Tom R. Bratton

Forward modeling of fracture-induced sonic anisotropy using a combination of borehole image and sonic logs

We develop a methodology to model and interpret borehole dipole sonic anisotropy related to the effect of geologic fractures, using a forward-modeling approach. We use a classical excess-compliance fracture model that relies on the orientation of the individual fractures, the elastic properties of the host rock, and the normal and tangential fracture-compliance parameters. Orientations of individual fractures are extracted from borehole-image log analysis. The model is validated using borehole-resistivity image and sonic logs in a gas-sand reservoir over a 160-ft (50 m) vertical interval of a well. Significant amounts of sonic anisotropy are observed at three zones, with a fast-shear azimuth (FSA) exhibiting 60° of variation and slowness difference between 2% and 16%. Numerous quasivertical fractures with varying dip azimuths are identified on the image log at the locations of strong sonic anisotropy. The maximum horizontal-stress direction, given by breakouts and drilling-induced fractures, is shown not ...

Application of Indirect Fracturing for efficient Stimulation of Coalbed Methane

Propped hydraulic fracture stimulation has been one of the primary completion methods for coalbed methane wellbores for more than twenty years. However direct fracturing of coal seams has been notoriously inefficient. High fracture pressures in coal seams, coal cleating and natural fractures can lead to shear slippage and inefficient non-planar fracturing which significantly underperforms the stimulation potential compared to conventional clastic rock fracture stimulation. In 2003 the concept of indirect fracturing was introduced to significantly increase Coalbed methane (CBM) fracturing efficiency by initiating fractures in lower stress clastic rock adjacent to coal seams and allowing these induced fractures to connect and grow into the coal seams. This paper presents several examples of the application of indirect fracturing for the stimulation of coal seams in the Rockies. This paper evaluates production results, fracture pressure analysis, as well as micro seismic results and frac tracer analysis for quantifying the effectiveness of indirect fracturing for the stimulation of CBM reservoirs. From this data we present guidelines for when and where indirect fracturing is applicable and just as important, where indirect fracturing is not

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.

Discrimination of fracture and stress effects using image and sonic logs for hydraulic fracturing design

Selecting and designing the proper completion in naturally fractured reservoirs is always a challenge because of the mechanical and flow heterogeneities due to the fractures. Furthermore, when hydraulic fracturing is used to enhance the recovery, the interplay between the 3D stress field and the existing natural fracture systems becomes an important factor. Three mechanical scenarios might occur while fracturing the medium (Figure 1). First, the natural fractures may have no influence and the hydraulic fracture will propagate in a direction orthogonal to the minimum principal stress as expected in a classical model (Figure 1a). Second, only the system of natural fractures will be reactivated and eventually extended (Figure 1b). Third, both newly generated hydraulic fractures and natural fractures will intersect and propagate in a complex manner (Figure 1c). The tortuosity of hydraulic fractures will be greatly controlled by the anisotropy of the effective elastic medium due to 3D stress and natural fractures.

Geomechanics for Unconventional Reservoirs

Abstract The discipline of geomechanics impacts nearly all phases of oilfield operations, from drilling to production. For unconventional reservoirs, the geomechanical properties of the subsurface greatly influence the processes of both drilling and hydraulic fracturing. A mechanical earth model (MEM) can be a useful tool for quantifying the subsurface elastic properties, rock strength, and earth stresses. Each component of the MEM is described in this chapter, followed by a discussion of the geomechanical implications of both drilling and hydraulic fracturing.

Estimation of formation horizontal stress magnitudes using radial profiles of shear slownesses

Summary A new inversion algorithm yields estimates of the maximum and minimum horizontal stresses using radial profiles of the three shear moduli referred to an orthogonal set of axes defined by the three principal stress directions. This algorithm inverts differences in the far-field shear moduli together with two difference equations obtained from the radial profiles of the dipole shear moduli C44 and C55, and borehole stresses in the near-wellbore region. Outputs from this inversion algorithm includes the maximum and minimum horizontal stress magnitudes, and an acoustoelastic parameter expressed in terms of two rock nonlinear constants referred to a local reference state. Difference equations in the shear moduli use radial positions away from the borehole surface that do not exhibit any plastic deformation. Results for estimated horizontal stress magnitudes obtained in a Piceance basin well are consistent with the post-fracture microseismic data suggesting nearly isotropic horizontal stresses in a sand