Abstract

In the rotary drilling process, mudcake starts to develop on the borehole walls while drilling. The deposition of a good quality mudcake is very important to prevent excessive amounts of fluid loss and internal filter cake formation. The mudcake should be easily removable to minimize formation damage to the pay-zone and also to accomplish a good cementation job.

Different methods are used by the industry to remove mudcake from the borehole walls including mechanical methods, physical methods, chemical methods or a combination of them. Hydrodynamic method is one type of physical methods where mudcake cleaning fluids are circulated at high flow rates to damage the mudcake matrix bonds and remove the cake from the borehole walls.

Due to the variation of different mud and mudcake compositions, removal efficiency of mudcakes may vary significantly under the same hydrodynamic conditions. This paper describes the removal efficiency of different mudcakes under the same hydrodynamic conditions and compares the removal efficiency by determining the erosional coefficients of the mudcakes. In the laboratory, mudcakes produced using different types of drilling fluids were tested under a constant hydrodynamic conditions using a newly developed test set up that simulates a scaled down version of the mudcake cleaning process under the hydrodynamic action of mud flow.

Experimental results showed that this method can assure reliable screening of washing fluids. For example, when bentonite mudcake was tested in fresh water, about 45%, 78% and 95% of the mudcake was removed in 20, 35 and 50 minutes respectively. Using these results, a mudcake erosion index was developed to describe the eroding effect of the washing fluid on the mudcake and to predict the removal efficiency of the mudcake.

This method gives a simple and reliable tool to help screening and selecting the best mudcake cleaning fluid that is compatible to the physics and chemistry of the mudcake. It also provides a guiding tool for testing and evaluation of different cleaning systems to select the best one and reject the inferior. The newly developed test method can play a key role in improving the primary cementing job, minimizing formation damage and enhancing the productivity of a well.

Introduction

Formation of filter cake on the borehole walls is important to minimize fluid invasion into formations while drilling. External filter cake can also help in minimizing the build-up of internal filter cake that is difficult to be removed using cleaning fluids. The formation of an internal mudcake increases the flow initiation pressure and reduces the productivity of a well. It is important to remove this filter cake after drilling the hole whether it is decided to have an open hole or a cased-hole completion. Filter cake removal is very important to improve the quality of primary cementing job, mitigate formation damage and minimize flow initiation pressure. Effective mudcake removal helps in enhancing cement job quality by improving the seal and bonding between the cement sheath and wellbore walls.

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