New Erosion Resistance PDC Bit Coating Eliminates Balling in Water-Based Drilling Fluids, Saudi Arabia

Abstract

The tendency of PDC (Polycrystalline Diamond Compact) bits to ball in soft shale formations when drilling with WBM is well documented, especially in deep/high-pressure applications. The capacity of shale to absorb water causes the formation to stick to the bit body and cutting structure compromising drilling efficiency. Balling also clogs the nozzles and junk slots reducing hydraulic effectiveness/cooling leading to accelerated cutter wear and premature bit failure. In Saudi Arabia's fields, a typical well requires approximately 1,600–2,200 ft of a 12-in. vertical borehole or 3,000 ft of a 12-in. directional borehole to be drilled through carbonates, shale and claystone lithologies. The middle part of the section is composed of mainly claystone, which is the most problematic zone. In recent wells, bit balling incidents through the claystone interval was reducing average rate of penetration (ROP) to less than 10 ft/hr, and in certain cases forced to pull out of hole (POOH). PDC bits with various hydraulics configurations and non-stick coatings were tested in an attempt to alleviate balling issues. The thin layer eroded before the bit entered the problematic zone, exposing the rough bit body. An R&D initiative determined mechanical and electrochemical sticking contributes to bit balling. The investigation revealed a coarse bit body increases surface area and adhesive force. When mud flow stops an electrostatic force can cause clay to stick to the bit surface. Based on these findings a new type nickel-phosphorus electroplating process was implemented that creates a thick/durable coating with an extremely strong chemical bond. This paper reviews the investigation process and findings of three case studies in the Saudi Arabian fields. The new anti balling coating was applied to a seven bladed PDC design and run on a powered point-the-bit rotary steerable system. The bottom-hole assembly (BHA) drilled the entire section achieving a field ROP record. Drill bits with the new anti-balling coating were also tested in vertical wells in different gas fields setting new bit performance benchmarks. Application Review In a large gas field in Saudi Arabia, containing several distinctive sub-fields, a 12 in. hole size section needs to be drilled through a mixed sequence of rocks that are comprised of limestones, dolomites, anhydrites, siltstones and shales. A particularly problematic section of the well occurs through a shale formation that is reactive to hydration. In this section, bit balling, as well as stabilizer balling, is a known occurrence with the water-based mud that is utilized. This causes a drastic reduction in ROP and sometimes bits are pulled prematurely but they are typically in good condition once seen at surface (Fig. 1). Field experience has shown that the cutting structure is partially balled up while drilling, causing the low rates of penetration. Use of drilling fluid additives to help reduce the potential risks of bit balling has been investigated and introduced into the application, showing some improvements. The overall drilling performance through this interval, especially the low ROP, is still a major concern for the operator in these wells.