Downhole cement

W. Pittman, W. C. Maurer, W. G. Deskins, F. Sabins, and D. Muller. Carbon-fiber technology for improved downhole cement performance final report. Gas Res Inst Rep GRI-96/0276, Gas Res Inst, August 1996. 

Sugama and Allan [5] used calcium aluminates (tricalcium aluminate, C -A, monocalcium aluminate, C A, or calcium dialuminate, C A2) as the cation donors and reacted them with an ammonium polyphosphate fertilizer solution and formed quicksetting cements. The purpose of this study was to develop cements that are not affected by the CO2 environment and are useful as downhole cements in geothermal wells (see Chapter 15). The composition of the fertilizer was 11.1 wt% N as ammonia, 37.0 wt% P2O3, 50.79 wt% water, and the rest trace elements. Differential scanning calorimetry (DSC) showed that the reaction rates of the three minerals are in the decreasing order ... 

American Petroleum Institute (API) has developed standards for testing downhole cements that are described in API recommended practice 10 [3]. In API specifications (API Spec. 10), if one assumes an ambient temperature of 80 °F (25°C), then the depth dependence of the downhole temperature is approximately given by... 

For the downhole cement to be effective, the bond between the cement and the casing steel as well as downhole rocks should be good. Wagh et al. [9] evaluated the Ceramicrete formulations given in Table 15.2 for bond strength. They used pipe sections made of mild steel API 5L for bond strength tests with the casing steel. The sections had an internal... 

The chemistry of cement slurries is complex. Additives will be used to ensure the slurry remains pumpable long enough at the prevailing downhole pressures and temperatures but sets (hardens) quickly enough to avoid unnecessary delays in the drilling of the next hole section. The cement also has to attain sufficient compressive strength to withstand the forces exerted by the formation over time. A spacer fluid is often pumped ahead of the slurry to clean the borehole of mudcake and thereby achieve a better cement bond between formation and cement. 

A spearhead or breakdown fluid followed by the cement slurry is circulated downhole with the packer by-pass open. This is done to avoid the squeezing of damaging fluids ahead of the slurry. A small amount of back pressure must be applied on the annulus to prevent the slurry fall caused by U tubing. If no tail pipe has been run, the packer by-pass must be closed 2 or 3 bbl before the slurry reaches the packer. If the cement is to be spotted in front of the perforations, with the packer unset, circulation is stopped as soon as the cement covers the desired zone, the tail pipe pulled out of the cement slurry and the packer set at the desired depth. The depth at which the packer is set must be carefully decided. 

The thermal conditions at site 504 are very different from those at site 505 (Table 8.4). Physical property measurements at both sites reveal systematic downhole changes and differences between the trends for the two sites. For example, Figure 8.18 shows the porosity-depth curve for the sediments at sites 504 and 505. It is evident that the porosity decreases more rapidly in the higher temperature site 504 calcareous sediments than in the lower temperature site 505 sediments. As would be anticipated from the porosity trends, the compressibility of the sediments decreases systematically with increasing depth. At a similar depth below the sea floor, site 505 sediments are more compressible than those at site 504. All the sediments are over-consolidated, a result of diagenetic cementation. 

The worldwide cement composition is —1.6 billion metric tons per year [2], approximately 3% of which is consumed by the oil and natural gas industry. Thus, the annual cement consumption by this industry is —48 million metric tons. The industry, till now, has depended on modified portland cement, but there are niche areas where conventional cement is not reliable. Portland cement has several shortcomings for borehole sealant. It does not set easily in permafrost temperatures, because the water in it will freeze even before the cement sets. Its bonding to earth materials in the presence of oily surfaces is poor. Inherently porous, it cannot form a good seal. A major ingredient, calcium oxide, is affected by downhole gases such as carbon dioxide as a result, cement performance can be poor. These problems can be overcome by a range of CBPC formulations because of their above-mentioned superior properties. 

Adding cenospheres and Styrofoam up to 10wt%, its thermal conductivity can be lowered to half that of conventional cement. When Ceramicrete-based permafrost sealant was cured in a CO2 environment, it set well, and when stored in CO2 for a week, it did not show any deterioration. Sugama and CarcieUo [8] predict that these sealants are durable up to 20 years in a downhole environment, compared to conventional cements that last for only a year. Unlike conventional cements, because CBS are neutral in pH and are not affected by downhole hydrocarbon gases, they are ideal for use in the gas hydrate regions in arctic climates. 

The stability relationships between calcite, dolomite and magnesite depend on the temperature and activity ratio of Mg " /Ca " (Fig. 5d). Lower Mg/Ca activity ratios are required to induce the dolomitization of calcite and to stabilize magnesite at the expense of dolomite (Fig. 5d) (Usdowski, 1994). Formation waters from the Norwegian North Sea reservoirs have an average log(an g -/ cz- ) - TO to 0.0 and thus fall within the stability field of dolomite. Nevertheless, both calcite and dolomite are common cements in these rocks, indicating that dolomitization is a kinetically controlled reaction. Further evidence of this is revealed from Recent sediments, such as the Fraser River delta in Canada (Simpson Hutcheon, 1995) (log (aMg2+/aca=+) -2.2 to h-1.0), where the pore waters are saturated with respect to dolomite, but it is calcite rather than dolomite that precipitates. Calcite rather than dolomite forms below the deep>-sea floor, yet the pore waters plot at shallow, near sea bottom temperatures in the stability field of dolomite and shift with an increase in depth towards the stability field of calcite (Fig. 5d). This shift is due to a diffusion-controlled, downhole decrease in Mg/Ca activity ratio caused by the incorporation of Mg in Mg-silicate that results from the alteration of volcanic material, a process which is coupled with the release of calcium (McDuff Gieskes, 1976). 

After placement, a static filtration process occurs, that is, the only flow is the filtration one, perpendicular to the bore hole surface, and the cake grows. If the cake is growing too much, it may bridge the annulus and prevent hydrostatic pressure to be transmitted downhole jeopardizing the control of the well (Figure 5). Finally, there are constraints that are related to the nature of the solid particles in suspension, that is, portland cement. 

When downhole heaters are to be used for bed ignition, the injection wells should not be completed by cementation, with subsequent perforation of casing (Fig. 65d). Because of thermal expansion, wells completed in this way may suffer damage. However, in cases in which ignition of the petroliferous bed occurs spontaneously within a limited zone, the completion by cementation and perforation of the casing is very suitable. 

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