Placing a maximum reservoir contact well in a thinly layered reservoir has always been a challenge. Experiences showed that the well trajectory could easily be steered out of the target, necessitating expensive plug-back and redrilling operations to ensure that the well is drilled as planned. With the deployment of advanced LWD technologies, such as density image (DI), resistivity image (RI) and directional deep resistivity (DDR) logging tools, and high speed real time satellite data transmission, well paths can be geosteered from anywhere and kept in a thinly layered reservoir.

The first Saudi Aramco field examples of utilizing RI and DDR are shown to demonstrate the added values of new technologies in geosteering difficult-to-drill wells. In some of the examples, images of density and resistivity are consistent and all could be used for geosteering. In other examples, wrong geosteering decisions would have been made had the DI been the only available tool. With the help of RI, reservoir contact of multi-lateral wells is increased. Examples also show that using DDR can prevent the well trajectory from being too close to the zero porosity rock layer or the underlying water.


The main oil producing reservoir in the XA field is a massive carbonate reservoir. At the top of this good quality reservoir, there is a thin heterogeneous reservoir interval (named L1Z1) with rock porosity ranging from less than 10 porosity unit (pu) to more than 20 pu (Fig. 1). This L1Z1 is sandwiched between an 8' thick overlaying zero porosity anhydrite unit and an underlying 2' thick zero porosity anhydrite or close to zero porosity anhydritic dolomite unit. The thickness of L1Z1 ranges from less than a few feet to more than 20', with a typical thickness from 4' to 8'.

Because of the rock quality, this thin layer has been difficult to target through traditional vertical producers. To improve oil recovery, maximum reservoir contact (MRC) multi-lateral (ML) horizontal wells have been drilled in the last few years; some drilled in new locations and others sidetracked from existing vertical wells.

Conventional Geosteering Tools

It is impossible to place a horizontal well in a thinly layered reservoir such as L1Z1 without the help of logging-while-drilling (LWD). Traditionally, LWD tools consist of gamma ray (GR), density, neutron, and resistivity (triple combo) measurements. Geosteering a horizontal well with only conventional LWD triple combo and GR are difficult because they do not provide direction of the measurement.

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