Abstract

Various workover methods for re-establishing oil production in heavy oil wells that are co-producing large quantities of sand have evolved in the last 20 years in Canada. Operators interested in profitable extraction from unconsolidated sands must pay particular attention to workover strategies, as these comprise a substantial part of the operating costs. Workover methods that apply large physical perturbations to the wellbore region and the surrounding reservoir appear to be the most successful in re-establishing production, but are more costly. Thus, a staged approach using cost-benefit estimates based on the behavioral history of individual wells and fields is the best approach for well workover planning. The need for comprehensive well reviews and autopsies, careful data collection, and history-based probable-cause analysis is fundamental, as there are no strong theories that can predict a priori which workover method is most likely to be cost-effective.

Introduction

CHOP (Cold Heavy Oil Production) is the method of producing heavy oil from unconsolidated sandstones by encouraging sand influx. It has been slowly but successfully implemented in more and more Canadian heavy oil fields (500 - > 10,000 cP; 400–900 m deep, 15–33 ∼C, 4 - >25 m thick, Ø ∼0.30, So ∼ 0.85, no free water leg or extensive gas cap) over the last 20 years, as operators have gradually evolved better methods of producing the well, to dispose of produced sand and fluids, and to re-establish well productivity (workovers).

New well development is now almost exclusively based on PCPs (Progressing Cavity Pumps). These were introduced in the early 1980's and have undergone great improvements based on practical factors associated with the need to lift mixtures of oil, gas, water and sand reliably for many months. PCPs have solved the lifting problem: slurries with up to 60–70% sand by weight can be reliably produced for months, although such high concentrations generally only occur immediately after initial completion, a workover, or prior to water break through. (Note that expressing the sand percent by volume of fluids entering the pump is difficult because the free gas volume in the fluid at the bottom-hole pump intake is generally unknown.) Typically, a good CHOP well will produce from 1 to 8% sand by weight of produced slurry for months and even years.

The problem of sand management has also been solved by gradual practical developments in the heavy oil fields around Lloydminster (Figure 1). Produced slurry is fed immediately into vertical gravity separators that are essentially insulated 100–150 m3 stock tanks heated to ∼60–80 ∼C. At this temperature, the sand settles out of the oil, dissolved methane evolves, water and oil stratify, and the stock tank is periodically drained of liquids and bottom sand. Sand is disposed of in various ways: by cleaning, by landfilling, by placement in dissolved salt caverns, by road spreading or by Slurry Fracture Injection ™ (SFI ™). An entire sub-industry to handle sand has developed in the Lloydminster area.

Heavy oil production using CHOP is now about 250,000 bbl/day; this could be easily doubled in a year if upgrading capacity was available.

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