Chapter 44 The Role of Heavy Mineral Analysis as a Geosteering Tool During Drilling of High-Angle Wells

The use of heavy mineral analysis on a ‘real-time’ basis at well site is a logical extension of the application of the method as a non-biostratigraphic correlation tool. To date, the method has been used to monitor the stratigraphy during drilling of horizontal wells in three fields on the UK continental shelf (Clair, Ross and Hannay), and in several cases heavy mineral data have been integral in deciding to amend well bore trajectories in order to remain within hydrocarbon-bearing zones, a process known as ‘geosteering’. Heavy mineral analysis now has a positive track record for geosteering horizontal wells in a variety of depositional environments, from fluvial/aeolian to shallow and deep marine, and can therefore be genuinely considered as an alternative geosteering tool in circumstances where biostratigraphic methods have inadequate resolution. Heavy mineral data have been used in the decision-making process in a variety of situations, including whether to maintain angle, to steer up or to steer down, to sidetrack and to terminate drilling. Application of heavy mineral analysis at the well site requires establishment of a robust correlation scheme prior to drilling horizontal wells, using samples from offset wells adjacent to the proposed well track. Parameters used to monitor the geology during horizontal drilling include those conventionally used for reservoir correlation (principally, ratios of stable minerals with similar hydrodynamic behaviour), but other parameters have also proved useful in the particular circumstance associated with individual fields. For example, the abundance of unstable minerals is locally useful in the Clair Field, and ratios of minerals with contrasting hydrodynamic behaviour are useful in the Ross and Hannay Fields.