Corrosion Inhibitor Selection Method Reduces Lease Operating Costs
Authors: Phillips Lydia A. , Cowden Jack R. 
Published: December 2007

This paper presents a method of corrosion inhibitor selection that will result in substantial savings in lease operating costs. A corrosion fatigue testing device was used to evaluate the inhibitors. Results are based on an inhibitor's ability to extend the corrosion fatigue life of steel specimens in produced fluids. A new corrosion treatment program was implemented based on test results. Changing to the most effective inhibitor has resulted in lease operating cost savings up to fifty-five percent.

Introduction

Effective corrosion inhibitor programs mean fewer subsurface equipment repairs, lower well servicing cost and less production downtime. The extent of gain in revenue as a result of these programs, is more apparent with rising equipment costs and crude oil prices In establishing corrosion treatment programs, it is essential to determine the most effective inhibitor for a given environment. In most cases, corrosion inhibitors are selected based on performance in the field with the rod strings serving as test coupons. Inhibitors and various inhibition treatments are tried until a combination is found that reduces failures and their inherent costs. This is a costly and time consuming process. Conoco Inc. holds a patent (U.S. Patent process. Conoco Inc. holds a patent (U.S. Patent 3,427,873) on the Plate Bending Corrosion Fatigue Machine known as the Box. This device permits accelerated corrosion fatigue testing in the actual well environment.

This paper presents the results obtained from three months of testing five inhibitors with the Box. A comparison of four test inhibitors that have been used in the field is presented, showing the savings as a result of reduced corrosion fatigue failures.

INHIBITORS EVALUATED

The evaluation program included five inhibitors that were being used in the field or had been recommended by local chemical companies. A generic description of the inhibitors tested is contained in Table 1.

CHARACTERISTICS OF TEST WELL

Tests were conducted on a well in the Gist Unitin Ector County, Texas. The Unit produced from the Grayburg formation at an average depth of 1310 m(4300 ft.). The primary corrosive element is hydrogensulfide and corrosion fatigue failures are frequent.

Gist Unit No. 84 was selected as an "ideal" well for the test because it is equipped with plastic coated rods. Since the well was not batch treat edduring the entire test series, the special rods provided protection against-rod failures that might have provided protection against-rod failures that might have aborted the test. In addition, the production is representative of the average unit well. The well produced 4.61 m3/d (29 bpd) of oil, 58 m3/d (365 bpd) produced 4.61 m3/d (29 bpd) of oil, 58 m3/d (365 bpd) of water and 56.62 m3/d (2 mcfpd) of gas with an H2Sconcentration of 5.47 mole %. (Refer to Table 2).The H2S concentration in the produced water was 780mg/kg (ppm). The composition of water and gas from the well are given in Tables 3 and 4, respectively.

FIELD INSTALLATION OF THE BOX

The Box is a small pressure vessel that is installed adjacent to the flowline of the test well as shown in Figure 1. The Box is isolated from the flow of well fluids by a valve at the inlet of the Box and one at the outlet. This installation allows the produced fluids of the well to flow through the Box and back into flowline without interrupting production. Four bending specimens can be tested in the production. Four bending specimens can be tested in the Box. The time to failure of the specimens is recorded by individual timers. Upon the failure of the last specimen, control circuits automatically shut down the Box and divert the fluids back through the flowline.

TEST PROCEDURE

Four carbon steel specimens (Table 5, Figure 2) are placed in the Box and brought to zero stress by aligning the upper grips (Figure 3). The eccentriccam is adjusted to apply the desired stress to the specimens.