Ultra-low Permeability Research Articles

The study of enhanced displacement efficiency in tight sandstone from the combination of spontaneous and dynamic imbibition

Spontaneous imbibition is critical in the oil recovery process for low permeability and tight reservoirs, which is driven by the capillary pressure. In the waterflooding process, dynamic imbibition has been determined as an important means of oil recovery for ultra-low permeability reservoirs. It functions significantly to the oil recovery in cyclic water injection. Unfortunately, some mechanisms are not fully understood within the spontaneous and dynamic imbibition such as the effect of pore size on imbibition and their collaboration on oil recovery. In this paper, three micro mechanisms were studied to understand their effects on oil recovery efficiency: spontaneous imbibition, dynamic imbibition and the combination of spontaneous and dynamic imbibition. The Nuclear Magnetic Resonance (NMR) technique was applied to investigate the sandstone formation of Yanchang group in Ordos basin. First, we analyzed the T2 spectrum characteristics of tight sandstone, by which the pore size was grouped into three along with the high-pressure mercury injection data. Second, we studied the quantitative effect of imbibition time and interfacial tension on the oil recovery efficiency of tight sandstone, with respect to various pore sizes during the spontaneous imbibition process. Simultaneously, the effect of interfacial tension on dynamic imbibition was evaluated at various pore sizes by means of displacement experiments and low-field NMR. Third, we proposed the methodology of the utilization of spontaneous and dynamic imbibition to enhance the displacement efficiency in tight sandstone, based on their characteristics. Meanwhile, the displacement characteristics were compared at various pore sizes from the three imbibition types. A feasible workflow was provided for field applications to enhance oil recovery by means of spontaneous and dynamic imbibition. This research fills the gap of waterflooding in tight sandstone and low permeability reservoirs and improves the waterflooding technique.

Mechanism of active silica nanofluids based on interface-regulated effect during spontaneous imbibition

The ultra-low permeability reservoir is regarded as an important energy source for oil and gas resource development and is attracting more and more attention. In this work, the active silica nanofluids were prepared by modified active silica nanoparticles and surfactant BSSB-12. The dispersion stability tests showed that the hydraulic radius of nanofluids was 58.59 nm and the zeta potential was − 48.39 mV. The active nanofluids can simultaneously regulate liquid–liquid interface and solid–liquid interface. The nanofluids can reduce the oil/water interfacial tension (IFT) from 23.5 to 6.7 mN/m, and the oil/water/solid contact angle was altered from 42° to 145°. The spontaneous imbibition tests showed that the oil recovery of 0.1 wt% active nanofluids was 20.5% and 8.5% higher than that of 3 wt% NaCl solution and 0.1 wt% BSSB-12 solution. Finally, the effects of nanofluids on dynamic contact angle, dynamic interfacial tension and moduli were studied from the adsorption behavior of nanofluids at solid–liquid and liquid–liquid interface. The oil detaching and transporting are completed by synergistic effect of wettability alteration and interfacial tension reduction. The findings of this study can help in better understanding of active nanofluids for EOR in ultra-low permeability reservoirs.

Reservoir characteristics and control factors of Tongbomiao formation in Hongqi sag

Hongqi sag is located in the north of Beiehur depression in Hailar Basin, which is a secondary structural unit in the peripheral depression of Hailar Basin, with great exploration potential. Based on SEM, conventional thin sheet, cast thin sheet, mercury pressure and physical property data, this paper discusses the reservoir characteristics, types and control factors of high quality reservoir in Tongbomiao formation, Hongqi sag, so as to provide a strong geological basis for the exploration of oil and gas in this area. The reservoir space of Hongqi sag is mainly composed of four types: primary intergranular pore, micro fracture, feldspar dissolution pore and debris dissolution pore. It is of the type of ultra low porosity and low permeability. Although Tongbomiao formation in Hongqi sag is of very low porosity and low permeability type, high quality reservoir is still developed, and its reason is mainly controlled by sedimentary environment, diagenesis and structural fracture.

A new slick water system for hydraulic fracturing in tight reservoir to enhance imbibition oil recovery

In order to give full play to the role of imbibition of capillary force and enhance oil recovery of ultralow permeability sandstone reservoir after hydraulic fracturing, the mixed water fracture technology based on functional slick water is described and successfully applied to several wells in oilfield. The core of the technology is determination of influence factors of imbibition oil recovery, the development of new functional slick water system and optimization of volume fracturing parameters. The imbibition results show that it is significant effect of interfacial tension, wetting on imbibition oil recovery. The interfacial tension decreases by an order of magnitude, the imbibition oil recovery reduces by more than 10%. The imbibition oil recovery increases with the contact angle decreasing. The emulsifying ability has no obvious effect on imbibition oil recovery. The functional slick water system considering imbibition is developed based on the solution rheology and polymer chemistry. The system has introduced the active group and temperature resistant group into the polymer molecules. The molecular weight is controlled in 1.5 million. The viscosity is greater than 2mPa·s after shearing 2h under 170s-1 and 100℃. The interfacial tension could decrease to 10-2mN/m. The contact angle decreased from 58° to 22° and the core damage rate is less than 12%. The imbibition oil recovery could reach to 43%. The fracturing process includes slick water stage and linear gel stage. 10% 100 mesh ceramists and 8% temporary plugging agents are carried into the formation by functional slick water. 40-70 mesh ceramists are carried by linear gel. The liquid volume ratio is about 4:1 and the displacement is controlled at 10-12m3/min. The sand content and fracturing fluid volumes of single stage are 80m3 and 2500 m3 respectively. Compared with conventional fracturing, due to imbibition oil recovery, there is only 25% of the fracturing fluid flowback rate when the crude oil flew out. When the oil well is in normal production, about 50% of the fracturing fluid is not returned. It is useful to maintain the formation energy and slow down the production decline. The average cumulative production of vertical wells is greater than 2800t, and the effective period is more than 2 years. This technology overcoming the problem of high horizontal stress difference and lack of natural fracture has been successfully applied in Jidong Oilfield ultralow permeability reservoir. The successful application of this technology not only helps to promote the effective use of ultralow permeability reservoirs, but also helps to further clarify the role of imbibition recovery, energy storage and oil-water replacement mechanism.

Research and Practice of CO2 Flooding Technology in Ultra-low Permeability Reservoir of XX Oilfield

CO2 flooding technology in ultra-low permeability reservoir is the key technology in modern oilfield technology development, which plays a key role in oilfield exploitation of ultra-low permeability reservoir. In this paper, CO2 flooding technology for ultra-low permeability reservoir in oilfield is analyzed and studied. The main characteristics of ultra-low permeability reservoir in oilfield are briefly analyzed, and the development of CO2 flooding technology is discussed. At the same time, the application of CO2 flooding technology in ultra-low permeability reservoir of XX Oilfield is studied and summarized.

Analysis and Quantitative Evaluation of Temperature Influence Mechanism of Multi-Cycle Water Huff-N-Puff in Ultra-Low Permeability Reservoirs

Analysis and Quantitative Evaluation of Temperature Influence Mechanism of Multi-Cycle Water Huff-N-Puff in Ultra-Low Permeability Reservoirs

On the Development of an Effective Pressure Driving System for Ultra-Low Permeability Reservoirs

On the Development of an Effective Pressure Driving System for Ultra-Low Permeability Reservoirs

Discussion on Fine Characterization Technology of Single Sand Body in Fluvial Reservoir

Fuyu oil layer of an oilfield is an ultra-low permeability oil layer. The remarkable characteristics of this oil layer are poor physical properties, low single well production, 2-4 layers of sand body longitudinally, large change frequency of plane upper layer and continuous characteristics. In order to realize effective oil layer development and ensure effective reservoir utilization. According to the poor physical properties of this oilfield, this paper analyzes the microscopic and macroscopic responsiveness of the reservoir, ensures effective permeability and thickness of permeable sand body, quantificationally depicts the plane distribution of permeable sand body, and effectively determines the distribution range of the reservoir according to the description of single sand body, thus providing a good technical means for subsequent scale production.

Effect and cognition of hole filling measures in ultra-low permeability reservoir block

In this paper, based on the application of water-flooded layer interpretation plate, multi-well comparison, and the combination of dynamic and static analysis methods to optimize the well selection of fill hole wells, and through the analysis of the production of fill hole wells in **block in recent years, we summarize the relationship between fill hole effect and fill hole well injection and extraction well spacing, connection relationship, water content before the measure, and the relationship between fill opening thickness of the patch opening, so as to provide reference for the next step to continue to carry out fill hole work and find fill hole potential.

Theoretical comparison between graphene-on-silver and gold-on-silver based surface plasmon resonance sensor

In this paper, we present a detailed theoretical analysis of a Kretschmann configuration based graphene-on-silver thin film. The advantage of silver is the better evanescent field enhancement and it possesses a narrow full width at half maximum of the plasmon resonance, hence silver providing higher resolution. On the other hand, the ultra-low permeability of graphene and the extremely low thickness make it a very good candidate to protect silver against oxidation without loss of resolution. The obtained results from numerical calculations clearly exhibit sharp reflectivity curve for graphene as protective layer in comparison with gold. These results suggest that graphene-on-silver based SPR biosensors have potential for use in high-precision measurements.

Stress sensitivity evaluation of ultra-low permeability sandstone reservoir and its influence on oilfield development

Luohe ultra-low permeability sandstone reservoir is a hot block in Yanchang oilfield, which is a potential point for increasing production and reservoir. In view of the current situation that there is no unified stress sensitivity evaluation standard for ultra-low permeability sandstone in the study area, taking the ultra-low permeability sandstone in Luohe district as the research object, the stress sensitivity evaluation of ultra-low permeability sandstone is carried out by using experimental analysis as the main means. The results show that it is more accurate to evaluate porosity by using pore stress sensitivity coefficient instead of pore compressibility coefficient. With the increase of net overburden pressure, the porosity stress sensitivity decreases gradually; the permeability stress sensitivity is evaluated by variable confining pressure. With the increase of confining pressure, the permeability damage decreases. With the decrease of confining pressure, the permeability damage increases, but it can not recover to the original value, so the permeability damage is irreversible; in the low bottom hole pressure stage, stress sensitivity has a greater impact on oil well productivity, while in the high bottom hole pressure stage, stress sensitivity has a smaller impact on oil well productivity; advanced water injection can reduce the adverse effect of stress sensitivity on the development of ultra-low permeability sandstone and maximize the economic benefits. The research results clarify the method of stress sensitivity evaluation, and provide guidance for efficient water injection in the next step.

An Emerging CO2 Storage Option: CO2 Storage and By-Product Oil Recovery from Shale Oil Formations

Recent studies, sponsored by the United States Energy Association (USEA) and prepared by Advanced Resources International, have identified an emerging CO2 storage option – injecting and storing CO2 in shale oil formations. Because this CO2 storage option also provides by-product oil recovery, it offers a source of revenues and thus a potential “business case” for pursuing CO2 capture and storage. Shale oil (also called tight oil) has become the dominant source of U.S. oil production, increasing from less than 1 million barrels per day (MMB/D) a decade ago to about 8 MMB/D at the end of 2019. In addition, large shale oil basins and resources exist in many parts of the world, including Argentina, Australia, Canada, China, North and South Africa, and other settings, as evaluated by our company for the U.S. Energy Information Administration. (World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States, April 2011) Because of ultra-low permeability, oil recovery efficiency from shale oil formations using primary (pressure depletion) practices is low, on the order of 5 to 10% of the original oil in-place (OOIP). This leaves behind a large resource target for application of more advanced shale oil recovery technologies. Use of advanced recovery practices involving the injection of CO2 into shale oil formations (Shale EOR) provides one such technology and thus an additional pathway for economically viable storage of CO2. While a series of CO2 injection pilots are underway in U.S. shale oil basins, industry has held the results of these pilots “close to the vest.” To overcome this lack of information, the USEA sponsored a series of studies, involving the use of geologic characterization and reservoir engineering, to examine the potential of Shale EOR in four major U.S. shale oil basins - the Bakken Shale, the Eagle Ford Shale, and the Wolfcamp Shale in the Midland and Delaware Basins of the Permian Basin. This work showed that cyclic injection of CO2 can lead to additional recovery of oil from these shale oil formations while trapping and storing significant volumes of CO2. The study involved building a geologic model for a representative portion of each shale oil formation, history matching reservoir properties with well completion practices against actual well performance, and using a compositional reservoir simulator (GEM) to calculate oil recovery and CO2 storage from Shale EOR. Shale reservoir characterization and modeling for a “type area” of the Wolfcamp Shale in the Midland Basin illustrates the methodology and results of the Shale EOR study. With 12 cycles of CO2 injection, soak and production (over 8.5 years), Shale EOR stored 102,000 metric tons of CO2 in a representative 180-acre well pattern of the Wolfcamp Shale, while producing 223,000 barrels of shale oil. The 223,000 barrels of incremental oil, in addition to 355,000 barrels from primary oil recovery, provided an uplift of 63% in oil recovery. Using an additional CO2 injection cycle followed by well shut-in, Shale EOR stored 155,000 metric tons of CO2 in this 180-acre Midland Basin’s Wolfcamp Shale “type pattern”, equal to 0.7 metric tons of CO2 stored for every barrel of incremental shale oil produced. Similar evaluations of cyclic CO2 injection in other shale basins also showed that with Shale EOR more CO2 could be stored than the CO2 content in the oil produced. The Bakken Shale had a CO2 stored to oil produced ratio of 0.62 metric tons per barrel, the Eagle Ford Shale had a ratio of 0.39, and the Wolfcamp Shale of the Delaware Basin had a ratio of 0.69. As such, given that the combusted portion of a barrel of oil creates 0.38 metric tons of CO2, Shale EOR enables more CO2 to be stored than the CO2 content in a barrel of oil when combusted. (Approximately 12% of an oil barrel is used for petrochemicals and other non-combustion sources.) The study noted that special geologic conditions and development practices are important for optimum application of the cyclic CO2 injection and storage process. Future pathways being evaluated for this emerging CO2 storage option and a potential new “business case” for CCUS include: (1) examining continuous, rather than cyclic, injection of CO2 and (2) using conformance and other CO2 storage enhancing technologies. These technologies would appreciably increase the volumes of CO2 able to be stored as well as improve the oil recovery efficiency of Shale EOR.

Reservoir characteristics of Chang 2 Member of Yanchang Formation in Area A, Ordos Basin

With the continuous enhancement of exploration and development in the Ordos Basin, in-depth research has been carried out on the petrological and reservoir characteristics of Chang 2 reservoir in Area A, which provides a geological basis for the efficient development of oil reservoirs. Comprehensive use of reservoir sandstone thin section identification casting analysis, mercury intrusion analysis, logging analysis and other methods to systematically study the petrological characteristics, pore characteristics and reservoir physical properties of Chang 2 reservoir in Area A. The results show that the reservoirs in the study area are dominated by fine-grained sandstones, with low component maturity and high structural maturity. They are all medium-low porosity, low-permeability and ultra-low permeability reservoirs. Primary intergranular pores and residual intergranular pores are developed, the reservoir drainage pressure is low, which is good-medium, and the mercury removal efficiency is high, indicating that the reservoir has good storage performance and seepage properties.

Effects of cyclic cryogenic treatment on rock physical and mechanical properties of Eagle Ford shale samples - An experimental study

Unconventional reservoirs have ultra-low permeability, and the combination of horizontal wells and multistage transverse hydraulic fracturing have unlocked economical hydrocarbon production from such formations. Fracturing fluid involves large volumes of water that is partially recovered during the flowback stage, and the rest remains in the formation damaging the matrix-fracture interface. In this paper, an alternative waterless stimulation technique has been studied. A cryogenic fluid, liquid nitrogen (LN2), was injected into cores from Eagle Ford formation. The effects of cyclic LN2 on rock physical and dynamic elastic properties were examined. Two cycles of injecting LN2 into cores at the reservoir temperature and pressurizing at different pressures (1.38, 2.76, 4.14 MPa) were implemented. Before and after each cycle, computerized tomography scan, porosity, permeability, and ultrasonic velocities tests were conducted on the samples. The results demonstrated that cyclic cryogenic treatment increases porosity and permeability and decreases ultrasonic velocities of the samples resulting from inducing new cracks and extending and widening the existing ones. Permeability enhancement, which results from the creation of high-permeable pathways allowing the gas to move rapidly across the tested samples, was noticeable (up to 2 orders of magnitude). Whilst, comparing to permeability, the porosity improvement was not significant (up to 26%) owing to the small increase in pore volume compared to the sample's bulk volume. The results showed a direct relationship between injection pressure and porosity and permeability enhancements after the completion of each cycle. Moreover, the reductions in ultrasonic velocities and dynamic elastic properties were insignificant compared to the changes in the porosity and permeability. Additionally, porosity and permeability enhancements and velocities reduction mainly occurred after completing the first cycle. This is because of lowering the thermal stress on samples, which results in reducing the possibility of creating new cracks or propagating and widening the current ones.

Investigation of Large-Diameter Borehole for Enhancing Permeability and Gas Extraction in Soft Coal Seam

The efficiency of gas extraction from the soft coal seam with ultralow permeability is low. Gas extraction with large-diameter borehole is proposed to deplete gas content for preventing gas outburst disaster in this study. The fractures around the large borehole will enhance the permeability in the damage area to promote gas extraction. We established a damage-stress-seepage coupling model for large-diameter borehole gas extraction in soft coal seam. This mathematical model contains governing equations of gases sorption and transport, coal deformation, and damage, reflecting the coupling responses between gas and coal seam. The model is solved by the finite element method to simulate the gas drainage large-diameter borehole through roadway. Distributions of elastic modulus, damage area, and maximum principal stress in soft coal seam with different borehole diameters including 94 mm, 133 mm, 200 mm, and 300 mm are analyzed. The gas pressure, gas content, and effective extraction area in soft coal seam are discussed. Results show that the shear failure zone appears around the large-diameter borehole, and its permeability rises sharply. This opens up the gas transport channel and is conducive to the rapid extraction. It is confirmed that gas extraction using large-diameter borehole (300 mm) can greatly improve the efficiency of the gas preextraction in soft coal seam by increasing gas extraction rate. These provide a foundation for guiding the operation of gas extraction with large borehole from the soft coal seam in the field.

Dynamic Response and Fractal Characteristics of a Pore-Fracture System in Ultralow Permeability Sandstone Based on Low-Field NMR

Quantitative characterization of the dynamic evolution of the pore-fracture system subjected to the variation of in situ stress is critical to the production of crude oil and CO2 sequestration in a...

Pressure performance of multi-stage fractured horizontal well considering stress sensitivity and dual permeability in fractured gas reservoirs

Generally, the horizontal well multi-stage fracturing technology is widely used to enhance single well production of tight or ultra-low permeability gas reservoirs. The deformation of the rock pore structure makes the permeability of the reservoir impossible to be a constant. The pressure transient analysis mathematical model of the multi-stage fractured horizontal wells (MFHWs) with consideration of stress sensitivity, varying conductivity and dual-porosity dual-permeability (DPDP) is established. Laplace transformation, pressure drop superposition and Gaussian elimination are utilized to obtain the wellbore pressure solution of MFHWs. Comparison result reaches a good agreement by model validation. On the basis of pseudo-pressure derivative curve characteristics, eight flow regimes can be diagnosed, namely, well storage, skin affection, bi-linear flow, early linear flow, early radial flow, middle linear flow, inter-porosity flow and later radial flow regime. The influences of some vital parameters on pseudo-pressure and its derivative curve is discussed in detail. The proposed model can help engineers understand the pressure dynamic characteristic of the MFHWs with stress sensitivity, varying conductivity and DPDP performance.

New Treatment for Improving the Productivity of Shale Reservoirs Using Thermochemical Fluids

AbstractShale reservoirs are characterized with very low productivity due to the high capillary pressure and the ultra-low rock permeability. This article presents an effective treatment to improve the hydrocarbon productivity for shale reservoirs by injecting thermochemical fluids. In this study, several measurements were carried out to determine the effectiveness of the presented treatment. Coreflood, rate transient analysis (RTA), and nuclear magnetic resonance (NMR) measurements were performed. The gas productivity was estimated, before and after the treatment, utilizing the gas flowrates and the pressure drop across the treated rocks. The improvement in gas productivity due to thermochemical fluids was estimated by calculating the productivity index (PI) and the absolute open flow (AOF) before and after the chemical injection. Also, the changes in the pore size distribution, due to chemical injection, were studied using NMR measurements. Results showed that thermochemical treatment can improve the gas productivity by 44%, increase the AOF by 450%, and reduce the capillary pressure by 47%. Also, NMR measurements showed that fractures were induced in the shale rocks after the treatment, which will improve the shale productivity. Ultimately, this study introduces, for the first time, the use of thermochemical fluids to improve the hydrocarbon productivity for shale reservoirs.

Pore structure and fluid saturation of near-oil source low-permeability turbidite sandstone of the Dongying Sag in the Bohai Bay Basin, east China

Pore structure and oil saturation are important parameters in petroleum exploration and development, and the relationship between them is closer in near-oil source reservoirs (with a short-distance migration of oil and gas) than in other reservoirs. Using epoxy-impregnated thin sections, scanning electron microscopy (SEM), conventional core analyses of porosity and permeability, mercury intrusion porosimetry, and well logging, the characteristics of the petrology, physical properties, and pore structure we studied, as well as the relationship between pore structure and fluid saturation (maximum mercury intrusion saturation Smax and oil saturation So) of the near-oil source low-permeability turbidite sandstone reservoirs in the Es3 member of the Dongying Sag, Bohai Bay Basin, eastern China. The results indicate that the turbidite sandstone reservoirs are mainly composed of fine sandstone and siltstone, with porosities and permeabilities ranging from 5 to 25% and 0.1 to 40 mD, respectively, and can be classified as medium to low porosity and extra-low to ultra-low permeability reservoirs. The pore types of high-quality reservoirs are mainly dominated by intergranular pores, both primary and dissolved, probably due to the influence of the acidic fluids generated from the cracking of organic matter. The pore structure is complex with respect to the pore-throat size distribution, and the overall pore-throat radius is small. Several pore structure parameters together affect the fluid saturation distribution, among which the medium pore-throat radius (R50) and medium saturation pressure (P50) are especially correlated with oil saturation. Different pore structure types have different characteristics of fluid saturation distribution, and the oil-bearing grade and oil saturation of the reservoirs above the oil-water contact are typically positively correlated with pore structure. This study will help to classify the pore structure in the wellbore profile and improve the accuracy of the oil saturation calculation by well logging.

Production Performance Analysis of Su-6 Well Block in Sulige Gas Field, Ordos Basin, China

As a tight sandstone gas reservoir of Su-6 well block in Sulige gas field, part of the gas wells have a poorer production performance with a faster decline rate of formation pressure and production, which is inconsistent with static reservoir parameters of gas well. In order to improve the productivity of gas well, on the base of static and dynamic classification of 232 gas wells, the reasons of inconformity between production performance and static reservoir parameters are analyzed from the four aspects of sand body connectivity, reservoir heterogeneity, gas saturation and fluid accumulation in wellbore. The results show that the main reasons causing the inconformity are the strong heterogeneity and poor sand body connectivity, and the secondary reasons are fluid accumulation in wellbore and low gas saturation. The results of this paper is of great significance to improve the productivity of gas wells and guide the development of ultra-low permeability gas reservoir.