Downhole temperature

Sohd materials, such as gilsonite and asphalt, and partially soluble sulfonated asphalt may also be added to plug small fractures in exposed shale surfaces and thereby limit water entry into the formation (105,124). The asphalts are oxidized or treated to impart partial solubiUty. These materials may be softened by the downhole temperature, causing them to deform and squeeze into small openings exposed to the borehole. Laboratory tests designed to evaluate shale-stabilizing muds have confirmed the beneficial action of these materials (125) (see also Soil STABILIZATION). 

Downhole temperature effect on the brine density—Table 4-69 [26]. 

Fluid Density Adjustment for Downhole Temperature Effect [26]... 

This is a prolific field for inhibitors although the main types remain as grouped by Bergmanm (see p. 17). In this application of inhibitors, probably more than in any other, the methods of introducing the inhibitor into the corrosive environment receive as much attention as the nature of the inhibitor. The most severe conditions are those met in acidising treatments, typically with 15-35% hydrochloric acid at high downhole temperatures. 

Fig. 2.39. Na /K+ atomic ratios of well discharges plotted at measured downhole temperatures. Curve A is the least squares fit of the data points above 80°C. Curve B is another emperical curve (from Truesdell, 1976). Curves C and D show the approximate locations of the low albite-microcline and high albite-sanidine lines derived from thermodynamic data (from Fournier, 1981). Small solid subaerial geothermal water Solid square Okinawa Jade Open square South Mariana Through Solid circle East Pacific Rise 11°N Open circle Mid Atlantic Ridge, TAG.

Stabilizing agents are used to maintain drilling fluid rheological properties at highly elevated downhole temperatures. Chromium and chromium-free lignosulfonates, polyglycol ethers, sodium polystyrene sulfonate-co-maleic anhydride), and a melanin polymer have been used in this application. Additives such as sodium diethyldi-thiocarbamate have been used to stabilize aqueous polysaccharides such as xanthan gum (18). 

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... 

The temperature (F) is measured in Fahrenheit, and depth d) in feet. This static temperature is that temperature, where the CBS will be placed and will set. The actual circulating temperature is considered to be the average of the downhole temperature and the surface temperature ... 

As discussed in Chapters 4 and 5, CBPC formation is governed by the oxide solubility. The solubility, in turn, is related to the Gibbs free energy, which is a function of temperature and pressure. As a result, the CBS formulation depends on the downhole temperature and pressure. The effect of the temperature on the solubility has already been discussed in Section 6.4. The pressure effect can be assessed in a similar manner, but as we shall see, it is negligibly small and can be ignored for all practical purposes. 

If we assume in Eq. 15.6 that the volume change AV does not depend on the temperature and pressure, we can write AVf = AVq, and the last two terms on the right-hand side reduce to (Pf — Pq)AVq. Calculations from this equation, based on the ionic radii of the right-hand side and the molecular radii of the left-hand side (see Ref. [4] for these data) of Eq. 15.3, indicate that AVq is very small even at a high pressure difference (Pf — Pq) Therefore, the pressure effects on the oxide solubility can be ignored, and the only variable one needs to consider while formulating the CBS slurry is the downhole temperature. 

The slurry cup is htted with a rotating vertical paddle. It rotates with a constant speed (150 rpm). The resistance on the paddle is measured by a pre-calibrated potentiometer. Consistency, which is a function of the nonlinear viscosity of the slurry, is measured in Bearden units (Be). The downhole temperature and pressure and Be are recorded by a chart recorder and also often on a computer. 

House et al. (1999) recognized that problems associated with thermal history could be circumvented by taking advantage of the fact that He ages of samples held close to the closure temperature will rapidly achieve the steady state age where He production and diffusive loss are in balance (Wolf et al. 1998). Provided a sample is in steady state, the measured age and downhole temperature provide a diffusivity-temperature pair that is completely independent of the laboratory measurements. Samples thought to be in steady state in boreholes from the Otway Basin, Australia, yielded diffusivities in excellent agreement with the extrapolated laboratory data (Fig. 2). Unfortunately the downhole temperatures from these industry wells are not very well known, so some uncertainty remains. 

When downhole temperature of 100-120°C is reached in the production wells, the steam injection is interrupted. At that time, intensive pumping of the liquids begins, first using the production wells of the block and then the injection wells. Production from the block continues for as long as it is economical, after which time the cycle is repeated. 

Electric heaters are the type most widely employed for the development of the in situ combustion front. They are fairly simple and convenient to use. The specific types employed for bottomhole heating can last a long time, working at downhole temperatures of more than 700°C. They range in power from 10 to 74 kw. To enhance heat transfer, the space between the electric heater and the walls of the well, as well as the the cracks and fissures around the well should be filled with material of high thermal conductivity, e.g., metal particles. 

Chen and Huang (1986) report that high-temperature deep wells are inevitable in most drilling operations. Therefore, it is always advisable that operating conditions are stepped up toward the actual field conditions during laboratory experiments. They studied the corrosion inhibition of magnetite and zinc carbonate on a deep well in southwestern Sichuan with a depth of 5.357 km and downhole temperature of 18rC-192°C. 

The shift in equilibriirm is so dramatic that path (2a) remains predominant even under downhole conditions where nitrate decomposition products are in a dense fluid state. For example, the heat of reaction at a downhole temperature between 500 and 700 K should not change by more than 10% relative to standard conditions. Estimates for the energy budget imder downhole conditions may be obtained without taking into accoimt variations in heat of reaction and product composition with temperature and pressure. 

Downhole temperature will be temporarily reduced when foreign fluids contact the formation oil. Once solid paraffin has begun to form (e.g., see fig. 3-4), to resolubihze, an increase in the oil temperature above the well temperature is usually required. Unfortimately, this is often difficult, if not impossible, to accomplish. Repeated treatment with solvent may be necessary. Removal treatment that includes a wax crystal modifier is most effective. 

Unbroken gel plugging may occur in carbonate acidizing procedures in which gelled acid is used. Monitoring and analysis of posttreatment flowback fluid samples may indicate unbroken gel, which will restrict flow of formation fluids. Gel breaking should be defined and controlled through pretreatment laboratory testing by the service company, at expected downhole temperature and residence time conditions. 

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