Borehole imaging for the geothermal and petroleum industry

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    Location: GNS, Avalon and Wairakei sites

    Organization: GNS Science

    Mobile: Yes

    Contact person: Mark Lawrence (Avalon), David McNamara (Wairakei)

    Email: m.lawrence@gns.cri.nz, D.McNamara@gns.cri.nz

     

    GNS Science has expertise in analysing information collected by borehole imaging devices used in petroleum and geothermal energy exploration. The data is acquired by lowering the imaging device down the borehole on a cable that comprises an outer steel section surrounding insulated conductor cables.  The conductor cables send the measured data to the surface where they are recorded. 

    This technique is known as wireline logging and is used in the petroleum and geothermal industries to obtain a wide range of information on the properties of rocks (e.g., resistivity, density, porosity, gamma ray attenuation) and constituent fluids at various depths.  Borehole imagers are one of a number of devices that can be deployed in this way.

    There are two broad types of wireline borehole imaging tools available:

    1. Electrical devices measure the electrical resistance of the rocks and fluids.  They are cylindrical but have a series of electrode covered pads on extendable arms. The pads must be in contact with the borehole wall to make measurements.
    2. Acoustic devices send out acoustic signals to the borehole wall and measure the reflected sound energy. The acoustic tool does not need to be in contact with the borehole wall.

    The vertical resolution of these wireline devices is of the order of 0.5-1.0cm.  This is higher precision than can be achieved with conventional borehole techniques (e.g., gamma ray, density, etc.) and provides more accurate geological information on which to base geological models. GNS Science has the in-house capability to process the raw data from both types of tool and generate images of the geology down the borehole (typically, but not limited to, false colour mapping of physical properties such as resistivity). 

    The technique can be used to produce stratigraphic logs, recognize lithologies, analyse structural features (e.g., faults), and reconstruct geomechanical features (e.g., in situ stress). The imaging can be combined with direct coring in order to calibrate/groundtruth the interpreted facies.

    The advent of new generation acoustic logging devices has boosted the power of this technique for evaluating geothermal wells in particular.  The images result from a combination of the physical properties of the rocks and the fluids with rock pores.  Interpretation of the images provides information about rock type, orientation of rock fabrics (e.g., bedding, fractures, faults, etc) and depositional history. This information can then be used by geoscientists to develop computer models of geothermal or petroleum reservoirs, which are crucial for optimal exploration and management of petroleum or geothermal fields.

    Most borehole imaging devices operate below about 175°C.  Recently, Tiger Energy Services, who have a base in Taupo, New Zealand, introduced an acoustic logging device that can operate at temperatures up to 300°C, specifically for use in geothermal wells.  It is one of the first of this new generation of instruments to be operated in the world.  

    Additionally to the techniques mentioned above, GNS staff members are contributing to the PEGI 11 project (run by the Ministry of Economic Development, MED). The main aim of this project is to provide a consistent suite of quality-controlled conventional logs for wells in New Zealand petroleum basins. In addition, some basic petrophysical properties (e.g., porosity, shale volume, saturations) were calculated from these raw data. The approach will be extended to outcrops, and can already be applied to actual cores.

    GNS Science has six staff trained to analyse and interpret borehole image data, including that acquired by the Tiger Energy’s new high-temperature acoustic logging tool. The data can analysed on site at the well, at a client’s office, or at the office of GNS Science.

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