Abstract
Corrosion control in downhole casing and tubing is a major concern for operators using carbon dioxide (CO2) in enhanced oil recovery schemes. A potential life-cycle cost advantage can be realized by utilizing polymeric liners in these applications. Conventional liners, however, have been shown to be problematic due to their propensity to collapse when the bore is rapidly de-pressurized. A novel solution to this problem has been the development of a grooved liner system. Grooved liners facilitate the venting of gases which permeate through the liner wall, thereby reducing the chance of liner buckling and collapse during blowdown. The objective of the current study was to verify the performance of a grooved liner system subjected to a supercritical CO2 environment. Full-scale tests were performed on liners made from three candidate thermoplastic polymers including high density polyethylene, cross-linked polyethylene and modified nylon.
The test sequence consisted of a series of repeated pressurization cycles with extended hold times to allow for steady-state gas permeation. In general, the tests on all grooved liner specimens were successful in that no liners collapsed during rapid depressurization of the bore. However, the grooved cross-linked polyethylene liner was clearly the most suitable candidate for the supercritical CO2 environment due to its ability to properly collect and vent the permeated gas products over the entire test period. In addition to the experimental results, a finite element model of the permeation behavior in the grooved liner system was developed. The predicted permeation rates of CO2 were found to be similar to those measured in the experiments.
Corrosion of oilfield tubing and casing, particularly due to wet CO2 environments, is a major financial and operational concern to the oil and gas industry. Of particular challenge is the ability to reduce corrosion problems in enhanced oil recovery (EOR) operations such as miscible flooding, where supercritical CO2 (i.e. at pressures greater than 1,070 psi at 88°F) is injected downhole to enhance the flow characteristics of the oil in the reservoir. Both the injection and producing wells in such operations can be subjected to highly corrosive environments. The two main methods currently used to mitigate these downhole corrosion problems are the use of corrosion resistant alloy (CRA) steels and chemical inhibitors [1,2]. Both options can be effective but are expensive to implement.
A novel approach to reduce corrosion problems in downhole tubing and casing is the use of fitted polymer liners. Polymer liners have been successfully used in reducing corrosion rates in oilfield piping systems (e.g. gathering lines, small diameter transmission lines) over the past two decades. The liners are simply continuous lengths of thermoplastic polymer tubing which are inserted into an existing pipeline system using a number of established installation methods. The liners are typically pre-fabricated by an extrusion process and supplied to the field in spools. Similar approaches have now been developed for lining pre-existing downhole tubing and casing strings [3].
The ability of the polymer liner to inhibit corrosion is due to the reduction of corrosion causing species at the inner surface of the steel tubular (e.g. liquid water and CO2/H2S). While the liner is effective at reducing corrosion rates, it has a secondary benefit of potentially improving the fluid flow characteristics in the well-bore since the friction coefficients of extruded polymer pipes are less than those made from steel [4].
Author:
Wolodko, J., Petersen, R., Cor, R., and Taylor, J.
Publisher:
NACE International Corrosion Conference
Year Published:
2004
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