Specialized Cement Design and Placement Procedures Prove Successful for Mitigating Casing Vent Flows—Case Histories

Abstract

This paper documents recent field cases in which attempts were made to mitigate casing vent flows (CVF's) on producing and abandoned wells by incorporating permeability-blocking gels with specialized cement blends. CVF's are defined in this paper as sustained gas pressure on the annuli of producing and surface casings. The amount of gas flow rate can vary from a few bubbles to cubic meters per day. However, when the annuli are shut-in, the gas pressure can build to a significant amount. The procedures detailed in this paper are the result of lab studies and postjob reviews of failed remedial attempts. Specific attention is given to why squeeze-cementing procedures can fail to provide long-term seals against source-gas production from casing vents. In most cases, rework of the abandonment procedures cost the operators over $200,000 CAD. The studies showed four possible causes of recurring CVF's:Development of thaumasite in the setting cementLeaking isolation toolsIncomplete long-term seal of source zonesIncorrect source detection or squeeze interval Cement-source quality and bulk handling methods were also investigated, but showed no evidence of probable cause. This paper explains the findings from laboratory studies and job reviews that lead to improved procedures. These improved solutions were conducted on 23 wells in the later part of 2001. Introduction The industry commonly encounters annular gas pressure on cemented casing annuli. However, this condition is often referred to in different terms based on the local interpretation of the problem. Terms such as sustained annular-casing pressure, annular gas pressure, casing vent flows, or annular gas flows refer to the same general problem. This problem exists when gas pressure builds on casing-by-casing annuli, the pressure is bled to zero, then the gas pressure returns over time. The amount of gas pressure can vary from slightly above atmospheric pressure to that of near deepgas reservoir pressure depending on the gas source and flow path from the source to the surface. Also, the amount of gas bled from the annuli can vary from a very slight flow to 1,000's of standard cubic meters per day. For uniformity in this paper, the described problem of gas pressure in the casing-by-casing annulus will be referred to as casing vent flows (CVF's). CVF's can be caused by several factors. However, the industry has recognized the following factors as the main causes:Poor mud displacement in the primary cement placement (Fig. 1)Cement sheath failure, resulting in sheath cracking (Fig. 2)Gas migration through the setting cement creating gas channels in the set cement (Fig. 3)Low cement top These factors are well documented, appearing frequently in past research. An overview is given in the following sections. Poor Mud Displacement. CVF can also achieve a firm foothold if mud displacement during the primary cementing operations is poor. A primary requisite for lowering the chances of CVF is effective mud displacement,1–3 which provides a relatively clean pipe and formation surface to which the cement slurry can bond. Generally, 90% mud displacement efficiency provides adequate zonal isolation, while 95% provides excellent zonal isolation.4 Lowering the drilled-solids content of the drilling mud, conditioning the hole, and reducing the long-term gel strength of the drilling mud helps obtain more efficient mud displacement.4 A properly designed cement system does not eliminate the need for proper mud conditioning or for following best cementing practices.2 These best practices include pipe movement, casing centralization, and spacer design in addition to mud conditioning as stated above. Poor Mud Displacement. CVF can also achieve a firm foothold if mud displacement during the primary cementing operations is poor. A primary requisite for lowering the chances of CVF is effective mud displacement,1–3 which provides a relatively clean pipe and formation surface to which the cement slurry can bond. Generally, 90% mud displacement efficiency provides adequate zonal isolation, while 95% provides excellent zonal isolation.4 Lowering the drilled-solids content of the drilling mud, conditioning the hole, and reducing the long-term gel strength of the drilling mud helps obtain more efficient mud displacement.4 A properly designed cement system does not eliminate the need for proper mud conditioning or for following best cementing practices.2 These best practices include pipe movement, casing centralization, and spacer design in addition to mud conditioning as stated above.

Review of integrity of existing wells in relation to CO2 geological storage: What do we know?

“…Incomplete mud removal often occurs in deviated wellbores (Keller et al, 1987), where a continuous mud channel may remain along the narrow section of the cemented annulus. Effective mud displacement is a primary requisite to prevent sustained casing pressure from occurring, with the main objective being to provide a relatively clean pipe and formation surface to which the cement slurry can bond (Watson et al, 2002). Before primary cementing, the wellbore must be properly conditioned by breaking the gel strength and by removing as much as possible of the filter cake (Ravi et al, 1992).…”