Development of a Geomechanical Ranking System to Investigate Hydraulic Fracturing Feasibility of an Unconventional Shale Oil/Gas Reservoir: Case Study from Middle East

Abstract

As part of a multi-disciplinary study to explore the unconventional shale oil/ gas potential within a prospective carbonate/shale formation in Middle East, geomechanical analysis was carried out to understand the formation characteristics in order to evaluate hydraulic stimulation feasibility. The main objective of the geomechanical analysis was to select the most suitable area for fracturing in the few fields under study. To understand the geomechanical setting across different fields, well based 1-D geomechanical models were built for dozens of wells from few adjacent fields. Rock mechanical tests specifically designed for unconventional reservoirs were performed in cores of selected wells and the results were incorporated in the calibration of geomechanical models. In general, a mild and very strong strike-slip stress regime was identified over target lime shale and bounding limestone formations, respectively, with very high horizontal stress magnitude and anisotropy noticed in low clay and low TOC layers. Different stimulation scenarios were investigated by using hydraulic fracturing simulation software. The results suggest that the high horizontal stress contrast between target shale and the bounding organic-free limestone formations provide a very strong stress barrier which reduces the risk of out of zone propagation of hydraulic fractures and therefore, increases the efficiency of fracturing job. However, depletion reported within certain layers of interbedded limestone formations in some of these fields in this study, reduces the stress contrast and fracture containment in those fields due to poro-elastic effects. A geomechanical ranking system was developed to evaluate the relative feasibility and effectiveness of hydraulic fracturing treatments in the prospective shale target and also to identify sweet spots across four fields in the study. In order to develop this ranking system, several properties have been defined and estimated, based on the derived rock mechanical properties and stress models. These parameters address the containment of the fracture within the target shale, complexity of generated fracture and also amount of estimated over-pressure in the target interval. A final geomechanical rank was calculated by assigning a weight multiplier for each ranking parameter based on its relative influence on feasibility and quality of stimulated hydraulic fracture. Geomechanical ranking (resource accessibility) along with petrophysical ranking (resource volume) have been considered together to produce an integrated "chance of success" statement for different areas within the subject fields. Composite risk maps were developed based on rankings of wells from the study fields. These maps provide information on sweet spots which in turn have been used to shortlist highest ranked areas for the subsequent unconventional development program.