The Effect of Capillary Condensation on the Geomechanical and Acoustic Properties of Tight Formations: An Experimental Study

Capillary condensation is the condensation of the gas inside nano-pore space at a pressure lower than the bulk dew point pressure as the result of multilayer adsorption due to the high capillary pressure inside the small pore throat of unconventional rocks. The condensation of liquid in nano-pore space of rock changes its mechanical and acoustic properties. Acoustic properties variation due to capillary condensation provides us a tool to monitor phase change in reservoir as a result of nano-confinement as well as mapping the area where phase change occurs as well as characterize pore size distribution. This is particularly important for tight formations where confinement has a strong effect on phase behavior that is challenging to measure experimentally. Theoretical studies have examined the effects of capillary condensation; however, these findings have not been verified experimentally. The main objective of this study is to experimentally investigate the effect of capillary condensation on the mechanical and acoustic properties of shale samples. The mechanical and acoustic characterization of the samples was carried out experimentally using a state-of-the-art tri-axial facility at the Colorado School of Mines. The experimental set-up is capable of the simultaneous acquisition of coupled stress, strain, resistivity, acoustic and flow data. Carbon dioxide was used as the pore pressure fluid in these experiments. After a comprehensive characterization of shale samples, experiments were conducted by increasing the pore pressure until condensation occurs while monitoring the mechanical and acoustic properties of the sample to quantify the effect of capillary condensation on the mechanical and acoustic properties of the sample. Experimental data show a 5% increase in Young's Modulus as condensation occurs. This increase is attributed to the increase in pore stiffness as condensation occurs reinforcing the grain contact. An initial decrease in compressional velocity was observed as pore pressure increases before condensation occurs which is attributed to the expansion of the pore volume when pore pressure increases. After this initial decrease, compressional velocity slightly increases at a pressure around 750 - 800 psi which is close to the condensation pressure. We also observed a noticeable increase in shear velocity when capillary condensation occurs, this could be due to the immobility of the condensed liquid phase at the pore throats. The changes of geomechanical and acoustic signatures were observed at around 750 - 800 psi at 27°C, which is the dew point pressure of the fluid in the nano-pore space of the sample at this temperature. While the unconfined bulk dew point pressure of carbon dioxide at the same temperature is 977 psi. Hence, this study marks the first measurement of the dew point of fluid in nano-pore space and potentially leads to the construction of the phase envelope of fluid under confinement.

High-pressure thermal conductivity and compressional velocity of NaCl in B1 and B2 phase

Sodium chloride (NaCl) is an important, commonly used pressure medium and pressure calibrant in diamond-anvil cell (DAC) experiments. Its thermal conductivity at high pressure–temperature (P–T) conditions is a critical parameter to model heat conduction and temperature distribution within an NaCl-loaded DAC. Here we couple ultrafast optical pump-probe methods with the DAC to study thermal conductivity and compressional velocity of NaCl in B1 and B2 phase to 66 GPa at room temperature. Using an externally-heated DAC, we further show that thermal conductivity of NaCl-B1 phase follows a typical T−1 dependence. The high P–T thermal conductivity of NaCl enables us to confirm the validity of Leibfried-Schlömann equation, a commonly used model for the P–T dependence of thermal conductivity, over a large compression range (~ 35% volume compression in NaCl-B1 phase, followed by ~ 20% compression in the polymorphic B2 phase). The compressional velocities of NaCl-B1 and B2 phase both scale approximately linearly with density, indicating the applicability of Birch’s law to NaCl within the density range we study. Our findings offer critical insights into the dominant physical mechanism of phonon transport in NaCl, as well as important data that significantly enhance the accuracy of modeling the spatiotemporal evolution of temperature within an NaCl-loaded DAC.

Log-based prediction of pore pressure and its relationship to compressional velocity, shear velocity and Poisson’s ratio in gas reservoirs; Case study from selected gas wells in Iran

A precise identification of pore fluid pressure (PP) is of great significance, specifically, in terms of drilling safety and reservoir management. Despite numerous work have been carried out for prediction of PP in oil reservoirs, but there still exists a tangible lack of such work in gas hosting rocks. The present study aims to discuss and evaluate the application of a number of existing methods for prediction of PP in two selected giant carbonate gas reservoirs in south Iran. For this purpose, PP was first estimated based on the available conventional log data and later compared with the PP suggested by Reservoir Formation Test (RFT) and other bore data. At the end, it has been revealed that while PP prediction is highly dependent on the type of litho logy in carbonates, the effect of fluid type is negligible. Moreover, the velocity correlations work more efficiently for the pure limestone/dolomite reservoirs compared with the mixed ones.

Geomechanical characterization of mechanical properties and in-situ stresses for predicting wellbore stability of ATG-field, A case study: Niger delta petroleum province, Nigeria

Well logs from ATG- field wells ATG-10 and ATG-11 were calibrated to develop Mechanical Earth Model (MEM) based on elastic parameter, failure parameters, in-situ stresses, pore pressure using well logs to predict wellbore failure. Poisson’s ratio derived from compressional and shear velocities interval transit time and density logs (RHOB), showed that the values ranges from 0.17 to 0.48 and 0.09 to 0.49, and the dynamic Young's Modulus derived from the Compressional and Shear velocity Logs, ranges from 6.0 GPa to 7.8 GPa and 3.6 GPa to 6.6 GPa, the dynamic shear modulus derived from dynamic young’s modulus and Poisson’s ratio which ranges from 3.8 GPa to 5.1 GPa and 2.1 GPa to 5.4 GPa, while the dynamic Bulk modulus ranges from 0.25 GPa to 1.67 GPa and 0.43 GPa to 1.18 GPa for wells ATG-10 and ATG-11 respectively. The calibrated failure parameters or rock strengths derived from compressional velocity logs include: the internal friction angle (ϕ) from Plumb’s correlation, these ranges from 20.869o to 65.5o and 20.869o to 45.61o, Unaxial compressive (UCS) strength ranges from 757.837 psi to 2505.836 psi and 4577.099 psi to 10512.876 psi, cohesion Strength (C) ranges from 205.697 psi to 355.308 psi and 70.652 psi to 390.32 psi and Tensile strength (To) varies from 17.141 psi to 29.609 psi and 5.885 psi to 32.527 psi for well ATG-10 and ATG-11 respectively. The elastic and rock strengths properties vary in a similar trend to the sonic logs as they are derived based on these values. These properties show increasing values with increasing depth, as a result of larger overburden stress, hence lower porosity or high compressional velocity of the formations. However, the elastic properties and formation strength may vary in different formations.

Reliability of velocity measurements made by monopole acoustic logging-while-drilling tools in fast formations

The accuracy of velocity measurements made using a monopole acoustic logging-while-drilling (ALWD) measurement tool is influenced by the eccentering of the tool due to complex drill string movements. We have used the velocity of collar flexural mode (at the source frequency range) as a reference and classified the fast formations into (1) fast-fast (FF) formations with compressional velocity far larger than the collar flexural velocity and (2) slow-fast (SF) formations with compressional velocity approaching that of the collar flexural velocity. We use a 3D finite-difference method to simulate the response of an eccentered monopole ALWD tool with different eccentering magnitudes (offsets) for the two types of formations to facilitate better interpretation of velocity measurements made in an actual drilling environment. We find that the collar extensional mode, existing in the centralized and eccentered tool cases, only affects the formation P-wave measurement and can be eliminated by using an isolator. The collar flexural mode, which is a shear motion in the collar and can only be excited in a centralized tool by a dipole source, is also excited when a monopole tool is eccentered, and it significantly affects the measurement of the compressional velocity in the SF formation and that of the shear velocity in the FF formation, even for small eccentering offsets. Thus, the uncorrected monopole ALWD tool provides unreliable formation velocities (either the compressional or shear velocities) in fast formations because of the significant influence of the tool offsets on the measurement. To minimize the influence of tool offset on the measurements, we compared the differences between the waveforms collected for different azimuths and tool offsets and the centralized monopole waveforms.