Fractured Core Plug Research Articles

Diagenesis including cataclastic band development in the Permian Penrith Sandstone, Vale of Eden Basin, England

Diagenesis in the Penrith Sandstone is characterized by early hematite and mixed illite/smectite cementation, followed by burial compaction, feldspar and two episodes of quartz cementation. The primary quartz overgrowths are characterized by heterogenous luminescence; however, the secondary quartz overgrowths are characterized by thinner and darker luminescence. Cataclasis post-dates primary quartz cementation, and significantly reduces grain and pore aperture size. The apex of mercury volume for a deformed sample was reduced up to three times in comparison with the undeformed rock. Dissolution occurred during cataclasis as authigenic hematite was removed in deformation zones, improving pore aperture size and consequently enhancing sandstone porosity and permeability. Recent fracturing created fractured-pores and improved porosity and permeability in tight cataclastic samples. Evidence for syn-cataclastic dissolution is observed in the samples studied. This is indicated by the dissolution of authigenic feldspars and the enhancement of poroperm parameters adjacent to cataclastic zones and the absence of hematite cement within them. Based on those the sequence of diagenetic evolution in the Penrith Sandstone can be reconstructed. Host samples were characterized by good to very good poroperm values, varying between 20 and 24% porosity and from 809÷1445 mD permeability. Cataclastic core plugs have 13÷21% porosity and 0.12÷39 mD permeability. The porosity reduction by cataclasis was estimated to be as much as 4.5% on average. Fracturing and dissolution improved poroperm parameters in studied samples. Recently fractured core plugs have about 292 mD permeability on average. Dissolution-cataclastic samples have 32% porosity and 18 mD permeability, whereas dissolution-host samples have 31% porosity and maximum permeability as much as 2469 mD on average. Cataclasis occurred in combination with in situ significant reservoir dissolution and fracturing may improve a sandstone reservoir in excellent quality.

Novel water shut off method based on temporary plugging agent and gel composition

This article presents the laboratory development of a novel rigless, self-selective water shut-off technique. The technique involves the design and pumping of three different fluids through fractured core plugs in a specific sequence. The first fluid serves to temporarily block porous medium while moving freely through fractures. The second fluid is a cross-linking polymer gelant, which is injected immediately after the first fluid at a pressure below the fracture pressure of the formation to block fractures. An enzyme-based chemical breaker solution, serving as the third fluid, is evaluated for removing the filter-forming materials. The treatment and flow studies are conducted using a high-pressure, high-temperature core flow setup. In conclusion, the developed water control technology presented in this study offers a low-cost solution for fractured and high-anomaly wells. The technology is particularly effective when there is a significant permeability contrast between the oil-bearing matrix and water-conductive fractures. The results demonstrate a successful blockage of fracture, with minimal contamination of porous medium. The study also highlights important factors to consider for the field application of this technology. It is recommended to conduct customized laboratory investigations under simulated reservoir conditions prior to implementing the technology in the field. This will help optimize the treatment design and ensure its effectiveness in real-world applications. Keywords: water shut-off; temporary blocking agent; gel composition; coreflood; permeability contrast.

Mini Review of Miscible Condition Evaluation and Experimental Methods of Gas Miscible Injection in Conventional and Fractured Reservoirs

Nowadays, one of the most efficient ways of enhanced oil recovery (EOR) is gas miscible injection (GMI). In GMI, minimum miscible pressure (MMP) is the most substantial parameter. The MMP can be estimated by different methods, such as laboratory methods, using the equation of states (EOS), empirical correlations, mathematical models, and artificial intelligence. Among these, laboratory methods of estimating MMP comprise slim tube displacement (STD), rising bubble apparatus (RBA), vanishing interfacial tension (VIT), X-ray computerized tomography (CT), fast fluorescence-based microfluidic (FFBM) method, magnetic resonance imaging (MRI), sonic response method (SRM), rapid pressure increase (RPI), oil droplet volume measurement (ODVM), and pressure–composition diagrams (PCDs), all of which are described in detail in this paper. Furthermore, experimental investigations performed using the mentioned laboratory methods have been presented. On the other hand, to perform miscible injection experiments in the matrix–fracture system, there are methods, such as fractured core plug (FCP), matrix–fracture system (MFS), modified core holder (MCH), and microconsolidation device (MCD), that the accomplished studies by the mentioned method have surveyed. Eventually, it can be deduced that, owing to the difference between the MMP in conventional and fractured reservoirs, a reliable method (laboratory or mathematical model) for estimating the MMP in fractured reservoirs is required.

Experimental and numerical study of flow and thermal transport in fractured rock

An experimental and numerical study was carried out to investigate matrix-fracture thermal transport in fractured rock. Two different synthetically fractured core plugs were used during the flow-through experiments while core plugs’ outer surface was maintained at two different constant temperatures. To investigate the matrix-fracture thermal transport, cold water was injected through the single fracture core plugs at different flow rates. A film type heat flux sensor was used in the fractured core plug to measure the fracture temperature and heat flux over the matrix-fracture interface. Experimental results showed that fracture temperature decreases with increasing flow rate while heat flux increases. At high flow rates, temperature difference over matrix-fracture interface increases due to large volume of water contacting the fracture at a given time. Water temperature decreased more in the case of fracture surrounded by rock matrix with low volumetric heat capacity and thermal conductivity. A numerical model calibrated using experimental values of the fracture temperature was developed to estimate thermal properties of rock matrix and to identify their importance on the matrix-fracture thermal transport. Using the calibrated numerical model, it has been shown that temperature in the fracture and fracture surface temperature are different contrary to previous studies assuming the same values for these two temperatures. Local and transient convective heat transfer coefficients were introduced with respect to local values of temperature difference and transient values of heat flux along matrix- fracture interface enabling accurate representation of matrix-fracture thermal transport.

Propagation of waves with a wide range of frequencies in digital core samples and dynamic strain anomaly detection: carbonate rock as a case study

SUMMARYRecent advancements in various types of seismic measurement methods, such as sonic logging, vertical seismic profiling (VSP) and surface seismic surveys, have allowed the high-quality measurement of seismic wave propagation over a broad frequency range. To elucidate the relationship between seismic wave propagation captured by various seismic methods at widely different frequencies and in highly heterogeneous zones (e.g. fractures, vuggy zones) developed in carbonate reservoirs, laboratory measurements have been conducted over a broad frequency range. However, existing laboratory methods measure the effective properties over an entire core sample. Furthermore, there are few laboratory measurements of individual fracture stiffnesses. We propose a method to indirectly estimate the local properties caused by local anomalies (e.g. fractures) in a core sample over a broad frequency range from the dynamic elastic properties of a dry core sample using synthetic seismic waveforms generated from a digital volume of the core sample. 3-D numerical simulations were conducted over a broad frequency range using a digital core model produced by X-ray computed tomography. The proposed method was applied to numerical models and two types (fractured and vuggy) of carbonate core plugs acquired in an Abu Dhabi oil field, with the frequency ranging from 200 Hz to 100 kHz in the tests. Local strain anomalies and their frequency dependencies were successfully detected in the fractured core plug. Such frequency-dependent local responses could be associated with the micromechanics of incomplete solid–solid contacts at fractures or the heterogeneity of core samples, and thus with the frequency-dependent fracture stiffness. It was also demonstrated that the heterogeneity-induced local strain in a vuggy core plug may affect the accuracy of existing low-frequency laboratory measurements.

Experimental investigation and theoretical modeling of stress-dependent permeability in naturally fractured tight gas reservoir

Stress-dependent permeability (SDP) of naturally fractured tight gas reservoir (NFTGR) has drawn considerable attention in recent decades. The SDP of natural fracture (NF) is quite different from that of the rock matrix (RM) due to the variance NF width and NF filling degree. The SDP of naturally fractures can be obtained by conducting permeability measurement experiments on naturally fractured core plugs (NFCP). However, it is usually impossible to obtain downhole NFCPs due to economic and technical limitations. Our research group obtained various types of naturally fractured downhole core plugs from a NFTGR (Tarim filed, China) and make it possible to investigate the effect of NFs on SDP. The NF types of the downhole core plugs include RM core plug (RMCP), unpenetrated-ring fractured core plug (URFCP), penetrated-vertical fractured core plug (PVFCP), penetrated-incline fractured core plug (PIFCP), unpenetrated-U-type fractured core plug (UUFCP), and multiple fractured core plug (MFCP). We conducted SDP experiments through the application of pressure-pulse-decay (PPD) method. Results showed that the initial permeability of RMCP and MFCP were higher than that of others, and the permeability vanished stress of them were also highest, indicating that the flow resistance of RMCP and MFCP were lower than those of others. It is also shown that the degree of permeability stress-dependent (DPSD) varied from NF types. A modified exponential permeability model was derived to evaluate DPSD of natural fractured core plugs. Two parameters (γσeff and γσeff') were used to describe the DSDP in NFTGR, and the range of which were obtained from the experimental data by regression analysis. The reasonable γσeff and γσeff' range of NFCP were also proposed in this paper. Sensitivity analysis showed that the influence γσeff' on SDP was more significant than that of γσeff. The proposed model can be integrated to reservoir simulators and help to improve of oil and gas production prediction accuracy in NFTGRs.

Bicarbonate flooding of homogeneous and heterogeneous cores from a carbonaceous petroleum reservoir

Two extensive core flooding experiments along with supplementary tests were conducted to investigate the effect of bicarbonate as a potential determining ion for improved oil recovery from Danish North Sea reservoir chalk samples. Different sequences of brines were injected to see the impact on tertiary oil recovery. The core flooding experiments were conducted using two kinds of core plugs, homogeneous and heterogeneous (fractured). As injection of bicarbonate solutions leads to increase in pressure drop across the homogeneous core plug during injection, it was inferred that permeability reduction may be caused by fines production. The same flooding scenario was repeated for a fractured core plug. The difference in recovery from homogeneous and heterogeneous core plugs was observed: a significantly higher additional recovery was obtained for the heterogeneous core plug. This effect cannot be explained just by changing the chemical interactions between oil and rock surface. Adding bicarbonate to the injection brine changed the equilibrium and caused the sulfate ion in seawater to precipitate. Mixing injection brine with seawater from secondary injection caused the fines migration in the porous space, while high pH of bicarbonate solutions causes emulsification. In the accompanying crude oil/bicarbonate brine contact tests, formation of dynamically stable emulsions was observed. It was hypothesized that the fines migration and emulsification are causes for the microscopic flow diversion to the oil saturated regions and additional recovery from the heterogeneous rocks.

Water Leakoff During Gel Placement in Fractures: Extension to Oil-Saturated Porous Media

SummaryCrosslinked polymers extrude through fractures during placement of many conformance-improvement treatments, as well as during hydraulic fracturing. Dehydration of polymer gel during extrusion through fractures has often been observed and was extensively investigated during recent decades. Injection of highly viscous gel increases the pressure in a fracture, which promotes gel dehydration by fluid leakoff into the adjacent matrix. The present comprehension of gel behavior dictates that the rate of fluid leakoff will be controlled by the gel and fracture properties and, to a lesser extent, be affected by the properties of an adjacent porous medium. However, several experimental results, presented in this work, indicate that fluid leakoff deviates from expected behavior when oil is present in the fracture-adjacent matrix. We investigated fluid leakoff from chromium (Cr)(III)-acetate hydrolyzed polyacrylamide (HPAM) gels during extrusion through oil-saturated, fractured core plugs. The matrix properties were varied to evaluate the effect of pore size, permeability, and heterogeneity on gel dehydration and leakoff rate. A deviating leakoff behavior during gel propagation through fractured, oil-saturated core plugs was observed, associated with the formation of a capillary driven displacement front in the matrix. Magnetic resonance imaging (MRI) was used to monitor water leakoff in a fractured, oil-saturated, carbonate core plug and verified the position and existence of a stable displacement front. The use of MRI also identified the presence of wormholes in the gel, during and after gel placement, which supports gel behavior similar to the previously proposed Seright filter-cake model. An explanation is offered for when the matrix affects gel dehydration and is supported by imaging. Our results show that the properties of a reservoir rock might affect gel dehydration, which, in turn, strongly affects the depth of gel penetration into a fracture network and the gel strength during chase floods.

English

The drilling operation cost represents 25% of the total oilfield exploitation cost. Drilling fluids represent 15 to 18% of the total cost of well petroleum drilling operations. The main drilling fluids problem is the loss into fractures and vugs. Mitigation of severe lost circulation is a main challenge while drilling in fractured formations where conventional lost circulation materials (LCM) will not cure these losses. Therefore, specialized fracture seal material (FSM) is required when drilling fractured formations. In this study, a promising FSM made from shredded waste car tyres was tested at laboratory for its ability to seal artificially fractured cores under High Temperature High Pressure (HT-HP) conditions similar to wellbore conditions. For this purpose, the conventional 500 ml HT-HP filtration cell was modified to accommodate a fractured core plug of length and diameter equal to 38.1 mm (1.5 inch) instead of the ceramic disc. Moreover, the cell outlet channel located below the fractured plug was increased from 1.0 mm diameter to 5.0 mm, easily allow the passage of the FSM in none effective fracture seal tests. Using the modified HT-HP filtration cell, shredded waste car tyres proved its ability to perfectly seal the artificially made fracture in the test core samples at overbalance pressures up to 900 psi and temperatures up to 80°C. The optimum mud composition was fresh water, 7% by weight bentonite, 7% shredded waste car tyres (a mixture ranging between 2.3 mm and less than 0.45 mm granule sizes) in weight bases. In addition to its great ability to seal fractured formation, the shredded waste car tyres material is cheap and locally available in commercially quantities. Additionally, the utilization of waste car tyres in drilling operations and other industrial applications can protect the environment from many hazards. Key words: HT-HP filtration, Fracture seal, shredded waste car tyres, drilling fluid, fractured core plug.

Fracture and vug characterization and carbonate rock type automatic classification using X-ray CT images

This paper presents a method of fracture and vug characterization and automatic classification of rock type in a naturally fractured vuggy carbonate reservoir using X-ray computed tomography (CT). First, using image processing technologies such as image segmentation, morphological processing and skeleton extraction, the fractures and vugs in the CT images of core plugs were characterized, and then, the characteristic parameters of the fractures and vugs were calculated. Then, a support vector machine (SVM) was utilized for automatic classification of the fractures and vugs. The shape and the eccentricity of fractures and vugs as the SVM training eigenvectors can effectively distinguish fractures from vugs with high classification confidence. Finally, with the characteristic parameters of fractures and vugs calculated from the CT images of core plugs, a new defined function of the carbonate rock index (CRI) was proposed, which can quantitatively classify the carbonate rock into three different types: matrix, fractured and vuggy. After analysis of more than 200 core plugs from the Y reservoir in the Middle East, the CRI values for matrix, vuggy and fractured carbonate rock were given and are 0<CRI<1, 1≤CRI≤10 and CRI>10, respectively. A high CRI value indicates a high fracture density, and a small CRI value indicates that the carbonate rock is mainly matrix without fractures and vugs. For the vuggy carbonate core plug, the CRI value is in between those of fractured core plugs and matrix core plugs. Therefore, the value of CRI can quantitatively evaluate the heterogeneity of carbonate rock due to fractures and vugs. The field application of this method demonstrated that this new method can effectively characterize fractures and vugs and quantitatively classify the rock type of the naturally fractured vuggy carbonate reservoir.

Superior fracture-seal material using crushed date palm seeds for oil and gas well drilling operations

Expenses for drilling represent 25% of the total oilfield exploitation cost. Drilling fluids represent 15–18% of the total cost of well petroleum drilling operations. The main drilling fluids problem is the loss into fractured and vugs. Prevention or mitigation of severe lost circulation is a main challenge while drilling in fractured formations where conventional lost circulation materials (LCM) will not cure these losses. Therefore, specialized treatment is required when drilling fractured formations.In this study, a superior LCM made from crushed date palm seeds was tested at laboratory for its ability to seal artificially fractured cores under High Temperature High Pressure (HT-HP) conditions. For this purpose, the conventional 500ml HT-HP filter press was modified to accommodate a fractured core plug of length and diameter equal to 38.1mm (1.5inch) instead of the ceramic disc.Using the modified HT-HP filter press, crushed date palm seeds proved its ability to completely seal the artificially made fracture in the test core samples at overbalance pressures up to 1000psi and temperatures up to 90°C. The optimum mud composition was fresh water, 7% by weight bentonite, 3.5% fine crushed date palm seeds, and 3.5% coarse crushed date palm seeds in weight bases. In addition to its superior ability to seal fractured formation, the crushed date palm seeds material is cheap, locally available in commercially quantities, and environmentally friendly material.

Utilization of Shredded Waste Car Tyres as a Fracture Seal Material (FSM) in Oil and Gas Drilling Operations

The drilling operation cost represents 25% of the total oilfield exploitation cost. Drilling fluids represent 15 to 18% of the total cost of well petroleum drilling operations. The main drilling fluids problem is the loss into fractures and vugs. Mitigation of severe lost circulation is a main challenge while drilling in fractured formations where conventional lost circulation materials (LCM) will not cure these losses. Therefore, specialized fracture seal material (FSM) is required when drilling fractured formations. In this study, a promising FSM made from shredded waste car tyres was tested at laboratory for its ability to seal artificially fractured cores under High Temperature High Pressure (HT-HP) conditions similar to wellbore conditions. For this purpose, the conventional 500 ml HT-HP filtration cell was modified to accommodate a fractured core plug of length and diameter equal to 38.1 mm (1.5 inches) instead of the ceramic disc. Moreover, the cell outlet channel located below the fractured plug was increased from 1.0 mm diameter to 5.0 mm to easily allow the passage of the FSM in none effective fracture seal tests. Using the modified HT-HP filtration cell shredded waste car tyres proved its ability to perfectly seal the artificially made fracture in the test core samples at overbalance pressures up to 900 psi and temperatures up to 80°C. The optimum mud composition was fresh water, 7% by weight bentonite, 7% shredded waste car tyres (a mixture ranging between 2.3 mm and less than 0.45 mm granule sizes) in weight bases. In addition to its great ability to seal fractured formation, the shredded waste car tyres material is cheap and locally available in commercial quantities. Additionally, the utilization of waste car tyres in drilling operations and other industrial applications can protect the environment from many hazards.

Visualization of Carbon Dioxide Enhanced Oil Recovery by Diffusion in Fractured Chalk

Summary This work demonstrates that diffusion may be a viable oil-recovery mechanism in fractured reservoirs during injection of carbon dioxide (CO2) for enhanced oil recovery, depending on the CO2 distribution within the fracture network and distance between fractures. High oil recovery was observed during miscible, supercritical CO2 injection (RF = 86% original oil in place) in the laboratory with a fractured chalk core plug with a large permeability contrast. Dynamic 3D fluid saturations from computed-tomography (CT) imaging made it possible to study the local oil displacement in the vicinity of the fracture, and to calculate an effective diffusion coefficient with analytical methods. The obtained diffusion coefficient varies between 0.83 and 1.2 ×10−9m2/s, depending on the method used for calculation. A numerical sensitivity analysis, with a validated numerical model that reproduced the experiments, showed that the rate of oil production during CO2 injection declined exponentially with increasing diffusion lengths from the CO2-filled fracture and oil-filled matrix. In a numerical upscaling effort, with the experimentally achieved diffusion coefficient, oil-recovery rates and local flow were studied in an inverted five-spot pattern in a heavily fractured carbonate reservoir.

Low-Salinity Chase Waterfloods Improve Performance of Cr(III)-Acetate Hydrolyzed Polyacrylamide Gel in Fractured Cores

Summary Polymer gels are frequently applied for conformance improvement in fractured reservoirs, where fluid channeling through fractures limits the success of waterflooding. Placement of polymer gel in fractures reduces fracture conductivity, thus increasing pressure gradients across matrix blocks during chase floods. A gel-filled fracture is reopened to fluid flow if the injection pressure during chase floods exceeds the gel-rupture pressure; thus, channeling through the fractures resumes. The success of a polymer-gel treatment, therefore, depends on the rupture pressure. Salinity differences between the gel network and surrounding water phase are known causes of gel swelling (e.g., observed in recent work on preformed particle gels). Gel swelling and its effect on fluid flow have, however, been less studied in conjunction with conventional polymer gels. By use of corefloods, this work demonstrates that low-salinity water can swell conventional Cr(III)-acetate hydrolyzed polyacrylamide (HPAM) gels, thereby significantly improving gel-blocking performance after gel rupture. Formed polymer gel was placed in fractured core plugs, and chase waterfloods were performed using four different brine compositions, of which three were low-salinity brines. The fluid flow rates through the matrix and differential pressures across the matrix and fracture were measured and shown to increase with decreasing salinity in the injected water phase. In some cores, the fractures were reblocked during low-salinity waterfloods, and gel-blocking capacity was increased above the initial level. Low-salinity water subsequently flooded the matrix during chase floods, which provided additional benefits to the waterflood. The improved blocking capacity of the gel was caused by a difference in salinity between the gel and injected water phase, which induced gel swelling. The results were reproducible through several experiments, and stable for long periods of time in both sandstone and carbonate outcrop core materials. Combining polymer gel placement in fractures with low-salinity chase floods is a promising approach in integrated enhanced oil recovery (IEOR).

Mobility control during CO2 EOR in fractured carbonates using foam: Laboratory evaluation and numerical simulations

Abstract Laboratory miscible CO 2 and CO 2 -foam injection tests were performed to study CO 2 EOR in fractured carbonate core plugs and to evaluate the feasibility of using foam for mobility control in fractured systems. A significant oil recovery was observed during CO 2 injections at miscible conditions ranging between 75% and 92% OOIP in cores with irreducible initial water saturation. In fractured core plugs viscous displacement was negligible and the recovery was mainly driven by diffusion. Injection of pre-generated CO 2 -foam accelerated oil recovery compared to pure CO 2 injection in fractured core plugs, by adding a viscous displacement in addition to diffusion. A conceptual numerical model was built to study the effect of reduced fracture conductivity on oil recovery efficiency and the impact of system size and orientation on diffusion dominated oil recovery during CO 2 injection in fractured systems.

A Comparative Experimental Study of Sweep Efficiency in Naturally Fractured Reservoirs

In this paper the performance of gel and polymer in fractured core plugs in laboratory is compared. The experimental results show that in a naturally fractured reservoir, the best solution to improve oil recovery should be such that the gel particle sizes are larger than the pore sizes, and the solution viscosity becomes very high after the particles contact with water. If the fracture is wide, the improvement of sweep efficiency by a polymer solution is limited because the polymer solution could flow through the fracture channel. The results in this paper provide us with guidelines to select proper polymers to improve sweep efficiency. Key words: Sweep efficiency; Naturally fractured reservoirs; Polymer solution; Oil recovery

イラン産天然フラクチャーコアを使ったミシブル置換効果の実験的検証

Determining the feasibility of miscible displacement process to increase oil recovery from Iranian naturally fractured reservoirs is the subject of this study. For that purpose, miscible displacement experiments including liquefied petroleum gas (LPG) injection were carried out under reservoir conditions on composite cores, consisting of reservoir fractured core plugs of the chosen rock type. Results of experimental studies indicated that LPG miscible flooding could maximize oil recovery. In this paper, results of the Iranian fractured core flooding experiments are presented.