Abstract

Sub-surface corrosion related to sulfate reducing bacteria has been noted in some of the oil-wells in Eastern Venezuela. Production is by electro-submersible pump (ESP), mechanical pumping or gas lift, from horizontal or standard gravel-packed completions. These wells have experienced an increase in water cut, coupled with increased corrosion rates. The corrosion is mainly associated with bacterial activity. The original purpose of the Tetrakishydroxymethylphosphonium sulfate (THPS) treatment was to reduce the corrosion levels associated with the bacterial activity, predominantly sulfate reducing bacteria (SRB). The squeeze type treatments incorporated THPS along with a non-ionic surfactant. These treatments have been successful in reducing corrosion levels and hydrogen sulfide (H2S) in the gas phase. In many cases, H2S levels have been reduced from, e.g. 330 ppm to 20 ppm. The corrosion rate in surface coupons, in some cases, has been reduced from, e.g. 70 mpy to 0.6 mpy. Well head coupons, employed to monitor corrosion rates, were also used to determine the amount of iron sulfide deposited on the coupon. A novel field evaluation method, discussed in this paper, was used to quantify the amount of iron sulfide deposited on the coupon, and thereby infer the bacterial activity and iron sulfide deposition in the completion.

An added benefit of the THPS squeezes, that was not anticipated, was the increase in oil production in several of the wells treated. These increases in oil production have ranged from 20% to more than 300 %.

This study will discuss the criteria used to select the candidate wells, the methodology employed in the application of the THPS, and corrosion rate and production rates obtained.

Introduction

The South Monagas Unit (SMU) is located In Eastern Venezuela, about 800 Km from Caracas. The SMU is producing from three oil fields; Uracoa, Bombal and Tucupita (see Fig.1). The reservoirs characteristics vary over a broad band of permeability, porosity and oil saturation. These fields exhibit a medium productivity index (PI), a wide range of gas/oil ratios (GOR), high water cuts (85% average), and semi-heavy crude (14 to 19 API degrees). The produced fluids contain CO2 and H2S in varying amounts. All SMU reservoirs are connected to a very active water drive. In general, the reservoir rock is non-consolidated sandstone, of good porosity and excellent permeability. The wells are completed vertical, deviated and horizontal with gravel packs and produce by artificial lift, such as: ESP, gas-lift (GL) and progressive cavity pump (PCP).

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