Obtaining high-resolution borehole images in oil-based mud (OBM) from logging-while-drilling (LWD) tools has been made possible through the recent development of ultrasonic imaging technologies. High-resolution acoustic impedance images enable reservoir evaluation through the identification of faults and fractures, bedding and laminations, and assessment of rock fabric. This paper presents examples of high-resolution images from a 4¾-in. ultrasonic imaging tool in OBM applications and discusses their value in assessing reservoir quality.
This paper provides details of field trials of an LWD ultrasonic imaging tool for use in boreholes ranging from 5¾ to 6¾ in. High-resolution images detailing both borehole caliper and acoustic impedance in both vertical and horizontal wellbores are shown, illustrating the high level of formation evaluation now available when OBM is used. The methodology used to address the impact of tool motion on the impedance images will also be covered. The value of real-time data on borehole stability assessment will be discussed, along with additional applications made possible from the real-time data, such as wellbore placement enhancement.
Both real-time and recorded data from field trials show the potential applications for the ultrasonic imaging tool. High-resolution impedance images covering different formations and lithologies show bedding planes and laminations and enable the calculation of stratigraphic dip, while the identification and assessment of fractures show the potential to aid operators during the development of their hydraulic fracturing program. Borehole caliper and shape assessment in real time can be used to modify the drilling parameters and to adjust mud weight, while providing an input into geomechanics assessment.
The LWD logs presented illustrate the factors that influence data quality and the methodology used to ensure high-resolution images are available in both vertical and high-angle wellbores using OBM. A direct comparison between data acquired while drilling and while re-logging sections is shown, highlighting the repeatability of the measurement while also illustrating the impact of time-since-drilled on the borehole. A comparison with wireline measurements highlights the potential for using the high-resolution LWD images as an alternative to wireline, where cost and risk of deploying the wireline may be high.
The ability to collect high-resolution images in OBM in wellbores ranging from 5¾ to 6¾ in. ensures that increased reservoir characterization is possible, leading to significant improvements in determining the viability of unconventional and other challenging reservoirs. The high-resolution amplitude images are comparable with those available on wireline technologies, and the real-time application of borehole size and shape for input into wellbore stability and geomechanics analysis ensures that common drilling hazards can be avoided.
Lu, Chuan (Department of Civil and Environmental Engineering, University of Alberta) | Brandl, Jakob (Department of Civil and Environmental Engineering, University of Alberta) | Deisman, Nathan (Department of Civil and Environmental Engineering, University of Alberta) | Chalaturnyk, Richard (Department of Civil and Environmental Engineering, University of Alberta)
In this study, a novel experimental system has been developed for static and dynamic elastic properties measurements at seismic frequencies under anisotropic stress and shear deformation conditions. This system focuses on static and seismic range frequencies dynamic (0.1 Hz to 20 Hz) elastic deformation properties of poorly consolidated oil sands and highly overconsolidated (clay) shales. The main body of the experimental system is a computer control servo-hydraulic system. A pair of laser displacement sensors measure nanometer scale displacement during the dynamic tests. A coarse scale and fine scale load cell system was developed for measuring force with high precision during dynamic testing. A novel triaxial cell for use with the loading system was also developed to simulate the reservoir stress and pore pressure condition during static and dynamic testing and allows permeability to be measured during testing. The loading system, dual load cell calibration procedure and results, and results for acrylic and 3D printed sand specimens are presented. The stable and reasonable results demonstrate the capacity of the new experimental system.
Flores, Juan Carlos (Baker Hughes, a GE company) | Patterson, Douglas John (Baker Hughes, a GE company) | Wakefield, John (Baker Hughes, a GE company) | Chace, David (Baker Hughes, a GE company) | Malbrel, Christophe (Baker Hughes, a GE company)
A new shape memory polymer (SMP) sandface completion is entering the market as an alternative to gravel packs. The SMP activation has been well established in the laboratory and successful field deployments has been reported. To confirm the SMP material is fully expanded and to enable long term stability assessment, a practical in-situ evaluation method is needed. Three acoustic and two nuclear services are evaluated for their ability to detect SMP expansion in a simulated formation.
A downhole equivalent in-situ test environment was built for purpose with a section of limestone blocks drilled to 8.5 in. gauge hole and a joint of unexpanded SMP screen placed within. Two cased-hole CBL services evaluated ultrasonic guided mode attenuation response in the base perforated pipe. These modes consisting of two lamb modes, the compressional (S0) and the asymmetric (A0 flexural), along with the shear horizontal (SH) mode. Additionally, a multipole acoustic tool was utilized to analyze the response of both the refracted modes, compressional and shear, and the guided modes, casing tube and flexural waves. Nuclear services employed pulsed neutron and compensated neutron techniques. The SMP was then activated and a second pass logged for each technique. The limestone was removed from the SMP to visually verify expansion. The logs before and after expansion are compared.
In conclusion, several measurements provided good indication of the SMP expansion. The ultrasonic guided mode analysis demonstrated attenuation response increases, with the largest change observed for the A0 mode when compared to the S0 compressional response. Of the lower frequency multipole acoustic methods, the guided casing tube wave demonstrated the greatest response, even greater than the ultrasonic, particularly in the low frequency range below 3 kHz. Generally, excitation modes with particle motion parallel with the base pipe were found to attenuate least while those with normal components displayed the largest change. This is likely due to compression of the base pipe and metal screen layers because the SMP begins to exert force in all directions when it reaches gauge hole, which is short of full expansion as designed.
The pulsed neutron (PN) logs provided a qualitative, yet clear verification of expansion when comparing measurements sensitive to hydrogen concentration. The SMP is an extremely low density organic molecule with hydrogen concentration less than that of water. Comparing before and after PN logs indicates that a change in the hydrogen concentration occurred as a result of SMP expansion. Compensated neutron did not show sensitivity to expansion of the polymer.
This is the industry’s first logging validation of shape memory polymer activation in the annulus of a simulated downhole environment. Prior attempts with a variety of logging services and a simpler test environment were inconclusive.
Processing acoustic data downhole as well as at the surface is necessary to transform the raw acoustic signals recorded by modern logging instruments into data suitable for interpretation and analysis. The goal of acoustic-data processing is to minimize the data noise while maximizing the petrophysical information. Data preprocessing reduces the influences of these sources, thus allowing extraction of the true formation signal. Following the rapid theoretical advances in acoustic-wave propagation made during the 1980s and 1990s, significant advances in data processing provided improved quality in slowness measurements and enabled a number of new applications using Stoneley and dipole-shear wave in open and cased holes. The combined interpretation of Stoneley and dipole-shear acoustic measurements with NMR and borehole imaging enhances formation evaluation.
Transmitting electrical current to the subsurface can create special considerations. Successful application of electromagnetic heating often requires a multi-disciplinary approach combining electric engineering and petroleum engineering. To assist petroleum engineers considering this approach, this article identifies some of the issues that an electrical engineer might normally anticipate and address. In most practical situations, we are concerned with fields that vary periodically in time (the sinusoidal steady state generally). In these cases the electrical phenomena are properly described by Maxwell equations in terms of complex vector field intensities of electric and magnetic fields (E and H); complex vector field electric, magnetic, and current densities (D,B,J); complex charge concentrations (ρc); and complex material parameters: conductivity, permittivity, and permeability (σ, ε, μM).
Locating fractures, recognizing fracture morphology, and identifying fluid-flow properties in the fracture system are important criteria in characterizing reservoirs that produce predominantly from fracture systems. Acoustic techniques can provide insight. Fracture identification and evaluation using conventional resistivity and compressional-wave acoustic logs is difficult, in part because fracture recognition is very dependent on the dip angle of fractures with respect to the borehole. Fractures are physical discontinuities that generate acoustic reflection, refraction, and mode conversion--all of which contribute to a loss of transmitted acoustic energy. In particular, compressional- and shear-wave amplitude and attenuation and Stoneley-wave attenuation are significantly affected by the presence of fractures.
Stoneley-wave velocity and attenuation are sensitive to formation and fracture permeability, particularly at low frequencies. Initial efforts (begun in the 1970s) to derive permeability information from Stoneley data were unsuccessful because neither the necessary low-frequency tools nor the appropriate processing methods had been developed. The parallel development of modern multipole array tools and sophisticated semblance- and inversion-processing methods enable computation of continuous profiles of formation permeability from monopole Stoneley-wave data. Typically, these methods first model the nonpermeability effects using the elastic-wave theory and then relate differences between the modeled and the measured data to formation permeability. Both traces have been shown to correlate well with permeability changes and compare well with core data, when it is available.
Low-frequency ( 1 kHz) dipole sources allow for shear-velocity determination that is much closer to seismic shear waves and permits acquisition of direct-shear velocities in slow and fast formations. However, increased noise (i.e., a lower signal-to-noise ratio) is one limitation of low-frequency operation. Noise has been reduced through improved acquisition electronics, the use of semi-rigid tool designs, and by choosing the operational mode of the dipole source. A semi-rigid tool body not only reduces the influence of the tool body on the measurement but also permits operation in deviated wells. At high frequencies, or when the borehole diameter is large, flexural-mode propagation is slower and a dispersion correction is needed to obtain the shear velocity from the measured flexural velocity. This dispersion correction is a function of mud compressional velocity, formation compressional and shear velocities, the ratio of formation and mud densities, and the product of borehole diameter and processing frequency. Few, if any corrections are required if the flexural wavelength (velocity/frequency) is at least three times the borehole diameter, which is why low frequencies ( 1 kHz) are used.
As seismic acoustic waves pass through rock, some of their energy will be lost to heat. For tight, hard rocks, this loss can be negligible. However, for most sedimentary rocks, this loss will be significant, particularly on seismic scales. In reality, all rocks are inelastic to some degree. This article discusses the calculations to account for this energy loss.