Abstract

In this study, a series of experiments was performed to investigate a procedure for killing sustained casing pressure (SCP) by the bleed-and-lube method of injecting heavy brine into the annulus. The procedure involves bleeding fluids from the annulus and lubricating in weighted fluids in order to displace annular fluid with the heavy brine. A pilot-scale model of the well annulus used in this study allowed measurements of hydrostatic pressure and density change resulting for the bleed-andlube method. Three concepts of annular fluid displacement were investigated: brine or mud lubricated into a water-filled annulus, brine into bentonite slurry, and immiscible displacement. The experiments showed that annular density increased after many injections of brine into the water-filled annulus. They also showed even more desirable performance using immiscible fluid. However, the experiment demonstrated an inability to displace drilling mud with brine due to a flocculation effect.

Introduction

The Minerals Management Service (MMS) defines SCP as a pressure measurable at the casing_head of a casing annulus that rebuilds when bled down and that is not due solely to temperature fluctuations and is not a pressure that has been deliberately applied. In contrast to SCP, an unsustained casing pressure determination is made if either the only casing pressure on a well is selfimposed (e.g., gas-lift pressure, gas- or water-injection pressure) or pressure is entirely thermally induced.

In the recent review of SCP problems, Bourgoyne, et al. (Bourgoyne, 2000) discuss various methods for SCP removal with and without using a drilling rig. In principle, the rig-less methods involve injecting highdensity fluid into the affected annulus in order to kill SCP.(1), (2) The fluid is injected either at the surface directly into the casing head (Lube-and-Bleed method) or through a flexible tubing inserted to a certain depth in the annulus (Casing Annulus Remediation System, CARS). The concept of these two methods is to replace the gas and liquids produced during the pressure bleed-off process with high-density brine, such as Zinc Bromide. The goal of these techniques is to gradually increase the hydrostatic pressure in the annulus.

The lube-and-bleed procedure involves bleeding small amounts of lightweight mixtures of gas and fluid from the annulus and lubricating in Zinc Bromide brine over several treatment cycles. A limited number of case histories reported the lube-and-bleed method as partially successful. In one of these cases, SCP in the 13- 3/8"casing was reduced from 4,500 psi to 3,000 psi. The operation took over a year with numerous cyclic injections, during which 118 bbls of 19.2 ppg Zinc Bromide brine replaced 152 bbls of the annular fluid (a gas-cut water-based mud having density of 7.4-9.5 ppg) (Hamrick and Landry, 1996).(3)

After trying the lube-and-bleed method for several years in several wells, the field results have not been as promising as first indicated. Many operators observed incomplete reduction in surface casing pressures after using this method.

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