Abstract

El Tordillo oil & gas field is located in the Gulf of Saint George basin, 40 km from the city of Comodoro Rivadavia, Chubut, Argentina. It currently has 380 active wells, using mechanical pumping, distributed over 11,500 hectares.

The produced fluids are collected in 20 batteries. The gas is distributed through a network of gas pipelines with working pressures between 4 kg/cm2 and 7 kg/cm2, depending on geographical location.

Temperatures in the winter can reach as low as −15 C.

The location of field in Argentina is shown in Figure 1.

The produced gas is processed for local sales, in addition to its use in heating and treating oil, and the generation of electrical energy.

This paper describes the studies and tasks performed in order to the capture gas from the casing-tubing annulus, the results obtained in the first stage of the project and the prospects for a wider-scale implementation.

Introduction

The wells in El Tordillo field which work with mechanical pumping produce a GOR on the order of 80–100 m3/m3 with average of water cut of 78%, with gas production totaling 300,000 m3/day. Chromatographic analysis of the captured gas reveals the following molar composition: N2:1.8%; CO2:0.9%; CH4:89.9%; C2H6:3.2%; C3H8:1.5%; iC4H10:0.3%; nC4H10:0.7%; iC5H12:0.3%; nC5H12:0.5%; others:0.9%.

The productive formations are El Trebol (1,400 mts.), Comodoro Rivadavia (1,700 mts.), and Mina El Carme (2,400 mts average depth). The production of hidrocarbons via mechanical pumping methods is complicated by the gas associated with the high liquid flow rates coming from more than 70 productive sands over a large vertical section.

The presence of liberated gas in the liquid stream extracted by the pump not only decreases the volumetric pump efficiency, but also generates stress conditions which shorten the life of the pump and the other materials and equipment in the system.

The alternatives to separate gathering & processing of the production gas are:

  1. Venting the gas to the atmosphere through the annular space, thereby producing contamination of the environment and loss of the gas, an option of limited viability owing to the new venting limits being established in the current legislation in Argentina (S.E. 236/93).

  2. Pumping the gas with the oil and water up through the tubing, with the already mentioned loss of efficiency and increase in costs.

Because of the undesireability of the alternatives and with an increased gas demand for oil treatment and generation of electrical energy, it was decided to analyze the possibility of capturing the gas independently from the annular space in the producing wells.

Well Testing

An intense well testing campaign was performed to measure and test gas flow rates, using an orifice meter in order to determine the availability of gas.

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