Rock-mechanical properties and in situ stress state of the Upper Cenomanian Mishrif limestone reservoir, Zubair oil field, southern Iraq

Abstract

Abstract Rock-mechanical properties and tectonic stress state have crucial implications for reservoir stability, drilling, and production optimization. We have developed the first ever in situ stress analysis focused on the Mishrif reservoir from the supergiant Zubair oil field, southern Iraq. We interpreted an overall hydrostatic pore pressure gradient of 9.8 MPa/km and a vertical stress (σv) gradient of 22.17 MPa/km in the Upper Cenomanian Mishrif limestone formation. We inferred a Poisson’s ratio of 0.27–0.31, a Young’s modulus of 27–38 MPa, a coefficient of internal friction range of 0.66–0.76, and estimated a uniaxial compressive strength (UCS) range of 35–70 MPa with a low cohesive strength of 7.8–9.8 MPa from compressional sonic slowness data. We established fitting relationships between the dynamic and static elastic properties from the core-based triaxial test measurements and used the same for estimating the minimum (σhmin) and maximum (σhmax) horizontal stress magnitudes by poroelastic horizontal strain model. We validated the reservoir σhmin with the mini-frac-based fracture closure pressure and interpreted its gradient as 14.5–15.9 MPa/km, whereas we calculated the σhmax gradient as 19.5–23 MPa/km. We interpreted C-quality breakouts from four-arm caliper data and deciphered a northeast–southwest σhmax orientation (N45°E–N70°E). Using the compressive failure criteria, we constrained the reservoir σhmax by stress polygon analysis against the estimated UCS ranges and concluded a normal fault to strike-slip transitional tectonic stress state (σv ≥ σhmax > σhmin) in the Mishrif reservoir.

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