Estimation of Near-Wellbore Alteration and Formation Stress Parameters From Borehole Sonic Data

Abstract

Highly depleted reservoirs exhibit sharply lower pore pressures and horizontal stress magnitudes than does the overlying shaly formation. Drilling through such depleted reservoirs can cause severe fluid loss and drilling-induced wellbore instability. Accurate and reliable estimates of horizontal stresses can provide an early warning of impending drilling problems that may be mitigated by appropriate drilling fluid design and drilling practices. We have developed a new multifrequency inversion algorithm for the estimation of maximum and minimum horizontal stress magnitudes using cross-dipole dispersions. Borehole sonic data for the case study presented in this paper was acquired by a cross-dipole sonic tool in a deepwater well, offshore Louisiana in the Gulf of Mexico (GOM). The logged interval spans 1,000 ft below the casing shoe. In addition, the Modular Dynamic Tester (MDT) (©Schlumberger) minifrac tests were performed at three depths in shale, thus yielding two minimum horizontal stress magnitudes. The borehole sonic data were suitable for the inversion of crossdipole dispersions at three depths in shale, as well as at a depth in a highly depleted sand reservoir. There was one depth in shale above the depleted sand where we could estimate the minimum horizontal stress magnitude with both the MDT minifrac tests and inversion of borehole sonic data. The results of the two techniques are consistent, providing encouragement for further validation of the multifrequency inversion of cross-dipole dispersions to estimate horizontal stresses. Even though the overburden stress is expected to increase with depth, both the maximum (S Hmax ) and minimum (S hmin ) horizontal stresses obtained from the inversion of borehole sonic data are significantly smaller in the depleted sand than in the overburden shale. However, both the horizontal stress magnitudes increase again in the shale below the depleted sand. Such rapid variations in horizontal stress magnitudes cause large fluctuations in the safe mud-weight window. This challenge in drilling through the depleted sand was successfully handled by using special drilling fluid to mitigate seepage losses and the differential sticking in the depleted sand and overlying shale. We have also performed dipole radial profiling (DRP) of formation shear slownesses using the measured cross-dipole dispersions at three depths in shale and one in highly depleted sand. Analysis of radial profiles in the two orthogonal directions indicates plastic yielding or stiffening of rock in the near-wellbore region. While plastic yielding increases the shear slowness, stiffening would reduce the shear slowness.

Overpressure Zones in Relation to In Situ Stress for the Krishna-Godavari Basin, Eastern Continental Margin of India: Implications for Hydrocarbon Prospectivity

“…The direction and magnitude of in situ stresses are required in planning for borehole stability during hydraulic fracturing for enhanced production and selective perforation for prevention of sanding during production as well as for directional drilling. So, accurate and reliable assessment of horizontal stress magnitudes can provide an early caution of impending drilling problems that may be mitigated by appropriate drilling fluid design and drilling practices (Sinha et al 2008). The PP gradient increases from the West Godavari sub-basin through the coastal part in the East Godavari sub-basin for KG-B offshore.…”

Synthesis and Plugging Performance of Poly (MMA-BA-ST) as a Plugging Agent in Oil-Based Drilling Fluid

“…However, the commonly used plugging materials in drilling fluids are generally in the micron-scale range in size, which cannot effectively seal nanopores. Drilling fluids will invade nano-fractures, leading to fracture expansion and poor formation stability, resulting in wellbore instability [11,12].…”

Study on Shear Velocity Profile Inversion Using an Improved High Frequency Constrained Algorithm

“…This is a compromise method between radial distance and velocity change, resulting in an average radial velocity profile that is closest to the actual situation [9,10]. Subsequently, this inversion method is successfully applied in well completions, stress parameters, and velocity estimations of Transversely Isotropic with a Vertical axis of symmetry (TIV) media [11][12][13]. For example, some researchers have improved this method by using a parametric velocity-characterization equation, and this method was successfully implemented in many cased wells [14,15].…”