Characterising and Defining Stimulation Zones in Tight Formations for Appraisal Wells Onshore U.A.E with the Aid of Integrated Standard and Novel Stress Determination Methods

Abstract

Several onshore concessions, currently under exploration by ADNOC, consist of tight laterally variable reservoirs that pose a significant challenge during the evaluation phase of exploration. Most tight hydrocarbon-bearing formations require fracture stimulation. As such, the evaluation phase of these resources comprises not only the characterisation of reservoir rock properties using petrophysical analysis but, crucially, the construction of 1-D Mechanical Earth Models which underpin the identification of stimulation intervals for both vertical and horizontal well completions. The 1-D MEMs discussed here were provided by different vendors and have been calibrated against interval pressure tests, that included standard "wet" straddle packer microfractures and novel "dry" Sleeve-Fracture tests. The microfracture test data used to calibrate the MEMs were obtained from different depth intervals in onshore Abu Dhabi E&A wells and exhibit non-ideal pressure decline "shut-in" behavior. This required re-analysis using different interpretation methods to identify the lower bound fracture closure pressures and minimum stress magnitudes. The identification of stimulation intervals from the 1-D MEMs highlighted the uncertainty in the minimum stress magnitude estimations from both the log-based models, and the microfrac interpretations. The uncertainty in the log-based minimum horizontal stresses can exceed 0.15 psi/ft (>17%), even after calibration with the microfracture tests. The uncertainty in the fracture closure pressure obtained from the microfracture test can also be as large as 1,600 psi (0.22 psi/ft and 30%). The identification of the sources of the uncertainty, their quantification and the re-evaluation of microfracture tests fed directly into updated 1-D MEMs, which led to improved recommendations for optimised injectivity tests and acid fracturing treatments. This, in turn, has translated into a successful fluid sampling and production appraisal programme.