Calibration, Repeatability and Interpretation of In-Situ Tectonic Stress Measurement From Wireline Straddle Packer MicroFrac Testing: Case Study of an Onshore Field in the UAE

Abstract

Wireline straddle packer microfrac tests for in-situ stress characterization have become an important technology to identify the best place to fracture during the drilling and completion of oil and gas wellbores. The microfrac testing procedure includes formation breakdown, fracture propagation, fracture reopening cycles, and pressure fall-off cycles for fracture closure identification. The objective of this microfrac testing was to validate and calibrate the horizontal stress profile in various intervals of the target formation. This paper will focus on the calibration and repeatability of microfrac tests to obtain quantitative downhole formation breakdown, fracture re-opening, fracture propagation and fracture closure pressure in a vertical borehole located onshore Abu Dhabi. A pre-job assessment was conducted to identify preferable intervals, which took into consideration the borehole conditions, the straddle packer capabilities and limitations so as to design a successful micro-fracturing job. Three fracture propagation and pressure fall-off cycles were analyzed to measure the fracture closure pressure. The results were used to calibrate the far-field minimum horizontal stress profile. Fracture closure was obtained by natural leak-off pressure decline behavior; no flow-back was required to induce fracture closure. The fracture closure pressure was identified by three different methods: (I) Square root of time (SRT) pressure decline analysis using the isolated interval pressure vs. square-root of shut-in time, (II) Log-Log pressure decline analysis using the pressure derivative of the delta pressure and delta time in log-log plot, and (III) G-function analysis by plotting the GdP/dG on a pressure vs G-time plot. The results show how crucial it is that the inflatable elements are positioned on layers with stress contrast in respect to the isolated formation interval so that optimum fracture containment and proper fracture propagation are obtained. This is important in order to avoid sleeve fracturing and early hydraulic communication between the fracture and the hydrostatic pressure. The repeatability and interpretation of the microfrac testing for validation and calibration of horizontal stresses and stress contrast are discussed as well as the importance of the static elastic properties of the rock to improve the lateral strain values in reproducing the fracturing pressure recorded on the microfrac tests.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015