Abstract

In instances of ultra-low bottom hole pressure (BHP) where the results of conventional cleanout methodologies range from inefficient to ineffective, wellbore vacuuming technology has proven to be a reliable alternative. Technology using concentric coil-in-coil tubing (CCT) design was developed in 1995 and is being used as an effective method to remove both unwanted liquids and solids from wells where formation pressure is prohibitively low for conventional cleanout techniques.

The process involves pumping a work fluid down the internal string through a venturi nozzle assembly. A localized pressure drawdown is created at the bottom hole assembly (BHA), drawing wellbore liquids and solids into the return flow with the work fluid. The combined flow is returned up the outer coiled tubing string to surface. This process does not expose the wellbore to the hydrostatic pressure of the workover fluids.

More than 600 wells have been treated to date with the wellbore vacuuming technology. Recently, the wellbore vacuuming technology has been revisited and extended to smaller internal diameter (ID) completions; this required a BHA and process redesign. Outlined in this paper are details of the research, development and testing of the 1.75" (44.5 mm) micro-vacuuming tool with recommended procedures, design criteria and operational practices for liquid and solids removal. Case histories from shallow gas wells <2,200 ft (670 m) and deep gas wells >10,000 ft (3050 m) with formation pressure gradients as low as 0.03 psi/ft (0.68 kPa/m) are presented. As well, engineering challenges, best practices and lessons learned from the micro-vacuuming process are summarized.

Introduction

Sand production is a common problem faced by many of oil and gas producers worldwide. Although a slotted liner acts as a partial barrier, grading effects at the slots often still result in sand migration into the wellbore and reduce production rates. The low production flow rates contribute to increased deposition of the sand within the wellbore.

Several cleanout options have been developed over the decades employing a number of different techniques approaches.1 However, coiled tubing (CT) or conventional jointed pipe often require the use of high circulation rates, special fluids or reverse circulation techniques to remove the solids. With the use of high rates and high specific gravity water based fluids, conventional sand cleanout methods apply excessive down hole pressure on the formation and result in lost circulation returns in the low formation pressure reservoirs. This makes sand removal virtually impossible and creates damage to the formation. Nitrogen can be used to reduce hydrostatic pressure, but this necessitates a very specific job design and execution and can require large amounts of liquid nitrogen in the case of highly deviated or horizontal wells, which may be located in some remote area.

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