Conventional horizontal drilling in thin pay zones has historically relied upon onsite geologic interpretation from drill cuttings and real-time logging-while-drilling (LWD) gamma ray data to confirm that the well bore is in the target formation and within the highest porosity of the target formation. Three issues can make this process difficult. First, the ability to make timely steering decisions from cuttings can be reduced because of sample lag and processing time. Second, it can be difficult to infer porosity from gamma ray data alone during drilling. Third, positioning a horizontal well within the highest porosity of a thin reservoir requires an extensive working knowledge of the area's geology, which is not always available from omni-directional gamma ray data.
Halliburton Sperry Drilling Services'new ALD azimuthal lithodensity imaging sensor can solve these problems when used with other standard logging tools. Combining it with a compensated thermal neutron tool with integral caliper, gamma ray sensor and propagation resistivity sensor (for early detection of approaching bed boundaries or fluid contacts) can significantly increase the percentage of the lateral drilled in the targeted porosity layer. This is accomplished by incorporating real-time azimuthal density information during active geosteering, which enables optimum penetration of the preferred porosity zones and determination of how the well bore is traversing up or down through the stratigraphic section.
Applications
The LWD imaging tool provides density and Pe logs with improved accuracy and precision, as well as valuable formation dip and borehole shape information for both geosteering and hole size and shape applications. Applications include:
Obtaining accurate density and Pe data, even in enlarged boreholes or with bi-center bits;
Acquiring real-time and recorded formation images for geosteering, structural dip determination in high-angle wells and detection of borehole breakout, washout and spiraling;
Determining lithology information;
Detecting pore pressure trends;
Pinpointing gas-bearing intervals (with neutron porosity); and
Acquiring formation mechanical and seismic properties.
Operation
Improved near and far detectors and electronics are incorporated in the density sensor along with a stabilizer blade to displace as much of the borehole fluid as possible. An acoustic standoff measurement in line with the detector blade is incorporated in the 63¼4-in. and 8-in. collar size tools to further improve accuracy of the measurements. Rotation of the bottomhole assembly (BHA) during drilling or on repeat passes allows formation and wellbore images to be created.
Images are created by rapidly sampling the data from near and far detectors along with the acoustic standoff sensor and then placing these rapid samples into "bins" that are referenced to either magnetic north or high side of the borehole and correspond to the position of the detector blade when that rapid sample was made. The data can be binned into as many as 16 radial sectors around the borehole. After binning the data for a conventional sample period, which is the period corresponding to how often data is stored in memory, the many rapid samples for each data type are averaged in each individual bin. Data in multiple bins can also be averaged to produce 8-bin data, 4-bin or quadrature data, 2-bin or up and down data and conventional density, which uses all the data in all the bins. From these multiple data sets, near density, far density, compensated density, delta rho, Pe and acoustic standoff are calculated for a variety of real-time and recorded applications.
The azimuthal density, Pe, delta rho and acoustic standoff data can be presented as log curves and as formation image logs in both real-time and in higher resolution from memory data after a bit trip. By selectively using quadrant data, the azimuthal data acquired can provide accurate density and Pe logs, even in enlarged boreholes or holes drilled with bi-center bits.
In horizontal wells, examination of the real-time azimuthal lithodensity sensor density image logs available in four or eight bins can identify the path of the well bore relative to bedding planes. The image permits the drilling plan to be actively modified to steer up or down within a geologic horizon to achieve maximum productive formation exposure and to have that exposure in the best porosity. Image logs including density and the other azimuthal measurements (delta rho, Pe and acoustic standoff) can also give information on hole quality such as presence of hole spiraling, washouts and stress-induced borehole breakout.
In addition to providing azimuthally binned data, the tool retains the rapid sampling statistical optimization technique originally pioneered with the stabilized lithodensity service for optimizing density and Pe data quality independent of azimuthal measurements. This technique identifies and segregates the portion of the count rates sampled when the standoff between the detector blade and formation was minimal during the rotation of the sensor. Since the segregation of count rates is statistical rather than azimuthal, high quality density measurements can be acquired in vertical wells where the tool does not preferentially orient to one side of the borehole throughout the run.
Case study
Recently, the imaging tool was used to geosteer a horizontal well in Canada. Although the most porous limestone and coarse-grained, dolomitized lithologies have been successfully produced using vertical wells, most production from the tighter, largely undolomitized rock were achieved with the introduction of horizontal drilling to the field.
The accepted practice for drilling horizontal wells in this area is to expose as much of the hole as possible in porous intervals, ideally above 9% log porosity. Real-time drilling information prior to drilling the latest wells was restricted to rate of penetration (ROP), wellbore inclination and azimuth, drilling mud gas data and gamma ray logs. Drillpipe conveyed neutron density and gamma ray logs were run once total depth was reached for post-drilling correlation of data. Resistivity logs run in a few earlier wells and in the horizontal well yielded little useful information. Wireline electrical micro imaging logs can provide excellent images and reveal details of fractures (including orientations), bedding dips and azimuths and presence of karst features; they have, however, been run minimally due to the cost to pipe convey the tools.
In a previous well, the objective was to drill parallel to the bedding plane, within a bed that was presumed to have excellent porosity due to a high average rate of penetration (ROP) and higher than average gas. Openhole logs were run after the drilling phase that revealed that only a small percentage of this interval transected rocks with above 9% log porosity. In fact, the best porosity occurred farther along in the horizontal leg after the inclination was changed and another stratigraphic level tested.
Following these disappointing results, the well was planned to employ a comprehensive formation evaluation while drilling (FEWD) system that included the azimuthal density tool, which was primarily used to help steer and position the horizontal portion of the well within the highest porosity of the targeted zone. A secondary benefit was the ability to generate a 360° image of the well bore along the entire horizontal section. These images were generated from both the azimuthally binned density data and acoustic standoff measurements.
Upon completion of drilling, the planned versus actual drilling days curve for the well was analyzed. Drilling times were similar to those planned, but the client felt that the well was drilled more effectively and that production results would eventually reflect the more effective positioning of the well bore in the higher porosity due to the evaluation while drilling system employed on the well. This comprehensive real-time FEWD system approach including neutron and azimuthal density logs can provide the geologist with the ability to consistently steer a well into "the sweet spot" and overcome the potential limitations of previous methods of steering.
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