Use of Unique Database of Good Quality Stress Data to Investigate Theories of Fracture Initiation, Fracture Propagation and the Stress State in the Subsurface

Andrews, J. S. (Statoil) | Fintland, T. G. (Statoil) | Helstrup, O. A. (Statoil) | Horsrud, P. (Statoil) | Raaen, A. M. (Statoil)

OnePetro 

Abstract:

Over the last few years Statoil has performed more than 200 high quality extended leak-off tests. This data has been used for determination of minimum stress and quantification of “formation integrity” parameters wrt well barriers and injection control. However the database includes many other parameters related to the initiation and propagation of a fracture with drilling mud. This represents a unique opportunity to investigate various claims in the industry relating to fracture gradient (FG) and formation integrity. For example, it is possible to compare fracture propagation pressures and determine whether there are any differences between water based and oil based muds. Actual formation breakdown pressure can be compared to theoretical models to determine the statistical validity of such models when used for FG generation. Statistical analysis on fracture propagation pressure permits another type of analysis which can be useful in a risk based approach for the estimation of FG during drilling. The data is also interesting in that it suggests that the subsurface on the Norwegian Continental Shelf (NCS) is not in fact in equilibrium state with frictional strength of faults. For the regions covered by the database, there appears to be other mechanisms that result in higher values of minimum stress than that which would be the case for frictional equilibrium. Finally, and in contrast to many other works, the work shows that NCS sediment packages are primarily characterized by a normal faulting regime.

Introduction

Statoil has put significant effort in performing high quality stress determination tests during the last decade or so [1-4]. Extended leak-off tests (XLOT) with several cycles are typically performed with drilling mud at casing shoes, and both shut-in and flowback phases are standard. Unlike earlier standard procedures, the flowback stage is performed on a constant choke and the volumes are measured. Interpretation is based on system stiffness approach. The basic procedure calls for a minimum of two cycles [1, 5]:

  1. Pump-in followed by a shut-in period of typically 15 minutes, followed by a flowback phase with constant choke setting.
  2. Pump-in followed directly by a flowback phase with constant choke setting.