Design and development of superior drilling and drill-in fluids and also 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 behaviour of mudcakes formed by different mud systems. Because of serious consequences of mudcake erosion on formation damage potential of drilling muds and mudcake cleaning prior to 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 behaviour 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 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 electrolyte significantly increases the erosion coefficient of bentonite mudcake. Ionic fluid loss additives such as PAC (polyanionic cellulose) and CMC (carboxymethyl cellulose) reduced the erosion behaviour of salt water-based bentonite mudcake significantly. However, the presence of polyanionic fluid loss additive PAC caused a drastic reduction in the erosion characteristic of NaCl-based salt water mud compared to the presence of anionic fluid loss additive CMC. The presence of non-ionic fluid loss additive, modified starch shows little improvement of erosional behaviour of salt water-based mudcake. It seems to be that the electro-chemical forces of interactions of ionic fluid loss additives play a vital role in making a mudcake non-erodible. The variation of the erosional characteristic of the mudcakes is attributable to the physical, chemical and electrical properties of the cake forming mud additives and also the nature of the fabrics formed by these additives within the mudcake matrix.

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