Although at-bit measurements provide rapid detection of changing lithology, real-time logging-while-drilling (LWD) services are helpful in steering a horizontal well through the pay zone. Furthermore, these services provide the petrophysical data for formation evaluation. As the picture becomes more complex, standard LWD tools provide information that may help keep a well on track if the tools are used in a proactive manner.
The responses of standard LWD resistivity and nuclear measurements have been well characterized in the literature. Therefore, these responses can be used to describe a reservoir, specifically the changing lithology and the geometrical structure of the formation. Forward modeling studies of 2- MHz LWD resistivity devices can identify changing geological facies. Neutron-density measurement, by the nature of the system, gives directionality to the measurement that identifies how the beds are approaching the tool and, therefore, the well.
Examples presented from recently drilled horizontal wells clearly illustrate the application of these standard measurements in a complex horizontal well environment.
Pay zone steering (PZS) is a geological steering technique that can help to (1) drive a well into a formation at the appropriate attitude and (2) recognize when the well is at or near the desired distance from a resistivity interface, such as the top of a formation, or fluid contact. In complex geological formations where faulting and stratigraphic features are present, these functions are performed by at-bit measuring tools, preferably with directional sensing tools. In simple circumstances, the PZS technique can use the standard, more economical, Logging-While-Drilling (LWD) services.
PZS differs from normal geological steering operations because it predicts the expected log response before drilling and uses this predicted log to identify critical landmarks during drilling operations. If the measured log compares well with the prediction, then the well is assumed to be on track.
Current measurement technology has advanced to the point where operators can seriously consider replacing wireline logs with LWD. Since a PZS operation uses standard LWD services, these services can also be used for a reservoir evaluation normally performed with wireline logs. An evaluation using LWD tools has several advantages.
savings in rig time for reservoir analysis
improved reservoir evaluation because of shorter exposure times, reduced invasion effects, better borehole conditions, and less formation damage
reduced risk inherent in LWD operations
quantifiable corrections and tool characterization
PZS depends on the operator's ability to identify different formation layers using LWD devices. This layering is the basis for most geological models of the earth; this assumes that the layers occur in a predictable sequence and each layer has distinct physical and petrophysical parameters. This earth model is defined by the geologist who uses various surveys along with a geological understanding of the field to identify the various strata (seismic surveys, mud logs, wireline logs).