Integration of Special Core Analysis, Wireline Logs and Multi-Scale Injection Data to Build Robust Mechanical Earth Models for Hydraulic Fracture Optimization Projects
Authors: Al Sadi S. S. , Merletti G. , Armitage P. , Gil I. , Sesetty V. , Jaboob M. , Casero A. , Gramin P. , Al Salmi H. , Al Hajri S. N. 
Published: October 2023

Reservoir deliverability of tight-gas reservoirs is underpinned by precise petrophysical evaluation and effective hydraulic fracture modeling. The Barik reservoir in Block 61 of the Sultanate of Oman is a giant Paleozoic tight-gas sandstone requiring hydraulic fracturing to produce wells at economic rates. Mechanical Earth Models (MEM) are critical for fracture design and optimization to target fracture containment between encasing non-reservoirs and to maximize lateral fracture penetration. This study describes the workflow applied to build an MEM for the Barik, its calibration to multi-scale closure pressure measurements and validation using fracture pressure diagnostics.

The MEM is built upon an Elastic Horizontal Strain model that uses specialized wireline logs, pressure decline analysis from injection tests, and geomechanical core tests on lithologies of different reservoir quality, including non-reservoir rocks (shales). Single-stage triaxial compression tests at in-situ effective stress yielded pseudo-static rock mechanical properties, whereas hydrostatic compression with application of pore pressure allowed the calculation of Biot's poroelastic coefficient. These discrete core measurements are correlated to equivalent properties computed from high-frequency wireline acoustic logs, while overburden stress is calculated by integrating the bulk density log with depth. Pore pressure is assessed by wireline formation testing tools which mimic mini-drillstem tests. The minimum horizontal stress is assessed by wireline MicroFracs delivering layer-by-layer closure stress and Diagnostic Fracture Injection Tests (DFIT) inclusive of After-Closure Analysis characterizing dozens of meters of interbedded lithologies. Horizontal tectonic strain values are approximated using closure stress and mechanical properties across diverse formation layers. Geomechanical analysis of breakouts and tensile fractures in borehole images delivers minimum to maximum horizontal stress ratio.

The MEM is validated via a Pseudo-3D hydraulic fracture numerical simulator by history matching the modeled and measured pressures during the fracture treatment, the instantaneous shut-in pressure, and the post treatment pressure decline. Additionally, radioactive tracers injected during the treatment help to validate hydraulic fracture height. Up to 25% difference in hydraulic fracture length between poorly- and fully-calibrated MEM in the Barik reservoir is observed. The simulated fracture geometry and proppant concentration at the end of the injection are key to defining the optimum number of fracture stages and fracture design to evenly stimulate every hydrocarbon bearing layer.

This paper demonstrates how increased reservoir deliverability can be achieved by improvements to hydraulic fracture optimization from interdisciplinary workflows, which process and analyze data to build and validate the MEM. The value of data processing and quality control in calculating the stress profile and its effect on history matching is demonstrated. Additionally, the impact of micro-fractured core plug and dispersive shear wave data on dynamic pseudo-elastic properties and the MEM is explored. The importance and limitations of radioactive tracer interpretations are discussed, with examples, and recommendations for other direct fracture diagnostic tools.