Abstract

The underbalanced drilling campaign undertaken by Shell U.K. Exploration and Production (Shell Expro) offshore in the Southern North Sea has been on the cutting edge of UBD technology since its inception in 1997. Electromagnetic (EM) telemetry systems were incapable of performing in the Southern North Sea due to the depth and nature of the formations. However, recent enhancements to the technology have greatly expanded the scope of EM telemetry. The use of this technology has potential for expanding the scope of underbalanced operations possible in the Southern North Sea.

A fundamental advantage of the EM telemetry system is the capability to transmit data independently of wellbore hydraulics. For example, down-hole pressure data can be transmitted to surface during pressure buildup tests and flow tests. Data can also be transmitted during trips, during connections, with pumps on or off, etc. The transmission of data is unaffected by loss of drilling fluid returns, the use of air, air-mist, foam or other multi-phase drilling fluid mediums generally associated with underbalanced drilling practices. The down-hole instrumentation can also be interrogated or reprogrammed at any time by direct communications protocol, which is completely independent of the rig or drilling activity.

A new, innovative twist to enhance the use of EM telemetry involves the use of insulated casing to reduce signal attenuation and further expand the application of the technology where it has been previously out of reach. This paper discusses rationale for the use of EM telemetry in the Southern North Sea basin and the results of a recent successful field trial of the technology.

Introduction

Electromagnetic MWD (EM-MWD) technology has made considerable advances since its introduction in 1987. The ability of the technology to circumvent some of the limitations of the more conventional mud-pulse telemetry systems has led to an increasing scope for application of the technology.

The use of EM-MWD has been identified as a key enabling technology that could have a significant impact on UBD procedures in the Southern North Sea. The Southern North Sea gas basin is a mature reservoir that is depleting. Technologies such as underbalanced drilling have served to increase the value of wells drilled in this region by improving productivity and overcoming conventional drilling challenges. Pore pressure will continue to decrease over time and there will be a greater need for enabling technologies such as underbalanced drilling to ensure continued exploitation of these reserves in a cost effective manner.

The primary objective is to expand the UBD operational envelope through the use of EM-MWD technology with the aim of increasing the value of the wells previously outside this window. The major advantage to the use of this technology is the ability to optimize the injection of gas down the drill string as a means of reducing bottom hole circulating pressure. This will also eliminate the consideration of signal transmission characteristics from the multi-phase drilling fluid design.

Currently, there is a limit to the bottom hole circulating pressures that can be achieved using conventional mud pulse MWD systems due to the limited ability to decode pressure pulses through a compressible, multi-phase drilling fluid. This can partially be compensated for by the injection of gas through a concentric annulus, however, this approach still limits the bottom hole pressure that can be achieved due to the high liquid rate required for hole cleaning and the depth of annular gas injection. There is a further limitation in available deck space to accommodate the additional equipment required.

The use of EM-MWD would allow a significant reduction in the liquid rate while greatly increasing the gas rate that can be pumped down the drill string. This combination must be designed to achieve the target bottom hole circulating pressure while satisfying hole cleaning requirements and optimizing down-hole motor performance.

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