This paper was selected for presentation by an SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE,

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.

No abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMany of the producing fields in the Cook Inlet area of Alaska are found in tightly folded, asymmetric, fault bounded, anticlinal structures that are not easily imaged with modern seismic. Historically, exploration wells have been drilled initially on the crest of these features, only to find at total depth that the bore holes had deviated off the crest and were several thousand feet down dip from the initial structural objective.In Cook Inlet, where difficult hole conditions prevail, a new approach to real time structural analysis was needed to evaluate complexly folded features as they were being drilled.Acquisition of structural dips while drilling a potentially complex structure was critical in evaluating the position of the well on the structure and steering it if necessary. Measurement while drilling (MWD), coupled with logging while drilling (LWD) provided the data necessary to enable a successful drilling operation and well completion.The Resistivity-at-the-Bit (RAB)* tool was used during drilling to acquire resistivity measurements close to the bit. These resistivity measurements were then used to produce images and calculate dips similar to a Fullbore Formation MicroImager (FMI)*. Modifications to the structural model were continually made based on interpretations from the RAB images, allowing the bore hole to be steered towards a preferred structural position.FMI and RAB images and interpreted dips were compared in a shallow interval to evaluate the quality of RAB data. Excellent agreement in measurements between the tools resulted in the continued use of RAB for the duration of the well.