Puff Cycle Research Articles

Experimental Study on CO2 Huff and Puff in the Daqing Fuyu Tight Oil Reservoir with Online NMR

Tight oil is a typical unconventional resource, and enhancing its recovery rate remains a challenge in current development efforts. In this study targeting the Daqing Fuyu tight oil reservoir, we combine a high-temperature and high-pressure long core physical simulation apparatus and a high-temperature and high-pressure online Nuclear Magnetic Resonance (NMR) testing system to conduct indoor simulation experiments on CO2 huff and puff in long cores. The results indicate that in the process, it is primarily the oil from micro-pores that is initially mobilized, but further along mobilization of fluids from a portion of sub-micro-pores and nanopores is enhanced, with an efficiency ranging from 25-33%. It was also found that there exist optimal conditions for huff and puff pressure, soaking time, and huff and puff cycles. Additionally, excessive extraction of light components by CO2 can prove disadvantageous for further improvement in huff and puff efficiency. This study can provide insight into the mechanisms of and data support for the development of CO2 huff and puff strategies in tight oil reservoirs, thereby contributing to the formulation of effective development plans.

Experimental study on influencing factors of CO2 huff and puff effect in the tight water-bearing reservoir

The presence of a water phase in tight oil reservoirs is inevitable. To investigate the effect and influencing factors of the CO2 huff and puff technique on enhancing oil recovery in water-bearing tight oil reservoirs, CO2–water–crude oil phase experiments were conducted. The results reveal that the solubility of CO2 and the expansion coefficient of crude oil decrease with increasing water content in the system. Furthermore, CO2 huff and puff experiments were performed to analyze the effects of the number of huff and puff cycles, soaking time, soaking pressure, water saturation, and permeability on the performance of CO2 huff and puff under saturated/unsaturated water conditions. The experimental findings indicate that the oil recovery increases logarithmically with the number of huff and puff cycles. Increasing soaking time and soaking pressure gradually enhances recovery until reaching optimal soaking time and pressure. However, further increasing soaking time and pressure beyond the optimal values has minimal effect on recovery. Compared to the cases under unsaturated water conditions, the optimal number of huff and puff cycles, soaking time, and pressure are higher under saturated water conditions. Additionally, the CO2 huff and puff effect deteriorates with increasing water saturation in low permeability reservoirs.

A compositional numerical study of vapor–liquid-adsorbed three-phase equilibrium calculation in a hydraulically fractured shale oil reservoir

The development of carbon capture, utilization, and storage technologies has notably advanced CO2-enhanced oil recovery (EOR) in shale oil reservoirs, which are characterized by abundant nanopores. These nanopores induce unique phase behaviors in hydrocarbons, challenging traditional phase equilibrium calculation methods. This paper presents a novel three-phase thermodynamic model (vapor–liquid-adsorbed three-phase equilibrium calculation) that addresses these challenges by considering the nanopore capillary pressure, critical parameter transitions, and material exchange between the adsorbed and bulk phases. Grounded in the multicomponent Langmuir–Freundlich adsorption equation and the Peng Robinson equation of state, this model is integrated into the MATLAB Reservoir Simulation Toolbox using an embedded discrete fracture model framework, enabling detailed study of CO2 and hydrocarbon phase behaviors within shale oil nanopores. The results reveal that there are significant nano-constrained effects on multicomponent fluid phase behavior, particularly in pores smaller than 20 nm, leading to notable changes in bubble and dew point pressures, as well as critical condensation pressures and temperatures. CO2 injection further complicates the system, enhancing interactions and expanding the coexistence region of the liquid and gas phases on the pressure–temperature diagram, especially across varying pore sizes. Optimization research on CO2 huff and puff technical parameters for shale oil reservoirs suggests the following optimal settings: a CO2 injection rate of 100 t/day, a shut-in time of 30 days, and six huff and puff cycles. The results of this study offer critical insights into CO2-EOR mechanisms in shale oil reservoirs and emphasize the importance of nanopore properties in EOR.

Integrated Optimization of Hybrid Steam-Solvent Injection in Post-CHOPS Reservoirs with Consideration of Wormhole Networks and Foamy Oil Behavior

Summary For this paper, integrated techniques have been developed to optimize the performance of the hybrid steam-solvent injection processes in a depleted post-cold heavy oil production with sand (CHOPS) reservoir with consideration of wormhole networks and foamy oil behavior. After a reservoir geological model has been built and calibrated with the measured production profiles, its wormhole network is inversely determined using the newly developed pressure-gradient-based (PGB) sand failure criterion. Such a calibrated reservoir geological model is then used to maximize the net present value (NPV) of a hybrid steam-solvent injection process by selecting injection time, soaking time, production time, injection rate, steam temperature, and steam quality as the controlling variables. The genetic algorithm (GA) has been integrated with orthogonal array (OA) and Tabu search to maximize the NPV by delaying the displacement front as well as extending the reservoir life under various strategies. Considering the wormhole network and foamy oil behavior and using the NPV as the objective function, such a modified algorithm can be used to allocate and optimize the production-injection strategies of each huff ‘n’ puff (HnP) cycle in a post-CHOPS reservoir with altered porosity and increased permeability within a unified, consistent, and efficient framework.

Effect of oxygen on the temperatures and conversion of tobacco in an electrically heated system

The goal of Heated Tobacco products (HTPs) is to devolatilize nicotine from the tobacco by a controlled electrical heating without inducing combustion and to thereby reduce the formation of harmful and potentially harmful compounds typically related to self-sustained combustion of tobacco as in conventional cigarettes. Here, we have instrumented a commercial puffing machine and an HTP with a micro-positioning system of a thin thermocouple (0.25 mm). The puffing was conducted under N2 or air. We provide the evolution of temperature as a function of time during standard puffing cycles and along the radial position within the tobacco plugs of tobacco sticks used as part of an electrically heated tobacco system (EHTS) during operation. The temperature profiles are nearly the same for the two atmospheres showing only minor influences of oxygen (maximum temperatures: 318 °C in air, 308 °C in N2, at the end of the puffing cycle). The thermal degradation of the tobacco remains always globally endothermic due to the high flow rate of the cold air flowing the tobacco during the puffing. To better understand the differences between air or N2 puffing, we have imaged the solid residues collected in the tobacco plugs by FTIR spectroscopy. The residues are clearly differentiated whatever their radial positions between air or N2 by principal component analysis of the FTIR spectra. This result highlights similar surface oxidation phenomena of the solid residue independent of the positions. Moreover, analysis of the FTIR spectra indicates that the heating of the tobacco in the EHTS is relatively uniform along the electrically-controlled heater without signs of local hot spots. These experiments were also completed by calorimetry of tobacco upon pyrolysis or oxidation, in a 3D sensor and under a fixed bed configuration (not in the EHTS). We evidence significant exothermic phenomena from about 230 °C mainly due to gas-phase oxidation of primary volatiles in the hot gas stream. During the operation of the EHTS, such potential exothermicity is counter-balanced by strong heat losses and the high flow rate of cold air flowing through the tobacco bed during the puffing. This leads to the observed net endothermic degradation of tobacco during EHTS operation.

Study on the production law and optimization parameters of CO2 huff ‘n’ puff for continental shale oil

AbstractIn this study, nuclear magnetic resonance and computed tomography scanning were used to analyze the production law and displacement mechanism of Jimusaer continental shale oil during CO2 huff ‘n’ puff, and the optimal parameters were determined. The results indicated that CO2 huff ‘n’ puff mainly produces crude oil in pore throats with 0.1–1 μm radii, while crude oil in pore throats with radii below 0.1 μm cannot be produced. Multiple CO2 huff ‘n’ puff cycles can connect fluids in fractures with fluids in large–medium pore throats, eliminate fracture effects on the oil recovery factor, and achieve coefficient development of both fractured and unfractured shales. In the CO2 huff ‘n’ puff process, core pressure change could be divided into three stages of injection–holding–depletion, and the oil displacement mode was the piston type. The study of huff ‘n’ puff parameters revealed that huff ‘n’ puff cycles and injection pressure have a great influence on the CO2 huff ‘n’ puff efficiency, while the injection timing and soaking time imposed relatively small effects. For continental shale in the Jimusaer Sag, the optimal CO2 huff ‘n’ puff parameters are five cycles, 4‐MPa injection timing, 25‐MPa injection pressure, and 12‐h soaking time.

Investigation of the Oil-Soluble Particulate Temporary Plugging Agent-Assisted Water Huff ‘n’ Puff Enhanced Oil Recovery in Tight Oil Reservoirs

Summary Water huff ‘n’ puff is an effective enhanced oil recovery (EOR) technology for tight oil reservoirs. However, the oil production of horizontal wells declines seriously after several huff ‘n’ puff cycles, and a large amount of oil is still trapped in the reservoir due to the heterogeneity of fracturing sections. The temporary plugging agent had been used for plugging high-permeability areas and thus diverting the following fluid into small permeability areas. It would improve the sweep efficiency of flooding fluid, enhancing oil recovery. However, the use of the oil-soluble particulate temporary plugging agent in the water huff ‘n’ puff application is barely reported. Therefore, the feasibility and influencing factors of oil-soluble particulate temporary plugging agent-assisted water huff ‘n’ puff (TAWHP) in enhancing oil recovery was investigated in this study. First, based on the evaluation of the performance of the oil-soluble particulate temporary plugging agent, the oil recovery of fractured core samples with different apertures for water huff ‘n’ puff and TAWHP was compared via the parallel-core experiment to verify the feasibility of TAWHP in enhancing oil recovery. The temporary plugging agent had good oil solubility, a low residual rate in the formation, and little damage to the formation. The oil recovery yielded by TAWHP was 5.17% higher than the traditional water huff ‘n’ puff process. More oil (i.e., about 1.71%) could be expelled from the fractured core samples with a small aperture. It indicated that the EOR performance yielded by water huff ‘n’ puff after several cycles could be enhanced by adding the oil-soluble particulate temporary plugging agent. After that, a mathematical model of TAWHP was established to investigate the effect of TAWHP parameters on EOR performance. The simulation results showed that the cumulative oil production increased with the increase in injection time of the temporary plugging agent solution, but the trend would level-off after 10 minutes. Moreover, as the diversion index increased, the effect of the injection rate on cumulative oil production gradually enhanced while the effect of the soaking time gradually weakened. Furthermore, the difference in cumulative oil production at different diversion indexes gradually increased as the huff ‘n’ puff cycle increased. This work could provide theoretical guidance for water huff ‘n’ puff enhancing oil recovery after several cycles.

Investigating Enhanced Oil Recovery in Unconventional Reservoirs Based on Field Case Review, Laboratory, and Simulation Studies

Aimed at advancing gas injection enhanced oil recovery (EOR) technologies in unconventional reservoirs, this study comprised a series of activities to bridge the gap between the theoretical study and actual field applications. Twenty-four EOR pilot tests were collected from the major unconventional plays in North America to evaluate the performance of different EOR technologies. Fit-for-purpose experiments and simulations were performed to investigate the effects of injection rate and pressure on EOR performance, as well as to reveal the effectiveness of huff “n” puff (HnP) cycles in actual field operations. The selection of injection rate and pressure as key parameters for investigation was based on field observations and communications with oil and gas operators because these two parameters play critical roles in both facility design and overall cost for an EOR project. Results showed that miscible EOR with a high gas injection rate and pressure is required for field operations because the injected gas needs to penetrate and extract oil from the tight matrix. Experimental results indicated that there is a correlation between oil recovery and the logarithm of core volume for miscible EOR. Immiscible gas EOR could not yield a satisfactory EOR response in actual reservoirs because the injected gas tends to flow through fractures instead of penetrating the matrix to interact with oil. Results also showed that reaching minimum miscibility pressure (MMP) does not guarantee an optimum EOR operation in unconventional reservoirs. Pressure higher than MMP is preferred in field operations. When designed properly, up to a tenfold oil production rate boost is achievable in field applications within a short period. However, such a high-performance operation is only effective in the first several HnP cycles due to the limited gas penetration depth into the rock matrix.

Investigation on pore structure, fluid mobility and water huff-n-puff oil recovery of tight volcanic oil reservoir

The pore structure of tight volcanic oil reservoirs is very complex and the permeability is extremely low. The oil recovery effect of traditional water-driven and gas-driven development is not ideal. In this paper, we first analyzed the pore structure characteristics and fluid movability characteristics of tight volcanic rocks. Then, the feasibility of water huff-n-puff in tight volcanic rocks was analyzed. Finally, we investigated the oil recovery characteristics, pore motility characteristics, and ORM of water huff-n-puff in tight volcanic rocks by using a high temperature and ultra-high pressure water huff-n-puff physical simulation experimental device under NMR monitoring. The study shows that the mineral composition of tight volcanic rocks is relatively simple, but the pore structure is very complex. The pores are mainly amygdule pores, and contain some vesicular pores and matrix pores. The fractures include structural fractures, dissolution fractures, and shrinkage fractures. Fractures are of great significance for communicating relatively independent pores and improving the seepage ability of fluids in tight volcanic rocks. The fluid movable saturation of tight volcanic rocks is 32.93%–83.75%, and the minimum movable pore radius is 0.0178 μm. The seepage capacity of large pores (r > 0.1 μm) is 2.19–7.75 times that of middle pores (0.001 μm < r < 0.1 μm). The oil recovery efficiency (the ratio of the volume of recovered oil to the volume of injected water) of the first two cycles of water huff-n-puff of tight volcanic rocks exceeds 20%, which is high-efficiency oil recovery period. However, with the increasing huff and puff cycles, the recovery factor and oil change rate both decrease rapidly. After five cycles of water huff-n-puff, the recovery factor of tight volcanic rocks increases by 29.13%. For the selected experimental core, middle pores are the main oil-producing pores, and their contribution rates in natural energy depletion and five cycles of water huff-n-puff are 59.665% and 78.173%. In five cycles of water huff-n-puff, the contribution rates of elastic displacement are 96.491%, 93.846%, 86.667%, 64.706%, and 55.556%, and the effect of imbibition displacement is relatively small. Increasing the water injection pressure is beneficial to water huff-n-puff oil recovery of tight volcanic rocks. The research can provide a good theoretical basis for the efficient development of tight volcanic oil reservoirs.

Experimental evaluation of enhanced shale oil recovery in pore scale by CO2 in Jimusar reservoir

With the domestic energy supply increase, exploitation of unconventional shale oil resources has drawn great attention from the industry. In this paper, laboratory studies were conducted to investigate the potential of cyclic CO2 injection process to enhanced oil recovery after primary production in shale oil reservoirs. Experimental results indicated that incremental oil production from CO2 huff-n-puff was obtained. The available literature provides limited information on examination of what pore size recovers the most incremental oil recovery by the applicability of CO2 huff-n-puff in shale formations. In this paper, the cyclic CO2 injection performance on three different reservoir rocks was investigated. The enhanced oil recovery by CO2 huff and puff is not limited to macropores, but the remaining oil in the 0.1-μm pore scale is recovered as well. The moveable oil in the large pores (1 μm<D < 10 μm) is mostly recovered at the first CO2 puff cycle. After the second cycle of CO2 huff-n-puff, oil recovery percentage from the large pores decreases significantly. The formation rocks were divided into 3 types based on the geological condition and petrophysical characteristics: type I、II and III. The incremental oil recovery from the type I rock is higher than type II and III. The oil recovery yields mostly from the pore size 0.1 μm <D < 1 μm.

The Evolving Temperature Field in a 1 m Methanol Pool Fire.

Thin filament pyrometry is used to measure the time-varying temperature field in a 1 m methanol pool fire. A digital camera with optical filters and zoom lens recorded the emission intensity of an array of 12 μm Silicon-Carbide filaments oriented horizontally at various heights across the steadily burning pool fire. A 50 μm diameter thermocouple measured the temperature at locations corresponding to the filament positions. A correlation was developed between the local probability density functions of the thermocouple time series measurements corrected for radiation and thermal inertia effects and the camera grayscale pixel intensity of the filaments. A regression analysis yields the local mean temperature and its variance. The time series of the temperature field is transformed into average values during consecutive phases of the fire's puffing cycle, providing quantitative insight into the complex and dynamic structure of a turbulent fire.

Effect of Dynamic Imbibition on the Development of Ultralow Permeability Reservoir

To explore the methodology for improving ultralow permeability reservoir recovery, cores of ultralow permeability reservoirs in China’s Ordos Basin were selected to study the dynamic imbibition micromechanism of crude oil in nanopore throat through core-flooding laboratory experiment and nuclear magnetic resonance (NMR) observation. In the meantime, the microimbibition characteristics and dynamic discharge of oil between matrix and fracture in partially closed boundary reservoirs were simulated to utmostly reflect the actual reservoir conditions. Our findings suggest that dynamic imbibition between fracture and matrix serves the core technology for improving the recovery of ultralow permeability reservoirs, while the main factors affecting dynamic imbibition efficiency include wettability, permeability, injection rate, fracture, water huff and puff cycles, and soaking time. Wettability, in particular, weighs the most, and imbibition can take place either on water-wet rocks or transformed oil-wet rocks with an imbibition agent added in during the waterflooding process. Meanwhile, the higher the permeability is in place, the greater the dynamic imbibition recovery might achieve. The experiments indicate that the dynamic imbibition recovery of a fractured core is 16.26% higher than that of a nonfractured core. Additionally, fractures can not only enhance imbibition recovery but also accelerate the occurrence of dynamic imbibition. The optimal water injection rate of dynamic imbibition is 0.1 mL/min; the reasonable huff and puff cycle of the ultralow permeability reservoirs tends to be two to three cycles; the optimal soaking time of ultralow permeability reservoir is speculated to be 30 days. Finally, the field practice shows that after Stimulated Reservoir Volume (SRV) and dynamic imbibition in 5 horizontal wells in An83 oilfield, there is a remarkable drop in water cut and a noticeable rise in oil production. This research underpins the significance of a dynamic imbibition effect in the development of ultralow permeability oilfield.

Thin Filament Pyrometry Field Measurements in a Medium-Scale Pool Fire

This paper presents the development of a thin filament pyrometry method to characterize the time-varying temperature field in a medium-scale pool fire burning in a quiescent environment. A digital camera with optical filters and zoom lens was used to record the high temperature emission intensity of 14 μm diameter, Silicon-Carbide filaments oriented horizontally at various heights above the center of a steadily burning 0.30 m diameter methyl alcohol (methanol; CH3OH) pool fire. Experiments collected 30 Hz video of the planar filament array. In a separate experiment, a 50 μm diameter thermocouple was used to acquire independent temperature measurements in the high temperature zone of the fire. A correlation was developed between the probability density functions of the radiation-corrected thermocouple measurements and the camera grayscale pixel intensity of the filaments. This arrangement enables measurement of the time-varying temperature field over a temperature range from about 1150 K to 1900 K with a spatial resolution of 160 μm, a temporal resolution of 0.033 s, and an expanded uncertainty of about 150 K (at a mean temperature of 1300 K). Measurements of the grayscale pixel intensities of the filaments were obtained. False color maps of the temperature field were produced to characterize the high temperature field as a function of time. Using statistical analysis, the local time-averaged temperatures and their variance for each location on the filaments were determined. Time-averaged temperatures were compared favorably to previously reported measurements. The dominant frequency of the puffing fire was determined. The temperature field time series was transformed to consider its character during consecutive phases of the fire's puffing cycle. The analysis emphasizes the cyclic nature of a pool fire, providing insight on its complex dynamic structure.

Impact of un-propped fracture conductivity on produced gas huff-n-puff performance in Montney liquid rich tight reservoirs

Studies have shown that the gas huff and puff injection potentially perform better than the continuous gas flooding in enhancing the hydrocarbon recovery in the liquid rich tight reservoirs. During the fracturing stimulation, only part of the induced hydraulic fractures is propped because proppants cannot be carried to the fracture tips. Moreover, some secondary and tertiary fractures may be too narrow to accommodate any proppants. The conductivity of the unpropped fractures is highly dependent on the variation of the in-situ pressure and may be open and close periodically during the huff-n-puff cycles. In this study, the stress-dependent fracture conductivity and its impact on the produced gas huff-n-puff performance are investigated in a liquid rich tight reservoir, considering the existence of the large amount of the unpropped fractures. The experimental data of stress-dependent fracture conductivity is employed first to simulate the dynamic conductivity during the depletion and the gas huff and puff cycles. A reservoir model is then constructed and history-matched based on the reservoir fluid samples and the field production data collected from the Montney liquid rich tight reservoir in Western Canada. Performance of the produced gas huff-n-puff is examined in the targeted reservoir and results show that contributions of the unpropped fractures cannot be ignored, which leads to 7.8% more condensate (i.e., oil) production and 2.8% higher in barrel of oil equivalent (BOE), compared to the case with propped fractures only. The effects of complex fracture geometry and the cluster completion are also investigated and results show that the unpropped fracture contributions towards the condensate production and BOE are even more pronounced in the complicated scenarios. The condensate oil and BOE are 42.0% and 22.9% higher in complex fracture geometry case and 12.4% and 5.6% higher in the fractures with multiple clusters than those scenarios with propped fractures only. This paper provides a better understanding on the potential performance of enhanced hydrocarbons recovery in liquid rich tight gas reservoirs via gas huff-n-puff operations.

A new organic fiber composite gel as a plugging agent for assisting CO2 huff and puff in water channeling reservoirs

The utilization of CO2 for enhancing oil recovery is an environmentally friendly way to dispose the industrial CO2 emissions. CO2 utilization ratio is defined as the volume ratio of produced oil to injected CO2. CO2 huff and puff technology can further improve oil recovery in high water cut reservoirs. However, as the huff and puff cycle increase, gas channeling is easy to happen and the utilization ratio of CO2 is reduced. Organic fiber composite gel is an effective plugging agent for gas channel. The major objective of this paper is to study the mechanisms of increasing CO2 utilization by injecting organic composite gel as the plugging agent. A large scale (40 cm*40 cm*3 cm) natural rock model with grooves is used to simulate the experimental conditions such as channeling channel and horizontal well. Two groups of experiments of direct CO2 huff and plugging agent assisted CO2 huff and puff were conducted. By comparing the pressure changes during the experiments, the mechanism of blocking the channel, balancing the CO2 displacement front and improving the utilization ratio of CO2 were analyzed. The experimental process was fitted by reservoir numerical simulation, and the remaining oil distribution, plugging agent distribution and CO2 distribution in CO2 huff and puff under different conditions were further demonstrated. Results show that compared with direct CO2 stimulation, the distribution of remaining oil saturation and reservoir pressure become more uniform and the utilization ratio of CO2 is greatly increased after injecting organic fiber composite gel. It is mainly due to the fact that the injected plugging agents flow preferentially into high water saturation region along the high permeability channels. Then, they can generate the organic fiber composite gel and remain in the channeling paths. As a result, the following injected CO2 will be forced to flow towards oil-rich regions radially and uniformly. Thereby, more CO2 can contact and react with the in-situ oil, resulting much higher oil recovery. In other words, the utilization ratio of CO2 is greatly improved. Taking Jidong oilfield as an example, it has been injected 19.2 × 104 t CO2 with the total oil production by 21.1 × 104 t. The utilization ratio of CO2 is calculated as high as 1.09. The observations of this study provide the theoretical basis for future application of organic fiber composite gel before CO2 flooding in the water channeling reservoirs.

Flow behavior of N2 huff and puff process for enhanced oil recovery in tight oil reservoirs

In the present work, the potential of N2 huff and puff process to enhance the recovery of tight oil reservoir was evaluated. N2 huff and puff experiments were performed in micromodels and cores to investigate the flow behaviors of different cycles. The results showed that, in the first cycle, N2 was dispersed in the oil, forming the foamy oil flow. In the second cycle, the dispersed gas bubbles gradually coalesced into the continuous gas phase. In the third cycle, N2 was produced in the form of continuous gas phase. The results from the coreflood tests showed that, the primary recovery was only 5.32%, while the recoveries for the three N2 huff and puff cycles were 15.1%, 8.53% and 3.22%, respectively.The recovery and the pressure gradient in the first cycle were high. With the increase of huff and puff cycles, and the oil recovery and the pressure gradient rapidly decreased. The oil recovery of N2 huff and puff has been found to increase as the N2 injection pressure and the soaking time increased. These results showed that, the properly designed and controlled N2 huff and puff process can lead to enhanced recovery of tight oil reservoirs.

Huff and Puff Induced Hydrate Dissociation Above and Below the Phase Equilibrium Point with Low Gas Saturation

In order to simulate the hydrate condition which is similar to the practical oceanic hydrate reservoir, hydrate samples with low gas saturation (8.4%) was obtained by the formation process of multi-step water injection. The huff and puff method (above the equilibrium pressure and below the equilibrium pressure) was employed for hydrate dissociation. When the injection temperature is lower than 20 oC, the system pressure rises with the increase of the huff and puff cycle for the huff and puff method which is above the equilibrium pressure. In addition, the gas production rate decreases over time and the water production rate is lower than that of water injection, indicating that the huff and puff method above the equilibrium pressure is unpractical for hydrate dissociation with low gas saturation. However, the huff and puff method below the equilibrium pressure can dissociate the hydrate in the reservoir completely.

Experimental and Numerical Study of Enhanced Condensate Recovery by Gas Injection in Shale Gas–Condensate Reservoirs

SummaryThis paper examines the potential of huff ’n’ puff gas-injection method to recover condensate in shale gas–condensate reservoirs by conducting experiments on a shale core. Numerical models were developed to verify experiment results. Our laboratory study shows that condensate recovery was increased to 25% by applying huff ’n’ puff gas injection on a shale core. Also, we compared the efficiency of huff ’n’ puff gas injection with that of gasflooding. At the end of same flooding with time that is the same as the time for five huff ’n’ puff cycles, the condensate recovery is 19%. From the experimental results, we found that huff ’n’ puff was more effective than gasflooding. During the experiment, condensate accumulated near the production-end region. In the huff ’n’ puff process, because the location for injection in the core was the same as that for production, the pressure in the condensate region built up faster than pressure in the flooding experiment. Also, because of the ultralow permeability, the pressure propagation was much slower in the shale core than in a conventional reservoir core, and the efficiency of gasflooding is much lower than that of the huff ’n’ puff.This study indicates that huff ’n’ puff has the potential to effectively enhance condensate recovery in shale gas–condensate reservoirs.

Near field characteristics of buoyant helium plumes

Puffing and entrainment characteristics of helium plumes emanating out into ambient air from a circular orifice are investigated in the present study. Velocity and density fields are measured across a diametric plane using Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) respectively in phase resolved manner. Experiments are performed in Froude numbers range 0.2–0.4 and for Reynolds numbers 58–248. Puffing frequency measurements reveal that the plume puffing frequencies are insensitive to the plume exit conditions, since the instability is buoyancy driven. The frequencies obtained in the present case are in agreement with frequencies obtained by Cetegen & Kasper (1996) for plumes originating from circular nozzles of various L/D ratios. Velocity and density measurements reveal that toroidal vortex formed during a puffing cycle entrains ambient air as it traverses downstream and this periodic engulfment governs the entrainment mechanism in pulsating plumes. The obtained velocity and density fields are used to calculate mass entrainment rates. It is revealed that though the flow is unsteady, the contribution of unsteady term in mass conservation to entrainment is negligible, and it becomes zero over a puff cycle. Finally, an empirical relation for variation of mass entrainment with height has been proposed, in which the non-dimensional mass entrainment is found to follow a power law with the non-dimensional height.

Effect of Reservoir Heterogeneity on Primary Recovery and CO2 Huff ‘n’ Puff Recovery in Shale-Oil Reservoirs

Summary An equation-of-state (EOS) -based compositional reservoir simulator, UT-COMP, is used to simulate both primary recovery and carbon dioxide (CO2) huff ‘n’ puff recovery in a shale matrix typical of the Bakken formation, to investigate the effect of reservoir heterogeneity on hydrocarbon recovery. Nonaqueous components are carefully lumped into seven pseudocomponents. Permeability fields with various heterogeneity and correlation lengths are generated. UT-COMP is able to solve the compositional model, despite the permeability difference between the fracture and matrix being six orders of magnitude. The effects of both primary recovery and CO2 huff ‘n’ puff recovery depend significantly on reservoir heterogeneity. In primary recovery, the recovery factor can be fit by a two-parameter exponential formula; higher heterogeneity reduces the rate coefficient in the formula. Permeability fields with identical or similar heterogeneity have similar rate coefficients, even if the correlation lengths are different, which implies that the recovery depends primarily on heterogeneity and is insensitive to correlation length. Multiple-cycle CO2 huff ‘n’ puff processes are simulated in both homogeneous and heterogeneous reservoirs. Recovery rate in the production stage rises to a peak value much higher than that in the primary recovery, and then declines dramatically. The peak recovery rate decreases with increasing huff ‘n’ puff cycles, resulting from depleted reservoir pressure and hydrocarbons. The final recovery factor in the huff ‘n’ puff recovery is lower than that in the primary recovery, because the incremental recovery in the production stage is unable to compensate the loss in the injection and shut-in stages. Use of a longer shut-in time does not help increase the recovery rate in the production stage, because CO2 migration into the shale matrix is very limited because of the low matrix permeability. Reservoir heterogeneity leads to a faster decline of recovery rate in the production stage.