The Effects of Formation Hardness, Abrasiveness, Heterogeneity and Hole Size on PDC Bit Performance

Abstract

Improvements in drilling efficiency and economics are continuously being achieved with PDC bits, specifically in medium-hard and non-abrasive formations - normal drilling. These gains have been aided by advancements in PDC bit technology, their selection practices and BHA design. However, PDC bit performance in harsh environments - hard, abrasive and heterogeneous formations - still lacks consistency and predictability. The problems associated with these applications are amplified when large hole sizes (diameters greater than 12") are drilled. Performance qualifiers such as footage, and especially rate of penetration (ROP), tend to be considerably lower in such instances. Considering the depths at which small hole sizes (diameters less than 9") are usually drilled, the likelihood of such hole sizes being drilled in harsh environments tends to be very high. It is evident that huge operational savings will be achieved if the performances of PDC bits are improved. This paper will discuss the effects formation hardness, abrasiveness and heterogeneity have on PDC bit performance. In addition, it will establish the influences different hole sizes have on PDC bits, especially in harsh environments. Drilling efficiency will be discussed in terms of operational parameters, vibrational behavior, durability equivalency (DEQ) and lithology differences. An engineered approach to PDC bit development and selection, that enhances performance in harsh environments and in large hole sizes, will be discussed. Background Researchers have identified stabilization1,2,3,4, durability5, directional efficiency6,7,8 and rate of penetration (ROP) as critical performance qualifiers (PQ) for PDC bits. Although the contributions of these qualifiers to operational success have normally been discussed in isolation, it is known that they exhibit strong dependencies. The importance of the different PQ's to PDC bit efficiency depends on the mechanical properties of the formations being drilled. Formation drillability9,10 (FD), characterized through drilling difficulty establishes the effects rock mechanical properties such as bulk compressibility, abrasivity, compressive strength and lithological composition have on the drilling process. Environments become harsh as their drilling difficulties increase. In some instances, high pore pressures, and associated higher mud weights, depositional depths and hole size compound the problems normally associated with harsh environments. These different PQs assume diverse roles when it comes to normal and harsh environment drilling. Most importantly, ROP's influence on operational efficiency in normal and harsh environments can be classified as direct and indirect respectively. This distinction establishes the groundwork needed to improve PDC bit performance in the challenging environments that will be described in this paper. Introduction Inaccurate evaluations of PDC bit dull grades have contributed to the known inefficiencies of these bits in harsh environments. In addition, inefficient interpretations of drilling parameters11,12, such as weight on bit (WOB), rotary speed (RPM), flow rate (Q) and HSI have also contributed to the performance inconsistencies of these bits.

PDC Bit Hydraulic & Mud Rheological simulation to model pressure drop across Bit

“…This is because the two nozzles generated asymmetrical flow which result in higher jet velocity and improved cutting transport [10]. For a similar total flow area (TFA) of nozzles, higher number of nozzles improves cuttings removal away from PDC bit as more nozzles provide a more-uniform fluid low distribution [11]. Bigger face volume of bit is at higher risk of bit balling when drilling at low ROP.…”

Drillability and Mechanical Specific Energy analysis on the example of drilling in the Pomeranian Basin

“…Therefore a drillability coefficient is introduced [2] as it characterizes this system: Drillability Z SP is a parameter describing the rock/bit system and always has positive values. This system is described with the use of two factors [4,11], abrasiveness index, which has an effect on the bit wear, and the index of rock strength, which describes its resistance to drilling. The wearing of a bit is negligible, therefore Z SP is interpreted as a rock property in this model.…”

Innovative Cutting Structure, With Staged ROP and Durability Characteristics, Extends PDC Bit Efficiency into Chert/Pyrite/Conglomerate Applications

“…Several authors 3,4,5 have investigated and presented processes that can be used to improve drilling efficiency. Whereas the effects of hardness and abrasiveness has been extensively studied and quantified in the drilling industry 6 , heterogeneity did not receive the same level of attention. However, this situation has changed in recent years.…”