A happy consequence of rising commodity prices and rig day rates is increased focus on improving drilling efficiency. Anything that improves drilling performance and safety can help reduce costs and risk. Service providers are working with oil companies to develop new technology or improvements to existing services with the ultimate objective of achieving technical limits. The payoff is too significant to ignore.
Two areas of recent emphasis are hole cleaning and drill bit efficiency. Effective hole cleaning has always been a priority, but has become more important because of the increasing number of large-diameter directional intervals. These require high rates of penetration (ROP) to meet build-rate requirements, while at the same time maintaining sufficient weight-on-bit (WOB) to deliver dogleg capability. Drill bit efficiency directly affects ROP and bit longevity, increasing the ability to drill shoe-to-shoe with no flat spots on the drilling curve.
Better hole cleaning
In general, a fine balance exists between the ability to maintain both high ROP and effective hole cleaning. This is particularly acute in large-diameter holes. Although maintaining ROP in deepwater surface and intermediate hole sections is usually not a problem, effective hole cleaning is. Not only is the mass of cuttings to be removed much greater, but the energy of the annular drilling fluid return flow is inversely proportional to the increases in hole diameter. Increasing mud flow rates to compensate is not always achievable. Additionally, increased mud flow rates can adversely affect equivalent circulating density (ECD) leading to formation damage or lost returns. Incorporating additional circulation time for better hole conditioning adds to drilling costs. On the other hand, inefficient hole cleaning can result in pack-offs or stuck pipe.
For some time, drillers have used downhole pressure-while-drilling (PWD) measurements to help them maintain optimum ECD. This can help them achieve the fine balance between drilling efficiency and hole cleaning. Data from PWD measurements are telemetered to surface together with other logging-while-drilling/measurement-while-drilling (LWD/MWD) data using traditional mud-pulse telemetry. But when circulation is stopped, to make a connection for example, no data are transmitted. Even though the time interval of the data gap is small, it can be significant, because when circulation is stopped pack-off is most likely to occur. For that short period, the driller is "blind."
The hole cleaning problem is compounded when motors are used in sliding mode because drillstring rotation cannot help move cuttings. The challenge of hole cleaning is further compounded when bi-center bits or other hole enlarging techniques are used due to increased cuttings load and reduced annular flow velocities.
An elegant solution
Early warning of incipient pack-off can be achieved with an innovative use of Sperry Drilling Services' existing "pumps-off" annular pressure measurement. In a typical pump cycle pressure profile, recorded (but not telemetered) annular and internal drillstring pressures follow a predictable pattern characterized by an immediate decrease, a swab-surge spike as the connection is made, and a rapid increase as pumping resumes. An additional surge is observed at the end of the cycle, as gels are "broken" in the fluid column (Figure 1). The character and amplitude of this surge can also be used to indicate the presence of a restricted annulus, the result of settled cuttings from the static interval. The resumption of circulation restarts mud pulse data transmission, and pressure data held in memory while the connection was being made is now available to the driller, albeit slightly delayed from real time. Nonetheless, with practice, drillers can learn to recognize the characteristic surge profiles that indicate impending pack-off conditions and adjust pre-connection circulation times, or other corrective measures, to compensate (Figure 2). Buildup of cuttings will also correlate roughly with drilling breaks, so drillers can train themselves to be especially observant of mud pressure profiles following these events. In the comparison shown, because of lost time remediating a pack-off, total drilling time for Well A (at left) was a full day longer than that for Well B (at right) notwithstanding the much longer circulation time intervals used for Well B.
To provide pumps-off data, the downhole computer identifies, records and telemeters three key data points acquired from the time the pumps are turned off to 1-minute after pumping is resumed. The three data points (minimum, average and maximum pumps-off annular pressure) are used to characterize the pump cycle pressure profile. The surface computer calculates equivalent pumps-off mud weight (EMW) that can be correlated with known pore and fracture pressure limits. Baseline data to indicate a "clean" annulus can be obtained by observing the surge character at the beginning of a drilling interval, after an extended period of circulation, or immediately following a leak-off or formation integrity test. Comparing baseline data to drilling data allows impending pack-off conditions to be identified earlier where traditional real time ECD techniques fail to provide ample warning.
Wasted energy
In drilling, any energy expended not making hole is wasted effort. So it is with drill bit vibration. Not only does vibration affect hole quality and ROP, but it also accelerates bit wear. Drillers have known this for some time and one of the early parameters measured with MWD devices was vibration. But usually this parameter was measured using a tri-axial accelometer. Thus axial and lateral vibration was recorded.
But one of the major impediments to drilling efficiency is vibration caused by the bit's rotation alternately sticking and releasing - so-called stick-slip, or torsional vibration (TV). Besides affecting bit efficiency, TV has a detrimental effect on the reliability of instrumented bottomhole assemblies (BHA) and rotary steerable systems (RSS). In the past, it was thought that TV could be measured effectively at surface, but recent study has shown that only low-frequency TV (<5 Hz) can reach surface sensors. High-frequency vibration is masked by the drillstring, and high frequency TV can be the most destructive.
Stick-slip stymied
A new torsional efficiency monitor (TEM), from Halliburton's Sperry Drilling Services uses a sensor embedded in the drive shaft of the company's RSS tools. Locating the sensor on the RSS drive shaft has two advantages: it is much closer to the bit than the tri-axial accelerometer, a consistent 12.5 ft (3.8 m)(as opposed to 50 ft to 100 ft or 15.25 m to 30.5 m for conventional systems), and it provides a more objective measurement of bit performance enabling a more direct determination of bit efficiency.
Torsional vibration will always be present. The trick is translating the TV into an efficiency index that can be used by the driller to maintain optimal drilling performance. To do this, Sperry engineers have developed a practical relationship between maximum and minimum revolutions-per-minute (DRPM) and torsional efficiency. Using a scale that runs between 100% and -50%, where 100% efficiency equates to no variation in RPM and 0% efficiency equates to the onset of a full-stall stick-slip condition, drilling efficiency can be calculated over its full range. On this scale, when DRPM reaches more than twice the mean RPM, full-blown stick-slip is occurring and a negative value of TV (-50%) is displayed. The reason for such a scale is that torsional efficiency is a non-linear function of RPM variation when DRPM reaches more than twice the mean RPM (Figure 3).
A "traffic light" display corresponding to the colors shown in Figure 3 alerts the driller when vibration needs to be monitored more carefully (yellow), when intervention is required (orange) and when drilling must be stopped immediately (red). Possible remedies include altering RPM or WOB or a combination of these, but it should be noted that picking up off bottom will cause dangerously high shock levels as torsional stress is suddenly relieved. The recommended practice is to allow excessive torsional stress to slowly dissipate by slowing down, stopping or easing off WOB. Because the TEM sensor is a consistent distance from the bit, its response will be essentially the same for all situations and all rigs, making it easy for drillers to acclimate themselves to using it.
In several projects recently concluded in the central North Sea, the operator compared results when using traditional surface and MWD vibration sensors to those obtained with the TEM sensor. The first project (without the sensor) resulted in numerous failures of the BHA and RSS. Half required additional pipe trips while damage to downhole tools was experienced that caused lost time for repairs. A different bit design was tried that showed some reduction in the severity of the vibration, but drilling efficiency was suboptimal. Two subsequent projects proved the value of the TEM sensor, the second doubling the historical endurance of the RSS while improving drilling efficiency by 34% and the third resulting in 100% RSS success with zero lost time.
While the presence of the TEM technology cannot guarantee there will be no stick-slip, its use has been proven to provide sufficient early warning of potentially destructive TV in time for the driller to take remedial action to prevent its occurrence. Because the TEM sensor is an integral part of Sperry's RSS tools, its implementation should be rapid and users should be able to see a marked improvement in drilling efficiency and reduction of non-productive time.

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