SELECTION OF THE PRODUCTIVE HORIZON IN WELLS

In multilayer oil and gas reservoirs, a single well often penetrates several productive formations with differing fluid and pressure characteristics. Effective isolation and selective exploitation of these layers are essential to ensure safe operations and optimized hydrocarbon recovery. During drilling, upper productive intervals are commonly isolated by installing casing strings and cementing the annular space, thereby preventing hydraulic communication between formations. As production proceeds, the lowest reservoir layer may experience progressive water encroachment, leading to a substantial decline in oil recovery efficiency and increased water production. Under such conditions, continued exploitation of the depleted interval becomes economically and technically unfavorable. To maintain well productivity, the lower watered-out layer is shut off using a cement plug, which provides a reliable barrier against further water inflow. This intervention enables the redirection of production to overlying reservoirs without drilling new wells. Since the upper layers remain isolated behind casing and cement, controlled perforation operations are applied to establish a flow path between the selected reservoir and the wellbore. The success of this approach largely depends on the accurate selection of the perforation interval. Incorrect interval choice, particularly the accidental perforation of water-bearing zones, can result in excessive water production and premature loss of well performance. Therefore, precise knowledge of the well’s geological section is a critical requirement for effective completion design. This knowledge is obtained through comprehensive well logging, including gamma ray, resistivity, and neutron logs, which together provide essential information on lithology, porosity, and fluid saturation. Integrating these data allows engineers to identify hydrocarbon-bearing intervals with high confidence and minimize production risks. Overall, the staged isolation and selective perforation of multilayer reservoirs represent a cost-effective strategy for extending well life and improving field development efficiency. Keywords: multilayer reservoir, zonal isolation, water shutoff, cement plug, perforation interval, well logging.