the application of MTC technology in the Changqing field is introduced. Introduction Conversion of drilling fluid (mud) into cement suitable for well cementing operations has been an area of interest in the petroleum industry for more than 50 years. Improved zonal isolation in the annular space between the casing and the borehole has been and continues to be the primary reason for pursuing this technology. For nearly a century, zonal isolation has been attempted largely by the placement of Portland cement formulations in the annular space. The widely practiced cementing process is plagued by variables that are often difficult to predict or control yet have a critical impact on the quality of the seal achieved. Two of the variables include effective removal of the drilling fluid occupying the annulus and the effects of mud or spacer fluid contamination on the cement formulation properties in the liquid (slurry) and solid states. With previous art methods and compositions, the displacement of the drilling fluid has been incomplete because of gelation and has often resulted in poor cement bonding or incomplete filling of the casing-to-wellbore annulus with homogeneous cement. Mud solidification has been pursued as a means to improve zonal isolation because of the following:better rheological compatibility between the cementing fluid and the mud that contributes to better mud displacement, lower impact of mud contamination on the performance properties of the converted mud in the fluid and solid state, and improved sealing in the annulus because of the potential for solidification of the filtercake and any undisplaced mud. 1 The conversion of drilling fluid to a cementitious slurry is not without some operational problems and undesirable compositional changes. For example, the addition of cementitious materials such as mixtures of lime and silica and alumina, lime and magnesia, silica and alumina and iron oxide, or cement materials such as calcium sulfate and Portland cements to aqueous drilling fluid can result in severe flocculation and substantially increase the viscosity of the fluid mixture. Efforts to circulate such mixtures through a wellbore can result in a very high equivalent circulating density or very low circulating rate, plugging of the wellbore annulus, breakdown of the earth formation in the vicinity of the wellbore and failure of the cement slurry to properly mix. Until the early 1990's, MTC technology has been of use to industry because of the development of a high quality dispersant. In-depth study and application of MTC technology have been conducted in America, Russia, Canada, and Norseland and elsewhere. But no studies of the rheology of the MTC slurry have been reported until now. Presently, research in this field is being carried out in China. This article is the first to introduce the dispersant's synthesis. The dispersant used for the design of MTC is a styrene sulfonic acid-maleic anhydride (SSMA) copolymer. On the basis of experiments, the effects of the amounts of dispersant and mud on the rheology of MTC slurry are studied and discussed. This work will bring mud solidification technology into wider use. Synthesizing Methods of SSMA The basic concept behind MTC technology is that drilling fluid (mud) and cement are made compatible through the use of a dispersant. The dispersant is the key element that allows the mud to be mixed with the cement and results in the preparation of a pumpable slurry. Many kinds of dispersants have been tried in MTC designs, but most are expensive. The dispersant used in this work is made by using fractions of C9 (the boiling point of which is between 328 and 473 K, and the styrene and methyl styrene content is 36.7%) which are a byproduct of the Daqing ethene production unit. The fractions of C9 and maleic anhydride are first polymerized. Then, the product is sulfonated and saponified. The final product is a SSMA dispersant suitable for use in a MTC design.
Reddy, B. R. (Aramco Services Company: Aramco Research Center-Houston) | Contreras, Elizabeth Q. (Aramco Services Company: Aramco Research Center-Houston) | Boul, Peter J. (Aramco Services Company: Aramco Research Center-Houston)
Abstract Achieving quality cement performance in deep water wells with large amounts of salt present requires fundamental understanding of the salt effects on the interactions of cement additives with cement on the slurry performance. Many problems encountered later in the life of the well can be traced to slurry performance issues during placement and the initial setting process. Cement slurry designs for an oilwell can range from simple to highly complex depending on the geology, lithology, placement logistics, wellbore conditions and long-term performance. Logically designed cement slurry formulations and dependable additive behavior are critical to meeting cement performance requirements. Formulations can include mineral admixtures added in substantial quantities and additives in smaller (<2% by weight of cement) quantities. The performance of the additives in cement slurries depends strongly on competitive adsorption on cement and mineral surfaces. Adsorption interactions are directly influenced by downhole temperature, the nature of the formation, and the mix water composition. These aspects underline the need for understanding additive interactions with cement under wellbore conditions. Of particular relevance to deep water cementing is the performance of additives in sea water containing monovalent and divalent salts with chloride and various other anions present in amounts close to 4% by weight. The problem of additive performance becomes even more of a concern when cement is placed against salt formations. High ionic strength of sea water or of a sea/fresh water slurry placed against a salt formation, can compress the electrical double layer of cement clinker particles, alter chain conformations of ionically charged or hydrophobically modified polymeric additives, or change the solubility of additives. Additives may perform extremely well in fresh water slurries, but may perform poorly in slurries with high salt content. Additionally, the salt itself can affect cement performance. For example, sodium chloride is a set accelerator in small quantities (≤13% by weight of cement) while it functions as a retarder in large quantities. The objective of this presentation is to gain molecular level understanding of relationships among additives based on chemical structures, adsorption onto cement surfaces, and the mix water ionic strength. In this study, typical additives for dispersion and retardation are contacted with Portland cement in deionized water, synthetic sea water, water containing 2% to saturation level of NaCl or divalent salts in a formation brine. The reactions were analyzed by isothermal calorimetry, UV/Visible spectroscopy and rheology. The interpretation and implications of laboratory results are presented.