Short Term Gas Migration Control in Well Cementing: Comparative Behavior of Fluid Loss Control Polymers

Abstract

Abstract Short term gas migration control is one of most critical aspects of zonal isolation that must be achieved through the cementing of a well. It has long been discussed and demonstrated that gas migration occurring either immediately at the placement or in the short-term (e.g. within few hours after plug bump) is a combination of many factors including cement design and density matching. In addition, it is generally considered that fluid loss control additives can play a significant role in the ability of a cement sheath to be considered as gas tight. It is of course mandatory to obtain low fluid loss values to hold cement initial design with adapted density, water content for full cement hydration and effective placement but even with identical fluid loss and rheology, very different gas permeability results can be obtained with different types of fluid loss control additives. This study is focused on the investigation of the relative performance of several fluid loss polymers and their ability to form a gas tight sheath. Systems that were considered for the study were conventional additives based on high molecular weight water soluble polymers such as so called "AMPS" based anionic copolymers and styrene butadiene latex which were compared to a newly developed block copolymer. This latter comprises first a strong adsorbing block, then a long second hydrophilic block providing filtration control. This technology was already demonstrated to be very effective in preventing performance losses in the presence of competitive formulation additives such as dispersant or retarders (see SPE173758). In addition in this work gas permeation through setting cement sheath was investigated and a systematic study on permeability and porosity within filter cake and setting cement using Mercury Intrusion Porosimetry (MIP) was carried out. MIP results were discussed versus direct scanning electron micrograph imaging. As expected, it was confirmed that low fluid loss is mandatory to successfully pass a gas migration test. In addition, it was shown that in spite of comparable standard fluid loss control performance, actual permeability and porosity reduction within the filter cake can be significantly different. As far as pore sizes within the setting cement are concerned, there was here very limited impact of the fluid loss additive system. It was confirmed as expected from industry standards that styrene butadiene latex very effectively reduce permeability but diblock copolymer can to a fair extent be considered as a very effective substitute to design gas tight slurries.