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When drilling horizontal wells, geological uncertainties have a much greater effect on the fulfillment of geological tasks than when drilling vertical wells. The combination of the classic geosteering methodology, complemented using logging while drilling (LWD) technologies, and rotary steerable systems, allows you to quickly make decisions on changing the well trajectory to optimize the entry into the target formation. During subsequent drilling within the target formation, the use of multilayer mapping technology in conjunction with an extended LWD and RSS system - helps to more accurately adjust the position of the horizontal section in the reservoir, thereby increasing the rate of reservoir penetration This article describes the effectiveness of the geosteering service to reduce structural uncertainties of landing of the transport section and while placing a horizontal section in the productive part of the S. Nurzhanov field. Structural uncertainties were mainly caused by the proximity of the regional fault to the horizontal section and the variability in the thickness of the target intervals in the offset wells. The provided geosteering service made it possible to complete all the tasks set for drilling the horizontal section, preventing the exit from going beyond the target zone and avoiding the unwanted approach of the wellbore to the unproductive rocks lying within its boundaries.

The objective of this article is to describe integrated approach, including planning and reasoning of well constructions based on field development modeling, calculation individual well trajectories taking into account full complex of knowledges in target formation geology in drilling area and well placement service usage at the drilling stage to elaborate formation geology and achieve planned well production. Results of horizontal sections trajectories planning are shown in this paper, as well as description of geosteering process features. Results of well positioning optimization, based on hydrodynamic modeling were used of well trajectory planning, as well as complex of geological information: structural surfaces, well-to-well correlation, fluid contacts, seismic profiles with inversion results and sector model for the drilling area. Well placement service at the drilling stage allowed to significantly decrease risks related to lithological and structural uncertainties and, therefore, maximize effective length of horizontal sections keeping total length fixed. Following measurements were used during the job: D&I (direction and inclination); GR, resistivity, neutron-density logging and density image. Those data were utilized for 2D structural model update and adjustment. Choice of optimal well trajectory for horizontal section effective length maximization in target sandstone reservoirs with low thicknesses and low permeability, along with well placement service usage resulted in significant well production increase. BHA with LWD tools, that was designed taking into account specific geological and technological conditions, was found as most appropriate for well placement methods sufficiency and determination of reservoir properties and saturation. Geological model adjusting during geosteering at the drilling stage allowed to achieve maximal NTG (net-to-gross ratio). A total of 19 horizontal sections were drilled for field development with cumulative total length more than 23,000 m and cumulative effective length more than 16,000 m. As a result of such approach, wells have productivity that exceeds planned. Final result of well drilling is gas production rate from 300,000 to 700,000 m3/day (summarized production rate is up to 3,000,000 m3/day). Integrated approach and close interaction between companies involved at all stages of well planning and drilling resulted in significant decrease of risks related to structural and lithological uncertainties. Increase of horizontal sections effective length had effect in production increase and, therefore, allowed to improve rate of return on investments.

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