A Novel Laboratory Method for Assessing the Erosional Characteristics of Mudcakes

摘要

Design and development of superior drilling and drill-in fluids and the assessment of the effectiveness of mudcake cleaning from the borehole wall by cleaning systems require a good understanding of the fundamental phenomena associated with the erosional behavior of mudcakes formed by different mud systems. Because of serious consequences of mudcake erosion on the formation damage potential of drilling muds and mudcake cleaning before a cementation job, it is important to have a suitable, simple, and field-applicable method for predicting the erosional potential of mudcakes formed by muds of different chemical composition. The prediction of erosion behavior is essential to screen and select appropriate mud additives and also to design new mud products for producing good-quality mudcakes on the borehole wall. The technique and technology could provide a suitable means to assess the effectiveness of mudcake cleaning systems and to help design such cleaning systems for removal of mudcake from the borehole wall. Cakes formed by several water-based muds were evaluated using the newly developed technique. The testing has been performed with the simulation of hydrodynamic conditions corresponding to the drill-collar borehole-annular section of a wellbore. The results indicate that the presence of electrolytes significantly increases the erosion coefficient of bentonite mudcake. Ionic fluid-loss additives such as polyanionic cellulose (PAC) and carboxy-methyl cellulose (CMC) reduced the erosion behavior of saltwater-based bentonite mudcake significantly. However, the presence of the polyanionic fluid-loss additive PAC caused a drastic reduction in the erosion characteristic of NaCl-based saltwater mud compared to the presence of the anionic fluid-loss additive CMC. The presence of the nonionic fluid-loss additive, modified starch, shows little improvement of erosional behavior of saltwater-based mudcake. It seems to be that the electrochemical forces of interactions of ionic fluid-loss additives play a vital role in making a mudcake nonerodible. The variation of the erosional characteristics of the mudcakes is attributable to the physical, chemical, and electrical properties of the cake-forming mud additives and also to the nature of the fabrics formed by these additives within the mudcake matrix.