Improving the efficiency of cleaning annular space with a new flushing fluid
Marcin Kremieniewski, Marcin Rzepka, Stanisław Stryczek, Rafał Wiśniowski
Vol. 35, no. 1 (2018), pp. 219-232, [1]
One of the most important conditions to be met when cementing casing columns is obtaining appropriate tightness of the sealed interval, otherwise gas will have to be released from the annular space due to the dangerously high pressure in the upper part of the well. Gas migrating along the well in the annular space can penetrate directly the soil, groundwater or atmosphere. To counteract these unfavorable effects the casing columns are sealed and the annular space filled with appropriate cement slurry. During injection, the slurry displaces mud from the annular space. However mud residue left out in the wellbore walls may cause that the slurry does not bond the rock sufficiently well and consequently gas may flow on the hardened cement slurry/rock/casing contact. This effect can be eliminated by thorough cleaning of the annular space from mud cake with flush fluid, followed by its complete displacement with buffer fluid and cement slurry. Hence it is crucial for the efficiency of cementing to precede this job with thorough cleaning of the annular space with appropriate flush fluids. Laboratory experiments were undertaken to work out a new flush fluid with which the efficiency of removal of filtration cake from the wellbore walls and the cleaning of the annular space will be improved. Tests were performed for a group of agents, which appropriately selected as far as their quantity and quality is concerned, could create bases for a new type of fluid with very good flushing properties. The efficiency of mud cake removal was analyzed in a mud flow simulator. The analysis of the results revealed that the efficiency of removal of the mud cake on the rock surface can be improved by synergic operation of surfactants and surface active agent, when they are used in good concentrations. The influence of the injection volume and duration of contact of the flush fluid with the rock were taken into account as the main emphasis was on how to increase the efficiency of the designed fluid on the near well zone. The works on the new recipe of flush fluid and its efficiency were based on the measurement of adhesiveness of hardened cement slurry and the rock sample, from which mud cake was removed with the use of flush fluid. The analysis of the laboratory results shows that the efficiency of mud cake removal was considerably increased when the presented flush fluid was applied.