An LWD Tool Suite for Formation Evaluation in HP/HT Environments

Abstract

The tool suite that is introduced in this paper provides Logging-While-Drilling (LWD) formation evaluation with tools rated up to 350°F (175°C) and 25,000 psi (172.4 MPa). These ratings qualify as High Pressure High Temperature (HPHT) environments for LWD services, where the standard ratings are much lower. The HPHT LWD suite consists of measurement-while-drilling (MWD), annular pressure, resistivity and nuclear porosity services. The MWD service provides surveys, optional gamma ray measurements and the mud-pulse telemetry connectivity to the surface. Changes to the real-time pulsed telemetry data streams can be requested from the MWD surface system to accommodate customer requirements for any sections logged within the wellbore. A retrievable MWD system is also available for high-risk drilling environments and conditions. The quartz pressure tool measures annular pressure, drill pipe pressure and mud temperature. The resistivity service comprises of a dual frequency (2MHz and 500 kHz) borehole compensated wave propagation tool with three transmitter-to-receiver coil spacings. The combination of dual frequency, triple transmitter coils and phase/attenuation measurements provides twelve resistivity curves with different diameters of investigation. Advanced resistivity analysis compute six dielectric assumption independent resistivities, and the related dielectric constants. In anisotropic formations in high angle wells, the horizontal and vertical resistivities are calculated. The porosity service is a density-neutron-stand-off-caliper tool. The spectral bulk density (Rhob) measurement is compensated for stand-off. Three neutron porosities are computed from the three neutron detectors. The more environmentally friendly Californium252 source emits a neutron output comparable with a conventional LWD Americium-Beryllium source but at a much lower radioactivity. The source is recyclable, thus removing the need for quasi-eternal storage for depleted sources. The tools in this suite are modular and can be configured in any order. A selection of Bottom Hole Assemblies (BHA) will be shown. Examples will show the range of measurements and the advanced applications. Introduction There is a wide variety of pressure and temperature ratings for the different M/LWD tools on the market1. It appears that ratings above 302°F (150°C) and above 20,000 psi (137.9 MPa) are considered high temperature and high pressure ratings. 350°F and 25,000 psi may not be considered HPHT in some drilling environments, nor would they be considered HPHT in wireline logging tools. In the LWD industry however, where the tools must withstand the temperature and pressure for extended periods of several hundred hours, these ratings indicate HPHT qualification. The M/LWD tool suite that is introduced here is rated up to 350°F and 25,000 psi. Temperature/Pressure Rating The rating is achieved by pre-screening all the electronic components and testing to 350°F. The tools are tested to 25,000 psi. One of the main challenges to meeting the 350°F requirement is the design of electronics. Components have to be carefully selected and individually tested to ensure that they are reliable and capable of operating at these high temperatures. In addition to component testing, rigorous testing of electronics at the system level is necessary to ensure the performance and reliability of the tools at high temperatures. In order to achieve a higher pressure rating, high tensile strength materials are used. Finite element stress analysis of the geometry of the components is also performed and suitable changes are made to accommodate the higher stresses associated with high pressure. The HPHT LWD suite consists of MWD survey, gamma ray and resistivity services for nominal collar sizes ranging from 4 ¾-in - 9 ½-in and in 4 ¾-in nominal collar size for MWD survey, gamma ray, annular pressure, resistivity and nuclear porosity services. The annular pressure service is also available for HP/HT environments in a 6 ¾-in nominal collar size.

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