The migration of gas to surface by means of the production casing/openhole and the production/surface casing annulus is a common occurrence in the petroleum industry. There are also situations wherein migrating gases will negotiate a route to surface outside the surface casing.The repair of these situations is a non-revenue generating exercise with the potential to reach significant expenditures. The recommended strategy will efficiently initiate and direct this process consequently minimizing the total cost of this intervention. The process commences with a logical technical approach to identify the gas source or sources that are responsible for the problem. The next step is to communicate with this gas source in a manner that enhances a remedial cementing activity. It concludes with the task of cement squeezing the source using a low rate cement squeeze technique to permanently seal the gas source thus preventing gas flow. This methodology has proven to be extremely successful and the subject paper describes in detail the recommended methods for identification, access to and sealing of the gas source responsible for these issues. Case histories will also be presented to illustrate strategies within the intervention.

In the western Canadian sedimentary basin, tens-of-thousands of wells currently are leaking gas between the surface and production casings. Often this leakage manifests itself as a surface vent. While much work has gone into preventing gas vents during primary cementing, little has been done to improve the chances of successfully sealing existing leaking wells. The work described in this paper focuses on new materials and techniques that have been developed to seal vent flows. The paper describes the theoretical physics behind the process and gives case histories to demonstrate the successful application of the technology. Introduction Microannular gaps as narrow as a few microns can allow gas leakage depending upon the differential pressure. The flow paths that allow for the leakage may be present at either the pipe/cement or cement/formation interfaces. In order to penetrate and seal such narrow gaps, special optimized microcement systems have been developed. The properties of a cement slurry required for placement into such narrow gaps are small particle size, efficient fluid loss control both axially and radially, a very thin filter cake, low rheology, zero free water and no sedimentation under down hole conditions. The set cement properties required for long-term sealing are extremely low permeability and mechanical properties sufficient to resist stress cracking. The placement technique of the slurry is also a key parameter in the success of sealing vent flows. Placing the slurry at extremely low rates, often less than 10 L/min, decreases the friction pressure generated in the gap. This in turn reduces the differential pressure across the slurry and decreases the probability of bridging. Once the slurry is in place it must remain undisturbed until it sets to form a permanent seal. This is accomplished by continuous pumping until the slurry thickening time has been reached and the cement sets. Changing Conventional Wisdom Cement squeezing has been defined as the process of forcing a cement slurry, under pressure, through holes or splits in the casing/wellbore annular space. When the slurry is forced against a permeable formation, the solid particles filter out on the formation face as the aqueous phase enters the formation matrix1. The difficulty in successfully carrying out this process is ensuring that the entire void space behind the casing is filled with cement prior to forcing sufficient water from the slurry to leave it unpumpable. This becomes particularly important when attempting to cure gas vent flows because if the entire conduit for the gas is not filled with cement then the seal is not effective and the problem is not solved. Cement Systems Cement systems for this application rely not only on small particle sizes to eliminate bridging in narrow gaps, but also on slurry properties of extremely low filtrate loss and very low viscosity. Filtrate loss must be controlled both perpendicular and parallel to the axis of the gap to prevent dehydration and bridging (Fig. 1). The filtrate loss must also be controlled by a mechanism that is not wall-building. The slurry viscosity must be kept very low, minimizing the pressure drop through the gap that leads to dehydration and bridging. Conventional Well-Dispersed Microcement Systems Microcements used in conventional Well-Dispersed Microcement Systems (WDMS) have a maximum particle diameter ranging from 13 to 30 microns, depending upon the manufacturer. The particle size distribution (PSD) of these cements is relatively narrow. In order to formulate a WDMS slurry that is easily pumpable, first the pore space between the cement particles must be filled with water. The water required for filling the pore space accounts for approximately 30 to 35% of the volume of the cement. Additional water must then be added to sufficiently separate the cement particles to prevent physical interactions between adjacent particles from impeding fluid flow (Fig. 2).

The age of the well, steamflood temperatures exercised and corrosion activity are some of the major factors influencing this abandonment process. Casing failures resulting from the operating conditions can present enormous problems and expense during the well abandonment. TX 75083-3836, U.S.A., fax 01-972-952-9435.