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.
Erosional resistance of mudcake under the hydrodynamic action of mud flow and mechanical disturbance of drill pipes is directly related to the chemical, physical and in-situ properties of the cake forming mud additives, depositional environment and the deposited mudcake texture. The chemical factors such as pH, base exchange capacity of suspended mud additives, ionic nature of the additives, nature and size of the ions, strength of their mutual interactions etc. directly influence the strength of the adhesive and cohesive bond transmitters formed between particles and/or flocs of the deposited mudcake (Amanullah 2003). Physical factors such as size of the flocs, nature of the structures, density of the particles, specific surface area etc. define their physio-mechanical resistance to external disturbance. In-situ mudcake properties such as packing density, porosity, permeability, nature of inter-particle contact, texture, smoothness and homogeneity of the mudcake surface etc. define the intensity of attack of flowing mud stream on the mudcake particles. Due to the involvement of a large number of factors with various generic origin, the theoretical determination of the erodibility may incur a significant margin of error. The consideration of only one type of factors such as either chemical or physical will cast an even greater doubt on the reliability of the output. In case of experimental study all these factors will combinely define the response of the mudcake under the external disturbance and thus expected to provide the most accurate and reliable out put values.