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Abstract

Inflows of gas into the primary cement slurry column of wells in the Ten Section field of Kern County have severely damaged the cement integrity. An inexpensive cement slurry and process design has been implemented that minimizes gas flows into the cement and eliminates the need of expensive remedial squeeze work.

Introduction

A. Problem Description

Gas invasion of a setting primary cement slurry is a very destructive process that results in a loss of good formation hydraulic isolation and expensive completions.

A great deal of gas migration research has been undertaken recently. Numerous publications define the mechanisms at work in a setting cement column that can allow gas intrusion.

Carter and Slagle recognized gas cutting effects on cement in 1972. They suggested that the cement column cannot effectively transmit full hydrostatic pressure to the zone containing the gas. They proposed that certain characteristics of the cement column (density, setting, gelatin, dehydration, and bridging) could all contribute to gas influx. They concluded that the following design criteria are critical for the prevention of gas migration:

  1. maintaining a hydrostatic head that exceeds formation gas pressure,

  2. preventing cement filtrate loss with appropriate fluid-loss controls,

  3. pipe movement while cementing,

  4. and proper casing centralization.

These were considered the most critical elements of preventing gas leakage.

C. E. Cooke et al. supported these theories with the placement of pressure and temperature sensors outside the casing at various depths. All information from the sensors was recorded as cement was placed behind pipe in various wells. With the analysis of the data from these experiments, they confirmed that the pressure at various depths in a cement column begins to decline shortly after the pumping of the cement is completed. The pressure loss, they postulated, can be explained in terms of a volume reduction of the cement and sufficient gel strength of the cement to prevent downward movement of the column.

A complete cement system design that limited annular gas flow problems was introduced by Hartog et al. in 1983. They emphasized the importance of understanding the mechanisms causing a loss of hydrostatic head and included a cement design that stressed certain cementing techniques that could limit gas migration, such as:

  1. drilling an on-gauge hole,

  2. conditioning mud,

  3. removing mudcake,

  4. use of spacers,

  5. use of highly-thinned scavenger cements,

  6. low fluid loss,

  7. good cement velocities,

  8. a set minimum contact time,

  9. pipe reciprocation, and

  10. good casing centralization.

They concluded that good cement designs based on these criteria would decrease the likelihood of gas intrusion and poor cement bonding.

In further investigation, Cheung and Beirute concluded that gas does not invade the cement matrix as long as the cement pore pressure remains above the formation gas pressure.

Two new exploratory gas wells were drilled at Ten Section field in September of 1985. Electric, sonic, and mud logs indicated economic reserves in multiple gas sands in both wells.

P. 593^

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