Wireline Microfracturing: Ultrahigh-Value Stress Measurements at Ultrahigh Pressures in an Ultradeep Lower Tertiary Play
Authors: Mishra V. K. , Guzman R. , Rylance M. , Carvajal E. I. , Castiblanco I. , Cañas J. , Garcia G. , Dumont H. , Rubio N. , Kayo A. , Alatrach S. 
Published: April 2016

The measurement of geomechanical properties of reservoir rock and caprock for completion optimization, enhanced oil recovery (EOR), and disposal/storage of any kind is becoming an integral and key aspect of asset evaluation and appraisal. One of the most important of these characteristics is an in-situ evaluation of the magnitude and variation of the minimum in-situ stress, the measurement of which is critical for geomechanical modelling and thereby a range of applications such as well construction, caprock integrity, and completion optimization.

A wireline formation testing (WFT) tool is a common approach for obtaining direct measurements of these stresses at a range of depths. This process is referred to as microfracturing and is most typically performed in an openhole environment. Typical toolstrings consist of a straddle packer arrangement, a pumping mechanism, gamma ray for accurate depth correlation, a motorized valve/manifold arrangement, and pressure/temperature gauges. To perform a stress test, a specific interval of the wellbore is isolated by inflating the straddle packers. The interval is then pressurized by incrementally pumping fluid until a tensile fracture has been initiated. In an open hole, the fracture will initiate and propagate normal to the minimum stress at the wellbore and multiple injection and falloff cycles are subsequently performed to ensure fracture growth beyond the influence of the hoop stress regime. The data are then analysed to determine fracture initiation, reopening, propagation and closure pressures. Additionally, it may be possible to approximate fracture orientation, if an image log is available.

This paper describes the process of obtaining minimum in-situ stress measurements using a WFT and advanced integrated stress analysis (ISA) process, in an ultradeep reservoir at ultrahigh pressures. Lessons learned and best practices are highlighted along with their importance for efficient job execution. The integrated geomechanical analysis covers subsequent generation of a calibrated stress model with minimum horizontal stress measured during microfracturing. Factors include evaluation of the stress contrast in the target formations and evaluation of the overburden gradient and mechanics for microfracturing job design for future operations (breakdown pressure) and lessons learned such as station selection, backup packer availability, and influence of stress cage material on breakdown, to name but a few.

Obtaining accurate knowledge of in-situ minimum stress values, based on actual measurements, is a key step on the road to effective execution, and the earlier that this is achieved, the more efficient the results of any development. This paper summarises the successful application of the WFT approach in delivering such data under extremely harsh depth and pressure conditions, but resulting in a measurement from which numerous subdisciplines can conduct their decision making and design.