Issues and Challenges in Geological and Rock Properties Modeling Using Log Data from High Angle and Horizontal (HA/HZ) Wells

Yin, Hezhu¹, Quinn Passey² (1) ExxonMobil Upstream Research Co, Houston, TX (2) ExxonMobil, Houston, TX

During the last fifteen years drilling technology has advanced to where HA/HZ wells have become common. Similarly, the formation evaluation sector has increased its ability to log these wells. Prior to routine drilling of HA/HZ wells, reservoirs were delineated by near-vertical wells, and geologic and reservoir models were constructed using the vertical wells. Formation properties were then interpolated between wells in the models using a variety of geostatistical interpolation techniques guided by seismic and stratigraphic interpretations. Initially, HA/HZ wells were used primarily for in-fill purposes to increase rate and improve recovery. The log data from subsequent infill HA/HZ wells were used primarily to geosteer and to select completion intervals. In general, data from HA/HZ wells were not used for reservoir delineation or assessment purposes. Currently, in some fields, operators have to rely on petrophysical information from HA/HZ wells as a primary source of data for geological and rock properties modeling. The lateral extension of HA/HZ wells provides new dimensions in modeling reservoir property variation along geological bodies, such as channel axis, stream bar, and shoreline deposits; however, the reliance on HA/HZ wells as a primary source of data remains problematic in many cases. Issues and challenges of deriving accurate rock properties from HA/HZ wells must be understood and addressed. First, in the absence of continuous and accurate survey data, depth control combined with the uncertainty in relative dip calculations impart a large uncertainty to the calculated formation thickness. Second, in the absence of thick isotropic formations, accurate interpretation of well log data is not guaranteed. Given that many formations are anisotropic, it is not unexpected that logging tools designed for vertical wells may respond differently in HA/HZ wellbores. This results in an increased uncertainty of all subsequent log-derived properties, such as porosity, water saturation, and hydrocarbon-pore-thickness.