Downhole temperature and pressure

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. 

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. 

Drilling surface data such as weight-on-bit and torque were difficult to interpret because they were loosely related to downhole values. MWD for the first time in the history of drilling gives values of parameters measured at the bit or close by. Rock strength, bit wear, drag and friction can be calculated in real time. Shocks, temperature and pressure can also be measured. 

By far the most widespread use of NMR in an on-line production environment is the utilization of downhole exploration tools by petroleum service companies such as Schlumberger, Halliburton, and Baker Hughes. Articles on these unilateral NMR systems are found in the patent databases, " academic literature, and on-line resources provided by the exploration companies. The references provided here are just a few examples in a very prolific field. The technique is applied in high-temperature and pressure situations and currently is used down to a depth of about 10 km (6 miles) to produce a picture of water/oil content in the adjacent rock formations as well as to derive permeability, diffusivity, and hydrocarbon chemistry information. Mobile unilateral NMR systems such as the NMR-MOUSE are also being developed in order to take benchtop NMR systems into the field to perform analysis of geological core samples at the drill site. NMR analyzers are also being developed to determine the bitumen content and properties in tar sand production and processing. " " ... 

Equation 15.6 is a generalization of Eq. (6.15) and includes the pressure effects. AHq is the change in enthalpy if the dissolution were to take place at normal temperature, while the rest of the terms on the right-hand side of Eq. 15.6 correspond to the change in enthalpy when the dissolution occurs at downhole temperamre and pressure. 

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. 

Ron et al. (2006) reported that downhole conditions may vary from one region to another. They studied that drilling pressures and temperatures can be from a range of low temperatures and pressures of 15 psi/75°F to HTHP conditions of about 40,000 psi/600°F especially in the hotter regions and deep reservoirs of the Gulf of Mexico Continental Shelf, northern India, Saudi Arabia, Brunei, Indonesia, Thailand, and northern Malaysia. 

FIG. 1—Subsurface variation in temperature and pressure in a well as a function of depth from downhole surveys [2]. 

Recognize corrosion problems in materials used at the site and make monitoring a normal part of the operation. Sour oil and gas operations are often conducted under high pressure and corrosive conditions. Therefore, in addition to temperature and pressure considerations, system designs for the wellhead, downhole equipment, and pipeUnes must have features to minimize the effects of corrosion and prevent an accidental release of H2S. Corrosion-inhibiting fluids can be used to prevent internal corrosion and cathodic protection can be used to prevent external corrosion. Also, during extended periods of shut-in and injection into pipelines, inhibitor applications may be beneficial. 

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. 

Sensors on the tree allow the control module to transmit data such as tubing head pressure, tubing head temperature, annulus pressure and production choke setting. Data from the downhole gauge is also received by the control module. With current subsea systems more and more data is being recorded and transmitted to the host facility. This allows operations staff to continuously monitor the performance of the subsea system. 

In continuous treatment Nowcorr 800 is added to the drilling fluid or to the acid blends. The concentration depends on downhole conditions temperature, total pressure and... 

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. 

G. M. Graham, S. J. Dyer, K. S. Sorbie, W. R. SableroUe, P. Shone, and D. Frigo. Scale inhibitor selection for continuous and downhole squeeze application in HP/HT (high pressure/high temperature) conditions. In Proceedings Volume, pages 645-659. Aimu SPE Tech Conf (New Orleans, LA, 9/27-9/30), 1998. 

The subscripts in Eq. 15.5 have the same meaning as in Eq. 15.4. The entropy is an explicit function of T, while the enthalpy depends on both T and P. The explicit temperature dependence on AG has been discussed in Chapter 6 and used to smdy its effect on alumina solubility in Chapter 11. Here, because the downhole pressure is very high, we extend the discussion of dependence of AG on P. 

The N2 used for well stimulation is transported to the well as a liquid in a cryogenic transport tank. The tank that contains the N2 is kept at a temperature of —196 °C and a pressure of 30 kPa. The liquid N2 is pumped with a cryogenic positive displacement pump into a heat exchanger where it is transformed from a liquid into a gas by raising the temperature (Figure 3). The dry gas is pumped into the liquid or slurry treatment line downstream from the liquid pumpers. At this point, the N2 is commingled with the treating fluid and creates a foam that is pumped downhole. 

Those factors that were previously mentioned that produce finer-textured foams also produce more stable foams. Factors such as surfactant type, concentration, increasing pressure, and higher inputs of mechanical energy generate more stable foams. For higher temperatures such as those that exist downhole, dynamic foam stability relies upon surfactant type and concentration rather than the addition of thickeners (polymer stabilizers). It is not known what rates are necessary to maintain dynamic stability in fractures, or whether those conditions typically exist. 

Changes in air consumption, pressure, and temperature with time during operation of downhole gas heater Curves 1-air input through annular space between the 65 and 127 mm columns 2-pressure measured in trap 3-pressure in the annular space between 127 and 203 mm columns 4-pressuie in the annular space between the 65 and 127 mm columns 5-pressure in 38 mm column 6-temperature of gases leaving the heater, 7-input of air injected through 38 mm column to sustain gas burning. 

An important part of this technology is the pressure-temperature optical monitoring systems (OMS). They are responsible for acquiring physical data (pressure and temperature) from the downhole environment of oil industries. OMS availabiUty analysis, mainly of the sensor component, is already done by Droguett etal. (2008). Moreover, Moura ei a/. (2008) developed optimal time maintenance poUcies for OMS systems. 

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