A New Method for Direct Measurement of In-Situ Stress Directions and Formation Rock Properties

Abstract

Summary This paper presents a new method to determine in-situ stress directions and in-situ formation rock properties with a newly developed downhole extensometer. The extensometer is used to measure wellbore deformation, pressure, and temperature during a microfracture stress test. This paper presents the method used to analyze the data this new device collects. Some of the parameters that can be determined are (1) principal stress directions, determined from the induced fracture orientation; (2) minimum in-situ stress magnitude, determined from the fracture width calculations and the conventional microfracture analysis; and (3) in-situ static rock properties, determined from the prefracture wellbore deformation. This paper emphasizes the theoretical methodology used in the data analysis. A field case example is included. Introduction Many production problems-including drilling and wellbore stability, fracturing (for vertical, deviated, or horizontal wells), sand production, and casing failure-require knowledge of the in-situ stress magnitudes and directions, and such in-situ rock properties as Young's modulus, shear modulus, and Poisson's ratio. Present ways of determining the fracture/in-situ stress orientation at great depths involve either testing of recovered oriented cores [e.g., an elastic strain relaxation, (ASR)] or microfracture tests during drilling and subsequent recovery of the oriented fractured core. A widely accepted method of determining closure pressure (minimum in-situ stress) is microfracture analysis. This type of analysis relies on the pressure-decline data to match a particular model. Several techniques, static and dynamic, exist for determining rock properties. Static methods usually involve laboratory tests that require carefully prepared samples and often are time-consuming. In addition, retrieved samples are no longer in the in-situ state, and the samples usually are very small compared with the formation being studied. On the other hand, dynamic methods, such as sonic logs, are based essentially on rapidly applied, nondestructive loads. They usually are performed under in-situ conditions. However, statically determined parameters are preferred in hydraulic fracture design because they represent actual loading conditions better.