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).

This paper was prepared for presentation at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.

A discussion is presented on the improvement of shallow gas well production through the use of advanced low-temperature breaker technology. A systematic field evaluation has shown that the use of this new breaker system has increased well production in spite of inherent problems in the area due to severe proppant embedment. Previous use of an encapsulated breaker (EB) designed for well temperatures above 125°F had shown some improvement in production over conventional dissolved breakers. However, the current study has shown that the new breaker system, designed for temperatures <125°F and closure stress <1000 psi, has been able to improve production by an additional 15 to 22% in direct offset comparisons. Study criteria included selection of a suitable area, maintenance of a consistent drilling and completion program, and variance only in the breaker system used. Laboratory development of the new breaker system also is discussed, including effects of temperature and closure stress on barrier properties, and fracture conductivity comparisons.

This paper discusses improvement in production rates of shallow gas wells through the use of improved low temperature breaker technology. Systematic field evaluation has shown that the use of a new breaker system has increased well production in spite of inherent problems in the area due to severe proppant embeddment Previous use of an encapsulated breaker (EB) designed for well temperatures above 500C had shown production improvement over conventional dissolved breakers The current study has shown that the new breaker system, specially designed for temperatures <500C and closure stress <7000 kPa has been able to improve production by an additional 15 to 22% in direct offset comparisons Study criteria included selection of a suitable area offering the ability to maintain consistent drilling and completion programs with the primary variable to be the breaker system used This paper also discusses laboratory development of the new breaker system including effects of temperature and closure stress on barrier properties, and fracture conductivity comparisons-INTRODUCTION Workovers and increased shallow drilling activity have stimulated continued interest in methods to improve the performance of low-temperature wells.Published data have shown improvement in these wells using encapsulated breakers, however, typical low-temperature conditions require that the breaker design be optimized to ensure achieving maximum efficiency/cost effectiveness. There is a significant amount of published literature regarding laboratory and field studies on the application and use of delayed release breakers for low to high temperature gas well conditions. However, there has been little if any systematic comparison of different encapsulated breaker designs on actual well production. This study specifically shows how optimized encapsulated breaker technology can significantly improve shallow gas well production. It also shows how a field evaluation can be accomplished in an area of typical marginal economics, provided prior planning in well selection and adequate study design are done.

No abstract