Within hydrocarbon containing formations the fluids within the pore space or the available space in fractures or cavities of the reservoir rock are under pressure (= “pore pressure”).
When a well is drilled into a formation, stressed solid material is removed. The borehole wall is then supported only by the fluid pressure (termed "mud weight") in the hole. The drilling mud has many functions including pressure control, wellbore stabilization, transport of cuttings to the surface, cooling and lubrication of the drill bit and as data transmission medium. The mud weight needs to give good control of formation pressures and wellbore stability without risking that the formation itself is fractured and the well goes on losses. This “lost circulation pressure” or “fracture gradient” is an important parameter for well design and it’s estimation for a priori and during the actual drilling progress is important.
The difference between the pore pressure and the fracturing pressure defines the mud weights that can be used during drilling. This ‘drilling window’ in Figure 1 shows that the mud weight must exceed the pore pressure but should also stay below the fracturing pressure. We use the term fracturing pressure/gradient here with the understanding that lost circulation can also be triggered by mechanisms other than induced fracturing at the wellbore.
The operational drilling window in the reservoir and near overburden will vary throughout a well’s life as a response to pore pressure and temperature changes caused by the extraction of the hydrocarbon fluids (Figure 2).
The ability to drill safely and efficiently in depleted reservoirs is important for achieving goals for oil and gas recovery. However, drilling and completing new wells in a depleted reservoir may be challenging since changes in the drilling window that arise from changes in pore pressure and stresses can be difficult to predict especially in layered reservoirs with different lithologies, differential depletion and where loss incidents may not be caused solely from fracture events.
Statoil is interested to get in touch with those having methods or able to propose innovative approaches to get to a better understanding of pressure profiles of complex and partially depleted reservoirs. Preferred methods will allow open-hole measurements or need only a limited amount of interruption of the drilling operation.