High Fracture Density Research Articles

A new type of high density fracturing fluid system

Deep oil and gas reservoirs enjoy great opulence in resources, and yet the ultra-deep burial, high temperature, high pressure and natural fractures in reservoirs bring great difficulties to propped fracturing. Given the challenge of propped fracturing in deep reservoirs, a new cost saving and high density fracturing fluid system was developed, of which the performance was evaluated and formulation was optimized, and with this fracturing fluid system, wells that cannot be treated before, can now be effectively stimulated. The advanced fracturing fluid system has been applied to eight layers in five wells, which all proved to be successful and effective, with remarkable reservoir stimulation efficacy. The post-production in the formation testing grew twice of the pre-fracturing production, and two wells were seen gas production exceeding one million cubic meters. The advanced fracturing fluid system greatly expands the well depth range of exploration and development, and lays a solid technical foundation for implementation of propped fracturing in deep reservoirs.

Integration of sonic and resistivity conventional logs for identification of fracture parameters in the carbonate reservoirs (A case study, Carbonate Asmari Formation, Zagros Basin, SW Iran)

Between all available methods for fractures evaluation, core and image logs are prime methods. However, both of these data are very expensive and practically available in less than 10% of drilled wells. Therefore, a less expensive and quick method would be very helpful for fracture analysis. The main aim of this study is to identify the fracture parameters in the carbonate reservoirs, i.e. fracture density and fracture aperture, using a combination of sonic, resistivity and other conventional logs which commonly available in all wells. For this aim, fractures and their parameters were precisely determined on the image logs in 6 wells from carbonate Asmari Formation as a typical fractured reservoir in the world. Then, the effect of fracture parameters on the sonic and resistivity logs were used to identify fracture parameters in the wells with no core and image logs. Results indicate that the zones with high fracture density and aperture are easily detectable by conventional logs. On the other hand, the behavior of these logs is a function of fracture parameters in the fractured intervals, especially in the zones with high fracture aperture. Separation between shallow (RLA2) and deep (RLA5) resistivity logs along with their cross plot are very reliable indicators for fractured intervals. Sonic transit time rapidly increases where fractures density or aperture increased as well. In this study, a combination of sonic and neutron porosity logs have also been used to determine the effect of fracture aperture, as the main effective factor on the reservoir properties. Furthermore, full waveforms in some studied wells combined with sonic transit time to improve the results of fractures evaluation. Using sonic and resistivity along with neutron porosity and gamma ray logs provides a reliable method for describing the fracture parameters which shows high correlation with image logs results. Due to the high impact of fracture aperture on both reservoir quality and conventional log responses, determination of this parameter is more crucial than other fracture parameters. This method is also applicable for OBM (Oil Base Mud) image tools which conventionally cannot detect fracture aperture.

Identification of Fracture Density and Orientation at “R” Geothermal Field Using Shear Wave Splitting Microearthquake Method

Identification of permeable zones is an important aspect in the development and monitoring of geothermal fields. Permeable zones are generally associated with subsurface stress conditions and the presence of structures such as fractures in the reservoir. One of the geophysical methods to detect the presence of permeable zones is the shear wave splitting (SWS) microearthquake method. The microearthquake data recorded by seismograms in the period of January - April 2018. The SWS phenomenon occurs when an S wave propagates through an anisotropy medium. The rotation correlation technique is used to determine the SWS parameters; direction of polarization (ϕ) and delay time (dt) of the S wave. The results of this study indicate that the center area of study has high fracture density. The fractures maybe associated with the presence of wells where the largest steam is produced in the field and the appearance of more complex geological structures. The dominant direction of fracture orientation is relatively following the trend of local structures NW-SE and NE-SW.

Assessment of sedimentary environment from PAHs and aliphatic biomarkers: The case study of Fahdene black shales in northern Tunisia

Polyaromatic and aliphatic biomarkers extracted from the Albian-Early Cenomanian source rock (Fahdene Formation) were evaluated by Gas Chromatography – Mass Spectrometry (GC-MS) and are compared to classic geochemical and mineralogical data. In addition to terpanes, hopanes, diasteranes and steranes, thirty-seven aromatic hydrocarbon elements were identified.The absence of the angiosperm-derived biomarker (18α(H)-Oleanane) and the low abundance of trimethylnaphthalenes isomers (1,2,5-TMN and 1,2,7-TMN) suggest that no land-plant traces were founded in the studied organic matter. In this fact, the Kerogene III identified by Rock Eval is not detected, and we can attribute these indices to the algae effect. The check of maturity by Tmax and the biomarker parameters (C27 18α-22,29,30-trinorneohopane (Ts), sterane isomerization ratio ββ/(αα+ββ), C32 22S/(22S + 22R) HH and the methylphenanthrene index (MPI-1)) indicates that these organic rich sediments are reached the early stage of oil window. In the other hand, aliphatic and polyaromatic biomarkers confirm classic data of X-ray diffraction and calcimetry and suggest a carbonate facies with some clayey intercalations in the middle part. The fracturing analysis of these carbonates prove the presence of high density of fractures with various sizes and directions. About filling, the fibrous calcite is the major element of mineralized veins, with the presence of bitumen traces. These fractures have two main orgin; the fluid pressure and the regional tectonic stress regime.Therefore, the Albian-Early Cenomanian fractured carbonates in the Mahjouba area were deposited, under suboxic conditions, in a marine environment affected by the eutrophication phenomenon, and have just reached the equilibrium stage of maturity.

Geophysical interpretation of the geology of the Stanthorpe region using aeromagnetic, gravity and radiometric data

This study provides an in-depth geophysical interpretation of the geology of the Stanthorpe region with a particular focus on hydrothermal alteration. The radiometric ternary map outlines the main lithological divisions in the region, which together with Th and Th/K ratio maps distinguish various granitic suites. Magnetic signatures delineate a non-magnetic zone within the low to moderately magnetic Stanthorpe Granite. The non-magnetic section, a north–northwest trending belt with a high density of faults and dykes is interpreted as a hydrothermal alteration zone that is narrower in the north, widens southwards, and is much more extensive at depth than is suggested by the surface alteration pattern. Modelling of gravity data reveals a massive low-density granitic intrusion extending to a depth of 10–15 km. The low-gravity section with modelled rock densities between 2450 and 2670 kg/m3 coincides with the low magnetic susceptibility zone in the magnetic model. High K, low Th and low Th/K also characterise the sections of the Stanthorpe Granite that have both low gravity and magnetic responses. Also conspicuous in the geophysical signatures within the non-magnetic zone is an ovoid feature of low magnetic response, high uranium content with moderate density, interpreted as a granitic pluton. Thus, decrease in magnetisation, changes in K concentration and corresponding low gravity with an associated high density of faults, fractures and dykes in the Stanthorpe region are the main features of the quartz–sericite–chlorite–muscovite hydrothermal alteration.

Chemical and Noble Gas Isotope Compositions of Formation Gases from a 3 km Deep Scientific Borehole in the Koyna Seismogenic Zone, Western India

A 3 km deep research borehole KFD1 was drilled in the Koyna reservoir-triggered seismicity region, Western India, between December 2016 and May 2017. The 1967M6.3 Koyna earthquake had generated a NNE-SSW trending surface fissure zone in the Nanel-Donichawadi-Kadoli sector. KFD1 is located ~5 km south of Kadoli along the trend of the Donichawadi fault zone. Online gas monitoring was carried out during drilling of KFD1 from 1315 m to 2831 m depth to sample and study the composition of crustal gases. Formation gases CO2, CH4, H2, and He were only observed during water flushing of ~100 m intervals following coring runs. Laboratory analyses of gas samples collected between 1737 m and 2831 m depth revealed concentrations of up to 1200 ppmv CO2, 186 ppmv CH4, 139 ppmv H2, and 12.8 ppmv He. Zones enriched in gases are mostly below the 2100 m depth with significant He enhancement ranging from 4.6 to 7.6 ppmv above the atmospheric value. The He-rich zones correlate well with the zones of anomalous physical and mechanical properties identified from geophysical logs and are characterized by high fracture density as revealed from borehole images, indicating that the borehole punctured multiple fracture zones. The helium concentrations are consistent with those previously observed over the surface fissures near Kadoli, suggesting a southward extension of the Donichawadi fault zone up to the KFD1 site and confirming that the fault zone is permeable even after 50 years of the 1967 Koyna earthquake.3He/4He ratios of eleven gas samples fall between0.426±0.022and0.912±0.059 Ra, with4He/20Ne values between0.3449±0.0091and0.751±0.020. Air-corrected helium isotope ratios indicate that helium is a mixture of atmospheric and crustal radiogenic components but no mantle contribution within 2σanalytical uncertainties.

Bayesian artificial intelligence for geologic prediction: Fracture case study, Horn River Basin

AbstractA Bayesian Belief Network (BN) has been developed to predict fractures in the subsurface during the early stages of oil and gas exploration. The probability of fractures provides a first-order proxy for spatial variations in fracture intensity at a regional scale. Nodes in the BN, representing geologic variables, were linked in a directed acyclic graph to capture key parameters influencing fracture generation over geologic time. The states of the nodes were defined by expert judgment and conditioned by available datasets. Using regional maps with public data from the Horn River Basin in British Columbia, Canada, predictions for spatial variations in the probability of fractures were generated for the Devonian Muskwa shale. The resulting BN analysis was linked to map-based predictions via a geographic information system. The automated process captures human reasoning and improves this through conditional probability calculations for a complex array of geologic influences. A comparison between inferred high fracture intensities and the locations of wells with high production rates suggests a close correspondence. While several factors could account for variations in production rates from the Muskwa shale, higher fracture densities are a likely influence. The process of constructing and cross-validating the BN supports a consistent approach to predict fracture intensities early in exploration and to prioritize data needed to improve the prediction. As such, BNs provide a mechanism to support alignment within exploration groups. As exploration proceeds, the BN can be used to rapidly update predictions. While the BN does not currently represent time-dependent processes and cannot be applied without adjustment to other regions, it offers a fast and flexible approach for fracture prediction in situations characterized by sparse data.

Tectonic Controls on Taupo Volcanic Zone Geothermal Expression: Insights From Te Mihi, Wairakei Geothermal Field

AbstractInformation on structure, stress, and their interrelationship is essential for understanding structurally controlled geothermal permeability. Active fault mapping, borehole image analysis, and well testing in the Te Mihi geothermal area, New Zealand, allows us to refine structural and fluid flow architecture of this resource. The Te Mihi area is structurally complex, comprising a set of NW dipping master faults containing pervasive SE dipping antithetic and splay structures in their hanging walls. These faults are also intersected by E‐W striking faults. A localized, N‐S striking structural trend is also observed at Te Mihi. In consideration with Global Navigation Satellite System velocity vectors, both active NE‐SW and E‐W striking faults create biaxial extension at Te Mihi, though the observed NE‐SW SHmax direction suggests that contemporary extension is NW‐SE dominated. Stress field perturbations coincide with structural complexities like fault splays and intersections and/or proximity to recently active E‐W and NE‐SW striking structures. Borehole fluid flow at Te Mihi is concentrated at NW dipping master fault intersections, travel time fractures on acoustic image logs, halo fractures on resistivity image logs, NE‐SW and E‐W striking fractures, intervals of high fracture density, and spatial concentrations of wide aperture fractures and recently active NE‐SW and E‐W striking fractures. This study suggests Te Mihi geothermal expression results from biaxial extension evident from active structural trend intersections and the predominance of NE‐SW and E‐W striking structures within permeable well zones. Biaxial extension is therefore an important control on crustal fluid flow within the Taupo Volcanic Zone and thus geothermal resource delineation.

Multiattribute framework analysis for the identification of carbonate mounds in the Brazilian presalt zone

Carbonate mounds, as described herein, often present seismic characteristics such as low amplitude and a high density of faults and fractures, which can easily be oversampled and blur other rock features in simple geobody extraction processes. We have developed a workflow for combining geometric attributes and hybrid spectral decomposition (HSD) to efficiently identify good-quality reservoirs in carbonate mounds within the complex environment of the Brazilian presalt zone. To better identify these reservoirs within the seismic volume of carbonate mounds, we divide our methodology into four stages: seismic data acquisition and processing overview, preconditioning of seismic data using structural-oriented filtering and imaging enhancement, calculation of seismic attributes, and classification of seismic facies. Although coherence and curvature attributes are often used to identify high-density fault and fracture zones, representing one of the most important features of carbonate mounds, HSD is necessary to discriminate low-amplitude carbonate mounds (good reservoir quality) from low-amplitude clay zones (nonreservoir). Finally, we use a multiattribute facies classification to generate a geologically significant outcome and to guide a final geobody extraction that is calibrated by well data and that can be used as a spatial indicator of the distribution of good reservoir quality for static modeling.

3D Vp, Vs, And Vp/Vs microseismic tomography imaging on “MA” geothermal field: fluid saturation condition analysis

“MA” Geothermal Field lies on andesitic volcano highlands in the center of Sunda Arc and has transition system between vapor and liquid-dominated reservoir. The reservoir characteristic has been changed over time caused by fluid injection and production. The objective of the research is to interpret the fluid saturation characteristic inside the reservoir based on 3D Vp, Vs, and Vp/Vs tomography image. Microearthquake (MEQ) activities in geothermal field may relate to changes in stress and fractures’ occurrence. P-and S-wave arrival time from 878 MEQ events are used for tomographic inversion. The initial hypocenters were determined using Geiger Adaptive Damping. To ensure the quality of data, we selected the events according to azimuthal gap and the recorded stations. The P-and S-wave arrival times are updated using waveform cross correlation. VELEST is used to determine 1D seismic velocities as input for SIMULPS12 tomographic inversion with hypocenter adjustment. The results show there are two clusters of MEQ events around “GB” and “WD” mountain. The distribution of MEQ around the fluid injection and production area may be related to weak zones and also reveal a structure with N-NE orientation. According to seismic velocity model, the production area is dominated with vapor (low Vp/Vs ratio value). Vapor dominated area has experienced pressure decrease caused by ongoing exploitation. There is also area that has high fracture density compared to other areas. Area that has high water saturation is situated around GB mountain (high Vp/Vs ratio value).

Coal seam porosity and fracture heterogeneity of marcolithotypes in the Fanzhuang Block, southern Qinshui Basin, China

Marcolithotypes play a significant role in coal seam porosity and fracture heterogeneity. The optical microscopy, CO2 and N2 adsorption, and X-ray CT were used for reconstructing the pore-fracture system of different marcolithotypes. The results show that the microfractures are well developed in the bright coal with the characteristics of high fracture density, good connectivity, and strongest fractal dimension, corresponding to the highest permeability and the best seepage capacity. The adsorption capacity measured with N2, CO2, and CH4 indicates that the specific surface area of micropores (<2 nm) increases linearly from the bright to the dull coal, and the bright coal is favorable for gas adsorption by providing more available surface sites. The difference in physical properties of various macrolithotypes causes different logging response characteristics, including the compensated density (DEN), the gamma ray (GR), and the acoustic time (AC). Based on that, the index (N = AC/(GR × DEN)) was put forward to forecast the distribution of coal macrolithotypes quantitatively (N ≤ 1.3, Partings; 1.3 < N ≤ 3, Dull coal; 3 < N ≤ 5, Semi-dull coal; 5 < N ≤ 8, Semi-bright coal; N > 8, Bright coal), and its accuracy was verified by coal cores from different areas, i.e. Fanzhuang, Zhengzhuang, and Hancheng blocks.

Recycling of Paleoplacer Gold through Mechanical and Postdepositional Mobilization in the Neoarchean Black Reef Formation, South Africa

The source of gold in the ca. 2.66 Ga Black Reef Formation (BRF) has been investigated and constrained through petrographic, mineralogical, geochemical, and high-resolution three-dimensional reflection seismic data combined with drill core and underground geological mapping. The BRF is a strong seismic marker and consists of carbonaceous shale, quartz arenite, and conglomerate. Gold grade in the BRF is primarily controlled by the nature of the host conglomerates. Most of the gold in the BRF conglomerate occurs in native form, and its morphology is highly heterogeneous. Gold was initially introduced through mechanical recycling of underlying Witwatersrand reefs, followed by short-range (millimeter- to centimeter-scale) postdepositional alteration/remobilization associated with the Bushveld Complex and the Vredefort meteorite impact. Although the BRF was subjected to high postdepositional fluid circulation facilitated by high fracture density, the volume of dissolved gold was probably too small to form a large gold deposit, except in areas around the Black Reef/Witwatersrand reefs subcrop positions. Findings from this study demonstrate the importance of both sedimentological controls and impact-related structures in the formation of paleoplacer gold deposits during Neoarchean times.

Structural mapping of rock walls imaged with a LiDAR mounted on an unmanned aircraft system

Structural mapping of rock walls to determine fracture orientation provides critical geological information in support of mining operations. A helicopter-style UAS (rotor diameter 2 m; take-off mass 35 kg; payload mass 11 kg) instrumented with a high-resolution LiDAR imaged a 75 m long and 10–15 m high series of four adjacent rock walls at the Canadian Wollastonite mine. A point cloud with a density of 484 point/m2 acquired at an angle of incidence of ∼41.7° from a flight altitude of 41.7 m above ground level was selected for structural mapping. The point cloud was first meshed using the Poisson surface reconstruction method and then remeshed to achieve an even element size distribution. Visualization of the remeshed Poisson mesh using a 360° hue–saturation–lightness colour wheel highlighted areas of higher fracture density, whereas visualization using a 180° colour wheel accentuated sliver-like geological features. Two joint sets were identified at 156/82 and 241/86 (strike/dip in degrees). A total of 18 virtual strike measurements and 13 virtual dip measurements were within 10% of manual compass measurements. This study demonstrated that the task of structural mapping of large rock walls can be automated by processing 3D images acquired with a LiDAR mounted on a UAS.

A novel radial jet drilling stimulation technique for enhancing heat recovery from fractured geothermal reservoirs

In this study, the application of the Radial Jet Drilling (RJD), a novel stimulating technique for enhancing productivity in the existing wells in deformable naturally fractured reservoirs was investigated using a robust three-dimensional finite element DFM (discrete fracture-matrix) model. Results showed that the RJD laterals were more effective in enhancing injectivity/productivity in cases with lower fracture density, i.e. lower equivalent permeability, while they had no significant effect on the heat production in these cases. In higher fracture density cases, the RJD laterals improved the heat production while had no significant effect on the injectivity/productivity. Results also showed that in reservoirs with very low permeability matrix, the RJD laterals can be used to connect the wells to the fracture network and hence enhance the well performance. The sensitivity analysis on the average net energy production rate with respect to the length of the RJD laterals showed that in the situations where the wells were not connected directly to the fractures, the length of RJD laterals played a crucial role in enhancing the average net energy rate. However, the 100 m laterals almost removed the dependency of the average net energy production rate on the well placement for low, medium and high fracture density cases.

Study on clues for gold prospecting in the Maizijing-Shulonggou area, Ningxia Hui autonomous region, China, using ALI, ASTER and WorldView-2 imagery

Several gold occurrences have been discovered in the Maizijing-Shulonggou area, Ningxia Hui autonomous region, China. The clues for gold prospecting in this area may relate to fractures and hydrothermal alterations, but it is very difficult to conduct a field investigation on the clues because of the alpine valleys. To explore the clues for gold prospecting, this study extracted geological information related to fracture zones and hydrothermal alterations using ALI, ASTER, and WorldView-2 data, and subsequently, explored potential relationships between extracted geological characteristics and gold mineralization. The WorldView-2 image in the study area was used for fracture interpretation. The azimuth and density of linear fractures were investigated, and the results showed that linear fractures with NE direction were most common, the gold occurrences except Au2 were all located in NE fracture zones, and all gold occurrences located in high fracture density areas (level 1–3 areas) where NE and NW fractures were the most common observations. This suggests that the high fracture density and NE and NW fractures are important indicators of potential gold mineralization. Hydrothermal alteration minerals in exposed bedrocks were mapped using ALI and ASTER imagery. Based on the spectral analysis, ASTER image in the study area were used to map quartz, illite and chlorite, while ALI image were used to map limonite. The spatial distribution of alteration minerals revealed regional specific alteration mineral in gold mineralization. In the northern Maizijing region, the composition of alteration mineral in the surrounding of Au1 was overwhelmed by quartz, while the alteration minerals in the surrounding of Au2-4 were dominated by limonite in the southern Shulonggou region, which showed different indicator minerals for gold prospecting in the two regions. As a result, the key areas for prospecting are the NE and NW fracture zones with quartz veins in the Maizijing region and the NE and NW fracture zones with limonitization in the Shulonggou region. In the Shulonggou region, pyrite in buried quartz-pyritization belt and illitization-pyritization belt is likely to be the precursor of limonite. Heavy mineral assemblages were analyzed for native gold content, and the results suggested high native gold content in the illitization-pyritization belt, followed by quartz-pyritization belt, which was consistent with the strong association of limonite and gold mineralization, thus the two belts shall be prioritized for gold prospecting in the Shulonggou region.

Use of seismic attributes and open-hole log data to characterize production variability in a fractured carbonate play: A case study from Madison County, Texas

Understanding natural fracture systems is key for tight carbonate plays, in which production is dependent on secondary interconnected porosity networks. Locating geographic areas and stratigraphic sections with high natural fracture density and optimizing well locations and perforations to connect these fractures can enhance well performance and asset value. There is substantial production variation in the Cretaceous stacked carbonate play in East Texas, despite similarities in well completion and perforated intervals. Petrophysical property models did not explain the significant variation in well production; therefore, we have developed a multidisciplinary workflow combining seismic and log data with the goal of identifying faulting and natural fractures and understanding their effect on production. We used seismic discontinuity to map faults as the main indicator of presence of fractures. We calibrated triple combo logs with an image log to generate an indicator curve to identify natural fractures. The fracture indicator curve provided a good prediction of where natural fractures may occur, and discontinuity maps revealed a good correlation to well production. Furthermore, we concluded that drilling too closely to large faults negatively impacted production and correlated with increased water production. The workflow developed here can be used to optimize well placement in the stacked carbonate play of Madison County, Texas, and it can be applied to other fractured carbonate reservoirs.

The effect of saturation conditions on fracture performance of different soundless cracking demolition agents (SCDAs) in geological reservoir rock formations

Fracture stimulation using soundless cracking demolition agents (SCDAs) is a potential alternative technique to induce high-density fractures in sedimentary reservoir-rock as an auxiliary technique to improve the efficiency of enhanced oil and gas recovery efficiencies. However, to date, its application has been limited to fracture stimulation in dry rock masses. Therefore, using modified SCDAs, which can be used for underwater rock fracturing, a series of experiments was conducted to investigate the fracturing performance of SCDAs in saturated rock masses. 18 coarse-grained sandstone specimens were saturated in water, oil, and NaCl brine and fractured using three different SCDA types: a standard SCDA (S1), and two modified for underwater application (S2) and accelerated reaction rate (S3). Then, the fractured samples were scanned in the Australian Synchrotron, and the fractures were quantified using Avizo 9.0.1. The fracture initiation time and the total fracture network length and volume were found to be dependent on the saturated pore fluid of rock. Water saturation of samples increased the fracture initiation time by 16.5%, 24.1% and 13.68% for S1, S2, and S3 type SCDAs respectively and reduced the fracturing potential of SCDA by 59.5%, 32.49% and 66.67% compared to dry samples. This reduction was less apparent in oil-saturated samples as the high pore fluid viscosity of oil-saturated samples aid fracturing, which is explained by the Poiseuille equation. Increasing salinity in the saturation fluid from 0% to 12.5% was favourable for the fracturing efficiency of SCDAs because of the formation of CaCl2 in the pore fluid, which accelerates the reaction of SCDA. Fracture orientation also changed depending on the saturation fluid, which was again governed by the variation in reaction rate in SCDAs under different saturation conditions.

A fully coupled fracture equivalent continuum-dual porosity model for hydro-mechanical process in fractured shale gas reservoirs

A fully coupled fracture equivalent continuum-dual porosity model is proposed to investigate hydro-mechanical coupling phenomena in fractured porous media. An improved fracture tensor is then developed to transform the local discontinuities at cell level to the anisotropic continuum at macro-scale. This model is incorporated into a set of nonlinear partial differential equations and numerically solved by multi-time-step finite element algorithm. The approach is then applied to a field-scale simulation of shale gas reservoir. Results indicate that the proposed method can be applied to insights into the field-scale hydro-mechanical coupling with high-density fractures in any arbitrary orientation within manageable computational cost.

Groundwater Occurrence from Hydrogeomorphological Study of Hard Rock Terrain of Part of Southwestern Nigeria

Abstract Studies of structural and hydrogeomorphological units (HGU) that are indicators of groundwater occurrence were carried out across an area extent of more than 700 km2 within the hard rock terrain of southwestern Nigeria. These studies integrated geological remote sensing techniques (RST) and geographical information system (GIS) methods to generate thematic maps that included elevation, drainage, lineaments and vegetation index for characterising the attributes of groundwater occurrence across the area. The results revealed that the lineament system is mainly rectilinear with major trends of NNW-SSE and NE-SW on the gneiss, NW-SE and NE-SW on porphyritic granite and NNE-SSW, NW-SE and E-W on migmatite. The discharge zones in the area are the lowland terrains underlain by gneiss and amphibolite. Similarly, variably directional discontinuities that are related to rock contacts are equally laden with groundwater. Conversely, the recharge areas are the high-lying terrains characterised by higher fracture density and underlain by porphyritic granite and migmatite. Additionally, there are evidences of groundwater seepage along the major river channels. Therefore, besides the rock structures, landform is another crucial factor that guides groundwater distribution in the study area.

Experimental investigation on water flooding and continued EOR techniques in buried-hill metamorphic fractured reservoirs

Fractured reservoirs are complicated due to the coexistence of matrix and fracture systems. In particular, the matrix of buried-hill metamorphic fractured reservoirs (BHMFRs) has plenty of micro-fractures and a few dissolution pores rather than common pores in sandstone. The storage and heterogeneity features, such as the storage ratio between matrix and fractures, and the distribution of fractures, significantly affect the performances of waterflooding and EOR techniques. With the objective of finding effective waterflooding mode and EOR techniques in such reservoirs, the BHMFR of JZ25-1S in Bohai Bay in China was taken as an example. First, static imbibition experiments using a novel imbibition cell were conducted. Then, 3D large-scale models of BHMFRs using outcrops were designed based on the similarity criterion and employed to conduct the experiments of waterflooding and followed EOR techniques. The results show that co-current imbibition is principal and gravity is the dominant force of imbibition for BHMFRs. Because imbibition is slow, most of the yield comes from the macro-fractures. Both water-free recovery and waterflooding recovery increase as the decrease of the storage ratio of matrix to fractures. Pulsing cyclic water-injection is better than intermittently cyclic water-injection. In BHMFRs with heterogeneity of fracture density, injection in the area with low fracture density and production in the area with high fracture density can achieve better effects than the reverse scenario for both waterflooding and followed EOR techniques. If gas flooding is selected as the EOR technique, producer should be perforated at the lower layers of the reservoir. Suitable gel particles not only get a good EOR effect by effective plugging, but also can create a more favorable condition for surfactant flooding. In our experiments, another 2–4% of OOIP was produced by PPG flooding. Surfactant can enhance the oil recovery by both removing the adsorbed oil on the fracture surface and promoting imbibition in matrix. 3–5% of OOIP was recovered by surfactant flooding and soaking in our experiments.