APPLICATION NOTE Accelerometers for Drilling Oil and gas extraction have tremendously evolved over the last century. The need to dig wells ever more deeply has required new processes and technologies. The use of sensors has allowed decreasing operating costs, and exploiting previously inaccessible resources. Of all these sensors, the accelerometer is one of the most widely used. Measurements While Drilling (MWD) or Logging While Drilling (LWD) methods involve embedding sensors in the Bottom Hole Assembly (BHA). These processes allow not only the position, in terms of depth and azimuth, but also the tilt of the drill to be controlled at all times. The purpose of MWD is to track and optimize the drilling path with high precision, enabling directional and horizontal drilling. There are four possible reasons that justify directional or horizontal drilling: 1) Inaccessible surface locations / Sidetracking for example, a mountain can block the vertical path. 2) Fault drilling the presence of a fault can often make the vertical method a lot more complicated. 3) Multiple target zones from a single wellbore to induce cost effective drilling. 4) Multiple wells generally applied offshore, it reduces the number of rigs down to a single structure. 1 2 3 4 Figure 1: The four directional drilling configurations www.colibrys.com Page 1 of 5 F +41 58 10 0 50 01
Directional drilling plays a major role in gas drilling. It is mostly combined with hydraulic fracking to access small pockets of gas trapped within a layer of shale. The direction drill tool helps to align the drilling path with the shale layer, thus improving the efficiency of the fracking. Architecture of the System MWD and LWD probes generally integrate four modules: a directional (or direction detector) module, a gamma module, a battery and a transmitter. The embedded electronics in the directional module compute the sensor output, compensate for all the positional errors and send the data to the transmitter module. Figure 2: A possible configuration of an MWD Probe The directional module consists of a cluster of sensors to measure the tool position, with three accelerometer sensors to acquire the tilt angle α, combined with three magnetometers for azimuth measurements β. Further information is shown in Figure 2. Figure 3: The principles of accelerometers and magnetometers www.colibrys.com Page 2 of 5 F +41 58 10 0 50 01
In addition, there are two possibilities that could apply to transmit data from an MWD to the surface: The Mud Pulse Telemetric (MPT) system: the transmitter at the end of the BHA generates pulses that are conducted to the receiver by the mud. This method is very common for its reliability and simplicity. The Electromagnetic (EM) system: the transmitter generates EM signals that travel through the ground to a receiver on the platform. The use of a relay is often necessary when the distance between the probe and the receiver exceeds a certain limit. Figure 4: The MWD Pulse communication Figure 5: The MWD EM communication Environmental Analysis Any sensor system embedded in, or linked to, the drill bit has to be highly resistant as it will face an extreme environment. The sensors must be reliable in harsh conditions subject to the combined effect of high temperatures, humidity and repetitive shocks. For every drop in depth of 100 meters, the temperature will increase by 3 C. For example, a sensor in a well around 6000 meters deep will need to be operational at temperatures up to 175 C. The shocks endured by the drill bit can also reach several hundred g easily, and the vibrations, an amplitude of 20g or more. Pulling the BHA back to the platform may take 8 to 10 hours, and the same amount of time is needed to reverse the operation. One day without drilling costs up to US$100 000. Therefore, a sensor with strong durability that resists the conditions stated above is absolutely necessary. A correct Mean Time Before Failure (MTBF) must be at least between 1000 and 2000 hours. Performance Required Logging is a process where a tolerance in the maximum tilt angle α of ±1 between the measured and the real path is allowed. Generally, the logging data are used by geologists. However, MWD demands a global precision below ±0.1 for the angle position α, corresponding to an acceleration performance below ±1.75 mg. The bias accuracy and repeatability of an accelerometer are thus priorities to avoid any adjustments being made while drilling. www.colibrys.com Page 3 of 5 F +41 58 10 0 50 01
Parameters such as thermal drift, noise, non-linearity, bias and scale factor repeatability and the misalignment of the sensors must be compensated electronically at the system level. A third order model is generally used for the Bias and Scale factor compensation. Long term stability and sensor thermal model repeatability are key to ensuring the overall accuracy requirements. MEMS Accelerometers and their Competitive Advantages There are two types of accelerometer technology used for directional drilling: MEMS capacitive and quartz sensors. Both solutions could resist both high temperatures and repetitive shocks. Even though the legacy quartz sensor is still the most commonly used for drilling applications, MEMS will progressively replace it, all thanks to their greater accuracy, improved reliability and cost effectiveness. Properties Quartz sensor MEMS sensor Max. temperature resistance 185 C 175 C Input range ±4g (min.),±20g ±1g (min.),±10g Max. shock resistance 500g 1500g Relative power consumption High Low Relative price High Low MEMS capacitive accelerometers are available in two different designs: bulk micromachining and surface micromachining. Bulk micromachining involves wet etching in a silicone structure. This technique gives more stability and precision to the accelerometers, due to a gap twice smaller and a larger capacitance (10 times) than surface micromachining. The Swiss Precision Colibrys exploits 3D bulk micromachining technology to its full potential, with the three crucial points necessary for developing accurate sensors: Highly stable 3D MEMS die fabrication Low noise, stable ASIC electronic designs Qualified, low stress assembly and hermetically packaged technology Figure 6: The TS1002T sensor After several years of research, Colibrys developed the high performance TS1000T sensor, dedicated specifically to drilling applications. It can function up to a temperature of 150 C on a continuous basis and up to 175 C intermittently. The sensor can also tolerate a maximum shock of 6000g, and 500 cycles of shocks at 1500g for 0.5 ms. These products are available at the following ranges: ±2g, ±5g and ±10g. The ranges of 5 and 10 g are generally used when the vibration of the MWD tool is high. www.colibrys.com Page 4 of 5 F +41 58 10 0 50 01
Figure 7 shows an example of the TS1002T (+/-2g) raw bias performance over the total operating temperature range. After applying the compensation model in figure 8, the sensor has a maximal residual thermal bias error less than ±600 µg. The overall residual Angle α accuracy over the temperature range -40 to 150 C, considering the scale factor, non-linearity, bias and Misalignment repeatability is better than ±0.1, which satisfies both LWD and MWD specifications. The residual bias at end of life is also estimated @ +/-2mg typ, by using Colibrys Long term stability plan 1. Figure 7: Raw bias over temperature Figure 8: Bias residual modelling error All sensors are delivered with hermetically sealed packages LCC20, and are guaranteed to work correctly for years in harsh environments. A self-test function is also integrated into each Colibrys sensor to check the reliability of the data. Glossary ASIC: Application-specific integrated circuit BHA: Bottom hole assembly EM: Electromagnetic LWD: Logging while drilling MEMS: Microelectromechanical system MPT: Mud pulse telemetry MTBF: Mean time before failure MWD: Measurements while drilling 1 The long-term stability represents the residual error defined after applying following environmental conditions: powered life test 500h @150 C, 60x temperature cycling -40 C to 150 C, random vibration @130 C (20grms / 10-2 000Hz), shock @130 C (100g / 2ms / 12 000 shocks). www.colibrys.com Page 5 of 5 F +41 58 10 0 50 01