200

* From Bannerman Davis.06 From Oil and Gas J. f Fluids are heat aged at 460 F in a rotating bomb.

In some field tests it was found that the addition of SSMA to lignosulfonate muds eliminated difficulties previously experienced in getting logging tools to bottom, even in geothermal wells with bottom-hole temperatures up to 700°F (371 °C).

Low molecular weight polymers provide good rheological properties at high temperatures, but do not provide adequate filtration control, which must be obtained with long chain polymer. Son et a/6615 described a vinylamide/vinylsulfo-nate with a molecular weight between one and two million, which maintained good rheological and filtration properties at 400°F (204°C). Test also showed that the copolymer prevented flocculation in muds containing 10% each of NaCl and CaCl2 and increased the tolerance of KC1 muds to drilled solids. In field tests, good rheological and filtration properties were maintained in two 20,000 ft plus wells with bottom hole temperatures in excess of 400°F (204°C).

Perricone et al660 investigated two other vinyl sulfonated copolymers with molecular weights in the range of 750,000 to 1,500,000. In laboratory tests, these copolymers were added to fresh, sea water, and lignosulfonate field muds, and hot-rolled for 16 hours. Low HTHP-500 psi (35 kg/cm2), 300°F (149°C)-filter losses were maintained in all cases, but in the tests with lignosulfonate field muds the rheological properties were higher than with the untreated base mud.

Field results with one of these copolymers have been remarkable: Low HTHP filter losses have been maintained in over thirty geothermal wells, many with bottom-hole temperatures in excess of 500°F (260°C). In one such well the HTHP filter loss increased only 2cc after logging for 72 hours. The copolymer was added to a bentonite-lignite deflocculated system, with SSMA as the defltx-culant.

The other copolymer has been used in a North Sea well, with a bottom-hole temperature of 350°F (177°C), to provide filtration control in a non-damaymg coring fluid.

Corrosion of Drill Pipe

Although the components of water-base drilling fluids are not unduly corrosive, degradation of organic additives by high temperature or bacteria may result in corrosive products. Also, contamination by acid gases (such as carbon dioxide and hydrogen sulphide) and by formation brines can cause severe corrosion. Under adverse conditions, the replacement of corroded drill pipe becomes an economic problem. A more severe problem arises if the corrosion is not detected and the pipe fails while drilling.

In this section, the several ways in which corrosion can occur and the necessary corrective measures are briefly discussed.

Electrochemical Reactions

If a metal is placed in a solution of one of its salts, the metal ions tend to pass into solution, leaving the metal negatively charged with respect to the solution. Only a minute amount of cations leaves the metal, and they are held close to the surface by its negative charge. An electrostatic double layer (similar to the electrostatic double layer discussed in Chapter 4) is thus formed, and the negative charge is the Nernst surface potential, the value of which is a function of the identity of the metal and the concentration—or rather the activity—of the salt solution.

Table 9-13 shows the Nernst potential of various metals when immersed in solutions of their own salts at unit activity, and referred to the potential of a hydrogen electrode, also at unit activity. This series is known as the electromotive series. The greater the tendency for a metal to ionize, the more negative its potential, and the more reactive it is in an aqueous environment. For instance, potassium reacts violently in pure water and zinc reacts in acid solutions whereas silver is inert even in concentrated acids. The underlying principle is that cations

copper strip where they are accepted by the copper ions, thus forming molecular copper. By this means, chemical energy, as represented by the equation

is converted into electrical energy, which can perform work. The device is known as an electrochemical cell; the metal strips are electrodes. The zinc is the anode and the copper is the cathode. Because zinc is higher on the electro motive series than copper, it is negative with respect to the copper.

Similar cells are set up between electrodes of any two metals when in contact with electrolyte solutions and connected by a conductor. Familiar examples are flashlight and automobile batteries. The action of the zinc-copper sulfate-copper cell is reversible, i.e., if a current is sent in the opposite direction by a battery, copper passes into solution and zinc is deposited on the zinc electrode. But if the cell contained, say, sulfuric acid instead of copper sulfate, hydrogen would be discharged at the cathode, and would bubble off as molecular hydrogen. Such a cell is irreversible.

Since electrode potential is a function of ionic activity, electrochemical cells are also set up if electrodes of the same metal are immersed in solutions of different ionic activity and connected by a conductor. Such cells are known as concentration cells.

Electrochemical activity is the fundamental cause of corrosion. Local differences in surface potential caused by lack of homogeneity of the metal prov ide sites for anodes and cathodes. The body of the metal is the conductor. Corrosion always takes place at the anode, as in the zinc-copper sulfate-copper cell discussed above, and the corrosion reaction products, such as hydrogen, are discharged at the cathode.

Corrosion cells are set up in drill pipe because steel is an alloy, and contains iron and iron carbide crystals. The iron crystals almost always act as anodes, the iron carbide crystals almost always act as cathodes, and the circuits are completed by aqueous drilling muds, causing general corrosion of the pipe surface. Patches of scale or deposits of any sort also provide cathodic sites, causing local corrosion or pitting. Local corrosion may also be caused by concentration cells set up by differences in ionic activity at the bare surface of the pipe and under barriers such as drill pipe protectors and patches of scale.70

Another type of electrochemical cell is set up by differences in oxidizing conditions, fundamentally because oxidation involves a gain of electrons, e.g., when ferrous iron is converted to ferric iron:

Potential differences caused by this mechanism are called redox potentials. In a drilling well, oxygen is inevitably entrained in the mud at the surface by mixing and conditioning operations. Downhole, the surface of the pipe is exposed to oxidizing conditions, but reducing conditions prevail under patches of rust or other barriers. An anode is therefore set up under the barrier as follows:71

Fe° — 2e— Fef f and a cathode at the surface of the pipe:

02 + 2H20 + 4e-4 OH depositing ferric hydroxide according to the equation: 4Fe+ + + 6H20 + 4e—4Fe(OH)3

Corrosion pits are thus formed under the scale, as shown in Figure 9-54. Note that corrosion will still occur even though there is some oxygen under the barrier. The essential condition is that less oxygen be present under the barrier than at the bare suface of the metal, thus establishing an oxygen concentration cell.

If the products of corrosion accumulate at the cathode, the flow of electrons is impeded, and the corrosion process slows up. The cathode is then said to be polarized. For example, cations of hydrogen may polarize a cathode by coating it with a layer of hydrogen atoms. If the hydrogen atoms unite and bubble off as molecular hydrogen, the cathode is said to be depolarized. Dissolved oxygen can act as a depolarizer by reacting with the hydrogen to form water, thus accelerating the corrosion process.

Stress Cracking

When a metal is subjected to cyclic stresses, it eventually fails, even though the applied stress may be well below the normal yield stress. Failure is caused by cracks, which start at points of high stress concentration in notches or other surface defects, and deepen with repeated cycling. Failures of this sort are commonly encountered in metallurgical engineering, and are known as fatigue failures.

Steel can endure a number of stress cycles before failure. The number decreases with the applied stress, the

Mud with dissoived O

Figure 9-54. Oxygen corrosion cell.

Mud with dissoived O

Figure 9-54. Oxygen corrosion cell.

hardness of the steel, and the corrosiveness of the environment (see Figure 9-55). In a drilling well, fatigue cracking is greatly accelerated by dissolved salts, oxygen, carbon dioxide, and hydrogen sulfide, because an anode develops at the bottom of the crack, and a cathode at the surface70 (see Figure 9-56). Thus, the propagation of the crack is accelerated by metal ions going into solution at the bottom of the crack. Corrosion-fatigue cracks are a major cause of washouts and pipe failures.

Another form of stress cracking is known as hydrogen embrittlement. " ! 1 4 It was stated previously in this section that hydrogen ions generated in a corrosion cell give up their charge at the cathode, and bubble off as molecular hydrogen. However, a certain amount of hydrogen remains in the atomic form and is able to penetrate the steel. Normally, the amount that penetrates is so small that no harm is done. In the presence of hydrogen sulfide, however, the formation of atomic hydrogen is enhanced, and a larger amount penetrates the steel, where it concentrates at points of maximum stress. When a critical concentration is reached, the crack grows rapidly and failure occurs. This type of failure is known as sulfide stress cracking.

The time to failure depends on the following three variables:

1. The stresses in the steel, either residual or imposed. The higher the stress, the shorter the failure time. Below a certain stress, whose value depends on the strength of the steel, failure will not occur.

0 0

Post a comment