New Mud Pulse Telemetry Techniques for Deepwater Applications and Improved Real-Time Data Capabilities

Abstract

Abstract Increasing water depth, total well depth, synthetic mud systems and increasing measurement complexity pose unique challenges for real-time data transmission via mud pulse telemetry. In deepwater environments, where the use of synthetic oil-based mud is prevalent, low water temperature significantly increases mud viscosity which reduces the signal strength at surface and makes detection of the signal more difficult. Noise within the mud channel further hinders transmission of downhole data. With rig rates approaching $350k per day and total well depths beginning to exceed 10700 m (35,000 ft), operators cannot afford to drill ahead without good quality real-time downhole data. On recent wells in deep water conditions in the Gulf of Mexico as many as seven different measurement while drilling/logging while drilling (MWD/LWD) tools have been run concurrently. Some of these tools may include the capability to produce real-time images. There is thus an increasing demand for higher data rates coupled with more reliable telemetry to transmit all this data to the surface in real-time. Recent advances in MWD tool design, signal strength prediction, and signal recovery on the surface, using advanced digital signal processing techniques, have made it possible to double telemetry data rates while also reducing error rates in the data received at the surface. Introduction Real-time transmission of data from sensors located downhole near the drill bit is a critical factor in safely and cost efficiently drilling wells that will maximize production from hydrocarbon reservoirs in the earth. Data from downhole sensors fall into two main categories: drilling data and formation evaluation data. The drilling data provide information for measuring and steering the trajectory of the well, such as survey data with the direction and inclination (D&I) of the drill bit and tool face orientation. Other important drilling data may include conditions in the well such as annular pressure and downhole temperature. The tools used for measuring directional information and generating pressure wave signals are usually called measurement while drilling (MWD) tools. If "wireline quality" logs are required in real-time, one measurement needs to be made for every 150 mm (6-in.) of hole drilled. When a typical "triple combo" service is run, eight variables (data words), of approximately 10 bits each, are transmitted for each measurement interval. At a rate of penetration of 30 m/hour (100 ft/hour) this would require a data transmission rate of greater than 4.5 bits/s. In this paper, we present a brief introduction to the generation, transmission and reception of mud pulse signals. This includes descriptions of how the signals are generated and the mud channel characteristics, such as attenuation and noise problems. We then discuss solutions available today to maximize the throughput and quality of real-time data transmitted from downhole MWD and LWD tools, especially in the context of drilling wells in deepwater applications and in wells that may have total lengths exceeding 10 700m (35,000 ft). Mud Pulse Telemetry Systems Mud pulse telemetry is still the most widely used and reliable method for transmitting data from downhole sensors to the surface while drilling. The concept of mud pulse telemetry for measurement while drilling applications is not new. Arps and Arps1 in 1964 described an early MWD system that used a plunger valve for generating discrete mud pulses. Data rates of less than 1 bit/s can be achieved using this type of modulator. Patton et al.2 in 1977 described a Mobil MWD system that used a rotating valve mechanism (also known as a mud siren) to generate continuous-wave telemetry using phase shift keying modulation. Data rates of up to 3 bits/s were claimed in the article.