When drilling horizontal drains in a thin layer environment, one of the main challenges is to maximise the reservoir exposure, which means remaining in the pay zone as long as possible and at the same time avoiding or at least identifying any possible water conductive events. Considering that an angular difference of only one degree between the well bore axis and the apparent dip of the formation along the well path results in about half a metre vertical shift every drilled stand, remaining within a one or two metres thick reservoir requires a better precision for an optimal reactivity in the geosteering process.
The first requirement in this type of application is an accurate geological model that ideally features the layer geometry as well as the distribution of reservoir characteristics such as resistivity, porosity, radioactivity, saturation… etc. A good knowledge of the structural scheme is necessary, including fault frequency with the order of magnitude of their vertical throw, the frequency of structural undulations with an idea of their wavelength… In addition, it is necessary to visualise in real time the well within the model. Then, it becomes possible to anticipate possible structural undulations and to pro-actively make the necessary decisions to properly steer the drain within the reservoir.
The industry proposes two types of LWD resistivity imaging tools which may be considered for this purpose:
• The azimuthal electromagnetic propagation resistivity imagers,
• The laterolog type resistivity borehole imagers
The laterolog type resistivity imagers have the essential drawback that they only work in conductive mud systems. But on the other hand, they are able to provide very high resolution images. This enables the accurate picking of sedimentary and structural events such as bedding, dips and fractures.
Al Khalij field, offshore Qatar, features a carbonate layer-cake of alternating oil bearing and water bearing limestone reservoirs. Each oil bearing reservoir layer is about 2m to 5m in thickness. Surrounding water-bearing layers show similar porosity values but much finer grain size and have no physical limits with oil bearing reservoirs. The present paper describes an integrated method developed to achieve an efficient geosteering of long horizontal drains which may reach 3,000m to 4500m in order to maximise the reservoir exposure along the well path.