The State of Stress in North Africa - A Conceptual Model Based on Borehole Image Data from Algerian Oil Wells

Abstract

Stress-induced features, such as borehole breakout and drilling-induced fractures, often observed on borehole image logs, reflect the orientation of the in situ stress in the formation. If a large database of stress data is available from a given region this may be used to create a “conceptual model”, forming a link between structural geology and geomechanics. This paper presents a conceptual model of the North African region based on data from 100 oil and gas wells across Algeria. Stress orientations show an overall NNW-SSE trend, which may be linked to the relative motion of the African and Eurasian plates. Local fluctuations of the stress field occur on all scales, ranging from large regional structures over local faults to localized deflections near fractures. Relatively flat surfaces, such as formation boundaries, also deflect the stress field if associated with a significant mechanical contrast. Examples of all have been observed on image logs. Borehole breakout is systematically located in mechanically strong rocks, such as sandstone reservoirs, but absent from softer shales, indicating that stresses are partitioned with higher magnitudes and/or high deviatoric stress in reservoirs. This has implications for borehole stability as well as for completion applications, such as hydraulic fracture stimulation.