High angle or horizontal (HA/Hz) wells are commonly drilled to increase reservoir exposure and enhance the overall production. Whereas reservoir navigation highly advance the correct placement of such wells, completion decisions for profit or recovery oriented production optimization require thorough interpretation of available shallow- and deep-reading formation evaluation measurements. Particularly, complex geological environments necessitate the use of reservoir maps and heterogeneity evaluations to understand the likely production behavior.
In such applications, formation evaluation measurements from logging-while-drilling (LWD) technology are affected by the relative geometry between well trajectory and the formation. With increasing depth of investigation of the measurements and geological complexity of the reservoirs, these effects become more severe and true properties become uncertain. The multi-propagation-resistivity (MPR) responses show so called polarization horn effects at bed boundaries when beds with a high resistivity contrast are encountered. During the process of inverting these resistivities to obtain true formation resistivity (Rt), inversion algorithms help to reconstruct the complex architecture of downhole formation geometries. This provides a better understanding of the geometrical reservoir complexity and provides insight into the potential future production behavior. Possible corrective actions in production and completion strategies can be derived from such insight. Additional complications like proximity to water transition zone and water coning can be predicted and certain corrective measures, for instance blanking particular zones or placing inflow control devices (ICDs) can be taken, potentially increasing the overall life of the well.
This case study presents a horizontal well in a layered Lower Burgan sandstone reservoir of West Kuwait Minagish Field, where a new approach of drilling horizontal sidetracks from abandoned and plugged old vertical wellbores attempted with the purpose of increasing reservoir productivity and tapping into unswept oil. A sidetrack was placed at the roof of the Lower Burgan and navigation while drilling within this complex reservoir was used to stay close to the lithological boundary. Represented by paleo river channel stacked sand bodies, the Lower Burgan formations are overlain by fluvio-tidal sequences of sandstone – siltstone – minor shale alternations of the Upper Burgan with an undulating nature of the lithological boundary. Steering and placing the well in close proximity to this undulating boundary was achieved. The production of this well is exceptionally high with minimum water cut despite a massively increased water transition zone over the last decade. The case demonstrates that an integrated interpretation of the reservoir architecture and heterogeneity along the lateral uncovers the reasoning behind the experienced production.
Overall, polarization horns were observed on the propagation resistivity curves. While the interpretation of log artefacts from sole MPR and azimuthal propagation resistivity (APR) data is challenging, the creation of a detailed structural Earth model from borehole images and the results from a 1D inversion algorithm were used to understand geometric effects on the logs qualitatively. The top of the reservoir was modeled as an uneven surface with open bed boundary positions, to represent the undulating nature of the erosional surface between the Lower Burgan and Upper Burgan formations. The used method is an iterative 1D resistivity inversion algorithm and is able to derive more accurate Rt along the lateral and allows to reconstruct and visualize the downhole geological complexity of the Lower Burgan reservoir. Reservoir heterogeneity was investigated by the distribution of flow and storage capacities along the lateral well using effective porosity and he permeability index from magnetic resonance data.
The combined interpretation of the reservoir map, Rt and the reservoir heterogeneity highlighted the likely main producing intervals along the well which were placed close to the lithological boundary and hence sufficiently far away from the water transition zone. Other intervals where the well is closer to the water zone are of reduced reservoir quality giving low risk of water production. Based on these insights, suggestions are given to optimizate the completion design to achieve increased and sustained hydrocarbon production.