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We study the propagation of a finite‐amplitude elastic pulse in a long thin bar of Berea sandstone. In previous work, this type of experiment has been conducted to quantify classical nonlinearity, based on the amplitude growth of the second harmonic as a function of propagation distance. To greatly expand on that early work, a noncontact scanning 3‐D laser Doppler vibrometer was used to track the evolution of the axial component of the particle velocity over the entire surface of the bar as functions of the propagation distance and source amplitude. With these new measurements, the combined effects of classical nonlinearity, hysteresis, and nonequilibrium dynamics have all been measured simultaneously. We show that the numerical resolution of the 1‐D wave equation with terms for classical nonlinearity and attenuation accurately captures the spectral features of the waves up to the second harmonic. However, for higher harmonics the spectral content is shown to be strongly influenced by hysteresis. This work also shows data which quantify not only classical nonlinearity but also the nonequilibrium dynamics based on the relative change in the arrival time of the elastic pulse as a function of strain and distance from the source. Finally, a comparison is made to a resonant bar measurement, a reference experiment used to quantify nonequilibrium dynamics, based on the relative shift of the resonance frequencies as a function of the maximum dynamic strain in the sample.

Kotabatak field, Sumatra, Indonesia is a heavily-faulted field undergoing an aggressive drilling and development campaign. Nine horizontal wells had been drilled with four more planned in 2008. One of the horizontal wells recently experienced well collapse (and sudden productivity decline) after some time on production, with cavings being flushed out during coil tubing workover operations. In addition to horizontal well drilling, feasibility of open horizontal well completions, hydraulic fracturing design and sanding onset prediction also warranted rock mechanics analyses. To make sound decisions on those issues, building a well-calibrated geomechanical model was critical. In this study, we reviewed the drilling, completion, logging and production information from several wells across the field. We found that (1) The Kotabatak field has a general maximum horizontal stress orientation of NESW. However, there could be localized stress orientation variations depending on structure complexity near a specific well. (2) There was no consistent evidence indicating a significant contrast between the maximum and minimum horizontal stresses. Using a maximum/minimum horizontal stress ratio of 1.05 yielded a consistent calibration result for the wells studied. (3) Sand minimum horizontal stress for the Kotabatak field was calibrated against available closure stresses from hydraulic fracturing and mini-frac data. (4) Rock mechanical properties were calculated with openhole logs based on a Rock Mechanics Algorithm that is closely linked to Chevron's worldwide rock mechanical property database. Consequently, even though there were no core test data available from the Kotabatak field to calibrate rock mechanical properties directly, the log data set provided the means to estimate reliable formation mechanical property values that are consistent with Chevron's worldwide database. Furthermore the entire geomechanical model was calibrated against offset drilling performance measures resulting in a high degree of confidence in the predicted values. Using the calibrated geomechanical model, horizontal well stability predictions were performed and indicated that horizontal sections can be drilled with low mud weight allowing the well to have some yield/failure. Open horizontal well sanding onset prediction indicated that the depth and width of a breakout (or plastic zone if reservoir sand behaves plastically) increase with increasing pressure drawdown. Since water flooding is used in the field to maintain reservoir pressure, sand control may not be needed if an appropriate Bottomhole Flowing Pressure (BHFP) is applied. Introduction The Kotabatak field, Sumatra, Indonesia is a heavily faulted field undergoing an aggressive drilling and development campaign ((Figures 1 and 2). Nine horizontal wells had been drilled (as of the end of 2007) with four more planned in 2008. One of the horizontal wells recently experienced well collapse (and sudden productivity decline) after some time on production, with cavings being flushed out during coil tubing workover operations. In addition to horizontal well drilling, feasibility of open horizontal well completions, hydraulic fracturing design and sanding onset prediction also warranted rock mechanics analyses. To make sound decisions on those issues, building a well-calibrated geomechanical model was critical.

Summary. This paper presents the results of a recent study conducted to determine application and operating requirements for polycrystalline diamond compact (PDC) bits in the Gulf of Mexico. This study evaluated PDC-bit usage in Miocene sections of the Gulf of Mexico and has resulted in a saving of more than $1.4 million based on 22 bit runs. As a result of this study, operational guidelines for PDC bits were established and drilling costs per foot were significantly reduced. In addition, a relationship was found between shale reactivity, strength, and density. This proved to be an effective aid in bit selection and determination of hydraulic requirements and verified the results of the study. Introduction While a well is being drilled, the rate of penetration (ROP) usually decreases with depth. ROP's with conventional rock bits are affected by tooth and bearing wear, whereas PDC bits are minimally affected. On the basis of advances made with PDC bits and new failure-mechanisms theories, PDC bits have successfully reduced cost per foot in basins outside the Gulf of Mexico.A majority of the footage drilled in the Gulf of Mexico was with rock bits. Past PDC applications were limited primarily to deep hole sections, smaller hole sizes, or wells drilled with oil-based muds. It was recognized that rock mechanics laws did not support the previously mentioned PDC limitations. PDC bits should out-perform rock bits because most shales fail easily in shear and PDC bits always operate in a shearing mode. in addition, PDC bits are known to drill better in water-based muds when shale shear strength is high. A paradox between physical law and practice existed. A more thorough understanding of the effect of mud type, formation composition, and hydraulics would be required to find a solution.A study was initiated in May 1985 to determine how to apply PDC technology in the Gulf of Mexico. The Tenneco Real Time Data Center was used to monitor and to collect drilling data for this study. This also allowed bit performance to be related to lithology and electric log data. The runs were made in a water-based drilling fluid with large- diameter cutter- and fishtail-type PDC bits. Typical savings, vs. those with conventional milled-tooth rock bits, ranged from $30,000 to $90,000 per run. The longest continuous run was 5,685 ft [1733 m] (6,665 ft to 12,350 ft [2031 to 3764 m]) at an average ROP of 90 ft/hr [27 m/h] (Table 1). The success experienced on these PDC bit runs was a result of matching proper bit design and hydraulics to formation composition, using the relationship between shale reactivity and bit performance. This paper examines the relationship between these previously mentioned variables with respect to PDC performance. It will also address design and operational considerations essential to successful water-based PDC applications in the Gulf of Mexico. Theorized Rock-Failure Modes The manner in which a rock fails is important in bit selection. This failure mode can be brittle or plastic depending on the confining stress. Conventional theories used in the calculation of confining stress assumed that the pore pressure within shale remained constant. If this were true, however, it is unlikely that the confining stresses would be high enough to cause the shale to become plastic. Recent data and rock failure theories support the position that most shales fail in a plastic condition. This plasticity was confirmed from field data.A theory explaining this behavior was presented in a paper by Warren and Smith, which examined localized stress conditions at the bit. This theory is predicated on stress relieving the original shale, which causes a PV increase. This stress relief is caused by replacement of the heavier overburden with a lighter mud. This is normally the case because most mud weights are less than 20 lbm/gal [less than 2397 kg/m3], which approximates overburden density. Because shale is impermeable, there is no pressure maintenance through fluid movement, and because the of PV increase, some pore-pressure loss is expected. This localized loss of pore pressure will increase confining stress on the shale near the bit. The magnitude of the confining stress depends on the fluid type trapped in the shale. If a compressible fluide.g., gasis contained in the pore spaces, the pore-pressure loss within the shale will be insignificant, If an incompressible fluide.g., wateris contained in the pore space, a significant amount of the original pore pressure is lost. This results in very high localized confining stress and causes the mechanical properties of the shale to change. This phenomenon is modeled with a Mohr envelope. Because these theories have established that most shales are plastic and fail easily in shear, it follows that a PDC bit, which is a drag-type bit, should be the optimum bit to use. Bit-Design Characteristics For the purpose of this study, PDC bits are broken down into three general classes:conventional PDC bits made with 1/2-in. [1.27-cm] cutters found on familiar diamond-bit profiles (Fig. 1),fishtail PDC bits made with 1/2-in. [1.27-cm] cutters on historic fishtail drag-bit profile (Fig. 2), andlarge-cutter PDC bits made with large (1, 1 1/2-, or 2-in. [2.54-, 3.81-, or 5.08-cm] -diameter) cutters with a nozzle for each cutter (Fig. 3). There are several styles with each of the three classes of bits, but all styles within a class will exhibit similar features in terms of overall design variables. The following general design variables are considered: Cutter Exposure. Exposure is defined as distance of the bit face to the cutter tip. Exposure can be achieved either with cutter size or with a structure on the bite.g., steps and blades-to elevate the cutters above the rest of the bit face. Conventional PDC bits used as a baseline show how the other two bit classes achieve an increase in cutter exposure. Fishtail bits use a drag blade, or exaggerated junk slot, to elevate the cutters from the bit body. Bits with large-diameter PDC cutters use the cutter size to increase the exposure. Profile. Profile is the shape of the bit or the structure on which the cutters are placed. Figs. 1 through 3 show a variety of profiles within each class. SPEDE P. 117^

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