Abstract: Using Leakoff Tests and Acoustic Logging to Estimate in situ Stresses at Deep Waters ? Campos Basin

Lima Neto, Francisco F. - Petrobras/Cenpes; Beneduzi, Carlos - Petrobras/E&P

Previous geothermal studies have shown how the rock thickness affects the thermal state with depth, at deep waters, in Campos basin. We found a similar behavior dealing with in situ stresses calculation, applying acoustic logs, measuring both shear and compressional waves, and leakoff test data results. The major control on in situ stresses in deep waters, in Campos Basin, is the sediment thickness, and not the depth itself.

The study was based on 105 leakoff tests (LOT) acquired in exploratory wells. The leakoff pressures obtained in these tests show increasing values with depth. Three groups of LOT were recognized: a shallow, an intermediate, and a deep one. The shallowest group (<1500 m) presents a linear fit with small dispersion. Between 1600 m and 3300 m occurs the largest dispersion; however, the increase of pressure with depth is maintained. Below 3300 m there are a few tests, all of them with very high values. These groups can be associated to different rock units. The shallow to the Emboré and Ubatuba Formations, the intermediate to Ubatuba and Macaé Formations, and the deep one, to the Macaé and Lagoa Feia Formations. This study considers the effects of lithology, overburden, wellbore diameter, and water depth on the in situ stress behavior. The LOT pressure increases inversely with the wellbore diameter, and directly with depth. The LOT data normalized by sediment thickness show variable LOT/thickness ratio at larger diameters, caused by the variation of the water depth. The water depth effect is more prominent in the wells with larger diameter (Fig. 01). The analysis of the data shows a Normal Fault Regime (SV>SHmax>SHmin) in the upper part of the sediments (<1500 m), where the overburden (SV) is the main stress. At shallow depth, the magnitudes of horizontal stresses are small and near the pore pressure. This could be explained by the water depth effect. Below 3500 m all tests show SHmax larger than SV, suggesting a compressional regime, which is interpreted, in this case, as a Strike-slip Fault System (Fig. 02). Between 1500 and 3500 m the data are not so clear regarding the stress regime, showing both Normal Fault and Strike Slip Fault Regimes. It is observed an increase in the calculated SHmax with depth. Sometimes the Shmax is greater than the overburden. The data are dispersed; therefore, it?s difficult to conclude about the actual stress regime in this interval.

The behavior of the shallow section is consistent with a weak material that couldn?t sustain high deviatoric stresses. On the other hand, the deepest tests indicate a material capable of sustaining very large stress differences reaching 60 MPa (Fig. 02). Those observations lead us to conclude that the regional stress field acts on the deeper and over-consolidated rocks. Local stresses, as those induced gravitationally by the tertiary slope, play a more important role at the shallow level. Probably, as previous studies of breakouts suggested, the ductile layer represented by the Lagoa Feia salt could have uncoupled the shallow stresses regime from the deeper regime (Fig. 03).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil