7.1 Measurements While Drilling and Logging While Drilling
During the drilling process measurements while drilling (MWD) are performed for col-lection of data, process control, and optimization. Examples of parameters that can be measured are annular pressure, temperature, shock and vibrations, intensity of natural gamma-rays, revolutions per minute (RPM), and steering “tool face”. In more compli-cated formations it is also normal to perform logging while drilling (LWD). Examples of measurements are density, porosity, resistivity, seismic, sonic, caliper, and formation pres-sure. In addition, it is possible to take samples of the formation fluid that can be brought to surface and perform measurements that can “look” into the formation around the borehole and in front of the bit [36]. The MWD and LWD tools are usually located in the BHA, and an example of an advanced BHA is shown inFig. 7.1. The data can be transmit-ted to surface by several different telemetry technologies. The available bandwidth of the telemetry system is shared between the different MWD/LWD services.Fig. 7.2shows the distribution of a case investigated by Klotz et al. [37].
Fig. 7.1– Example of an advanced BHA [38].
Normally the power to run the MWD and LWD tools comes from downhole lithium batter-ies or turbines. With turbines, the hydraulic energy of the drilling mud is transformed into
Fig. 7.2– Bandwidth distribution of a case investigated by [37].
electrical energy used to power the downhole tools in the BHA. The method is effective, but the system has a high risk of wear and failure, as there are many moving parts and large amounts of fluids running through. The use of battery also has several challenges, including limitations for temperature and battery lifetime, required personnel to handle the batteries at rig site, and requirements for environmentally proper disposal with the asso-ciated costs. The complexity, cost, and length of the BHA are increased by the addition of downhole batteries and turbines. Adding to the length of the BHA means that some important MWD and LWD tools are pushed further up the string, which can lead to de-creased quality of the measurements and difficulties when it comes to directional drilling and steering of the BHA [36].
High data rates for telemetry are important during drilling to be able to utilize data from MWD and LWD tools, and thereby optimize drilling parameters, wellbore conditions, and drilling navigation. Increasing the amount of data that can be transmitted by telemetry will allow for more real-time data, for instance by higher log data density. More information available can help make well-educated decisions that can improve the drilling process, increase reservoir coverage, minimize drilling costs, and lead to a safer operation. Getting critical downhole information transmitted quickly to surface is also a safety measure, as dangerous situations can be detected rapidly [37]. The reduction in drilling cost can for example be accomplished by the improved data rates enabling an increase in ROP [38].
Steerable Bottom Hole Assemblies
To perform directional drilling a steerable BHA is utilized. The conventional approach was using a mud motor, together with a bent sub. The drillstring slides in the well while the mud motor rotates the drill bit. Building inclination or azimuth is accomplished by the angle in the bent sub. The method has issues with accuracy, restricted drilling distances, and irregular well trajectories. An alternative method, rotary steerable systems (RSS), was developed in the late 1980s and early 1990s. When drilling with RSS the entire drillstring is rotated by the top drive. Inclination and azimuth are controlled by rotating mechanisms
within the RSS tool. Downlinking is used to change the settings of the RSS tool during drilling. Steering commands are transmitted by series of mud pulses propagating down in the drillstring. The mud pulses are created by adjusting the flow rate at the mud pumps.
The RSS tool processes and decodes the pressure signal and adjusts the current settings accordingly, before sending a confirming signal back to surface using MWD technology.
Data is also stored within the memory of the RSS tool and can be accessed after the run is completed and the tool is retrieved at surface. To allow for the RSS tool settings to change the bit can be pulled off bottom, but downlinking can also be performed while drilling ahead, if the ROP is sufficiently low.Fig. 7.3shows examples of common RSS downlink sequence scenarios. Analyzing the RSS downlinking data can be used in evaluation of key performance indicators of the drilling process. Operational and economical aspects are considered when choosing between the two steerable BHA methods. The day rate of the RSS assemblies are usually higher than the day rate of the conventional assemblies. Which method is operationally favorable depends on the conditions of each specific scenario [39].
Fig. 7.3– Common RSS downlink sequence scenarios [39].
An example of how implementation of downhole tools led to process optimization is the improved directional drilling. Before the introduction of MWD technology, directional drilling was a very time-consuming process. To perform a survey, drilling had to be stopped and the string pulled off bottom and set in slips. The survey was then conducted by running a survey tool on slickline in the well, dropping a survey tool into the drillstring, or if the well was deviated, a survey tool had to be pumped down the string. The survey tools
dropped or pumped down the well had to be retrieved at surface by pulling the drillstring before drilling could continue [40].
7.2 Telemetry Technologies
There are two main categories of transmission technology for downhole data to surface, which are wired-pipe telemetry and wireless telemetry. Wireless systems can be electro-magnetic, acoustic and by mud pulsing [41].
In electromagnetic telemetry electromagnetic waves are transmitted and conducted to sur-face through the formation. An advantage of the system is that it has no moving parts, making it more robust and enduring, which reduces the need for maintenance and in-creases reliability. A disadvantage is that the system cannot be used in all types of wells due to limited transmission distance and severe signal attenuation, especially in offshore locations [41]. Mud resistivity and formation resistivity also leads to difficulties for signal transmission [40]. A lot of research has been performed on electromagnetic telemetry, but other technologies, such as mud pulsing, have advanced in parallel and showed greater overall potential [41].
The acoustic telemetry is still in the research and development phase. The transmitted acoustic waves traveling along the drillstring suffers from severe signal dispersion due to multipath reflections, which limits the transmission distance and bit rate. Whereas mud pulsing technology in 2015 could be applied to distances exceeding 10 700 m, acoustic telemetry could only be applied to distances shorter than 2 500 m. The short distance problem could be solved by introducing acoustic repeaters, but this increases the com-plexity and cost of the system and the need for maintenance [41]. Another challenge with acoustic telemetry is the interaction between the drillstring and the formation, which causes significant disturbances to the transmitted signal [40].
Mud pulse telemetry is described in chapter 8 and wired drill pipe in chapter 9.