TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Antonio J. Bermudez basin in Southern México is a low porosity massive Jurassic and Cretaceous carbonate reservoir that is extensively faulted and fractured due to post depositional salt intrusions. The natural fractures create many drilling challenges and obstacles. Underbalanced drilling with foam fluid systems has been used to minimize mud losses in these fracture systems. The underbalance drilling, and drilling with casing greatly improves well construction, but these drilling techniques also create many formation evaluation challenges. For example, open hole sonic logs require a liquid filled borehole. Also, formation resistivity is such that lateral logs would be the preferred resistivity device, but they require a conductive borehole fluid.Artificial Neural Network (ANN) methods have traditionally been used for reconstruction of petrophysical data due to tool pulls and poor borehole conditions. Drilling obstacles, in this field, sometimes prohibit the running of conventional open hole logs. Thus, an ANN technique has been developed that uses cased hole pulsed neutron log (PNL) data to synthetically generate a conventional, open hole, triple combo log. These logs are used for well-to-well correlations and for petrophysical evaluation. Still, due to the synthetic smoothing of an ANN result based on casing formation evaluation data (i.e., PNL), detection of natural fractures remains problematic using ANN-based synthetic density data. On the other hand, cased hole dipole sonic anisotropy analysis is routinely used successfully to identify natural fracture systems and far field stress orientation for geomechanical applications. Intervals to be perforated are selected by combining through-casing sonic information with petrophysical analysis of synthetic open hole logs.Data and production results from several wells will be presented and discussed in the paper.
Crossed dipole sonic logging data has been used to improve reservoir understanding in the Southern/Marine areas of Mexico. The anisotropy analysis of the in-line and cross-line dipole waveform data provides the fast and slow shear wave travel time data and their corresponding orientation. These slowness values provide the input to a model that calculates the minimum and maximum principal stresses and the far-field stress orientation. The far-field stress orientation and stress anisotropy combined with 3D hydraulic fracture modeling is used to optimize the stimulation treatment design. The far-field stress information is also used for optimizing the perforated interval and orientation of the perforations. Natural fractures provide many challenges for drilling, zonal isolation, and production enhancement. The detection and orientation of natural fractures is a needed key for reservoir management, and can be determined from the cross dipole anisotropy analysis. Inflow analysis from production logs has verified that the fractures identified by anisotropy analysis are open and are a major source of production in the Cretaceous section in the Marine area and Cretaceous/Jurassic in the Southern area. Mapping of the fracture orientation obtained from the sonic anisotropy analysis is used for well planning. Crossed dipole log examples in carbonate formations in Mexico will be used to illustrate how reservoir understanding can be improved with the use of this interpretation process. Introduction The geology of Southern Mexico is complex. The major fields of southeastern Mexico occur in two areas: the onshore Reforma Trend in Chiapas and Tabasco and offshore on the Campeche shelf (Figure 1 and Figure 2).This region accounts for more than 90% of the approximately 3 million BOPD produced in Mexico. Very thick Tertiary sediments, which are mostly clastics, characterize the onshore southeastern region of Mexico. Underlying the Tertiary are the Cretaceous and Jurassic carbonates. The thickness of the Cretaceous is approximately 2800 meters1. Productive formations range in depth from 2000 to 4500 meters offshore in the Bay of Campeche, with Cantarell being the largest oil field in Mexico. These formations occur much deeper (3500 to 5500 meters) in the land operations in Comalcalco and Reforma. The interpretation of seismic data is a key to successful exploration and development programs. VSP and sonic-log derived time depth correlations are used extensively since localized porosity developments, and lithology changes from limestone to dolomite can greatly alter time-to-depth correlations and reflectivity. Synthetic seismograms from sonic and bulk density data; and AVO analysis (which uses P wave, and shear wave data with bulk density) which take into account lithology and porosity changes greatly aid the seismic interpretation. The combined effects of poor acoustic impedance contrasts in the Jurassic, and lower Jurassic salt deposit make seismic interpretation of the Jurassic problematic. Four tectonic styles can be identified in the basins of southern Mexico: extensional, wrench (transform) compressional, thrust and fault2. Trap types include: reefal developments, anticlines, fold belt events, syn-depositional rollovers and salt-related traps. These tectonic events have resulted in significant stress induced anisotropy in the sediments. The softer formations frequently exhibit stress anisotropy, while the harder/brittle formations often have regions of natural fracturing on the crossed dipole analysis. The tectonic stress orientation is not consistent; it changes with both the type of tectonic event, and with depth.
Crossed dipole sonic logging data has traditionally been used in the openhole environment to:identify natural fracture systems and their orientation,evaluate far field stress data for 3D stimulation modeling, andseismic correlations and synthetic modeling. Materials presented inthis paper will show open hole and cased hole comparisons of flexural/shearwave slowness data and the anisotropy analysis in both fast and slowformations. The paper will show through-casing fracture identification examplesfrom the Cretaceous-Jurassic formations in Southern Mexico that have beenconfirmed with inflow analysis. The paper will also show applications ofthrough-casing anisotropy analysis combined with oriented perforating in theBurgos Basin in North Eastern Mexico. These techniques have resulted in lowerbreakdown pressures on stimulation treatments due to minimizing near wellboretortuosity. 2D and 3D analytical and finite-difference wave propagationsimulators have been developed to verify the open-hole and cased-holescenarios. The finite-difference simulations are used as a predictive tool toidentify cased-hole conditions that are either favorable, or unfavorable forcased-hole dipole acquisition. Introduction Dipole logging requires the propagation of a low frequency out-of-phaseflexural wave that is detected by the acoustic receivers on opposite sides ofthe logging tool. To accomplish this symmetry is required. The wireline dipolesonic tool must be centralized in a near round borehole. For through casingdipole logging, the casing should also be centralized, and the material in thecasing-formation annulus must be radially uniform. The effects of boreholeovality, tool centralization, or casing centralization on waveform propagationare illustrated in Fig. 1 for a 1.5 kHz center frequency flexural wave. Lowerfrequency flexural/dipole transmitted pulses have longer wave lengths, thanhigher frequency flexural wave propagation, thereby slightly reducing the phaseshift due to borehole geometry effects. An elliptical borehole is common indeviated wells due to eccentralization of the drill string and bottom holeassembly. Acquiring crossed dipole sonic data at low frequency and in two axes(X and Y) often minimize these environmentally induced effects. The crossed dipole sonic logging tool is frequently run open hole in what isreferred to as a "quad-combo" (natural gamma ray, eccentered dual spacedneutron, density, dual induction, or dual lateralog with appropriate standoffs, centralized dipole sonic, and navigation tool). One such logging toolstring is configured such that the X-dipole measurement is aligned with thedensity pad, which due to gravity effects will seek the low side of theborehole in deviated well conditions. Therefore, the X dipole measurement maybe affected by tool positioning effects in the angle build sections of deviatedwellbore. The Y-dipole measurement is orthogonal to the density pad and in poorborehole conditions often provides quality shear wave slowness data. Aspreviously discussed, centralization is required, but in certain bore holeconditions this may not be operationally possible. Acquiring crossed dipoledata through casing is a viable option, provided the borehole conditions arefavorable. Unfavorable conditions for through casing dipole logging areexcessive washouts, poor radial placement of cement, lack of casingcentralization, and poorly consolidated formations. Advanced waveformprocessing, such as slowness anisotropy analysis, which is used to determinethe fast and slow shear wave travel times and their corresponding orientationrequires that a navigation device be run simultaneously with the dipole sonictool.1 In open hole logging conditions magnetometers andaccelerometers are utilized. In deviated cased hole logging conditionsmulti-axes accelerometers can be used. In near vertical cased hole loggingconditions a gyroscope is required. Anisotropy analysis can be performedwithout tool navigation data, to determine the magnitude of the anisotropy, butthe orientation of the fast and slow shear waves is referenced to the toolface, which can rotate while logging. This is frequently done in geologicconditions where the anisotropy is know to be a result of natural fracturing, and fracture orientation is not desired.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Antonio J. Bermudez basin in Southern México is a low porosity massive Jurassic and Cretaceous carbonate reservoir that is extensively faulted and fractured due to post depositional salt intrusions. The natural fractures create many drilling challenges and obstacles. Underbalanced drilling with foam fluid systems has been used to minimize mud losses in these fracture systems. The underbalance drilling, and drilling with casing greatly improves well construction, but these drilling techniques also create many formation evaluation challenges. For example, open hole sonic logs require a liquid filled borehole. Also, formation resistivity is such that lateral logs would be the preferred resistivity device, but they require a conductive borehole fluid.Artificial Neural Network (ANN) methods have traditionally been used for reconstruction of petrophysical data due to tool pulls and poor borehole conditions. Drilling obstacles, in this field, sometimes prohibit the running of conventional open hole logs. Thus, an ANN technique has been developed that uses cased hole pulsed neutron log (PNL) data to synthetically generate a conventional, open hole, triple combo log. These logs are used for well-to-well correlations and for petrophysical evaluation. Still, due to the synthetic smoothing of an ANN result based on casing formation evaluation data (i.e., PNL), detection of natural fractures remains problematic using ANN-based synthetic density data. On the other hand, cased hole dipole sonic anisotropy analysis is routinely used successfully to identify natural fracture systems and far field stress orientation for geomechanical applications. Intervals to be perforated are selected by combining through-casing sonic information with petrophysical analysis of synthetic open hole logs.Data and production results from several wells will be presented and discussed in the paper.
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