ABSTRACT:
The paper presents a theory for determining the in-situ stress state from multiple fracturing data and induced fractures from image logs. A solution can be obtained with a minimum of three data sets. However, using an inversion technique, a solution can be o btained with any number of data sets, as the solution is over determined. The magnitude of the stresses is mainly determined from the fracturing data. Fracture information from image logs is mainly used to determine the geographic direction of the principal in-situ stress. In the paper, plots of the Effective Fracture Pressure Ratio, the Fracture Angle and the Fracture Trace Angle gives a good overview how these three quantities behave, as a function of the borehole inclination and azimuth. This knowledge has advantages in planning new oil wells. The mathematical technique is to describe the general fracture equations in terms of effective stress ratios, resulting in a fracture criterion, which is independent of the borehole fracture angle.
The data are leak-off data from oil wells. They are recorded in wells with different inclinations and azimuths, a requirement for a robust inversion. However, there is a non -uniqueness problem in fracturing modelling as the position on the wall where the fracture initiate, is usually not known. By using image logs, this uniqueness can be removed. The fracture trace on the image log is also helpful in finding the directions of the in-situ stresses, whether horizontal/vertical or inclined. The model presented in this paper gives opportunity to use the information of the fracture position and direction directly in the in-situ stress calculations.
INTRODUCTION
The application of rock mechanics in the petroleum industry has increased in later years. Due to the increasing complexity of petroleum wells, borehole stability issues have become challenges that have to be handled. Borehole collapse is one class of problems, whereas circulation losses due to unexpected fracturing accounts for significant additional expenditures. In general, drilling cannot proceed before mud losses are healed.
It has become clear that assessment of the in-situ stress state is very fundamental for all modelling work of borehole stability. Data used for stress modelling includes: Leak-Off Tests at each casing shoe (usually 3 in each well) pore pressure and overburden pre ssure, and lithology. In vertical exploration wells we may also deduce the minimum horizontal stress direction from borehole breakouts.
Aadnøy [1] developed an inversion technique in 1990. Since directional wells have different orientations (inclinations and azimuths), independent fracturing equations were derived. These were organized as an over determined system of equations and the in-situ stress state were solved in an inversion routine. In addition to determining the magnitude of the two horizontal stresses, it determines the direction of the stress field. Okabe et. al. [2] developed an inversion technique for data taken in the same borehole. Djurhuus and Aadnøy [3] presented a general solution to the problem and showed that the linearized version produces good solutions to the in-situ stresses and their directions.
