Abstract

At the present time, Rosneft Oil Company is applying various technologies to minimize scaling problem. One of these techniques is, for example, a continuous injection of inhibitors using surface dosing systems to protect downhole equipment from damage caused by scale deposits (more than 1200 wells are equipped with dosimeters). It has been discovered that this method has some disadvantages, in particular an abnormal high rate of downhole equipment corrosion. As a result, wells are destroyed within less than a year. Our estimation shows that for some cases the corrosion rate is higher than 20 mm/year. Corrosion types and mechanisms have been determined. Corrosivity testing of the production from the above wells has been performed using coupons made of the least corrosion resistant material in the injection system, and an industrial adaptation of the material followed the examination. The corrosivity has been studied in a wide temperature and partial pressure CO2 range. Testing and selection of chemicals for corrosion prevention technologies have been carried out.

Introduction

The third stage of development is characteristic of most deposits in Western Siberia, and the fields are developed under conditions of a high watercut. Water injected to maintain reservoir pressure causes a number of problems accompanying the production. One this problem is scale damage in the bottom-hole zone, downhole and surface facilities.

Various technologies are used for scale management in these wells, including, for example, continuous inhibitor dosing using surface dosing systems [1]

A wide application of this technology allowed to significantly reduce scaling in downhole equipment. However, it has been discovered that this technology has a significant disadvantage: a sharp growth of corrosion of metal downhole equipment contacting with the chemical. Taking into account that at present about 1200 wells of OAO "Yugansknftegas" fields are equipped with dosing devices, the scope of corrosion problem of downhole equipment is quite substantial.

Experiments and methods

Chemicals testing for corrosiveness was carried out using a standard method: by determining the corrosion rate of low-carbon steel using a gravimetric method, polarization curves, LPR babble test and partitioning test (dynamic and static conditions) [2].

Corrosion damage

We have studied several dozens of wells of Mamontovskoye, Teplovskoye, Malo-Balykskoye, Yaraneiskoye and Dvurechenskoye fields. As a result of this research, the most typical types of corrosion damage have been discovered. These are mainly cases of tubing coupling, casing and electric centrifugal pump (ECP) cable corrosion. Figure 1 shows a picture of tubing coupling, due to the corrosion of which the parting of the tubing string took place. The coupling had been working for 145 days before the destruction. When working, the coupling was above the flowing level. The surface shows metal fracture traces that are 73 mm in diameter in the form of a track which is 35–38 mm wide and up to 9 mm deep. The track formed in the area where the power cable adjoins the coupling, and it repeats the form of the cable armoring. In some places the coupling material has a very deep destruction. There is crack running along the whole length of the coupling, which probably appeared as a result of material strength weakening. The coupling failure led to the separation of the tubing below the above mentioned coupling and to the fall of the pumping unit down to the well bottom.

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