The main results concerning the relationships between fracturation gradient, shales sonic transit time and related to undercompaction pressures, through sand-shale depositional environments, can be resumed as follows: within the normally compacted zone, a direct relationship can be made between the shales sonic compaction value (T) and the fracturation gradient to vertical stress ratio (FG/σ1). This ratio tends to 1 upwards, above 500 m. At top of undercompaction, this ratio exceeds always 0.8. Within the undercompacted zone, the FG/σ1 ratio increases with the formation pressure (P), and tends to 1 as soon as the pressure exceeds 1.65 g/c EMW or 13.8 lb/gal. In addition to the interest in fracturation gradient estimation before drilling from seismic data, the results should improve the understanding of stress-pressure relationships.
The two essential parameters needed to optimize the technical profile are: fluid pressure and fracturation gradient. A related to undercompaction pressures profile can be approached before drilling using the low frequency synthetic sonic obtained from the stacking velocities of the conventional seismic. Therefore, a project was carried out in order to establish the possible relationships between fracturation gradient, sonic transit time and pressure. As the leak off test (LOT) data constitute the only in-situ measurements, able to approach the fracturation gradient, a total of 247 LOT was selected from 96 wells belonging to varied sand-shale Tertiary sedimentary basins (Angola, Cameroon, Malaysia, Brunei, Norway). The LOT statistical distribution was studied regarding to the following parameters: depth, shales sonic transit time, pressure and overburden gradient. Different steps to be considered will be as follows:
General qualitative relationship between fracturation gradient and depth
Fracturation gradient to shales sonic transit time relationship within the normally compacted zone
Influence of the formation pressure on the fracturation gradient
Fracturation gradient significance.
From hydraulic fracturing and LOT data, some relationships between horizontal total stress and depth were provided by Breckels and Van Eekelen (1981) in different zones (Gulf Coast, Venezuela and Brunei). Using a similar approach, the LOT distribution was analyzed on each area and compared each other in order to obtain a better understanding of the parameters in the Fracturation Gradient depth evolution. This study confirmed the impossibility to extrapolate the LOT Depth relationship from a specific geological context to another. However, on a qualitative point of view, the depth dependent LOT profile, whatever the geographical zone considered, showed a lot of similitudes through both normally and undercompacted zones. A synthetic sketch, reported on Figure I, resumes this general evolution and its relation with some major parameters such as vertical stress (01), pressure (P) and shale sonic transit t me (T).
The constants "a" and "n" are specific to each sand-shale depositional environment. Within the upper part of the normally compacted section, a large dispersion of the LOT values can be observed and FG tends to reach the vertical stress value σ1.
