Corrosion and Stress-Corrosion Cracking of Aluminum Alloy Drillpipe in a Water-Based, Low-Solids, Nondispersed Drilling Mud

Abstract

Summary. Corrosion and stress-corrosion cracking (SCC) tests were performed on aluminum drillpipe Alloy 2014-T6 in a performed on aluminum drillpipe Alloy 2014-T6 in a water-based, low-solids, nondispersed (LSND) drilling mud. Variables examined included chloride concentration, temperature, state of aeration, and effects of galvanic coupling to tool-joint steel. Electrochemical corrosion test results suggested that the aluminum alloy has adequate corrosion resistance to at least 71 degrees C f 160 degrees F, even when it is galvanically coupled to steel in aerated muds. However, SCC susceptibility of the alloy in aerated muds was clearly demonstrated by slow-strain-rate and rising-stress-intensity/SCC tests. SCC did not occur in deaerated muds. We concluded that oxygen scavenging and monitoring are important to the successful performance of aluminum alloy drillstrings in chloride-containing, water-based, LSND drilling muds. These conclusions were confirmed by excellent field performance. Introduction A highly deviated well (Fig. 1) was recently drilled from a fixed offshore platform. The well was kicked off at 427 m [1,400 ft], and angle was built at a rate of 1.5 degrees /30 m [1.5 degrees /100 ft] to an inclination of 58 degrees from vertical at 1615 m [5,300 ft] measured depth (MD) and 1416 m [4,846 ft] true vertical depth (TVD). The inclination angle was held at 58 degrees to 1640 m [5,380 ft] MD and 1429 m [4,688 ft] TVD, where 340-mm [13yg-in.) casing was set. Drilling continued below the 340-mm [13%-in.] casing shoe, holding the inclination angle between 57 and 60 degrees to 2830 m [9,285 ft] MD and 2054 m [6,739 ft] TVD, where 245-mm [9%-in.] casing was set. As will be explained later, continued drilling of the well required that sections of the steel drillstring be replaced with aluminum drillpipe. Thus, at the 245-mm [9%-in.] casing point, approximately 1372 m [4,500 ft] of 127-mm [5-in.] aluminum drillpipe was substituted for the steel pipe used earlier. The aluminum pipe was positioned immediately above the heavyweight drillpipe of the bottomhole positioned immediately above the heavyweight drillpipe of the bottomhole assembly (BHA). The remainder of the drillstring above the aluminum drillpipe to surface was 127-mm [5-in.] steel drillpipe. Drilling continued below the 245-mm [9%-in.] casing shoe with the combined aluminum/steel drillstring, with the inclination angle held at about 59 degrees to 3155 m [10,351 ft] MD and 2206 m [7,236 ft] TVD. At this point, an additional 457 m [1,500 ft] of steel drillpipe was removed from the drillstring and replaced with 127-mm [5-in.] aluminum drillpipe in the same manner as described earlier. Drilling then continued at an inclination angle between 58 and 64 degrees to the final total depth (TD) of 4007 m [13,145 ft] MD and 2635 m [8,645 ft] TVD. The total departure at this final TD was 2642 m [8,667 ft] from the surface location. The aluminum pipe used in this project was fitted with internal-flush, steel tool joints. Approximately 250 joints (2286 m [7,500 ft]) of the aluminum drillpipe were obtained on a rental basis. Of these, 191 joints were new and 59 joints were used but in premium condition. The seamless drillpipe was specified as Alloy 2014-T6, with a minimum yield strength of 400 MPa [58 ksi]. OD and ID of the aluminum drillpipe were 131 and 104 mm [5.15 and 4.10 in.], respectively. Pipe-body wall thickness was 13.3 mm [0.525 in.], and the OD of the steel tool joints was 178 mm [7 in.]. Corrected weight of the aluminum drillpipe in air, including the tool joints, was 19.6 kg/m [13.2 Ibm/ft]. Aluminum drillpipe was required to drill the well because experience in the area indicated that formation friction factors would be high. Previously drilled wells had not exceeded 1830 m [6,000 ft] in departure because of the 27, 100-J [20,000-lbf-ft] rig rotary-table torque limitation and excessive drag. Computer simulations of the well made before drilling also had indicate that high torque and drag levels would be encountered. The predicted levels were sufficiently high that the rotary-table torque limitation of the rig would be exceeded unless aluminum drillpipe were incorporated into the drillstring. A chloride-containing, freshwater, LSND mud was used during drilling. Because galvanic corrosion of aluminum near the tool joint connection is sometimes observed in such muds and because SCC of Alloy 2014-T6 is possible in aqueous brines, a laboratory investigation of these potential problems was performed before drilling began. Chemical composition, hardness, and mechanical properties of the aluminum pipe were also determined. The following section summarizes the results of this study and suggests that corrosion control was an important contributor to the success of the drilling operations, Experimental Procedure Electrochemical Corrosion Testing. Electrochemical testing was performed to determine rapidly which parameters had the greatest performed to determine rapidly which parameters had the greatest effects on corrosion behavior. These tests were performed in stirred, water-based drilling fluids. Table 1 lists the chemical composition and physical properties. The fluid was modified to simulate different levels of chloride and aeration. Test temperature was either ambient or 71 degrees C [160 degrees F], the approximate maximum downhole temperature. Aeration was obtained by continuously purging air at ambient pressure. Deaeration was accomplished by continuously purging with nitrogen and by periodically adding small doses of purging with nitrogen and by periodically adding small doses of ammonium bisulfite oxygen scavenger. Cylindrical specimens were machined from a section of drillpipe and an attached tool joint. The longitudinal axes of specimens were oriented parallel to the pipe axes. Following machining, specimens were polished to 600 grit with silicon carbide abrasive. Specimens were then rinsed in distilled water and acetone. A brush-applied lacquer was used to mask the top and bottom faces of the cylindrical specimens to avoid end-grain effects. All electrochemical tests were performed in Pyrex TM glassware with saturated calomel reference electrodes (SCE) and graphite counterelectrodes. Potentials in this report are given in millivolts vs. SCE. Potentials in this report are given in millivolts vs. SCE. Two different electrochemical tests were performed: cyclic polarization and galvanic coupling. In the cyclic polarization tests, a polarization and galvanic coupling. In the cyclic polarization tests, a controlled voltage (potential) was applied to the specimen, and the resultant current was monitored. Controlling the potential effectively controls the driving force for corrosion. These tests were used to determine the ability of the aluminum to passivate and thereby resist corrosion in the drilling fluid. SPEDE P. 135