A unique test procedure and a novel test facility have been developed for experimental determination of the adhesive-cohesive bond strength (ACBS) and the adhesion-cohesion modulus (ACM) of mudcakes formed by different mud systems. The test set up consists of a Wykeham Farrance stepless compression machine to embed a cylindrical foot up to a particular depth of the mudcake matrix and then to apply a pulling force to unstuck the cylindrical foot from the mudcake matrix, a digital dial gauge to register the depth of embedment and upward displacement during loading and unstucking process, an electronic balance to register the driving/pulling force during embedment and pulling stages of the test as a function of displacement, a lab jack to elevate the mudcake to a desired height and a PC-based automatic data acquisition system to record the data and monitor the test.
Experimental results indicate significant variations in ACBS and ACM of mudcakes for an identical depth of embedment depending on the composition of the mudcakes. The magnitude of the pulling force and the shape of the force-versus decrease in embedded area curve are influenced by adhesive and cohesive bonding potential of the mudcake material and the nature of the adhesive and cohesive bonds. The presence of electrolyte significantly influences the metal to mudcake and mineral to mineral bonding. The hydrated ionic diameter of the cations also has some effect on the adhesion and cohesion properties of mudcakes. The presence of barite within the mudcake matrix causes a significant increase in ACBS and ACM of the mudcakes probably due to the filler and mass adhesion effects of barite particles along with the molecular forces of interactions. The effect of three fluid loss additives such as modified starch, CMC and PAC on the ACBS properties of NaCl-Bentonite mudcakes has also been investigated. The presence of CMC and modified starch in NaCl-bentonite muds has little effect in the ACBS and ACM of NaCl-bentonite mudcakes. However, the presence of PAC in NaCl-bentonite mudcake causes some increase in ACBS but a significant reduction in ACM. The results of the test could provide useful information regarding bonding potential of different mudcakes to drill pipes, reamers, stabilisers and drill bits and also drill cuttings in case of cuttings bed formation in directional and horizontal wellbores. The test method can be used for mechanical screening of drilling mud additives and the results can be used in designing drilling muds to produce desirable mudcake quality.