8

<x3, Least principal horizontal stress

Figure 9-41. Diagram illustrating mechanics of squeezing lost circulation material into a vertical fracture.

Natural Open Fractures

As already mentioned, open fractures occur when one of the principal stresses is tensile. As there is at present no means of measuring subsurface rock stresses in drilling wells, there is no means of differentiating between natural open fractures and induced fractures. Since the remedy would be the same—high pressure

Initial fracture at point of maximum overpressure

Squeeze pressure i Subsequent

Initial fracture at point of maximum overpressure

Squeeze pressure i Subsequent fractures J—induced while squeezing squeezes to increase compressive hoop stresses round the bore hole—open fractures may well have been encountered but not recognized.

High Temperatures

Geothermal Gradients

The increase in temperature with depth in the earth's crust is called the geothermal gradient, and is expressed in °F/100 ft (°C/km). The heat flow in the upper crust is derived from two sources: (1) heat conducted from the lower crust and mantle, and (2) radiogenic heat in the upper crust.52 Conducted heat is low in regions of ancient tectonism, e.g., the eastern half of the United States, and high in regions of recent tectonism, e.g., the mountainous regions of the West (see Figure 9-42). Temperature gradients vary widely within each region depending on:

1. The amount of radiogenic heat in the upper crust.

2. Structural features: gradients are high at structural highs.

3. Thermal conductivity of the formation: gradients are low in conductive formations such as sandstones, and high in low conductivity ones such as shales.

4. Convective flow: in thick permeable beds, water circulates by convection, causing high temperatures at relatively shallow depths.

5. Pore pressure: temperature gradients are higher in geopressured formations.53

Because of these factors, geothermal gradients vary from 0.44°F/100 ft (8°C/ km) to 2.7°F/100 ft (50°C/km) according to location, in the United States.54 In the Gulf Coast, they vary from 1.2°F/100 ft (22°C/km) to 2.2°F/100 ft (40°C/ km) (see Figure 9-43).55 Very high gradients are found in the steam wells in the Salton Sea area of California. Here, a thick aquifer overlies a local upthrust of igneous rock. Temperatures of water circulating by convections rise as high as 680°F (360°C) at depths of about 5,000 ft (1,500 m), which equals a gradient of 12.5°F/100 ft (230°C/km) from the surface to the top of the aquifer.56

Detailed surveys have shown that geothermal gradients are not linear with depth, but vary according to the factors listed above, i.e., formation, pore pressure, etc. In Louisiana's Manchester Field, for instance, the gradient is 1.3°F/ 100 ft (23.6°C/km) in the normally pressured zone above 10,500 ft (3,200 m) and 2.1 °F/100 ft (38°C/km) in the geopressure zone below.53 Elsewhere in the Gulf Coast, gradients as high as 6°F/100 ft (109°C/km) have been observed in the geopressured zone.57

The bottom hole temperatures of drilling wells are always less than the virgin formation temperature. For example, the maximum temperature logged in a

Figure 9-42. Map showing probable extent of hot (stippled), normal (white), and cold (doited) crustai regions or tne unuea

Slates. Physiographic provinces do not necessarily represent heat flow provinces. (From Diment" Courtesy U S, Dept.

Figure 9-42. Map showing probable extent of hot (stippled), normal (white), and cold (doited) crustai regions or tne unuea

Slates. Physiographic provinces do not necessarily represent heat flow provinces. (From Diment" Courtesy U S, Dept.

Figure 9-43. Contour map of isothermal gradients in southwest U.S. (From Moses.55, Courtesy of API.)

geothermal well drilled to 4,600 ft in the Imperial Valley of California was 430°F (221 °C) after an eight hour shut-down, but the well subsequently produced steam at a temperature at 680°F (360°C).

The reason for the difference between bottom hole and formation temperature is that the mud, while circulating, cools the formation around the lower part of the hole, transfers the heat to formations around the upper part of the hole, and loses it to the atmosphere at the surface. Figure 9-44 shows the change in mud and formation temperatures with time of circulation as calculated by Raymond''8 for a hypothetical well with a virgin formation temperature of 400°F. During a round trip, the temperature of the mud at the bottom of the hole rises, but under normal circumstances does not have time to reach virgin formation temperature (see Figure 9-45). Note that only the mud in the bottom half of the hole increases in temperature: in the rest of the hole and at the surface, it decreases in temperature. Thus, the average temperature of the mud is always substantially lower than the bottom hole logged temperature (a fact which should be remembered when making and evaluating high temperature stability tests).

The hydrostatic pressure of the mud column in a well depends on the density of the mud in the hole, which differs from the density at the surface because of increases in temperature and pressure with depth. Therefore, calculating hydrostatic pressures from surface densities will result in error.

McMordie el measured changes in density with temperature and pressure in a variable pressure autoclave. Figure 9-46 shows the results for a fresh water

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