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Aberdeen Evening Lecture March

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The Copthorne Hotel, Aberdeen

March 21


“Navigating the drilling window, improving drilling safety and assessing seal breach” with Richard Swarbrick or Swarbrick GeoPressure Consultancy

All wells require prior “knowledge” of the expected pore pressures to be penetrated by the drill bit, especially permeable geological formations, and the strength of the borehole wall to withstand fracturing from high pressure fluids.   The pressures of drilling muds (directly related to “mud weight”) are selected to navigate between the pore pressure and the fracture strength in the “open hole” section, i.e. prior to protecting the borehole with steel casing (Figure 1).    Some wells receive unwelcome surprises – higher pore pressures or weaker formations than expected – with consequences which can vary from managed “shut-in” and subsequent well control to propagation of a fracture to the surface and the birth of a mud volcano (in the case of a well in Java in 2006).   A “kick” is an instantaneous indication of drilling under-balanced (mud weight lower than pore pressure).  Many kicks are experienced in fields where injected fluids have unexpectedly found their way to unintended strata.  Of course, nature can duplicate these fluid movements too, leading to underground blow-outs, as well as seepage to surface.     

Much of the technology to predict pressures was developed using data from the Gulf of Mexico about 50 years ago, including the popular Eaton method for pore pressures and the Mathews & Kelly method to predict fracture pressure.   In the past 20 years focussed research, within the oil and gas industry and supported by academia, has helped improve the capability to predict pore pressure, offering global solutions to handle deep-water, tectonically compressed, and high pressure-high temperature plays such as mid-Norway and the North Sea.   Even so, pressure prediction remains a significant challenge in these areas.    Amazingly it is only in the past few years that comparisons between the Gulf of Mexico and the “rest of the world” have exposed the unsuitability of the Gulf of Mexico as a developmental area for pore pressure prediction!   But whist there has been improvement in predicting pore pressure there have been much less evolution of understanding as to how to predict fracture pressure. 

Fracture pressure prediction involves understanding rock stresses in buried rocks, and the direction and relative magnitude of those stresses.    Most predictive solutions involve a relationship between the overburden (vertical stress) and the horizontal stress, as well as incorporating a pore pressure term.  We also know from analysis of “Leak-Off” data (testing fracture strength at a casing shoe) that fracture pressure is coupled to pore pressure.  Published coupling values of 0.6 to 0.8 can now be shown to be unrealistically high, on account of the approach to quantify the coupling as well as assumptions about the overburden stress.   A new approach to fracture pressure determination (currently in process of being published and incorporating realistic coupling values) is proposed, with implications for fracture stimulation (Figure 2) and in-fill drilling in producing assets where reservoirs are heavily depleted, including the HPHT play.  Moreover, success in predicting fracture pressure improves seal breach risking, which will be illustrated by a case study from the Central North Sea.

The significance of the subject of this talk can be compared to geo-steering – i.e. the ability to successfully navigate to a target and subsequently complete a well.  Not all of them do reach their target, and several high-profile wells around the globe in recent years which failed to reach their objective on account of a drilling window which was not as expected.  

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