Paleosols are fossil soils, and provide additional challenges to drilling and completion operations as they are generally weaker and more friable than rocks such as shale or sandstone. This weakness often results in borehole degradation, wash- or breakouts and even formation collapse, which poses challenges to well operations. It is therefore important to identify paleosol horizons whilst drilling and perform operations in a manner that will minimize damage to these formations. Conventional log data is in many cases unable to uniquely identify paleosols.
Recent analysis of real-time GR capture spectroscopy, which is available in logging-while-drilling, has assisted in identifying these horizons and aided the successful drilling and completion of several wells on the Gullfaks Satellite fields. Previous work demonstrated that paleosols could be characterized using iron content. Spectroscopy data was used to determine the concentration of a suite of elements whilst drilling. By combining conventional log data with measurements of iron content it has been possible to identify weaker paleosol horizons based on an increased iron content, high Gamma Ray signature, and high bulk densities. This data has been utilized in real-time to avoid excessive work of the drillstring over the paleosol zones, particularly with respect to stabilisers and the bit, and to optimize the position of the bottom hole assembly (BHA) during formation pressure testing.
In order to reduce formation damage whilst drilling, a Low Energy Drilling Operations (LEDO) strategy in which the drilling parameters are only changed one at a time in order to establish trends, off bottom circulation is minimized and ROP is limited has been used. This technique was combined with traditional LWD triple combo data along with GR spectroscopy, measured matrix properties and borehole images. This approach has made it possible to successfully drill several wells through paleosol- rich formations.