Bit balling and balling on other bottom hole assembly (BHA) components is a common concern when drilling shale with water-based mud (WBM) because it limits drilling efficiency or in extreme cases can stop drilling, causing costly non-productive time (NPT).
Among many strategies which have been created to mitigate the above balling problem, an effective approach is coating the tool surfaces with hydrophobic materials that are generally characterized by high water contact angle. This approach has been pursued by bit manufacturers with some success; however, poor coating durability is still a common concern. Additionally, previous applications are often guided by surface hydrophobicity evaluated at ambient conditions. Elevated temperatures and wellbore pressures can potentially cause the behavior to change and limit effectiveness. Therefore, evaluating the surface hydrophobicity under conditions that closely simulate downhole conditions becomes essential.
To address these challenges, this paper reports the recent success on a new anti-balling coating technology and the novel high-pressure, high-temperature (HPHT) laboratory tests used to develop it. The laboratory-level HPHT apparatus developed in this study is capable of measuring key parameters including the contact angle and interfacial tension at extreme conditions of up to 500 °F and 30,000 psi. It is also demonstrated how pressure and temperature have an impact on the surface hydrophobicity.
The newly developed coating technology combines good surface hydrophobicity at HPHT conditions with superior wear performance. The development results were later demonstrated in several shale applications (Marcellus Shale, West Virginia in 2014; East Cameron, Gulf of Mexico in 2015), where the coating technology was applied onto 8 ½" diamond bits that at one time drilled up to 10,000 ft in a single run without requiring a trip while still maintaining coating functionality.
This development work provides drilling operations with an effective solution to improve the performance in balling-prone shale applications. Additionally, the body of work demonstrates the importance of material testing in conditions matching real-world applications and how this approach leads to improved material selection for solving downhole problems.
