Wireline Straddle Packer Microfrac Testing Enables Tectonic Lateral Strain Calibration in Carbonate Reservoirs
Authors: Ihab Tarek , Naial Radwan , Moronkeji Dee A. , Franquet Javier A. , Smith Steven S. 
Published: May 2016

Wireline straddle packer microfrac tests have become an important technology in creating microfractures to measure in-situ formation breakdown pressure, fracture re-opening pressure, fracture closure pressure and stress contrast between reservoir and non-reservoir intervals. The formation tensile strength can also be estimated from the difference between breakdown and re-opening downhole pressures.

This case study describes the use of microfrac tests measurements to validate and calibrate the horizontal stress profile in various intervals of the carbonate reservoir.

Well-injection plans, cap-rock integrity assessment, shale reservoir fracture containment, stress contrast and minimum and maximum horizontal stress estimations can all be quantified from microfrac test measurement.

Six straddle packer stations were tested for microfracturing in this study well. One microfrac test was repeated in one formation due to observed poroelastic effects in the fracture re-opening pressure responses. Poroelastic effects around the borehole occur when the pore pressure near the borehole increases with the injection cycles, thereby making it more difficult to effectively re-open the pre-existing induced fracture. When poroelastic effects are evident, it is important and recommended to record the first pressure fall-off cycle after the formation breakdown for fracture-closure identification. Subsequent cycles will indicate higher fracture closure pressures and therefore overestimate the minimum horizontal stress in the interval.

This paper describes the pre-job modeling, real-time monitoring and post-job interpretation of straddle packer microfrac testing for recalibration of the geomechanical model to provide continuous logs of in-situ horizontal stress profiles over the entire interval.

Introduction

The well described in this field was drilled in a carbonate reservoir field. It is a new field that is located approximately 260 km south of Abu Dhabi city, and about 80 km southeast of ASAB field. Figure 1 shows the map of this field and the approximate well location. The shallow interval of this field consists of organic rich carbonates because it is in a basinal setting [1], while the deeper formation is the main oil production formation in onshore Abu Dhabi [2].

The objective of the microfrac testing was to measure the fracture initiation, propagation, reopening, and closure pressure in various intervals of the carbonate reservoir to validate and calibrate the horizontal stress profile to aid in the study of stress-field anisotropy, the impact of the stress-field on productivity and cap-rock integrity.

The microfrac testing procedure includes formation breakdown, fracture propagation, two fracture reopening cycles, and three pressure fall-off for fracture closure identification. Fracture closure was observed by natural leak-off pressure decline behavior and no flow-back was required to induce fracture closure. Borehole quality is an important factor to consider during the positioning of the wireline straddle packer. The sealing capacity of the inflatable elements against the formation can be compromised by borehole washouts and extensive breakouts [3]. Extreme borehole wall rugosity can also deteriorate the rubber during inflating and deforming the elements. The wireline straddle packer microfrac testing follows the sequence of pre-job assessment, real-time monitoring and post acquisition pressure decline analysis.