Case Study of Condensate Dropout Effect in Unconventional Gas/Condensate Reservoirs with Hydraulically Fractured Wells

Condensate banking is a major issue in the production operations of gas condensate reservoirs. Increase in liquid saturation in the near-wellbore zone due to pressure decline below dew point, decreases well deliverability and the produced condensate-gas ratio (CGR). This paper investigates the effects of condensate banking on the deliverability of hydraulically fractured wells producing from ultralow permeability (0.001 to 0.1 mD) gas condensate reservoirs. Cases where condensate dropout occurs over a large volume of the reservoir, not only near the fracture face, were examined by a detailed numerical reservoir simulation. A commercial compositional simulator with local grid refinement (LGR) around the fracture was used to quantify condensate dropout as a result of reservoir pressure decline and its impact on well productivity index (PI). The effects of gas production rate and reservoir permeability were investigated. Numerical simulation results showed a significant change in fluid compositions and relative permeability to gas over a large reservoir volume due to pressure decline during reservoir depletion. Results further illustrated the complications in understanding the PI evolution of hydraulically fractured wells in "unconventional" gas condensate reservoirs and illustrate how to correctly evaluate fracture performance in such a situation. The findings of our study and novel approach help to more accurately predict post-fracture performance. They provide a better understanding of the hydrocarbon phase change not only near the wellbore and fracture, but also deep in the reservoir, which is critical in unconventional gas condensate reservoirs. The optimization of both fracture spacing in horizontal wells and well spacing for vertical well developments can be achieved by improving the ability of production engineers to generate more realistic predictions of gas and condensate production over time.

Disentangling the contributions of orographic waves, boundary-layer coupling, and aerosol to the occurrence of ice in mixed-phase clouds

<p>We will present a study on the impacts of orographic waves, surface coupling, and aerosol load on the frequency of heterogeneous ice formation in stratiform clouds using ground-based remote-sensing observations. Disentangling the convoluted effects of vertical motions and aerosols is critical for the understanding of heterogeneous ice formation and requires comprehensive observations. For the study, multi-year datasets from Punta Arenas (53.1°S 70.9°W, Chile, >2 years) and the northern hemispheric sites of Leipzig (51.4°N 12.4°E, Germany, 2.6 years) and Limassol (34.7°N 33.0°E, Cyprus, 1.5 years) were obtained by the same set of ground-based instruments (35-GHz cloud radar, Raman polarization lidar, 14-channel microwave radiometer, Doppler lidar, and disdrometer). The datasets at Limassol and Punta Arenas resemble the first multi-year ground-based remote-sensing datasets in the Eastern Mediterranean and in the western part of the Southern Ocean, respectively.</p> <p>The cloud properties were extracted from the synergistic dataset and the following key results on the efficiency of heterogeneous ice formation emerged:<br />The apparent lack of ice forming clouds at Punta Arenas below -15 <strong>°</strong>C can be related to orographic gravity waves, which allow persistent liquid saturation. These clouds could be identified by the autocorrelation function of the in-cloud vertical air velocity. Additionally, a correlation between the surface-coupling of a cloud and the likelihood of ice formation was found for Punta Arenas and Leipzig. At T>-10°C clouds coupled to the aerosol-rich boundary layer, were found to contain ice more frequently. Taking both effects into account, free-tropospheric, fully turbulent clouds at Punta Arenas form ice less frequently than their northern-hemispheric counterparts. This difference is linked to the lower abundance of INP in the free troposphere over the Southern Ocean.</p>

Theoretical and Methodological Features for Gas-condensate PVT Fluid Modelling with Limited Data

Majority of gas-condensate reservoir discoveries in Dnieper-Donets Basin (Ukraine), is characterized by limited composition only up to C5+, phase behavior studied by non-equilibrium, so called differential condensation PVT experiment, combined with the uncertainty in condensate production allocation to individual wells, makes the direct application of the results in modern PVT modeling software not possible. The new method, based on the Engler distillation test (ASTM86) for definition of pseudo-components combined with synthetic creation of liquid saturation curve for CVD experiment, was proposed and successfully applied for different gas-condensate reservoirs in the area of study. The quality control (QC) of the PVT model is further performed by applying material-balance method on a single-cell simulation model for exported black-oil PVT formulation when needed. The method proved being useful for modeling of multiple gas-condensate reservoirs of Dnieper-Donets Basin with different potential condensate yields varying from 30 to 700 g/m3 and as an example presented for two reservoir fluids with 108 and 536 g/m3. Results of numerical simulation studies were within the engineering accuracy in comparison to historically observed values. The investigation showed that a representative fluid model can be create in the cases when no detailed fluid composition or required laboratory experiments are available. PVT model can be efficiently validated and QC-ed by performing material-balance type numeric simulation constructed with one cell. However, the proper fluid sampling and PVT cell laboratory experiments are still major requirements for precise reservoir fluid characterization and equation of state (EOS) modeling.

Measured Solid State and Sub-Cooled Liquid Vapour Pressures of Benzaldehydes Using Knudsen Effusion Mass Spectrometry

Benzaldehydes are components of atmospheric aerosol that are poorly represented in current vapour pressure predictive techniques. In this study the solid state (PSsat) and sub-cooled liquid saturation vapour pressures (PLsat) were measured over a range of temperatures (298–328 K) for a chemically diverse group of benzaldehydes. The selected benzaldehydes allowed for the effects of varied geometric isomers and functionalities on saturation vapour pressure (Psat) to be probed. PSsat was measured using Knudsen effusion mass spectrometry (KEMS) and PLsat was obtained via a sub-cooled correction utilising experimental enthalpy of fusion and melting point values measured using differential scanning calorimetry (DSC). The strength of the hydrogen bond (H-bond) was the most important factor for determining PLsat when a H-bond was present and the polarisability of the compound was the most important factor when a H-bond was not present. Typically compounds capable of hydrogen bonding had PLsat 1 to 2 orders of magnitude lower than those that could not H-bond. The PLsat were compared to estimated values using three different predictive techniques (Nannoolal et al. vapour pressure method, Myrdal and Yalkowsky method, and SIMPOL). The Nannoolal et al. vapour pressure method and the Myrdal and Yalkowsky method require the use of a boiling point method to predict Psat. For the compounds in this study the Nannoolal et al. boiling point method showed the best performance. All three predictive techniques showed less than an order of magnitude error in PLsat on average, however more significant errors were within these methods. Such errors will have important implications for studies trying to ascertain the role of these compounds on aerosol growth and human health impacts. SIMPOL predicted PLsat the closest to the experimentally determined values.

Condensing Solvent Processes: In Search of the Production Function

Summary The heavy-oil- and bitumen-recovery process by injection of a pure condensing solvent in a solvent vapor chamber provides an alternative to steam-based recovery techniques such as steam-assisted gravity drainage (SAGD). Because of the lower operating temperature between 40 and 80°C, the process uses a much lower energy budget than a steam process and thus results in significantly reduced greenhouse-gas emissions. This paper describes the route to a successful production function with the physical processes at play and using analytical tools. Physical relationships are derived for the solvent/bitumen (S/B) ratio, the bitumen drainage from the roof of the solvent vapor chamber, and for bitumen extraction from both sides of the solvent chamber by the draining condensed solvent. The fast diffusion of bitumen into this narrow liquid solvent zone is likely subtly enhanced by transverse dispersion. The speed of bitumen extraction from the roof of the solvent vapor chamber is constrained by the gas/oil capillary pressure. Extraction from the side of the chamber is approximately three times faster by the action of the thin gravity-draining liquid solvent film. Several equations are provided to enable creation of a heat balance for this condensing solvent process. Laboratory and field observations are matched, including the rates, the heat balance, and the S/B ratio. The model can explain constrained production performance by identifying the rate-limiting steps (e.g., when insufficient solvent condenses). The model predicts high solvent holdup during the rise of the solvent chamber. A method to estimate this solvent liquid saturation is provided. The S/B ratio depends on injector-wellbore heat losses, the (high) liquid saturation in the rising solvent chamber, and the process properties (operating temperature), reservoir properties (heat capacity, porosity, and oil saturation), and solvent properties (density and latent heat). In the existing body of literature, no satisfactory analytical model was available; this new approach helps to constrain production performance and to estimate solvent and heat requirements. The methods in this paper can be used in the future for subsurface project design and performance predictions.

Capillary transport in paper porous materials at low saturation levels: normal, fast or superfast?

The problem of capillary transport in fibrous porous materials at low levels of liquid saturation has been addressed. It has been demonstrated that the process of liquid spreading in this type of porous material at low saturation can be described macroscopically by a similar super-fast, nonlinear diffusion model to that which had been previously identified in experiments and simulations in particulate porous media. The macroscopic diffusion model has been underpinned by simulations using a microscopic network model. The theoretical results have been qualitatively compared with available experimental observations within the witness card technique using persistent liquids. The long-term evolution of the wetting spots was found to be truly universal and fully in line with the mathematical model developed. The result has important repercussions for the witness card technique used in field measurements of the dissemination of various low-volatility agents in imposing severe restrictions on collection and measurement times.

Measured solid state and subcooled liquid vapour pressures of nitroaromatics using Knudsen effusion mass spectrometry

Knudsen effusion mass spectrometry (KEMS) was used to measure the solid state saturation vapour pressure (PSsat) of a range of atmospherically relevant nitroaromatic compounds over the temperature range from 298 to 328 K. The selection of species analysed contained a range of geometric isomers and differing functionalities, allowing for the impacts of these factors on saturation vapour pressure (Psat) to be probed. Three subsets of nitroaromatics were investigated: nitrophenols, nitrobenzaldehydes and nitrobenzoic acids. The PSsat values were converted to subcooled liquid saturation vapour pressure (PLsat) values using experimental enthalpy of fusion and melting point values measured using differential scanning calorimetry (DSC). The PLsat values were compared to those estimated by predictive techniques and, with a few exceptions, were found to be up to 7 orders of magnitude lower. The large differences between the estimated PLsat and the experimental values can be attributed to the predictive techniques not containing parameters to adequately account for functional group positioning around an aromatic ring, or the interactions between said groups. When comparing the experimental PSsat of the measured compounds, the ability to hydrogen bond (H bond) and the strength of the H bond formed appear to have the strongest influence on the magnitude of the Psat, with steric effects and molecular weight also being major factors. Comparisons were made between the KEMS system and data from diffusion-controlled evaporation rates of single particles in an electrodynamic balance (EDB). The KEMS and the EDB showed good agreement with each other for the compounds investigated.