Resumen de Borehole images for assessing present day stresses
Arnaud Etchecopar, Tetsushi Yamada, Philip Cheung
Borehole images, which are now commonly acquired in the oil industry highlight very precisely damage that affects the borehole wall. Two damage types exist. The first corresponds to drilling operations: corkscrew effect, keyseat, cave etc. The second is related to present day stresses: induced fractures, breakouts or shear on preexisting planes. In addition to the real borehole damage, the acquisition and the processing of the image data may bring confusion in the identification of what is due to stresses. A new processing and filtering method is proposed for identifying the stress induced features and measuring precisely their geometry. Borehole images of two wells drilled in a shaly-sand reservoir particularly tight of the Timimoun basin (Algeria) clearly show that the main parameter that controls the damage geometry is the lithology. The stiffer and tighter is the rock the wider and deeper are the breakouts. To explain this strange behavior it is proposed that long term plasticity redistributes the horizontal tectonic stresses between the layers as a function of their shaliness. Another borehole image from Hassi Messaoud reservoir (Algeria) exhibits strong and progressive variation of the stress state where the well crosses a fault. By analogy to outcrops this perturbation is interpreted as due to a local deviation of the fault plane geometry and not to the average fault orientation itself. Such perturbations are unpredictable apart from images acquired while drilling.
Once stress induced features are clearly identified we examine how they can be used for assessing the present day stresses. In a vertical well it is well known that breakouts or induced fractures just provide the azimuth of the principal horizontal stresses as the vertical stress, parallel to the well surface, has no effect on the damage location. In a deviated well the three principal stresses influence breakouts and induced fracture location. Mastin [1988] has clearly demonstrated where breakouts will appear in deviated wells. This location depends on the three following parameters: the horizontal stress azimuth, which stress is vertical and R the shape of the stress tensor. These parameters are exactly the ones that also control the orientation of the shear on a preexisting plane. As a consequence, the observation of a breakout or an induced fracture in a single deviated well does not bring conclusive information on the stress state. But the combination of observations in multiple deviation situations may lead to a clear stress state definition. This is similar to the methods promoted by Jacques Angelier in which slickenside inversion is used for determining the stress tensor. The breakout inversion method applied to 7 wells of a north Ural field drilled with very different azimuth and deviation leads to clear definition of the local stress state. Once the stress tensor parameters are clearly identified, it becomes possible to predict borehole or perforation stability for any well deviations.
