Advances in micro-nano scale seepage in unconventional reservoirs: Experiments, simulations, and modeling perspectives.

The Far East must fuel its growing economies while oil production has remained flat since 2000. With more than half of the region's conventional oil already depleted, industry hopes to increase liquid supplies through the development of unconventional resources. Difficult to manage unconventional reservoirs, though, challenge the ability to transform even huge in-place resources like extra-heavy oil and oil shales to supplies. The Far East is endowed with estimated 268 Bb of in-place heavy oil and bitumen resources. Drilling has found 229 fields with estimated 6.2 Bbo of 2P recoverable heavy oil. China dominates with 4,082 MMb and heavy oil reserves have been discovered in India, Indonesia, Brunei and Malaysia. Unconventional igneous, basement and volcanic reservoir rocks yield important oil and gas production in the Far East. Igneous and basement rocks account for 3,658 MMbo and 1,359 Boe of gas resources. Vietnam accounts for the most 2P resources in this category. Led by China and Indonesia, volcanic reservoirs contribute 471 MMbo plus 1,394 MMboe of gas to the region. Oil shale is the region's sleeping giant. China commenced oil shale processing in 1928 and has ramped up three projects, Fushun, Maoming and Huadian, where it produced about 2.1 MMb during 2007 and targets 35 MMb recovery by 2020. Petrochina has boosted China's recoverable shale oil resources to 241 Bb. Technology is the key to unlock these resources. It may be only a matter of time before China tests the in situ processes being evaluated by Shell, Chevron and EGL in the Colorado Plateau or recently licensed by Raytheon to Schlumberger. The paper reviews the evolving plays and technologies that impact the development of the Far East's unconventional oil resources. Introduction Unconventional oil resources are increasingly important in the quest for energy security. Global liquids discoveries have failed to replace oil production for more than twenty years. There is concern that conventional oil resources will not be able to meet growing supply requirements to fuel worldwide economic growth. This concern has triggered a scramble to secure long-term oil supplies. The identified, 7,590 billion barrels (Bb) of in-place bitumen, "heavy" oil and oil shale resources are about the same as estimated global in-place conventional liquids resource. In a high oil price environment, unconventional resources are viewed as important and economically attractive components of future oil supplies. But there is a "catch" to the role of unconventional oil in the global oil supply perspective. While in-place unconventional oil resources are about the same as the conventional resources the unconventional resources currently account for only about 2.5% of global oil production. Nevertheless, large International Oil Companies (IOCs) and National Oil Companies (NOCs) have increased their unconventional oil holdings as a means to secure large, long-life oil supplies. They also have increased their R&D budgets to develop new technologies to boost recoveries from unconventional resources.

Re-recognition of “unconventional” in unconventional oil and gas

Correlation and difference between conventional and unconventional reservoirs and their unified genetic classification

Fluid flow in porous media is the key scientific problem in the development of oil and gas reservoirs. The traditional mechanics of fluid flow in porous media which based on the continuum hypothesis and Darcy′s law plays an important role in developing conventional oil and gas resources. In recent years, unconventional reservoirs are drawing more and more attention all over the world, therefore the development theory and technology, especially the corresponding flow mechanisms have become the hot research issues. The unconventional reservoirs exhibit distinct multiscale characteristics, even with six orders of magnitude difference. In addition, the application of massive multi-stage hydraulic fracturing can induce strong stress interactions. Therefore, the traditional theory of fluid flow in porous media cannot accurately describe the flow characteristics in unconventional reservoirs. In essence, the development of unconventional oil and gas resources involves multiphase fluids (e.g. oil, water and gas) flow in multi-scale porous media with multi-field coupling and various flow patterns. Therefore, the concept of modern system of multiphase flow in porous media is proposed, which means multiphase fluids flowing in multi-scale porous media with multi-field coupling and various flow patterns. The research status and development tendency are reviewed from the aspects of: (1) micro- and nanoscale oil and gas flow simulation; (2) upscaling for reservoir simulation, (3) macroscale flow simulation of unconventional oil and gas reservoirs; (4) simulation of flow in large scale fractured and vuggy carbonate reservoirs and (5) physical simulation of hydrocarbon transport in porous media. More specifically, in nanoscale the density functional theory and molecular simulation method can be used to study the interfacial phenomena to understand the hydrocarbon transport behavior in nanopores and provide key parameters for mesoscale flow simulation. The current study of nanoscale simulation mainly focuses on developing more realistic molecular structure model to represent the heterogeneous shale samples. Microscale simulation methods involve pore network model, lattice Boltzmann method, direct simulation of Navies-Stokes equation, level-set method and smoothed particle hydrodynamics, etc. Digital core and pore network model are the fundamental research platforms. Various methods can be used to reconstruct digital cores with multiscale pore structures and mineral compositions. The complex physicochemical phenomena namely adsorption/desorption, wettability change and boundary effect should be considered in the microscale flow simulations and extensive works have been done in microscale gas flow simulations. The future work on microscale simulation should focus on the multiphase flow mechanisms with multi-field coupling. The multiscale characteristics of unconventional reservoirs indicate the necessity of upscaling process to introduce the microscale flow mechanisms to macroscale. Homogenization theory and volume averaging method are the main upscaling approaches. Current upscaling methods are mostly based on the periodic boundary condition and are unreliable to be used in complex oil and gas reservoirs, which needs further study. In addition, more research needs to be conducted on the upscaling from molecular scale to mesoscale. In macroscale simulations of unconventional oil and gas reservoirs, the fluid-structure interaction should be considered and high efficiency numerical algorithm needs to be established. For large scale fractured and vuggy carbonate reservoirs, the non-Darcy flow characteristics and different flow regimes in vugs and fractures should be taken into account during flow simulation. Physical simulations of hydrocarbon transport in porous media are conducted at two scales: macroscale, nano- and microscale. Macroscale physical simulations aim at monitoring the dynamic saturation and pressure fields change under the realistic reservoir conditions. Nano- and microscale physical simulations are mainly applied to study the fluid transport mechanisms in single pore or throat. In summary, the proposed theory of multiphase fluids flowing in multi-scale porous media with multi-field coupling and various flow patterns can be applied to study the fluid flow problems in unconventional oil and gas industries.

Fracture stimulation fundamentals

As conventional gas resources are depleted, unconventional gas (UG) resources (gas from tight sands, coal beds, and shale) are becoming increasingly important to U.S. and world energy supply. The volume of UG resources is generally unknown in most basins outside North America. However, in 25 mature North American basins, UG resources have been produced for decades, and resources and reserves are well characterized. The objective of this work was to evaluate recoverable UG resources and determine the quantitative relations between known conventional and unconventional hydrocarbon resources in mature North American basins, with the goal of using these relations to estimate unconventional hydrocarbon resources in basin outside North America, which we call frontier basins.We used data from the U.S. Geological Survey, Potential Gas Committee, Energy Information Administration, National Petroleum Council, and Gas Technology Institute to evaluate relations among hydrocarbon resource types in the Appalachian, Black Warrior, Greater Green River, Illinois, San Juan, Uinta-Piceance, and Wind River basins. We chose these seven basins for preliminary analysis of relations between conventional and unconventional gas resources because they are mature basins for both conventional and unconventional oil and gas production. To conduct this analysis, we wrote a computer program that we call PRISE (Petroleum Resources Investigation Summary and Evaluation). Input data for PRISE, obtained from the published data sources, were the values of technically recoverable resources, which were the sum of the following resource categories: cumulative production, proved reserves, growth, and undiscovered recoverable resources. We then compared technically recoverable conventional and unconventional resources for each basin to evaluate relationship between conventional and unconventional resource volumes in these seven basins.For the seven basins studied, we found that 10% of the total recoverable hydrocarbon resources are conventional oil and gas, whereas 90% of the recoverable hydrocarbons are unconventional resources. We propose that the results of this study may be used to estimate recoverable resources from unconventional gas reservoirs in target (frontier) basins, where conventional oil and gas resources are known but unconventional resources have not been evaluated.

The Shanxi Formation layers in the northeast of the Zhoukou Depression, Southern North China Basin, mainly consist of dark mudstone interbed with tight stone and widely developed coal seam, which is a promising target for unconventional oil and gas exploration. A series of geochemical and geological methods were used to analyze the characterization and controls of the pores structural heterogeneity in low-thermal-maturity shale. These methods include the Rock-Eval analysis, total organic carbon (TOC) analysis, scanning electron microscope observation with an energy-dispersive spectrometer (SEM-EDS), X-ray diffraction, and low-pressure N2 adsorption. Based on these measurements, the pore diameter, specific surface area (SSA), and fractal dimension (D) were calculated, and then, the pore structure heterogeneity was analyzed. The result shows the pores of Shanxi Formation shale are mainly interparticle pores with low porosity and low permeability, and the pore structure is highly complex. The average fractal dimension of the micropore and the macropore are both 2.77, but that of the mesopore is 2.65, indicating a less-complex mesopore structure than the micropore and macropore. The S2, S1, and TOC exhibit no clear correlation with SSA and fractal dimension of pores, which proved the little impact of organic matter on the heterogeneity of pore structure in the low-maturity shale of the research area. The illite has a strong effect on the pore structural heterogeneity of Shanxi Formation shale. The samples with high content of illite show higher SSA, better physical properties, and low fractal dimension, reflecting low pore structural heterogeneity. However, the quartz and clay minerals show a slight correlation with SSA and no obvious relationship with the fractal dimension, indicating a little effect of them on the pore structure heterogeneity. The pore structural heterogeneity decreases along with the increase in porosity, while the permeability influenced by a variety of reasons under the compaction shows a poor relationship with SSA and fractal dimension. On the whole, the pore structural heterogeneity decreases for low-thermal-maturity shale with high content of illite and high porosity, which should be considered to be the better unconventional oil and gas reservoir in the research area.

The unconventional oil and gas reservoirs have been playing a significantly increasing role in US. The drilling and hydraulic fracturing technologies that unconventional resource developments comprehensively depend on are under increased scrutiny due to the uncertainties involved on their potential contamination to groundwater as well as their contributions to air and other pollutions. The potential distortion to natural habitat by the required infrastructure also one of the consequences of large developments in most often nearby wildlife preserves as well as highly populated urban areas. The missing fundamental technical knowledge on the stimulated reservoir volume and drained gas slows down the implementation and enhanced recovery effort costing oil and gas companies significant revenues and reserve recovery. Moreover, the heterogeneities and high level of anisotropy contained in unconventional resources make it impossible to use the existing conventional techniques for many applications due to higher precision information required in nano/micro scale and core scale. On contrary, the production history match and reservoir simulation effort uses averaged effective reservoir parameters as input for drainage patterns, gas —in‐place, production rate and field design which does not take into consideration these anisotropy and heterogeneity in the models. The corresponding predictive modeling effort give several decades off from what happens in unconventional reservoirs and require immediate attention not only from characterization perspective, but also having a better understanding on the fundamentals of fracturing operations in unconventional reservoirs and its impact on environment and groundwater.

Technology Focus In my last two Technology Focus columns, I discussed CO2-enhanced oil recovery (EOR) and the challenges it faces in conventional oil reservoirs. In this entry, my focus is on its applications in unconventional reservoirs. Oil and gas production from unconventional resources has changed the dynamics of the world oil supply, particularly in the US. This has changed the US from a declining oil producer to one of the highest oil producers in the world. Oil production from unconventional reservoirs is still a challenge and depends on a number of factors, including brute force, for drilling and hydraulic fracturing. Production from these reservoirs declines rapidly, and more wells have to be drilled to keep production at reasonable levels. Recovery, by some estimates, can be less (sometimes much less) than 10%. Currently, the number of wells drilled in unconventional reservoirs exceeds 100,000, and many are producing just a trickle of hydrocarbons. In recent years, some effort has been made to use EOR techniques, particularly CO2 injection, to extract additional oil and gas from unconventional resources. This is by no means a trivial feat. It has the potential to change the dynamics (again) of oil production from these tight and difficult reservoirs. Considerable research and laboratory studies have been conducted addressing the use and potential of CO2 in extracting hydrocarbons from unconventional reservoirs. Estimates of oil recovery range from an additional 10% up to more than 50%. Very few field trials have been conducted, but the use of CO2 in these reservoirs is promising. The recommended papers that follow present examples of laboratory studies, taking the results to the field, and mechanistic studies that elucidate some of the factors to consider and the pros and cons of CO2-EOR in unconventionals. They are meant to be a starting point for better understanding and further research. What the industry needs at this stage is more-daring EOR field trials reminiscent of the risks taken by the pioneers of unconventional resources at the beginning of this century. Recommended additional reading at OnePetro: www.onepetro.org. SPE 191780 Enhanced Oil Recovery in Eagle Ford: Opportunities Using Huff ’n’ Puff Technique in Unconventional Reservoirs by Piyush Pankaj, Schlumberger, et al. OTC 28973 Recent Advances in Enhanced-Oil-Recovery Technologies for Unconventional Oil Reservoirs by S. Balasubramanian, University of Houston, et al. SPE 192734 Miscibility Effects on Performance of Cyclic CO2 Injection in Hysteretic Tight Oil Reservoirs by Yasaman Assef, University of Calgary, et al.

A unified model for the formation and distribution of both conventional and unconventional hydrocarbon reservoirs

Development of shale reservoirs: Knowledge gained from developments in North America

In recent years interests in the North America oil/gas industry have shifted to and focused on tight/shale liquid-rich gas/oil reservoirs because of the low price of natural gas. It is evident that, beside gas permeability, permeability to hydrocarbon liquids must be understood to properly evaluate liquid production potentials. Previous studies have used gas to determine the intrinsically “true permeability” with the assumption that the “true permeability” corrected from gas permeability is equivalent to the liquid permeability for shale or tight reservoirs with microporous fabric. The microporous fabric with pores or pore-throats in the nanometer size range causes multiple co-existing gas transport mechanisms (continuum/viscous flow, slip flow, transitional and knudsen diffusion). Several studies have shown that the conventional klinkenberg correction to gas permeability is no longer appropriate for microporous medium, implying that the permeability to hydrocarbon liquid is likely also different from the gas-based “true permeability”. We envision that for unconventional rocks with nano-scale pores or pore-throats, the intrinsically “true permeability” from gas does not exist alone anymore because the permeability becomes a parameter that measures both the effectiveness of pore-network connectivity and the strong interaction between the specific fluid and the pore structure. In this study, the gas-based “true permeability” of multiple samples of the montney formation from the Western Canada Sedimentary basin were measured with different gases (including helium and argon) using different techniques. Permeability to hydrocarbon liquid (decane) was also tested on duplicate samples. The results show that liquid permeability is significantly lower than the gas permeability, even with klinkenberg effect corrections. Gas and oil porosity, pore structure and lithology of the samples were also tested using pycnometry, highpressure mercury-injection porosimetry, scanning electron microscopy, and x-ray diffraction. Integration of the data provides a comprehensive characterization of the permeability and porosity of the samples and sheds insights into our understanding of gas and liquid permeability of unconventional rocks. The implications of the results, the importance of appropriately designed laboratory programs for permeability testing and proper utilization of measured permeability data to evaluate the unconventional gas and oil production potentials are discussed. To order the full paper, visit https://www.onepetro.org/conference-paper/SPE-168730-MS

Management Unconventional resources have transformed the landscape of the oil and gas industry. The primary oil recovery factor ranges from 2–8% for the various shale plays throughout the US. Hence, it is imperative to develop the vast potential of unconventional reservoirs and increase the recovery factors beyond primary depletion by implementing IOR/EOR methods. This article describes the role of effective reservoir management and summarizes the detailed review of the advances in IOR/EOR technologies applied to unconventional oil reservoirs, as performed by the Energy Industry Partnership Team (EIP) at the University of Houston (Balasubramanian et al. 2018). That review•included: A thorough review of the pertinent published literature on IOR/EOR Results of EOR application to unconventionals shared by various operators in their investor presentations and from press reports Classifying IOR/EOR studies as laboratory experiments, numerical modeling, and field laboratory trials•(pilots) Analysis of field trials based on the representative shale plays. Most studies performed for the application of EOR technologies to unconventional oil reservoirs have been limited to experimental investigations and numerical simulation studies. The research revealed that miscible gas injection (produced field gases, CO2, etc.) is the most promising method among the EOR techniques, including miscible gas, waterflooding, surfactant, chemical, and polymer. Experimental studies showed the following: CO2 injection had the highest potential of improved recovery in unconventionals followed by produced gas injection. Surfactant injection showed the next best potential to increase oil recovery by altering the wettability of rock in laboratory experiments. The produced field gas injection pilots showed that sufficient injectivity was achieved mainly due to the injection-induced fractures and did not exhibit any significant effect of diffusion. Conformance control remains a big challenge due to the channeling of the gas through the fractures. Produced field gas injection pilots in the Eagle Ford formation have demonstrated the greatest success in increasing oil recovery. Many inconsistencies exist between laboratory investigations and field trials that need reconciliation and further research to bridge the gap. This methodical study elicits the learnings and challenges from the application of different IOR/EOR technologies to unconventionals at various scales (micro to macro to field scale). In addition, ideas for future research are recommended to improve the understanding of the complex mechanisms of EOR in unconventional oil reservoirs. Unconventional resources have changed the landscape of oil and gas industry in the US and the world. Oil production from unconventional tight oil reservoirs have accounted for more than 50% of total oil production in the US in the recent years. Todd et al. reported that unconventional oil reservoirs contributed to more than a 4 million•B/D increase in production between 2011 and•2014.

The combination of extended-length horizontal drilling and high volume hydraulic fracturing has led to previously unimaginable production increases, yet the recovery potential of unconventional oil and gas resources remains largely unrealized. Recovery factors for unconventional oil and gas wells are typically reported at < 20% in gas shale reservoirs and < 10% in the oil plays.Neutrally buoyant ultra-lightweight proppants have been demonstrated to effectively provide production from fracture area that is otherwise unpropped and thus, non-contributive with conventional sand/slickwater hydraulic fracturing processes. Production simulations illustrate that treatment designs incorporating neutrally buoyant ULW proppant treatment designs tailored for contemporary unconventional well stimulations deliver cumulative production increases of 30% to over 50% compared to the typical large volume sand/slickwater treatments. Unfortunately, production simulation results may not sufficiently lessen risk uncertainties for operators planning high-cost multi-stage horizontal stimulations. Therefore, several field trial projects using the neutrally buoyant ULW proppant in extended-length horizontal unconventional wells are currently in progress to validate the production simulations.Since the initial 4-stage fracturing stimulation incorporating neutrally buoyant ultra-lightweight proppant in 2007, deployment has occurred in fracture stimulating hundreds of oil and gas wells spanning multiple basins and reservoirs. Most of the wells are vertical or relatively short lateral wells common to asset development practices predating the unconventional shale completions mania, but many were targeted at the same unconventional reservoirs as the current multi-stage horizontal completions. Several published case histories have documented the production enhancement benefits afforded by the legacy ULW proppant wells, but questions remained as to how those lessons might be correlated to provide engineers confidence in the current production simulations.Well completion and production information was mined from the various accessible databases for the neutrally buoyant ULW proppant wells. The scope of the legacy data compiled for analysis was limited to the reservoirs common to the current field trials and production simulations, ie. unconventional oil and gas shale reservoirs. Production performance contributions of neutrally buoyant ULW proppant in past applications were compared with the production uplift observed in applications and/or simulated application of neutrally buoyant ultra-lightweight proppant fracturing treatments in current multi-stage horizontal reservoirs.The lessons learned from this investigation provide the practicing engineer the means to confidently assess production simulation data for multi-stage horizontal unconventional completions incorporating neutrally buoyant ulw proppant in the treatment designs.

Unconventional resources or unconventional reservoirs (UR) refer to a category of underground hydrocarbon deposits which are different in operations and methods of recovery compared to conventional reservoirs.