Summary The crystalline nature of hydrated Portland cement is dependent primarily on temperature. The calcium silicate hydrate (CSH) gel is produced at low temperatures and, upon curing at higher temperatures, will convert to one or more crystalline phases. The better cementing compositions contain a low lime-to-silica (C/S) ratio. Xonotlite is a phase commonly produced above 150 deg. C (302 deg. F) when approximately 35% fine silica is added to Portland cement. Generally, it has good strength but moderate permeability, Truscottite, produced when an even larger quantity of silica is added to the cement, has lower permeability than xonotlite but is slightly more difficult to produce and to stabilize. Pectolite can be produced by introducing sodium into a truscottite-type formulation. Once formed, pectolite is very stable but typically has high permeability. The addition of carbonate to any of these formulations may produce scawtite. Scawtite appears to be an inferior phase by itself, but in small quantities it can be helpful in strength development. Introduction The failure of wells in several geothermal fields has been directly attributed to degradation of cement. This implies that the cementing materials used to complete geothermal wells had not been sufficiently evaluated. For the past 3 years, under the auspices of the U.S. DOE, we have studied geothermal cementing materials in an attempt to identify suitable systems. A major portion of this study was devoted to research on the behavior of calcium silicate hydrates at the high temperatures found in geothermal zones. The literature contains many references pertaining to calcium silicate hydrates in wells at temperatures up to 150 deg. C (302 deg. F), but little has been published concerning higher temperatures. Portland cement is the material normally used to seal steel pipe in a borehole. Originally designed for hydration at or near atmospheric temperature, Portland cement can be adapted for use in petroleum or geothermal wells with bottomhole temperatures approaching 370 deg. C (700 deg. F). The hydration chemistry and phase equilibria of Portland and similar calcium silicate cements change with increasing temperature. At atmospheric temperatures, tricalcium silicate (C3S)* and dicalcium silicate (C2S), which comprise about 75% of the dry Portland cement composition, react with water to form a CSH gel with variable composition and calcium hydroxide (CH). A cement slurry becomes rigid when less than one-half, and sometimes less than one-fourth, of the cement has hydrated. At this point, pores begin to close and free movement of water is no longer possible within the cement. Consequently, a true gel is formed that is strong and impermeable. Calcium ions migrate from C3S and C2S particles into the water trapped in pores. Silica migrates from quartz (sand) grains into the water at various locations. The resulting calcium silicate reaction products are high in calcium at one point and high in silica at another. Aluminum ions released by another important compound in Portland cement, tricalcium aluminate (C3A), are also of concern. Considering the number of calcium silicate compounds and aluminum substitutions possible, it is surprising that reasonably pure cement phases are commonly obtained. As temperature increases to about 120 deg. C (247 deg. F), CSH gel converts to other crystalline forms. If excess calcium hydroxide is present, alpha dicalcium silicate hydrate (alpha-C2SH), a very weak and porous material, is produced. Fine silica is normally added to Portland cement to prevent this. If at least 35% silica is added to Portland cement, to bermorite (C5S6H5 approximately), also a strong and impermeable binder, usually is formed. JPT P. 1373^
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Neat Portland cement systems lose strength and become permeable at temperatures above 250 degrees F. This deterioration usually is more extreme over the first few days or month of heating but is usually not severe enough to cause disintegration of the neat cement. After this initial regression many neat Portland systems will regain a portion of Portland systems will regain a portion of their strength and reduce in permeability. This temperature regression of cement can largely be prevented by using about 35 per cent very fine silica sand. The strength can be maintained or increased but permeability will increase. Most additives for oil well cements can be included in such a silica stabilized system without extensive effect due to temperatures up to 600 degrees F. An exception to this is fly ashes and, to some degree, natural pozzolans. These are stable at 450 degrees but are losing strength and recrystalizing 600 degrees F. Introduction The deterioration of Neat Portland cement at temperatures above 250 degrees F (120 degrees C) has been known for many years. Menzel proved that fine silica added to a Portland cement paste would improve the strength of cements cured at elevated temperatures. An increasing number of wells are being subjected to these elevated temperatures each year. Deeper and hotter oil and gas wells are being drilled and other wells are being subjected to hot water, steam, or fire flood methods. High temperature geothermal wells are also increasing in number each year. The cement in these wells will be subjected to elevated temperatures from bottom to top. It is probable, therefore, that the surface and intermediate strings on many of these wells are not adequately protected from deterioration.
Work performed under the auspices of U.S. Department of Energy, Contract No. EG-77-C-02-4190 and coordinated by Brookhaven National Laboratories. By acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a non-exclusive royalty free license in and to any copyright covering this paper. Abstract Portland cements require the addition of fine silica (quartz) to maintain reasonable strength and permeability at elevated temperatures. The production permeability at elevated temperatures. The production of the desired high temperature stable silicate, xonotlite, is dependent on the calcium to silica composition of the cement, curing temperature, particle size of silica, particle size of cement, and fluid (fresh water, brine, etc.) contacted by the cement. Greater deterioration is generally observed when compounds having higher calcium to silica ratios than xonotlite are produced. The production of one of these compounds, kilchoanite, is favored by using a fine grind cement with a coarser silica at elevated temperatures in contact with a heavy brine. A relevant factor is that a coarse silica has a greatly reduced rate of solution in a heavy brine at elevated temperatures. Sand fractions from approximately 175 micron (80 mesh) to 3.2 average micron range were tested and compared with commercial fine silica and silica flour as stabilizers for Portland cement. The finer fractions, including silica flour, produced lower permeabilities and usually higher strengths. Addition of permeabilities and usually higher strengths. Addition of 35 percent silica flour by weight of Portland cement is sufficient to stabilize the hydration products of Portland cement under usual geothermal conditions up Portland cement under usual geothermal conditions up to 325 degrees C (617 degrees F), the highest temperature in this study. Coarser silica may be used where temperatures are lower, steam or light brines are encountered, or coarser cements with extra silica are used to prepare the slurries. Introduction The research reported in this paper was performed in connection with a Department of Energy (DOE) contract to investigate current geothermal cementing technology and develop improved cementing materials for geothermal wells. The chemistry of silica stabilized Portland cements exposed to dense brines at geothermal temperatures is unusual, and we find that this aspect of cement chemistry has not been previously mentioned in literature. previously mentioned in literature. Fine silica has been used to prevent deterioration of Portland cement at temperatures above 120 degrees C (250 degrees F) for a number of years. Today the development of wells at higher temperatures for recovery of geothermal fluids, deep gas, geopressured fluids, and artificially heated steam or fire flood wells places increased stresses on the cements. The behavior of Portland cements under such conditions has been published recently. Included are the behavior of geothermal cements under simulated and actual well conditions. The reported temperatures have varied from 150 degrees C (300 degrees F) to over 425 degrees C (800 degrees F). The environments have varied from static fresh water at saturated steam pressures to geothermal fluids (steam to 10 percent brine) at flowing conditions. An extensive series of potential geothermal cements has been evaluated in the present project. In a preliminary screening program, many cement systems failed to meet strength and permeability requirements and were dropped from further testing. This included cements not reaching a compressive strength of 3500 kPa (500 psi) in 24 hours or 7000 kPa (1,000 psi) after 7 days. Likewise, normal weight cements with permeabilities greater than 0.1 md and lightweight permeabilities greater than 0.1 md and lightweight systems greater than 0.25 md were eliminated. During this phase of the screening program it was noted that the behavior of silica stabilized Portland cements cured in heavy (25 percent solids) geothermal brines was dramatically affected by the particle size of the silica originally added to the cement.
Steamflood and fireflood wells present special challenges when one designs a cement slurry for such wells. In most cases, the cement slurry is subjected to relatively low temperatures during the cement job and early curing. However, after the cement sets, if must be able to withstand the thermal shock associated with the initiation of steamflooding or fire flooding. Additionally, the cement must be able to preserve adequate compressive strength and low permeability despite the potentially disruptive crystalline changes that occur at high temperatures. Another complicating factor is the weak or incompetent formations often encountered with thermal recovery wells. This paper discusses the chemical and phase equilibria relationships which prevail when cements are exposed to the high temperatures associated with fire flood and steamflood wells. The CaO-SiO2 H2O and CaO-Al2O3-H2O systems, which respectively pertain to Portland cement and high-alumina cement, are discussed. In addition, an overview of methods for preparing slurries suitable for high temperatures, yet lightweight enough to prevent damage of weak formations and lost circulation, is present. The use of hollow-glass or ceramic microspheres to extend cement and foamed cement are shown to be particularly advantageous for cementing thermal recovery wells. Introduction The application of heat to stimulate oil production has been practiced for over 50 years. Methods such as in-situ combustion (fireflood), downhole heaters, hot fluid injection and steam stimulation have been utilized. In-situ combustion and steam injection are the most popular methods practiced today. These techniques have been the salvation of many oil fields with high viscosity crudes., and essentially involve the trading of BTUs for viscosity reduction(1). For reliable long-term performance of the fireflood and steam flood wells, a good cement job is essential. High levels of stress are built up in the pipe and the cement sheath, and the strongest possible pipe/cement and cement formation bonds are necessary. Failure of the bonds could allow interzonal communication and pipe expansion. The ultimate result would be casing failure by buckling or telescoping(2). In addition, the cement itself must be able to withstand the elevated temperature exposure and thermal cycling associated with steamflood and fireflood wells. The cement must be able to maintain sufficient compressive and bonding strength, low permeability and preferably low thermal conductivity. Good cementing techniques, such as the use of spacers and washes for adequate mud removal, casing centralization, mud conditioning and pipe movement are extremely important. However, such meticulous preparation and effort are wasted unless the cement is properly designed for long-term stability and adequate performance characteristics. This paper is a discussion of the pertinent parameters one must consider in the design of the cement for a thermal recovery well. These parameters can be divided into two principal categories:chemistry and phase equilibria relationships, andphysical characteristics of the well and the formation. A discussion of these parameters is followed by some examples of cement systems which are suitable for various types of thermal recovery wells.
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