The conventional way to assess drilling performance in the oil field is to compare actual performance to statistical standards derived from offset records. By their nature, these standards are subjective and variable. While they are ideal to monitor short-range performance and trends in well-known, older fields, they lack the power of physical models to establish absolute, technical performance standards. Several authors have proposed models in which the drilling process is treated as a mechanical, energy-balanced system. Mechanical specific energy input, drilling efficiency, and a minimum specific energy equal to the rock strength are the three key elements of these systems. Full-scale simulator tests were conducted under controlled laboratory conditions to develop and validate an energy-balanced model for drilling under hydrostatic pressure. In these tests, wide variations in drilling efficiency from a few percent to three and four times were observed in the spectrum of common drilling practices. They provided new insights into the drilling process and the nature of common drilling problems. Analysis of field data revealed good correlation between simulator and field results. Using mechanical efficiency, specific energy input, and a bit-specific coefficient of sliding friction as key indexes of drilling performance makes bit selection and the diagnosis of failures and drilling practices more accurate and less ambiguous. Introduction Teale derives the following equation for specific energy in rotary drilling:He also introduces the concept of minimum specific energy and/or maximum mechanical efficiency. The minimum specific energy is reached when the specific energy approaches, or is roughly equal to, the compressive strength of the rock being drilled, i.e.,The mechanical efficiency is thenand the maximum efficiency is reached whenTeale recognized that the specific energy cannot be represented by a single, accurate number since the drilling process is characterized by wide fluctuations of the drilling variables due to its complex dynamics and the heterogeneous nature of rock. Approximate and mean values were found to be sufficiently accurate for a model to predict and analyze drilling performance. P. 373^

Dynamic instabilities of the bottomhole assembly (BHA) such as whirl, stick/slip, and bit bounce have been the focus of the majority of research studies in drilling vibrations. Recently, elevated levels of high-frequency rotational vibrations were recorded in the field with a high-bandwidth downhole vibration monitoring device. New experimental and numerical insights into this relatively unexplored phenomenon of high-frequency torsional oscillations are presented and discussed in this paper. A suite of laboratory and field tests were conducted to understand and characterize the phenomenon. Computer modeling with in-house drillstring dynamics software corroborated well with the field measurements and validated the research team approach. The investigation concluded that the drilling process excites BHA torsional vibrations. These vibrations occur at much higher frequencies than drill collar resonance and are significantly dependent on the drilled formation. New insights are gained into drilling dynamics in general and high-frequency torsional oscillations in particular. The paper presents worldwide field case studies to illustrate the phenomenon. Details are provided of testing where the drilling environment is strategically controlled to characterize the dependence of these high-frequency torsional oscillations on bit design, operating parameters, and formation properties. The implications on drilling performance are also discussed in an attempt to satisfy the quest for efficient and reliable drilling.

Summary Early concepts of hybrid drill bits go back to the 1930s, but the development of a viable drilling tool has become feasible only with the recent advances in polycrystalline-diamond-compact (PDC) cutter technology. This paper describes a new generation of hybrid bits that is based on proven PDC-bit designs with rolling cutters on the periphery of the bit. Laboratory and field results will be presented that compare the performance of hybrid bits with that of conventional PDC and roller-cone bits. A hybrid bit can drill shale and other plastically behaving formations two to four times faster than a roller cone bit by being more aggressive and efficient. The penetration rate of a hybrid bit responds linearly to revolutions per minute (RPM) unlike that of roller-cone bits, which exhibit an exponential response with an exponent of less than unity. In other words, the hybrid bit will drill significantly faster than a comparable roller-cone bit in motor applications. Another benefit is the effect of the rolling cutters on the bit dynamics. Compared with conventional PDC bits, torsional oscillations are as much as 50% lower, and stick/slip is reduced at low RPM and whirl at high RPM. This gives the hybrid bit a wider operating window and greatly improves toolface control in directional drilling. The hybrid drill bit is a highly application-specific drill bit aimed at (1) traditional roller-cone applications that are rate-of-penetration (ROP) limited, (2) large-diameter PDC-bit and roller-cone-bit applications that are torque or weight-on-bit (WOB) limited, (3) highly interbedded formations where high torque fluctuations can cause premature failures and limit the mean operating torque, and (4) motor and/or directional applications where a higher ROP and better build rates and toolface control are desired.

fax 01-972-952-9435. AbstractROP enhancer chemistry for water-based muds combined with polished cutter PDC (Polycrystalline Diamond Compact) bit technology is shown to: 1) increase penetration rates and lower costs drilling high pressured shales by reducing bit balling and improving drilling efficiency and 2) improve borehole stability by reducing exposure time and reducing near wellbore fracturing which minimizes pore pressure elevation and loss of borehole integrity. Laboratory drilling simulator and pore pressure transmission results, a case history, caliper logs and summary economics from 16 offshore wells are presented documenting the effectiveness of this technology.