A technique has been developed by which strain relaxation measurements can be used to determine the direction and magnitude of the stress field in the Danian Chalk formation. Direction of induced fractures is determined by the direction of the least principal stress, therefore, this information is needed to determine well locations, inclination, and in the case of horizontal drilling, the azimuth. Pressures required to propagate the fracture are primarily dictated by the magnitude of the minimum principal stress. A nine-axis strain relaxation instrument has been developed to obtain measurements needed for determination of stress direction and magnitude. The process is as listed below.
An oriented core is obtained from the formation. As the core is drilled, a scribe in the coring tool marks a line along the axis of the core. The position of this line makes possible the correct directional orientation of the core once it is removed from the well. Reactions of the core to stimuli applied are assumed to be similar to in-situ reactions of the chalk formations.
In the laboratory, cores are subjected to hydrostatic pressure in an attempt to reproduce deformations that could be related to relieved stress fields. Deformations recovering after relief of hydrostatic pressure are monitored by the strain relaxation measurement device.
Measurements are analyzed to determine (a) direction of least principal stress, and (b) ratios between principal stress magnitudes. The Danian Chalk is one of the most important sources of hydrocarbons in the North Sea area, and is often stimulated by hydraulic fracturing or by acid fracturing to increase its permeability, which leads to increased productivity. Cores from the Danian Chalk have exhibited similar relaxation behavior, and distinctive patterns have been observed. Directions determined by each core sample have been definitive and results have agreed within an acceptable magnitude. Measurements have suggested that the maximum principal stress is slightly deviated from vertical. Magnitude of minimal principal stress measured in the same zone by small volume hydraulic fracturing (microfrac) tests have been within 4% of magnitudes determined by strain relaxation testing. This paper presents the theory, configuration, and capability of the strain relaxation instrument. Examples from a North Sea well are presented to show close correlations of stress magnitudes determined by strain relaxation to those results observed by hydraulic fracturing tests.
The state of stress, magnitude, and orientation, in deep reservoir rocks such as the Danian Chalk, has considerable significance for stimulation and production processes. For stimulation technology wherein producivity is enhanced by hydraulic fracturing or acidizing, the stress field distribution is the principal factor that controls the fracture orientation and height. It was established by Hubbert and Willis (1957) that hydraulically induced fracture propagates perpendicular to be minimum principal stress σ min. Furthermore, variation in σ min, which is assumed to act horizontally in deep reservoirs, is the overriding criterion to restrict vertical fracture growth. Knowledge of the stress field has become a high priority requirement in wells when drilled horizontally or deviated from vertical.
