Gas channeling often occurs soon after the placement of a conventional cement slurry in wells having gas. The slurry, during and immediately after placement, will act as a true fluid transmitting hydrostatic pressure onto the gas-bearing formation, until the cement begins to gel or set. As long as the hydrostatic pressure remains above the gas formation pressure, no gas will penetrate the cement column. However, as the cement begins to set (changing from a fluid to a rigid material), it gradually loses its ability to transmit hydrostatic pressure. This process is often called the "transition period." Shrinkage compounds the problem, leading to poor bonding between the cement and formation, thereby allowing gas to flow at the interface.
A field study has been conducted on a design approach which incorporates a unique cement slurry which interacts with incoming gas to form an impermeable barrier in the cement pore spaces, thereby inhibiting further gas flow.
Seventy-seven wells in Oklahoma and the Texas Panhandle using this cement slurry were reviewed. Based on success being defined as no gas migration to surface or no indication of interzonal communication, 86% of these wells were classified as successful.
Case histories of five wells giving qualitative and well parameters used in the design are discussed, along with results.