MPT has been the most used technique for transmitting LWD and MWD real-time data to surface since the 1980s. This is largely due to the simple design, the robustness of the system, and its ability to handle various and changing conditions [42].
The principle of MPT can be seen inFig. 8.1. In the drillstring downhole, drilling fluid passes through a restriction, often represented by a moving valve. The generated coded pressure waves are transmitted up the fluid in the drillstring and recorded at surface by a receiver [37]. There are three types of generated mud pulses, positive, negative, and continuous. These can be seen inFig. 8.2. Positive and negative pulses can be generated by poppet pulsers and are defined as discrete signals. Continuous, periodic pulses can be generated by siren pulsers. Rotationally oscillating shear valves can generate both discrete and continuous signals [41].
Fig. 8.1– The principle of MPT [37].
To achieve high data rates MPT systems must be flexible and able to handle different sit-uations and changing parameters [37]. If the MPT system cannot handle changing mud channel conditions, there is a risk of having to pull out of hole to reconfigure the trans-mitter. For a deep offshore well a bit trip can easily lead to 24 hours of downtime. It is also important for the MPT system to be robust and enduring for managing drilling of long wells and hard formations [38].
Fig. 8.2– Types of generated mud pulses: positive (a), negative (b) and continuous (c) [41].
There are many factors limiting the MPT data rates. The most important are maximum downhole signal strength, signal attenuation, surface-induced noise, and signal reflections.
Many of these can be hard to adjust for and manage as they are hard to predict and can be prone to change during the drilling process. The MPT signal is affected by the mud pumps and pulsation dampeners, drillstring components and surface piping, the location of the pressure transducer, well depth, and the properties of the drilling mud [37]. The drilling process can generate downhole noise due to bit vibrations, drilling motors stalling out, the drillstring stalling out and interactions between the drillstring and the formation, such as pipe joints hitting a ledge [41]. Eq. 8.1 expresses a simple data transmission formula [38].
r(t) = s(t)∗h(t) + n(t) (8.1)
where r(t) is the signal at the receiver, s(t) is the signal at the transmitter, h(t) is the channel distortion and n(t) is the additional noise, which is mainly pump noise. All parameters are valid at time t.Fig. 8.3shows a schematic of the mathematical model.
Fig. 8.3– Mathematical model of MPT [38].
The generated MPT signals do not only propagate upward in the well but also downward.
The signals that travel down in the drillstring are reflected, for instance at the drill bit, and travel back up in the mud. Signals traveling upwards can also be reflected on drillstring components or by solid particles in the mud. The multiple reflected signals are added to the main transmitted signal by superposition. The interference can either be constructive and enhance the signal strength, or destructive and limit the transmission distance and data rate [41].
The reciprocating motion of the pump pistons create pressure signals traveling down the well, in opposite direction of the MPT signal. Pulsation dampeners are utilized to smooth the pressure pulses from the pump, but complete removal is not accomplished. The fre-quency of pump generated noise is calculated by Eq. 8.2 [41]. Harmonics are generated due to the operation of the mud pumps and their physical nature [43]. Harmonics lead to noise peaks, and the harmonics giving the highest amplitudes is generally found by Eq.
8.3 [41].
fn= n∗s
60 (8.2)
n = m∗Nc∗a (8.3)
where fnis the frequency of the nthharmonic, n is the harmonic number (n∈ {1, 2, 3, ...}), s is the pump stroke rate, m is the positive integer number, Nc is the number of pump cylinders and a is the pump action (1=single, 2=double).
During underbalanced drilling, compressible gas is injected into the drilling mud to de-crease the density. The mud compressibility is inde-creased, which causes high signal atten-uation. Formation gas entering the drilling mud will have the same effect. Small, solid formation particles generated during drilling will also change the properties of the mud.
Other factors that affect signal attenuation are drilling mud type, number of drillstring joints, drillstring inner diameter, borehole depth, and signal operating frequency. Higher frequencies are usually subjected to more attenuation than lower frequencies. In addition, the pressure drop along the well can lead to weakening of the transmitted signal. [41].
Drilling deep wells with drilling mud of high density and viscosity is especially chal-lenging when it comes to MPT due to signal attenuation and distortion [38]. With some rheological conditions, such as foam, MPT might not be possible [40].
A surface receiver must be able to automatically detect and synchronize with the transmit-ted signal. Further processing of the signal by performing noise cancellation and interfer-ence suppression is often conducted. It is crucial that the surface processing does not slow down the signal considerably, as important real-time information may be delayed [37].
Klotz et al. [37, 42] presented a new MPT system, with a novel, advanced, and reliable mud pulser design and a new surface data acquisition unit with enhanced signal processing
capabilities. Based on continuous measurements of the mud channel conditions the sys-tem can automatically adjust decoding parameters to ensure high-speed telemetry during drilling. The system had been developed and tested for 7 years and could show data rates up to 20 bits per second (bps).
The downhole transmitter was designed to handle varying conditions ranging from shallow wells drilled with water-based mud (WBM) to deep wells drilled with highly compress-ible oil-based mud (OBM). To achieve this, the pulser supports baseband modulation with underlying pulse position encoding. The systems also include three different car-rier frequency modulation schemes, which are frequency shift keying (FSK), phase shift keying (PSK), and amplitude shift keying (ASK). Shift keying modulations can position the signal around specific carrier frequencies. Finding the correct frequency is critical for optimizing the SNR. For example, ASK can be used to minimize signal attenuation by producing low-frequency signals. The system can switch carrier frequencies without changing the data rates, and this flexibility improves system reliability. At lower data rates a higher number of frequencies can be supported [37].
The designed downhole transmitter in the system is a shear valve consisting of a stator and an oscillating rotor, which can be seen inFig. 8.4. Pressure waves are superimposed on the mud by oscillating the rotor in different positions and at different speeds. To produce baseband signals the rotor is oscillated at lower speeds and carrier modulated signals are created by oscillating the rotor at higher speeds. Maximum pulse pressure is achieved by decreasing the flow area through the valve. The valve can also be kept fully open. This is an advantage when pumping lost circulation material (LCM), as the erosion of the valve is decreased by increasing the flow area. Downhole pulse pressure at the valve with different mud flow areas can be seen inFig. 8.5. Relevant parameters, such as signaling type, rotor angle, and transmission frequencies can be defined at surface by surface tool programming and can later be changed by performing downlinking [37].
Fig. 8.4– Shear valve consisting of a stator and an oscillating rotor [42].
Fig. 8.5– Downhole pulse pressure at the valve with different mud flow areas (second from right: closed valve, first from right: fully open valve) [37].
Instead of only using one surface receiver, the system uses two, with one located in the standpipe and one located further upstream the same pipeline. This allows for removal of interfering surface-induced signals by diversity processing algorithms. The interfering signals can have energy in orders of magnitude greater than the MPT signals transmitted from downhole, and it is therefore important to separate them. A data acquisition system converts the pressure signals from the two receivers to digital data. It is then processed with noise reduction, synchronization, signal conversion, and demodulation, before being used as real-time data.Fig. 8.6shows a simple model of the signal flow [37].
Fig. 8.6– Simple model of the signal flow [42].
Fig. 8.7shows the surface signal processing stages of the system. Pump noise cancella-tion, diversity processing, and band filtering are used to improve SNR. The equalization filter reduces interferences caused by reflections, and demodulation shifts the signal from carrier frequency to 0.0 Hz before the signal can be utilized as real-time data [37]. Pre-viously the noise cancellation was carried out by pre-run modeling based on known input parameters [42]. An example of a disadvantage with this method is that gradual wear on the mud pumps will lead to changes over time which can be challenging for pre-run mod-els [38]. The new system continuously measures the mud channel conditions and adjusts the processing algorithms accordingly. To do the calibration, short times slots are used for transmitting test signals [42].
Fig. 8.7– Surface signal processing stages [37].
Fig. 8.8shows an example of change in signal quality. The test signal was recorded off-shore Norway in 2006, with a data rate of 10 bps, transmission location at approximately 5500 meters MD and carrier frequency of 30 Hz. The top screen shows the raw signal as recorded by one of the receivers. The deterministic pressure fluctuations induced by the transmitter are hard to detect. The middle screen shows the signal after noise cancellation.
The signal is cleaner, and the sine wave shape is an indication of good signal transmission.
The final output signal, which is a binary signal, can be seen in the bottom screen. The beginning of each bit is marked by a red line and the middle by a cross. The crosses above the horizontal line represent a 1, and the crosses below represents a 0 [42].
Emmerich et al. [38] studied the new MPT system developed by Klotz et al. [37, 42] and found three improvements that could be applied to the system. A new system of high speed mud pulse telemetry (HSMPT) was presented. The first point was improving detection of the training sequence (what Klotz et al. called test section) and the adaptive filter coeffi-cients calculated from the training sequence by advanced algorithms. Both parts proved unreliable under difficult conditions. Successful adjustments were made to the training sequence signal and signal processing, and it led to the system being more reliable and efficient. The training sequence signal was shortened by almost 20 %, which means more time available for transmitting important downhole information.
The second point was the addition of an adaptive filter database. For every set of adap-tive filter coefficients, the decoding quality is estimated and saved. The data is assembled, together with information on operative parameters such as pressure, flow rate and active pumps. Adding the database is part of automating the process, by replacing manual work and eliminating double data entries. It also provides valuable information [38].
Fig. 8.8– Example of change in signal quality [42].
The third point is to maximize the profits of having the database, by enhancing system functionality. The system should be easy to use and not require extensive training and advanced knowledge on the system prior to use. This allows for selection of the best co-efficients for the current conditions based on the information given in the database [38].
Fig. 8.9shows the changes that were made by Emmerich et al. [38] to the surface signal processing. The light gray components are unchanged, the grey components are adjusted, and the black components are new.
Fig. 8.10 shows the worldwide utilization of MPT in 2016. It is mostly applied off-shore, where economics allow for costly and advanced telemetry technology. Due to re-cent advances in MPT technology it is now possible for MPT systems to handle conditions that were previously considered too challenging, such as high-pressure, high-temperature (HPHT) environments [38].
Fig. 8.9– Changes made to surface signal processing by Emmerich et al. [37]. Light gray:
unchanged, gray: adjusted and black: new.
Fig. 8.10– Worldwide utilization of MPT [38].
There are still some conditions were MPT telemetry proves insufficient. In narrow mud weight window situations, the pressure fluctuations imposed by the system can lead to formation damage or unintended influx of formation fluids. Exceeding the mud weight window can have dangerous consequences, and the limitations using MPT technology in such formations can lead to the decision of not going through with projects, due to the high associated risk. Another challenge with MPT is the dependence of a minimum flow for signal transmission. If there is an influx of formation fluids or losses are encountered it can be difficult to achieve the flow rate required for MPT to function. In such a situation there is no data received from downhole and no commands can be sent to the BHA tools [44].