No abstract

No abstract

This paper defines current geosteering practices in the Middle Bakken formation using conventional gamma-ray detectors and discusses the use of a new azimuthally sensitive gamma-ray sensor for more accurate well placement in the target formation. Two wells were drilled in the Middle Bakken formation using both a conventional gamma-ray detector and a new azimuthally sensitive gamma-ray sensor equipped with an inclinometer to provide continuous inclination. Because omnidirectional gamma-ray measurements can be ambiguous, especially when markers have similar gamma-ray readings, azimuthal gamma-ray wellbore images were used to infer the relative stratigraphic movement of the drilling assembly through the formation. The two lateral wells presented in this study were drilled entirely inside the target zone between the objective gamma-ray markers. Both real-time and memory data were analyzed using borehole imaging software. Relative and true formation dip angles were calculated after estimating the best depth-of- image. The use of azimuthal gamma-ray images made it possible to determine if the wellbore was moving up or down stratigraphically. A more accurate geological correlation based on the image interpretation, together with an enhanced survey using continuous inclination, made it possible to map the top and base Middle Bakken surface locations. A comparison was established with laterolog resistivity images from the same area that were of higher resolution and higher cost, validating the use of azimuthal gamma-ray for dip calculation. The azimuthal gamma-ray measurement provides insight into stratigraphic well placement beyond what can be determined from omnidirectional measurements. Data acquired from the gamma-ray image, when combined with enhanced directional-survey measurements, can help to further refine geosteering decisions and reduce uncertainty in wellbore placement. This paper presents an application of a new, low-cost measurement to allow for better unconventional well placement in difficult-to-interpret situations. As this technique is more widely applied, it may lead to a step change in how lateral wellbores are positioned stratigraphically to increase reservoir exposure.

Geosteering long lateral wells in narrow target windows demands accurate stratigraphic positioning, true vertical depth (TVD) positioning, and apparent dip determinations. Using positive displacement motors with alternating slide and rotate drilling modes to control the well trajectory, along with standard 90-ft surveys and single-detector azimuthal gamma ray (GR) tools, have proven to be inadequate. To overcome these issues, a new method has been developed that combines continuous measurements and uses a four-detector azimuthal GR tool. The bottomhole assembly (BHA) used here is equipped with two inclinometer packages and a four-detector azimuthal GR logging-while-drilling (LWD) tool. A high-resolution survey is calculated using a combination of the two continuous inclination survey data sets for the deeper portion of the well and the stationary survey data from the shallow (low inclination) portion of the well, which provides a more accurate well path that better reflects the TVD positioning of the wellbore. The four-detector azimuthal GR tool is used to generate high-quality wellbore images, both while sliding and rotating. This enables more accurate structural dip angles to be determined from the continuous GR images. It also leads to better stratigraphic and structural positioning of the wellbore and a better understanding of changes in stratigraphy across the length the lateral section. Combining more accurate surveys with complete GR images and more accurate dip picking enables a better determination of the stratigraphic position of the wellbore and its path through stratigraphic layers. The wellbore can be divided into various up- and down-drilled sections that can be compared side-by-side using true vertical thickness (TVT) methods to show lateral continuity of the beds.

No abstract