Duvernay Keeps Pushing Boundaries Of Conventional Geomechanics
Updated: Dec 27, 2020
At least in some regions, I like to think of the Duvernay Formation as a half-asleep mountain lion that wildly roars back with a steaming breath and trembles the earth beneath when awakened. The aggressive/sensitive behavior of this formation during drilling and fracturing is because it is extremely over-pressured, critically stressed and highly fractured and faulted. This fierce temper, however, has different levels of severity in different regions of the Duvernay making this play a curious case for geomechanical experts. The reality is (similar to some other unconventional plays), with its special characteristics, the Duvernay play has been challenging the status quo of some of the very common problem-solving approaches in petroleum geomechanics. In the following, very briefly, I will discuss some of these observations at a very high level. The provided references contain details for the interested readers*.
Figure 1. Detailed studies on the Duvernay show that the conventional problem-solving approaches have serious challenges explaining geomechanical behavior of this play.
Figure 2. Map of Woodbend/Leduc reef complexes, platforms and basins showing the location of the Duvernay play (Source: Switzer et al., 1994).
Figure 3. Schematic cross-section of reef, shelf/platform and basin-fill within Woodbend intervals, west-central Alberta (Source: Switzer et al., 1994).
Where high pressure rules geomechanics
The extremely high pore pressure (PP) (close to twice as large as hydrostatic PP in some regions) in some areas of this play has influenced almost every geomechanical characteristics of this play. For instance, as a result, stresses are in a highly sensitive equilibrium state and can be easily disturbed by underground operations. Likewise rock properties and fracture fabrics in this play are strongly affected by the excessive pore pressure in the formation.
Even minimum stress measurement can be difficult
Pressurized fracturing tests such as DFIT are supposed to provide closure pressure (CP) which is usually considered as the best proxy for minimum stress (read more about pressurized tests in this article on my blog). As its name suggests, CP is the measured pressure when the created fracture is closing. Fracture closure in highly overpressured formations, if ever happens, will take a very long time far more than the patience of the test operators. This is probably the reason that, in several interpretations of CP for the Duvernay, this parameter does not show a significant difference with PP. If valid, this observation (i.e., small difference between PP and CP) can challenge the applicability of common theories of stress characterization such as frictional equilibrium state or poroelasticity. The specific relations between PP and stresses can be related to the fact that in highly overpressured and low permeability rocks, tectonic stresses are mostly borne by the fluid rather than the rock matrix, a topic that I will discuss in a coming post soon.
Seismically at the edge both in-zone and out-of-zone
The proximity to the tectonic front and high PP in some areas such as Fox Creek region have brought the half-dormant faults in the Duvernay on the brink of reactivation and made induced earthquakes with magnitudes as high as 4.4 possible. Surprisingly, several of the recorded induced seismic events are interpreted to occur in the zones other than the targeted play (similar observations exist for some other formations), sometimes as far as several meters deeper. Although some believe this issue could be related to the level of accuracy in interpretation of event locations, some others, accepting the validity of locations, have tried to investigate the potential reasons behind these observations. In these presentation and article, more details on this controversial observation have been provided.
Figure 4. A profile of seismicity induced by hydraulic fracturing showing that the events are not confined to the targeted fracturing zone in the Duvernay Formation and occur far deeper. (Source: Bao and Eaton, 2016).
Rock properties are influenced by high pressure