A Simpler, More Effective Method for Cementing Surface Casings in Shallow Gas Zones: A Case History

Abstract

Abstract Cementing the upper casing strings in wells that traverse shallow gas sands has often been a troublesome process. The top zones are often weak and this prevents the use of high-density slurries that are gas-tight. When a combination of low-density lead slurry and high-density tail slurry is used to reduce the hydrostatic to prevent lost circulation, the concentrations of cement additives required to make the lead slurry gas-tight are usually too high, and cost can be prohibitive. There are also problems associated with the use of the combination of extended lead/neat tail slurries. While such jobs are routinely done, the transition point where the cementers need to switch slurry density from low-density lead to high-density tail can be a weak link as far as smooth density control and job continuity are concerned. Eliminating this transition point potentially improves the entire operation by maintaining consistent slurry density, a parameter widely recognized as critical to cement properties. This is done by utilizing a single, high-performance, reduced-density slurry that is made gas-tight by two additives, A highly efficient, filming fluid loss agent, combined with a lightweight, active particulate, provide both short and long-term gas stopping properties. This prevents primary gas migration and minimizes the risk of long term leakage due to secondary migration through the set cement column. The not-insignificant permeability of conventional extended cements is a key point that is often overlooked. Yet, it is the principal reason for gas build-up in the annuli of large numbers of wells. Considering the grave consequences of gas leakage around offshore installations, from the standpoint of safety, as well as environmental and economic concerns, any system that helps minimize the problem is advantageous. This paper presents case histories where the process of cementing surface casings in shallow gas was made simpler and more effective by the approach described.