Gustavo Ugueto

Integrated shared earth model: 3D pore-pressure prediction and uncertainty analysis

Quantitative subsurface prediction and uncertainty analysis are critical success factors in all phases of exploration, development, and production projects, including regional evaluation, leasing, prospect maturation and drilling, appraisal and development planning, system selection, and reservoir management. Today, the E&P industry is moving into more frontier oil and gas provinces, such as deepwater and very deep plays, where well costs are very high. Consequently, the regional and in-field well densities are generally very low compared with historical exploration and development. Such sparseness of well control poses a challenge for quantitative subsurface analysis because it is difficult to establish spatial correlations of rock and reservoir properties with confidence, when the distances between wells are too large to assume geostatistical stationarity of earth model parameters.

Near-Wellbore Alteration and Formation Stress Parameters Using Borehole Sonic Data

Highly depleted reservoirs exhibit sharply lower po re pressures and horizontal stress magnitudes than in the overlying shaly formation. Drilling through such de pleted reservoirs can cause severe fluid loss and drilling -induced wellbore instability. Accurate and reliable estima tes of horizontal stresses can provide early warning of im pending drilling problems that may be mitigated by appropri ate drilling fluid design and drilling practices. We have develo ped a new multi-frequency inversion algorithm for the estimat ion of maximum and minimum horizontal stress magnitudes using cross-dipole dispersions. Borehole sonic data for t he case study presented in this paper was acquired by a cro ss-dipole sonic tool in a deep-water well, offshore Louisiana in the Gulf of Mexico (GOM). The logged interval spans 1000 ft below the casing shoe. In addition, the Modular Dynamic T ester (MDT) 1 mini-frac tests were performed at three depths in shale, yielding two minimum horizontal stress magnitudes. The borehole sonic data was suitable for inversion of crossdipole dispersions at three depths in shale and als o 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 using both the MDT mini-frac tests and inversion of borehole sonic dat a. The results of the two techniques are consistent, provi ding encouragement for further validation of the multi-f requency inversion of cross-dipole dispersions to estimate h orizontal stresses. Even though the overburden stress is expe cted to increase with depth, both the maximum (SHmax) and minimum (Shmin) horizontal stresses obtained from the inversion of borehole sonic data are significantly smaller in the depleted sand than in the overburden shale. How ever, both the horizontal stress magnitudes increase again in the shale below the depleted sand. Such rapid variations in h orizontal stress magnitudes cause large fluctuations in the s afe mud weight window. This challenge in drilling through t he depleted sand was successfully handled by using spe cial drilling fluid to mitigate seepage losses and diffe rential sticking in the depleted sand and overlying shale. We have also performed Dipole Radial Profiling (DRP) of for mation shear slownesses using the measured cross-dipole di spersions at three depths in shale and one in the highly depl eted sand. Analysis of radial profiles in the two orthogonal d irections indicates plastic yielding or stiffening of rock in the nearwellbore region. While plastic yielding increases t he shear slowness, stiffening would reduce the shear slownes s.