Successful Field Application of an Electro-Negative 'Coating' to Reduce Bit Balling Tendencies in Water Based Mud
Authors: Smith Lee , Mody F.K. , Hale Arthur , Romslo Nils 
Published: December 2007

In response to a documented need to improve drilling performance of PDC bits in soft shale formations when drilling with water based fluids, a new bit manufacturing process has been developed to alleviate the tendency of the bit to "ball" under these drilling conditions.

Based on laboratory research which indicated that imparting a negative charge to steel cutters would repel negative ions of shale, a process was developed to apply a permanent negative charge to PDC bits in an attempt to reduce "bit balling" and improve penetration rates in soft shale formations drilled with water based muds.

Successful application of this technology is documented in field data from more than 20 bit runs in which the "coated" bits were used. They include hole sizes from 8-1/2" to 17-1/2", primarily in applications in the North Sea and Gulf of Mexico.

Finally, several factors are discussed with regard to successful application of this technology for improving PDC bit performance in water based muds. They include bit design, hydraulics recommendations, operating parameters and geologic information.

Background

Approximately 60 to 80% of formation drilled in the oil and gas industry is shale or shaly type formations which are mechanically weak and easily penetrated. Long term problems of borehole stability and more immediate problems with compromised drilling efficiency arise from the capacity of shale to absorb water, which creates swelling and stickiness. This leads to problems in the long term of bore hole stability, and in the short term, of cuttings sticking to the bit, reducing drilling efficiency.

Specifically, bit balling reduces the rate of penetration (ROP) of the bit because cuttings accumulating around the cutter or bit teeth can change the distribution of applied forces for which the bit was originally designed. Bit balling also creates considerable problems with cuttings being recrushed, due to poor cleaning on the bit face. In addition, accumulation of cuttings in the waterways of the bit face can reduce the flow of cooling mud, leading to accelerated wear and premature bit failure.

It has been estimated that lost time and extra material costs associated with "shale drilling problems" approach approximately $500 million annually. Any steps that can be taken to reduce bit balling or clear a balled bit are therefore of significant economic importance, both in terms of the direct consequence of drilling performance, and in reducing potential trouble time.

Present Solutions

A number of solutions to the problems of bit balling and borehole instability have been developed, most of which address the problem in terms of the mud system used. They include various oil- and water-based systems, as well as mud additives designed to change the chemical properties of the mud.

Oil Based Systems. At present, one method of preventing bit balling is use of an oil based mud system, in which the oil acts as a coating to the metal of the bit, reducing the ability of the cuttings to stick. In this case, the oil also can coat the cuttings and borehole wall, thereby reducing their absorption of water and resultant stickiness.

Both conventional and invert emulsions may be used, and it is advantageous to balance the activity of the water in the aqueous phase so there is no osmotic pressure to allow water to pass into the cuttings. This can be accomplished by adding salts, such as potassium chloride, to the aqueous phase.

Unfortunately, use of oil based muds is subject to severe restrictions, with the result that their use has been substantially reduced. In cases where they are considered essential, highly purified paraffin oils, or even biodegradable vegetable oils are used, substantially increasing the cost of the drilling operation.

Water Based Muds. The various water based muds developed to combat bit balling generally are designed to prevent the absorption of water by the shale.

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