Strategies in High-Data-Rate MWD Mud Pulse Telemetry Yinao Su1, Limin Sheng1, Lin Li1, Hailong Bian1, Rong Shi1, Xiaoying Zhuang2, and Wilson Chin2 ,* 1China National Petroleum Corporation (CNPC), Beijing, China 2Stratamagnetic Software, LLC, Houston, Texas, United States Received August 15, 2014; Accepted November 5, 2014 Abstract: Low data rates, typically 1-3 bits/sec or less, are the norm in “mud pulse” Measurement-While-Drilling (MWD) systems. For example, “sirens” offering high carrier frequencies produce low-amplitude signals subject to severe attenuation; and strong signals from “positive pulsers” are slow since large forces are required to overcome mud inertia. Moreover, refl ections, high pump noise, erosion and rapid power consumption affect all pulsers. This paper describes an integrated systems approach to high-data-rate telemetry. Source strength is enhanced using downhole constructive interference, and surface signal processing eliminates downward refl ections and pump noise through “directional fi lters,” both drawing on wave- based methods. Hydraulic properties associated with torque, erosion, aerodynamic stability and turbine performance, and subtleties found in large-scale acoustic wave interactions, are studied using dynamic similarity. In particular, short and long wind tunnels are introduced with signifi cantly reduced test times, cost and required labor. Experimental facilities, prototypes and signal processing methods are presented in detail. Keywords: Echo cancellation, mud pump noise, mud siren, MWD, noise removal 1 Summary Measurement-While-Drilling (MWD) systems presently employing mud pulse telemetry transmit no faster than 1-3 bits/sec from deep wells containing highly attenuative mud. The reasons – “positive pulsers” create strong signals but large axial fl ow forces impede fast reciprocation, while “mud sirens” provide high data rates but are severely lacking in signal strength. China National Petroleum Corporation research in MWD telemetry focuses on improved formation *Corresponding author: [email protected] DOI: 10.7569/JSEE.2014.629515 J. Sustainable Energy Eng., Vol. 2, No. 3, December 2014 269 Yinao Su et al.: High-Data-Rate MWD Telemetry evaluation and drilling safety in deep exploration wells. A high-data-rate sys- tem providing 10 bits/sec and operable up to 30,000 ft is described, which cre- ates strong source signals by using downhole constructive wave interference in two novel ways. First, telemetry schemes, frequencies and pulser locations in the MWD drill collar are selected for positive wave phasing, and second, sirens-in-series are used to create additive signals without incurring power and erosion penalties. Also, the positions normally occupied by pulsers and tur- bines are reversed. A systems design approach is undertaken, e.g., strong source signals are augmented with new multiple-transducer surface signal process- ing methods to remove mudpump noise and signal refl ections at both pump and desurger, and mud, bottomhole assembly and drill pipe properties, to the extent possible in practice, are controlled to reduce attenuation. Special scaling methods based on dynamic similarity are developed to extrapolate wind tun- nel results to real muds fl owing at any downhole speed. We also describe the results of detailed acoustic modeling in realistic drilling telemetry channels, and introduce by way of photographs, CNPC’s “short wind tunnel” for signal strength, torque, erosion and jamming testing, “very long wind tunnel” (over 1,000 feet) for telemetry evaluation, new siren concept prototype hardware and also typical acoustic test results. Movies demonstrating new test capabilities are available upon request. 1.1 High Data Rates and Energy Sustainability Predictions that the world would “run out of oil” have worried consumers and governments for decades; however, these concerns have always proven unfounded. Although the planet’s supply of fossil-based energy is limited, this resource is likely to be plentiful in the long term if prudent management practices are in place. Sustainability can be enhanced by deepening our understanding of geological processes and by augmenting this advantage with detailed real-time data obtained during drilling and exploration, e.g., see Boyd, Darling, Tabanou, Davis, Lyon, Flaum, Klein, Sneider, Sibbit, and Singer [1]. This is easier said than done. For example, in the early days of well logging, formation evaluation dictated that gas or oil-fi lled rocks have higher resistivities than water-fi lled rocks. Interpretation was simple. But through the years, “low- resistivity pay” has become a worldwide phenomenon; with lower oil prices driv- ing the re-exploration of mature fi elds, new methods of interpreting resistivity readings have proliferated. Answers depend on many factors. It is known, for instance, that properties iso- tropic at small scales may be anisotropic macroscopically. Many logging tools lack the resolution needed to resolve values for individual thin beds of sand and shale, often taking averages that conceal useful reservoir information. Ideally, instru- ments would sample properties at multiple scales and return their fi ndings to on- site geologists. Similar arguments apply to formation tester, acoustic, nuclear and NMR logs – more data, especially if available during drilling, is always helpful. DOI: 10.7569/JSEE.2014.629515 270 J. Sustainable Energy Eng., Vol. 2, No. 3, December 2014 Yinao Su et al.: High-Data-Rate MWD Telemetry The bottleneck in this quest for increasing information – again, aimed at enhanc- ing sustainability by reducing the likelihood of bypassing productive zones – is a direct consequence of low data transfer rates supported by present MWD technol- ogy. In a digital age where “megabytes per second” defi ne the norm, data rates during drilling, typically just 1-3 bits/sec or less, are limited by high mud attenu- ation, poor signal strength at the source, and highly reverberant and distortive environments, all leading to decreases in signal-to-noise ratio. Moreover, problems associated with high levels of sand erosion and excessive power demands add to already diffi cult engineering challenges. Little effort has been undertaken in the past thirty years to approach MWD mud pulse telemetry as a science based on acoustics and fl uid-dynamical prin- ciples. The present authors have approached this research systematically over the past decade and results are reported in their recent book, i.e., Chin, Su, Sheng, Li, Bian, and Shi [2]. This paper presents an overview of the new methodologies plus a major update on “direction-based” (as opposed to frequency-oriented) signal processing techniques for surface enhancement. 1.2 Introduction The petroleum industry has long acknowledged the need for high-data-rate MWD mud pulse telemetry in oil and gas exploration. This need is driven by several demand factors: high density logging data collected by more and more sensors, drilling safety for modern managed pressure drilling and real-time decision- making, and management of economic risk by enabling more accurate formation evaluation information. Yet, despite three decades of industry experience, data rates are no better than they were at the inception of mud pulse technology. To be sure, major strides in reliability and other incremental improvements have been made. But siren data rates are still 3 bits/sec in shallower wells and positive pulser rates still perform at a dismal 1 bit/sec or less. Recent claims for data rates exceeding tens of bits/ sec are usually offered without detailed basis or description, e.g., the types of mud used and the corresponding hole depths are rarely quoted. From a business perspective, there is little incentive for existing oil service com- panies to improve the technology. They monopolize the logging industry, main- tain millions of dollars in tool inventory, and understandably prefer the status quo. Then again, high data rates are not easily achieved. Subtleties abound. Quadrupling a 3 bits/sec signal under a 12 Hz carrier wave, as we will fi nd, involves much more than running a 48 Hz carrier with all else unchanged. Moreover, there exist valid theoretical considerations (via Joukowski’s classic formula) that limit the ultimate signal possible from sirens. Innovative mechanical designs for positive pulsers have been proposed by others and tested. Some offer extremely strong signals, although they are not agile enough for high data rates. Unfortunately, the lack of complementary telemetry schemes and surface signal processing methods renders them hostage to strong reverberations and signal distortions at desurgers. DOI: 10.7569/JSEE.2014.629515 J. Sustainable Energy Eng., Vol. 2, No. 3, December 2014 271 Yinao Su et al.: High-Data-Rate MWD Telemetry Figure 1 Prototype single-siren tool (assembled). Figure 2 Prototype single-siren tool (disassembled). One might surmise that good “back of the envelope” planning, from a systems engineering perspective underscoring the importance of both downhole and sur- face components, is all that is needed, at least in a fi rst pass. Acoustic modeling in itself, while not trivial, is a well-developed science in many engineering applica- tions. For example, highly refi ned theoretical and numerical models are available for industrial ultrasonics, telephonic voice fi ltering, medical imaging, underwater sonar for submarine detection, sonic boom analysis for aircraft signature minimi- zation, and so on; several of them deal with complicated three-dimensional, short- wave