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Differences between observed and forward modelled flexural wave dispersion characteristics in cross dipole sonic logging in homogeneous formations are useful for evaluating mud invasion and for differentiating between stress-induced and fabricrelated anisotropy. In this paper we investigate the sensitivity of the dispersion effect to borehole size and mud slowness, and show the importance of taking account of the tool's mechanical properties in the forward model -particularly when the tool diameter is a substantial fraction of the well diameter. We demonstrate that a new generation of small diameter cross dipole tool has a small tool-effect in common hole sizes, and extend the dispersion model results down to hole diameters less than 4 inches; we verify the calculations with real data sets acquired in sub 4-inch wells, as well as larger wells up to a 12 inches diameter. The results are also used to examine the size of dispersion corrections needed to calculate shear slowness in situations where low frequencies from the broadband transmitter are coupled relatively weakly to the formation. IntroductionVelocity logs are a key component in generating seismic ties, in the derivation of data used to populate mechanical earth models, and for fluids identification and related petrophysical applications. In fast formations, refracted shear waves are generally present in monopole waveforms, but shear velocity in slow (as well as fast) formations is commonly derived from flexural waves excited by dipole sources mounted in a wireline tool. Borehole flexural waves are dispersive, however. In the low frequency limit flexural waves travel at the formation shear velocity, but in the high frequency range they travel more slowly. Very often the processed shear velocity needs to be corrected to the true shear velocity according to its dispersion characteristics -Kimball (1998). In model-based dispersion correction the flexural velocity is computed in a narrow lowfrequency band and then corrected for residual dispersion effects using a theoretical model. Additionally, the dispersive behaviour can be exploited to evaluate formation alternation due to mud invasion, borehole damage or stress -in this case by comparing the observed flexural wave dispersion characteristics with the modelling results in an assumed homogeneous formation. For both applications the accuracy of the end product depends on the quality of the forward model, and an important factor in the model is the mechanical behaviour of the tool itself within the borehole.In this paper we analyze the tool effect for a new generation of small diameter cross-dipole wireline tool, and include results for holes as small as 3.8 inches (96mm) -for which we also have field data. The primary objective is to characterize the full dispersion curve as the foundation of a future study of formation stress, but we also show the utility of the result in the basic processing of shear slowness (inverse velocity).
Differences between observed and forward modelled flexural wave dispersion characteristics in cross dipole sonic logging in homogeneous formations are useful for evaluating mud invasion and for differentiating between stress-induced and fabricrelated anisotropy. In this paper we investigate the sensitivity of the dispersion effect to borehole size and mud slowness, and show the importance of taking account of the tool's mechanical properties in the forward model -particularly when the tool diameter is a substantial fraction of the well diameter. We demonstrate that a new generation of small diameter cross dipole tool has a small tool-effect in common hole sizes, and extend the dispersion model results down to hole diameters less than 4 inches; we verify the calculations with real data sets acquired in sub 4-inch wells, as well as larger wells up to a 12 inches diameter. The results are also used to examine the size of dispersion corrections needed to calculate shear slowness in situations where low frequencies from the broadband transmitter are coupled relatively weakly to the formation. IntroductionVelocity logs are a key component in generating seismic ties, in the derivation of data used to populate mechanical earth models, and for fluids identification and related petrophysical applications. In fast formations, refracted shear waves are generally present in monopole waveforms, but shear velocity in slow (as well as fast) formations is commonly derived from flexural waves excited by dipole sources mounted in a wireline tool. Borehole flexural waves are dispersive, however. In the low frequency limit flexural waves travel at the formation shear velocity, but in the high frequency range they travel more slowly. Very often the processed shear velocity needs to be corrected to the true shear velocity according to its dispersion characteristics -Kimball (1998). In model-based dispersion correction the flexural velocity is computed in a narrow lowfrequency band and then corrected for residual dispersion effects using a theoretical model. Additionally, the dispersive behaviour can be exploited to evaluate formation alternation due to mud invasion, borehole damage or stress -in this case by comparing the observed flexural wave dispersion characteristics with the modelling results in an assumed homogeneous formation. For both applications the accuracy of the end product depends on the quality of the forward model, and an important factor in the model is the mechanical behaviour of the tool itself within the borehole.In this paper we analyze the tool effect for a new generation of small diameter cross-dipole wireline tool, and include results for holes as small as 3.8 inches (96mm) -for which we also have field data. The primary objective is to characterize the full dispersion curve as the foundation of a future study of formation stress, but we also show the utility of the result in the basic processing of shear slowness (inverse velocity).
It is estimated that only one third of the remaining worldwide oil and gas reserves are conventional, the remainder being in unconventional reservoirs whose evaluation requires appropriate measurements delivered in a cost-effective way. In the case of shales and other tight reservoirs, the defining characteristics are low matrix porosity and low or ultra-low permeability which requires artificial stimulation to encourage production. The optimum stimulation strategy for a particular reservoir is strongly dependent on the distribution of organic material, and on the mechanical and geometrical properties of the rock, and the associated stress field. It is essential to quantify these to an appropriate level of certainty, and well logs are the primary source of such data. Until recently the options for acquiring appropriate logs in high angle and horizontal wells have been constrained either by the limited available sensors or tool conveyance methods. However, the introduction of memory capable small diameter specialized tools and multiple innovative conveyance options has changed the cost-benefit balance for the better. This paper reviews the current status of open hole log measurements with full spectrum conveyance options, and how they impact the evaluation of these challenging reservoirs.
Post-drilling open-hole log measurements made by wireline tools are most commonly transmitted to surface via wireline, but the trend is towards increased use of memory-capable tools with greater operational flexibility. The primary purpose of the original memory tools was to operate on wireline in intermediate and TD sections while offering a low-risk alternative to conventional tools in horizontal wells, bad hole conditions and other challenging well scenarios. This wide operational envelope required the tools to be small-diameter to allow protected conveyance inside drill pipe, but it also required that measurement performance and associated environmental characterization be on a par with conventionally sized tools that generated the majority of the historical record. The extent to which these goals have been achieved, using evidence from the over 150,000 wells logged with the most highly developed of the small-diameter tool types, is reviewed. Standard response modelling is essential but insufficient to ensure optimum measurement performance. Findings from the performance in thousands of wells have been used to refine models, hardware designs and processing algorithms, including more precise control on the position of tools in the wellbore. Log data from test wells and commercial wells are provided showing the measurement quality from small-diameter tools matches that from conventionally sized tools for the large majority of operations. Log data is also provided from wells that had presented insurmountable challenges for traditional logging tools which demonstrate the conveyance flexibility and the reduced acquisition risk in using the small-diameter tools. The significance of the findings is that the performance of the most highly developed small-diameter tools has evolved to the point where measurement quality is not a differentiating factor for the large majority of operations, allowing job planning to be driven by the other considerations, in particular the minimization of acquisition risk.
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