Ultra-low Permeability Research Articles

Study on Sedimentary Facies and Reservoir in Taiyuan Formation in Qingshimao area

Recent exploration results show that there is an industrial airflow in Taiyuan Formation in Qingshimao area of Ordos Basin. The Marine quartz sandstone reservoir developed in Taiyuan Formation is the target reservoir after He 8 and Shan 1 Members in Qingshimao area. In order to study the sedimentary facies characteristics of Taiyuan Formation in Qingshimao area, this paper analyzed the well logging, single well facies and connected well facies on the basis of core observation. In addition, using the data of casting thin section, scanning electron microscope and cathode luminescence, the response relationship between sedimentary facies and well logging facies is established, thus the sedimentary facies in the study area is divided and the reservoir characteristic is studied. The results show that the Taiyuan Formation mainly developed barrier coastal sedimentary facies, which can be subdivided into four sub-facies: marsh, tidal flat, barrier island and lagoon. Lithic quartz sandstone is mainly developed in Taiyuan Formation, followed by quartz sandstone and lithic sandstone. The reservoir space types include intergranular pores, dissolved pores, intergranular pores and micro-fractures. Extra-ultra-low porosity and ultra-low permeability reservoir is developed in Taiyuan Formation in study area.

Dramatically Enhancing Oil Recovery via High-Efficient Re-Fracturing Horizontal Wells in Ultra-Low Permeability Reservoirs: A Case Study in HQ Oilfield, Ordos Basin, China

The ultra-low permeability oil reservoir in the HQ oilfield within the Ordos Basin exemplifies a classic “three-low” oil reservoir characterized by low pressure, low permeability, and low abundance. Upon the commencement of oil well production, substantial initial production decline and inadequate pressure maintenance levels are encountered. Consequently, these wells commonly face persistent low production issues resulting from ineffective water injection. Currently, the available technical approaches for repeated stimulation of such wells to enhance production and efficiency are limited, thereby restricting the effective utilization of the remaining oil reserves. In response to this challenge, this article presents an innovative technology tailored for high-efficiency re-fracturing to improve oil recovery in ultra-low permeability reservoirs. Grounded in the theory of multiple flow couplings and volume development, this technology introduces a novel integrated process encompassing seepage field reconstruction, fracturing, and oil displacement. This comprehensive approach culminates in an integrated energy replenishment methodology applicable throughout the entire reservoir’s life cycle. The proposed technology materializes a novel development method for ultra-low permeability reservoirs, centered on the principles of “seepage field reconstruction, integration of fracturing and oil displacement, multi-mode energy replenishment, and all-round displacement”. This integrated strategy ensures the efficient development of ultra-low permeability reservoirs. The successful implementation of this technology in the HQ oilfield is a notable achievement. Notably, the daily oil production of directional wells and horizontal wells significantly increased from 0.5 tons and 1.8 tons to 2.4 tons and more than 10 tons, respectively. Furthermore, the estimated ultimate recovery rate witnessed a substantial improvement from 5.2% to 17.3%. These compelling results underscore the potential of this technology in revitalizing the production of low-yield wells in ultra-low permeability reservoirs.

Geomechanical Characterization of the Barren Measure Formation in the Raniganj Sub-basin of Damodar Basin, India

Abstract This paper presents laboratory investigations on the vertically drilled borehole specimens belonging to the Barren Measure Formation shales in the Raniganj sub-basin, India. These shales were examined for their mineralogical makeup and geomechanical properties through a series of laboratory experiments viz. X-ray diffraction, uniaxial tests, single-stage triaxial tests, tensile strength tests, porosity, and permeability measurements. The mineralogical assessment has revealed dominant clay minerals and moderate silica content. The mechanical tests yielded a high Young’s modulus and low Poisson’s ratio in these shales with brittle to semi-brittle type of failure. The compositional, elastic and tensile brittleness of these shales indicate values that are in good agreement with the brittle nature of gas shales. Furthermore, these shales report low porosity and ultra-low permeability. The Barren Measure Formation in the Raniganj sub-basin is organically rich and was targeted during India’s first exploratory well, yielding natural gas. However, these shales are still in their infancy in terms of their geomechanical attributes. This study also attempted to generate basic data on properties like compressibility, elastic moduli, and tensile strength, which are among the critical parameters controlling the reservoir simulation program. The results of brittleness and failure parameters (elastic moduli, internal friction angle, cohesion, compressive strength and tensile strength) of Barren Measure Formation in Raniganj sub-basin of Damodar Basin, India show high similarity with those of Roseneath Formation in Cooper Basin, Australia revealing their analogous origin.

Hydrocarbon accumulation characteristics in basement reservoirs and exploration targets of deep basement reservoirs in onshore China

Based on the global basement reservoir database and the dissection of basement reservoirs in China, the characteristics of hydrocarbon accumulation in basement reservoirs are analyzed, and the favorable conditions for hydrocarbon accumulation in deep basement reservoirs are investigated to highlight the exploration targets. The discovered basement reservoirs worldwide are mainly buried in the Archean and Precambrian granitic and metamorphic formations with depths less than 4 500 m, and the relatively large reservoirs have been found in rift, back-arc and foreland basins in tectonic active zones of the Meso-Cenozoic plates. The hydrocarbon accumulation in basement reservoirs exhibits the characteristics in three aspects. First, the porous-fractured reservoirs with low porosity and ultra-low permeability are dominant, where extensive hydrocarbon accumulation occurred during the weathering denudation and later tectonic reworking of the basin basement. High resistance to compaction allows the physical properties of these highly heterogeneous reservoirs to be independent of the buried depth. Second, the hydrocarbons were sourced from the formations outside the basement. The source-reservoir assemblages are divided into contacted source rock-basement and separated source rock-basement patterns. Third, the abnormal high pressure in the source rock and the normal–low pressure in the basement reservoirs cause a large pressure difference between the source rock and the reservoirs, which is conducive to the pumping effect of hydrocarbons in the deep basement. The deep basement prospects are mainly evaluated by the factors such as tectonic activity of basement, source-reservoir combination, development of large deep faults (especially strike-slip faults), and regional seals. The Precambrian crystalline basements at the margin of the intracontinental rifts in cratonic basins, as well as the Paleozoic folded basements and the Meso-Cenozoic fault-block basements adjacent to the hydrocarbon generation depressions, have favorable conditions for hydrocarbon accumulation, and thus they are considered as the main targets for future exploration of deep basement reservoirs.

Evaluation of Saturation Interpretation Methods for Ultra-Low Permeability Argillaceous Sandstone Gas Reservoirs: A Case Study of the Huangliu Formation in the Dongfang Area

Ultra-low permeability argillaceous sandstone reservoirs have become a significant focus for exploration and development. Saturation is a crucial parameter in evaluating such reservoirs. Due to the low porosity, low permeability, complex pore structure, and strong heterogeneity in ultra-low permeability argillaceous sandstone reservoirs, traditional evaluation methods are unable to achieve the required level of interpretation accuracy. To improve the accuracy of gas saturation calculations in ultra-low permeability argillaceous sandstone gas reservoirs, the conductivity characteristics of the ultra-low permeability argillaceous sandstone gas reservoirs in the Huangliu Formation in the Dongfang area, China, were analyzed through rock physics experimental data and geological information. The results revealed the clay content in the study area to range from 6% to 33.4%. Influenced by burial depth and temperature, kaolinite and montmorillonite transform into illite and chlorite, and the cation exchange capacity is not correlated with the clay content. This suggests that the effects of cation-attached conductivity can be ignored. The regional variation in rock electrical parameters is significant. When the lithology coefficient is a = 1, the consolidation index m varies between 1.25 and 1.75. When the lithology coefficient is b = 1, the saturation index n varies between 1.6 and 1.96. Under the influence of high temperature and pressure, the reservoirs in the study area exhibit two distinct characteristics on the capillary pressure curve, fractal dimension, and effective porosity index intersection diagram, (1) In Class I reservoirs, there is a strong correlation between the formation factor and natural gamma, as well as porosity, and a logarithmic relationship between the saturation exponent and the formation factor; and (2) in Class II reservoirs, there is a strong power-law relationship between the formation factor and porosity, as well as a logarithmic relationship between the saturation exponent and formation factor. Based on experimental and logging data, reservoir classification was conducted using core-scale logging and principal component analysis. Additionally, a saturation interpretation model was developed using multivariate regression, based on rock electrical parameters. The actual application results demonstrate that compared to the fixed rock resistivity saturation model, this model has reduced the average absolute error by 5.9%. The calculated values are consistent with the gas saturation analysis of the core at the pure gas section as determined by cable testing sampling. This model meets the requirements of practical production. The study of this interpretation method is of great significance for the formulation of development plans for ultra-low permeability offshore argillaceous sandstone gas reservoirs.

Study on Pore Structure Characteristics of tight Sandstone Reservoirs -- Taking Chang 8 in XX block of Ordos Basin as an example

The XX block of Ordos Basin has recently achieved a major exploration breakthrough, however the reservoir is more heterogeneous and the single well productivity is more different. Based on drilling, core, analysis and testing data, the rock characteristics, physical characteristics and microscopic pore structure characteristics of the area are comprehensively analyzed. A set of reservoir classification criteria with strong applicability is established. The results show that the reservoir lithology in the study area is complex, and the types and geometric forms of reservoir space are diverse. The porosity is 0.15%~13.65%, and the permeability is) 0.001~83.5 (×10-3μm2. The distribution range of physical properties is wide, and the overall performance is low porosity and ultra-low permeability. The heterogeneity is strong, and a few large pores have an important influence on reservoir permeability. A set of reservoir classification and evaluation criteria is established. The study area mainly develops type II reservoirs. The classification and evaluation criteria can provide reference for the later exploration and development of the study area.

Optimization of Gas-Flooding Fracturing Development in Ultra-Low Permeability Reservoirs

Optimization of Gas-Flooding Fracturing Development in Ultra-Low Permeability Reservoirs

The flooding mechanism and oil recovery of nanoemulsion on the fractured/non-fractured tight sandstone based on online LF-NMR experiments

Hydraulic fracturing and water/nanoemulsion flooding technology are effective stimulation measures for tight oil production. The complex pore structure of ultra-low permeability sandstone greatly affects oil recovery. At present, scholars have done a lot of research on the characteristics of oil-water flow in micro-nano pores of ultra-low permeability sandstone reservoirs. However, few scholars have comprehensively studied the whole process of ultra-low permeability sandstone reservoirs from hydraulic fracturing to enhanced oil recovery. In this paper, the whole process from fracturing to production of ultra-low permeability sandstone reservoirs is comprehensively studied at the laboratory scale by combining triaxial fracturing, CT scanning, online LF-NMR and other technologies, aiming to promote the understanding of tight oil development. The results show that the high confining pressure (45 MPa) inhibits the propagation of fractures in the rock samples more than the low confining pressure (15 MPa) during the triaxial fracturing process. The oil recovery of the rock sample with high fracture volume (1213.42 mm3) is 16.32 % higher than that of the rock sample with low fracture volume (674.22 mm3). Whether it is fractured rock or non-fractured rock, nanoemulsion can further improve the oil recovery of micropores on the basis of water flooding, with a maximum increase of 28.21 %. The oil recovery of the nanoemulsion with a concentration of 0.3 wt% was 25.34 % higher than that of the nanoemulsion with a concentration of 0.05 wt%. Confining pressure conditions have an important impact on oil recovery. The oil recovery of non-fractured rock samples with initial confining pressure of 5 MPa is 8.15 % lower than that of non-fractured rock samples with initial confining pressure of 30 MPa. At the same time, we propose a new T1-T2 map region division scheme based on sandstone to better characterize oil displacement behavior at the pore scale. The research results can guide the exploitation of ultra-low permeability sandstone reservoirs.

A highly selective and high-performance sulfonated poly(ether ether ketone)-based hybrid membrane enabled by complexed UiO-66-NH2 and sulfonated graphitic carbon nitride for vanadium flow batteries

Complexed UiO-66-NH2 and s-g-C3N4 nanofillers contribute ultralow permeability and ultrahigh selectivity to SPEEK/NF-1 : 1 and ensure outstanding VFB efficiencies owing to manipulated acid–base interactions and synergistic ion barriers.

Pulse power plasma stimulation: A technique for waterless fracturing, enhancing the near wellbore permeability, and increasing the EUR of unconventional reservoirs

The inherent characteristic of unconventional and ultra-low permeability reservoirs is a small drainage area that negatively affects the production rate and ultimate recovery (UR). Drilling horizontal wells, coupled with a stimulation process such as hydraulic fracturing, are applied to wells in these reservoirs to enhance well productivity. Although hydraulic fracturing has been the most famous stimulation technique in these reservoirs and has contributed to the increased US oil and gas production during the last several years, other stimulation techniques may be competitive with (or complementary to) hydraulic fracturing. This is because the main contribution to production in these reservoirs would be from complex fracture networks (often micro-cracks) combined with enhanced formation permeability rather than the classical two-wing fractures. Hence, a near-wellbore stimulation technique for improving the permeability of these reservoirs by activating the existing natural fractures and creating secondary microfractures would be highly desired. This paper investigates the pulse plasma stimulation technique to investigate such an approach of waterless stimulation/fracturing that creates multiple fractures and is also a permeability enhancement method. In this method, plasma is generated by discharging a high voltage and high current electrical energy in a very short time into two electrodes submerged in a wellbore filled with aqueous fluid. This study is divided into two sections. The first part presents experimental and numerical examples of stimulation/fracturing induced by pulse plasma in different rocks. Several parameters, including the effect of discharge with and without wire, input energy, confining stress, and rock type, on the induced fractures are studied. The second part uses the insights and results from the first part to numerically investigate pulse plasma's effects on production. Measurement of the formation permeability around the generated fractures indicated that the shock wave generated multiple fractures and increased the rock permeability by more than one or two orders of magnitude. The second part of the numerical section discusses the increased estimated rate and ultimate recovery (EUR) of horizontal wellbores due to fracturing and modified permeability resulting from pulse plasma. We present a sensitivity analysis on the magnitude and extent of the improved permeability region and its impact on EUR.

Robust water-borne multi-layered superhydrophobic coating on concrete with ultra-low permeability

Concrete always suffers from deterioration under harsh conditions due to its hydrophilicity and porosity. Water and aggressive ions can easily penetrate the concrete which may result in safety concerns and tremendous economic loss. Coating protection can effectively prevent concrete from aggressive media penetration and improve concrete durability. Here, we fabricated a multi-layered superhydrophobic (MLS) coating with excellent superhydrophobicity and low permeability. The coating can maintain superhydrophobicity after heat treatment at 300 °C, 115-day outdoor exposure, and 500 cycles of Taber abrasion under a 1-kg load. Moreover, it has excellent resistance to chloride ions. We believe that the MLS coating has an interesting application prospect in concrete anticorrosion in bridges, ships, and other fields.

Preparation of carbon-based active nanofluids with temperature and salt resistance for enhanced oil recovery

The abundant hydrophilic surface groups endow carbon-based nanoparticles (CDs) with intrinsic great dispersibility, however, the strong hydrophilicity limits the oil–water interface activity, restraining the performance in enhanced oil recovery. Developing CDs with simultaneously ultra-small sizes, high temperature and salt tolerance, as well as superior interfacial activity is of great significance. In this work, CDs with the size of 3.3 nm and excellent temperature and salt resistance were prepared by hydrothermal reaction, using citric acid (CA) and ethylenediamine (EDA) as precursors. Carboxyl ether carboxylate-6Na (APEC-6Na) was selected as the compound agent to construct carbon-based active nanofluids (CANs) with excellent oil–water and oil-solid interfacial activities. CANs can reduce the oil–water interfacial tension to 8.9 × 10-4 mN m−1, withstand a temperature up to 90°C and a salinity up to 6.3 × 104 mg·L-1. Even at the highest tolerant salinity, the underwater oil contact angle of 155° can be achieved, demonstrating excellent wettability control capability, which could be attributed to the synergistic effect of the compound CANs system. The increase of the contact angle of underwater oil droplets facilitates the coalescence of oil film on the lipophilic surface to form oil droplets. The electrostatic repulsion generated by the adsorption of CDs and APEC-6Na at the oil–water interface and rock surface makes the coalesced oil droplets more easily detached from the rock surface, and the oil film reduction rate exceeds 97.7 % within 24 h. The excellent oil–water and oil-solid interface properties and the structural separation pressure endows CANs with remarkable performance in core displacement experiments. Compared with simulated formation water, CANs can improve the recovery rate by 26.3 %, exhibiting important application value for improving the recovery rate of ultra-low permeability reservoirs.

Applications of Two-Dimensional Laboratory Higher-Frequency NMR in Unconventional Shale Characterization

The complexity of the microstructure and fluids in unconventional reservoirs presents challenges to the traditional approaches to the evaluation of geological formations and petrophysical properties due to the low porosity, ultralow permeability, complex lithology, and fluid composition. Nuclear magnetic resonance (NMR) techniques have been playing major roles in unconventional shale characterization in the last decades as NMR can provide critical information about the reservoirs for quantifying their petrophysical parameters and fluid properties and estimating productivity. Laboratory NMR techniques at higher frequency (HF), e.g., 23 MHz, especially two-dimensional (2D) T1-T2 mapping, and their applications have been essential for the noninvasive characterization of tight rock samples for identifying kerogen, bitumen, heavy or light hydrocarbons, and bound or capillary water. Traditional T2 cutoffs, established with low frequency (LF) NMR, no longer apply and need new definitions to reflect the inferences from water and hydrocarbons separately. The crushed rock analysis method, as applied to unconventional formations, has been successful in evaluating total porosity and water saturation but also suffers from inconsistency in results due to desiccation and solvent effects. In the past decade, the oil and gas industry has witnessed significant development of HF NMR techniques that couple advances in petrophysics, petroleum engineering, and geochemistry with a broad range of applications. It is necessary to review such technological advances and draw conclusions to benefit unconventional core analysis programs. This article will summarize key advances in laboratory NMR applications in unconventional shale characterization, including monitoring processes of liquids equilibrium, desiccation, and imbibition in fresh shale samples, determination of activation energy of hydrocarbons in shales, monitoring changes in a shale sample during liquid flooding experiments, and direct measurements on kerogen.

Experimental study of the influencing factors and mechanisms of the pressure-reduction and augmented injection effect by nanoparticles in ultra-low permeability reservoirs

Nanoparticles (NPs) have gained significant attention as a functional material due to their ability to effectively enhance pressure reduction in injection processes in ultra-low permeability reservoirs. NPs are typically studied in controlled laboratory conditions, and their behavior in real-world, complex environments such as ultra-low permeability reservoirs, is not well understood due to the limited scope of their applications. This study investigates the efficacy and underlying mechanisms of NPs in decreasing injection pressure under various injection conditions (25–85 °C, 10–25 MPa). The results reveal that under optimal injection conditions, NPs effectively reduce injection pressure by a maximum of 22.77% in core experiment. The pressure reduction rate is found to be positively correlated with oil saturation and permeability, and negatively correlated with temperature and salinity. Furthermore, particle image velocimetry (PIV) experiments (25 °C, atmospheric pressure) indicate that the pressure reduction is achieved by NPs through the reduction of wall shear resistance and wettability change. This work has important implications for the design of water injection strategies in ultra-low permeability reservoirs.

Potential of nonionic polyether surfactant-assisted CO2 huff-n-puff for enhanced oil recovery and CO2 storage in ultra-low permeability unconventional reservoirs

Sequestering CO2 in oil reservoirs using CO2 huff-n-puff technology while maximizing oil recovery is critical for the efficient development of shale/tight reservoirs. In this work, intrinsic reasons for the limited EOR efficiency of CO2 huff-n-puff technology and the potential of a nonionic polyether surfactant C4(PO)3-assisted CO2 huff-n-puff process in further enhancing oil recovery and increasing CO2 storage capability in shale/tight reservoirs were investigated by physical modeling experiments combined with nuclear magnetic resonance techniques. The EOR performance, the mobilization of oil from different pore size ranges, and CO2 storage capability were analyzed in the CO2 huff-n-puff and CO2–C4(PO)3 mixed fluid huff-n-puff experiments. The results showed that the EOR efficiency of CO2 huff-n-puff was limited by the rapidly declining cyclic recovery and the meager recoveries from micropores and deep matrix regions. Applying CO2–C4(PO)3 mixed fluid with a surfactant concentration of 0.5 wt% resulted in a 9.7%–13.2% increase in the cumulative matrix oil recovery relative to CO2 under the same huff-n-puff cycles. Moreover, relative increases in oil recoveries from micropores, mesopores, and macropores reached 113.4%–142.2%, 14.2%–41.1%, and 0.8%–17.6%, respectively. When CO2 no longer enhanced production after the 4–5 huff-n-puff cycles, approximately 14%–19% of the residual oil was effectively recovered by applying the C4(PO)3-assisted CO2 huff-n-puff stimulation. Simultaneously, the ultimate CO2 storage capability in the examined cores increased by 22.3%–32.2% after the CO2–C4(PO)3 mixed fluid huff-n-puff process. The enhancement of CO2 injection and diffusion capabilities induced by the CO2–C4(PO)3 mixed fluid improved CO2 sweep and oil displacement efficiencies in shale/tight reservoirs, leading to a significant increase in the oil mobilization within micropores and an extended effective stimulation cycle, as well as achieving a higher CO2 storage capability. The increase of CO2 injection and diffusion amounts induced by nonionic polyether surfactants during the huff-n-puff process was attributed to the reduction in capillary resistance caused by the lowered CO2–oil interfacial tension and the increase of dissolved CO2 in oil resulting from the improved CO2–oil miscibility. This paper presents new strategies for advancing the sustainable and efficient development of shale/tight reservoirs with highly developed nanopores.

Crosslinked PVA based polymer coatings with shear-thinning behaviour and ultralow hydrogen permeability to prevent hydrogen embrittlement

An economic transition from natural gas to a hydrogen economy will require redeployment of steel pipelines and other infrastructure. Mechanisms are urgently needed to prevent embrittlement of the steel in hydrogen service. In this work, we show that poly vinyl alcohol (PVA) and poly (ethylene glycol) diglycidyl ether (PEGDGE) crosslinked polymer materials can be used as internal coatings to dramatically reduce hydrogen permeation to the steel surface. Unlike other crosslinked PVA systems, these materials are shear-thinning and thixotropic, which is an essential requirement for facile, in situ application onto existing infrastructure. A hydrogen permeability of 0.01 Barrer is achieved, which is up to 100 times lower than commercially available coating materials. Experiments show that increasing concentrations of the alkali catalyst (KOH) do not impact the permeability of the crosslinked films but can benefit the rheology by shortening the reaction time. Higher reaction ratios of PVA to PEGDGE give lower hydrogen permeability due to a higher degree of polymer crystallinity, but less favorable rheology due to slower reaction kinetics. PVA with higher molecular weight gives lower hydrogen permeability and promotes shear-thinning behaviour, while PEGDGE with higher molecular weight increased the film permeability but enhances the shear-thinning behavior with shorter reaction times.

Unconventional Reservoir Characterization and Formation Evaluation: A Case Study of a Tight Sandstone Reservoir in West Africa

Unconventional reservoirs, including gas shales and tight gas sands, have gained prominence in the energy sector due to technological advancements and escalating energy demands. The oil industry is eagerly refining techniques to decipher these reservoirs, aiming to reduce data collection costs and uncertainties in reserve estimations. Characteristically, tight reservoirs exhibit low matrix porosity and ultra-low permeability, necessitating artificial stimulation for enhanced production. The efficacy of the stimulation hinges on the organic material distribution, the rock’s mechanical attributes, and the prevailing stress field. Comprehensive petrophysical analysis, integrating standard and specialized logs, core analyses, and dynamic data, is pivotal for a nuanced understanding of these reservoirs. This ensures a reduction in prediction uncertainties, with parameters like shale volume, porosity, and permeability being vital. This article delves into an intricate petrophysical evaluation of the Nene field, a West African unconventional reservoir. It underscores the geological intricacies of the field, the pivotal role of data acquisition, and introduces avant-garde methodologies for depth matching, rock typing, and the estimation of permeability. This research highlights the significance of unconventional reservoir exploration in today’s energy milieu, offering a granular understanding of the Nene field’s geological challenges and proffering a blueprint for analogous future endeavours in unconventional reservoirs.

Lithological and diagenetic variation of mixed depositional units in the middle permian saline lacustrine deposition, Junggar Basin, NW China

Mixed depositional reservoirs are widely distributed in Junggar Basin, NW China. These reservoirs are featured by complex lithology, ultra-low permeability and extremely heterogeneous pore structures that markedly impact field development efficiency. This study illustrated the distribution of lithological combinations within a single sand bar in the Middle Permian Lucaogou Formation, Jimsar Sag, Junggar Basin and revealed the diagenetic control on the reservoir quality of a single sand bar based on thin sections, scanning electron microscopy (SEM), helium porosity and air permeability measurement, pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), X-ray computed tomography (CT), and collected and published data. The results show that the heterogeneity of microscopic pore structures within a single sandbar is controlled by the distribution of various lithological combinations and different levels of diagenetic alteration. The results show that there are mainly three types of lithology developed in a single sand bar. The reservoir quality of dolomitic siltstone reservoirs is the best, followed by silty dolomite reservoirs. The reservoirs quality of dolomicrite reservoirs is the worst. Three main lithological combinations can be identified within a single sand bar. Controlled by climate, lake level fluctuation and provenance, lithological combination A, characterized by blended mixing of dolomitic siltstone, silty dolomite and dolomicrite, is mainly developed in the middle of single sand bar. Lithological combination B, characterized by saltatory mixing of interbedded dolomitic siltstone and dolomicrite, and lithological combination A developed on the side of the sand bar near the shallow lake, while lithological combination C with blended mixing of interbedded dolomitic siltstone and silty dolomite developed on the side near provenance. Strong compaction is the main factor of the decrease of reservoir quality of sand bar reservoirs. Carbonate cementation promotes the densification of reservoirs. The irregular flaky clay minerals lead to the exponential decline of permeability, and dissolution is the main kind of diagenesis to improve reservoir quality. The dolomitic siltstone reservoirs in the middle of a single sand bar have the best reservoir quality because the overlying dolomicrite layers resist compaction, resulted in a certain amount of primary pores remained. Besides, the dolomitic siltstone reservoirs are far from the sand-mudstone interfaces, which leads to the low carbonate cement content. Furthermore, abundant dissolution pores in dolomitic siltstone reservoirs improve the reservoir quality. These research results are crucial to reservoir evaluation and development in similar mixed depositional tight reservoirs.

Fracture identification and its application of ultra-low permeability carbonate reservoir in Fauqi North oilfield, Iraq

Fracture identification and its application of ultra-low permeability carbonate reservoir in Fauqi North oilfield, Iraq

Establishing a water baseline for the unconventional gas industry: A multiple environmental isotopes assessment (18O, 2H, 3H, 13C, and 14C) of surface and groundwater in the São Francisco Basin, Brazil

Unconventional hydrocarbon production has become the target of an intensive environmental debate due to the risks it poses to water resources. Fracking, while enabling the extraction of oil and gas from ultra-low permeability reservoirs, also possesses the risk of polluting water systems through failures from hydraulic fracturing and its associated procedures. The need to foster national industrial development with a transitional energy matrix has led Brazil to discuss the environmental suitability before producing its large unconventional reserves. Many studies have highlighted the need for a robust environmental characterization before the development of the unconventional industry. In this sense, multiple environmental isotopes may work as a proxy for identifying water contamination right from the early stages. Environmental isotopes may also be applied to enhance the understanding of the natural geochemical processes intrinsic to a given area. This study presents an environmental isotopes baseline for the groundwater and riverine water systems within the São Francisco Basin, a proven tight gas reservoir in Brazil, in a pre-operational context. δ18O, δ2H, 3H, δ13C, and Δ14C were evaluated in three different seasons in groundwater and surface water samples, along with other auxiliary parameters such as physical-chemical parameters (in situ), major ions, and d-excess. The δ2H and δ18O in surface water shows an upstream → downstream enrichment trend, with some variations suggesting baseflow interactions in the surface water systems. An evaporation line for the study area was defined as δ2H = 4.6903 δ18O + 10.362. δ13C indicates a mutual dissolution of silicates and carbonates in the groundwater system and suggests a group of samples highly related to the recharge areas. Groundwater dating denotes the Serra da Saudade Formation as a modern fractured aquifer with a strong recharge capacity. These findings support stakeholders in environmental monitoring and management of the unconventional gas industry.