Comparison of Wormhole Profiles on Carbonate Rocks Using Emulsified and Single-Phase Retarded Acids

Acid retardation through emulsification is commonly used in reservoir stimulation operations, however, emulsified acid are viscous fluids, thus require additional equipment at field for preparation and pumping requirements. Mixture of HCl with organic acids and/or chemical retarders have been used developed to retard acid reaction with carbonate, however, lower dissolving power. Development of low viscosity and high dissolving retarded acid recipes (e.g., equivalent to 15-26 wt.% HCl) addresses the drawbacks of emulsified acids and HCl acid mixtures with weaker organic acids. The objective of this study is to compare wormhole profile generated as a result of injecting acids in Indian limestone cores using 28 wt.% emulsified acid and single-phase retarded acids at comparable dissolving power at 200 and 300°F. Coreflood analysis testing was conducted using Indiana limestone core plugs to assess the pore volume profile of retarded acid at temperatures of 200 and 300° F. This test is supported by Computed Tomography to evaluate the propagation behavior as a result of the fluid/rock reaction. Wider wormholes were observed with 28 wt.% emulsified acid at 200°F when compared to test results conducted at 300°F. The optimum injection rate was 1 cm3/min at 200 and 300°F based on wormhole profile and examined flow rates. Generally, face-dissolution and wider wormholes were observed with emulsified acids, especially at 200°F. Narrower wormholes were formed as a result of injecting retarded acids into Indiana limestone cores compared to 28 wt.% emulsified acid. Breakthrough was not achieved with retarded acid recipe at 300°F and flow rates of 1 and 3 cm3/min, suggesting higher flow rates (e.g., > 3 cm3/min) are required for the retarded acid to be more effective at 300°F.

Dolomite Stimulation with Retarded Acids

Globally, dolomite formations are important reservoirs for oil and gas. Acid stimulation is commonly used to extend the life of carbonate reservoirs, and a good understanding of the fluid performance is essential for effective treatment design. Three acids, hydrochloric acid (HCl), emulsified HCl, and a single-phase retarded acid based on HCl, were assessed for their ability to create wormholes in Silurian dolomite under laboratory conditions using a standard core flow experiment. Select cores were imaged by X-ray computed tomography to visualize the wormhole morphology. Similar experiments in Indiana limestone was used as a control. The core flow experiments showed that the pore volume to break-through (PVbt) values for the retarded acids in Indiana limestone were less sensitive to changes in temperature overall than unmodified HCl. For Silurian dolomite though, the opposite is observed. HCl has uniformly high PVbt values at lower (200 °F) and higher (325 °F). The emulsified acid and the single-phase retarded acid are more efficient than HCl, but the difference is smaller at 325 °F. Core images revealed that all three fluids had some degree of wormhole branching at 200 °F and much less branching at 325 °F. By visual inspection, the single-phase retarded acid has less ramification than HCl and the emulsified acid. Overall, the results show that retarded acids should make effective stimulation fluids for dolomite reservoirs.

Surfactant-Polymer Formulations for EOR in High Temperature High Salinity Carbonate Reservoirs

Harsh conditions, such as high temperature (>100 oC) and high salinity (>50,000 ppm TDS), can make the application of chemical enhanced oil recovery (EOR) challenging by causing many surfactants and polymers to degrade. Carbonate reservoirs also tend to have higher concentrations of divalent cations as well as positive surface charges that contribute to chemical degradation and surfactant adsorption. The objective of this work is to develop a surfactant-polymer (SP) formulation that can be injected with available hard brine, achieve ultra-low IFT in these harsh conditions, and yield low surfactant retention. Phase behavior experiments were performed to identify effective SP formulations. A combination of anionic and zwitterionic surfactants, cosolvents, brine, and oil was implemented in these tests. High molecular weight polymer was used in conjunction with the surfactant to provide a high viscosity and stable displacement during the chemical flood. Effective surfactant formulations were determined and five chemical floods were performed to test the oil recovery potential. The first two floods were performed using sandpacks from ground Indiana limestone while the other three floods used Indiana limestone cores. The sandpack experiments showed high oil recovery proving the effectiveness of the formulations, but the oil recovery was lower in the cores due to complex pore structure. The surfactant retention was high in the sandpacks, but it was lower in Indiana Limestone cores (0.29-0.39 mg/gm of rock). About 0.4 PV of surfactant slug was enough to achieve the oil recovery. A preflush of sodium polyacrylate improved the oil recovery.

Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone

<p>Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. To date, investigation of sample heterogeneity, specifically tensile strength and strength anisotropy has focused on layered rocks, such as shales, sandstones and gneisses. Data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites, even though these frequently occur in geo-energy settings.</p><p>We have performed Brazilian Disc tests on limestone samples containing planar heterogeneities, investigating Brazilian test Strength (BtS) and the effects of orientation on strength. We used prefractured Indiana limestone to represent a planar heterogeneity without cohesion and Treuchtlinger Marmor samples with central stylolites to represent a planar heterogeneity of unknown strength (as an example of a sealed fracture). The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ (steep angle) is parallel to the principal loading direction, and 0⁰ (low angle) to the horizontal axis of the sample. All experiments were filmed, and where possible Particle Image Velocimetry was used to determine internal particle motion. Moreover, we used a 2D Comsol model in which we simplified the stylolite surface as a sinusoid. The model was used to qualitatively determine how i) a different period of the sinusoid and ii) relative strength of sinusoid/matrix affect the results.</p><p>Our results show that all imperfect samples are weaker than intact samples. The 2D Comsol model indicates that the qualitative results remain unaffected by changing the period (assumed to be representative of roughness) of the cohesive heterogeneity, nor by the relative strength contrast: the location of the first fracture remains unaffected. For both heterogeneity types, the fracture patterns can be divided into four categories, with two clear endmembers, and a more diffusive subdivision in between.</p><p>For a cohesion-less heterogeneity:</p><ul><li>steep angles lead to frictional sliding along the interface, and only a small hypothesized permeability increase.</li> <li>Intermediate angles lead to a combination of tensile failure of the matrix and sliding along the interface, where for steeper angles more new fractures form which follow the path of the existing fracture.</li> <li>Low angles lead to closure of the old fracture and new tensile failure.</li> </ul><p>For a cohesive heterogeneity of unknown cohesion:</p><ul><li>Steep angles lead to intensive failure of the heterogeneous zone, attributed to the presence of a stress concentrator.</li> <li>Intermediate angles lead to partial failure along the heterogeneous zone, and the formation of new fractures in the matrix, potentially instigated by mode II failure to accommodate motion.</li> <li>Low angles lead to the formation of a new fracture plus opening within the heterogeneous zone.</li> </ul><p>These results imply that hydrofracture (i.e. creating tensile stresses) of a stylolite-rich zone will lead to more fractures than fractures in a homogeneous zone, where the orientation of the stylolites and bedding will control the orientation of the permeable pathways.</p>

Direct Visualization of Ultrasonication Induced Asphaltenes Removal in Carbonate Rock Using Confocal Imaging

Asphaltenes deposition is a major issue in the petroleum industry as it can have a detrimental impact on hydrocarbon recovery efficiency. Therefore, it is imperative to study the fundamental mechanisms controlling the asphaltenes flocculation and deposition in reservoirs allowing us to prevent and possibly eliminate such problem. Hitherto many studies have highlighted ultrasonication as a potential remediation technique but no investigation has been able to provide direct visual evidence of the phenomena. The primary objective of this study is to visualize the deposition of asphaltenes and their subsequent removal by ultrasonication in Indiana Limestone using state of the art confocal microscopy. To do so, we performed a comprehensive series of experiments by flooding Indiana Limestone core samples with crude oil and later passing ultrasonic waves through the flooded sample. Four core samples of Indiana Limestone each displaying different permeability were used, these are referred to as A2, B2, C4, and D4. At each stage of experiment series of images were captured by confocal microscopy depicting asphaltenes deposition and it’s post-sonication distribution. The images were further segmented allowing us to compute changes in the asphaltenes content before and after sonication. The comparison of confocal scans reveals that the ultrasonic irradiation is highly efficient in removing asphaltenes from the low permeability core samples, whereas in the case of highly permeable cores, rather than preventing it promoted the asphaltenes flocculation. Surprisingly, an increase in asphaltenes content was observed after ultrasonication in high permeability core samples.

A New Cationic Polymer System That Improves Acid Diversion in Heterogeneous Carbonate Reservoirs

Summary In-situ gelled acids are used for acid diversion in heterogeneous carbonate reservoirs. However, most of the gelled systems are based on anionic polymers that are difficult to clean up after the acid treatments. Residual polymer deposition leads to formation damage by blocking pore throats in the matrix. This work evaluates a new cationic-polymer acid system with self-breaking ability for application as an acid diverter in carbonate reservoirs. Experimental studies have been conducted to examine the rheological properties of these polymer-based acid systems. The apparent viscosities of the live and the partially neutralized acids at pH from 0 to 5 were measured against the shear rate (0 to 1000 s−1). The effects of salinity and temperature (80 to 250°F) on the rheological properties of the acid system were also studied. The viscoelastic properties of the gelled acid system were evaluated using an oscillatory rheometer. Dynamic sweep tests were used to determine the elastic (G′) and viscous (G″) moduli of the system. Single-coreflood experiments were conducted on Indiana limestone cores to study the nature of diversion caused by the polymer-acid system. The effect of permeability contrast on the process of diversion was investigated by conducting dual-coreflood experiments on Indiana limestone cores that had permeability contrasts of 1.5 to 20. Computed tomography (CT) scans were conducted to study wormhole propagation after acid injection for both single and dual cores. The live acid system displayed a non-Newtonian shear-thinning behavior with the viscosity declining as temperature increased. For 5 wt% hydrochloric acid (HCl) and 20 gal/t polymer content at 10 s−1, the viscosity decreased from 230 to 40 cp as the temperature increased from 88 to 250°F. Acid-spending tests demonstrated that the acid generated a gel with improved viscosity of 260 cp (at 250°F and 10 s−1) after it reached a pH of 2. The highly viscous gel plugged the wormhole and forced the acid that followed to the next higher-permeability zone. The viscosity of the gel continued to increase until it broke down to 69 cp (at 250°F and 10 s−1) at a pH of 4.8, which indicates a self-breaking system and more thorough cleanup potential. Coreflood studies indicated that the wormhole and the diversion process are dependent on the temperature and the flow rate. There was no indication of any damage caused by the system. The injected acid pore volume to breakthrough (PVBT) decreased from 2.2 to 1.4 when the temperature increased from 150 to 250°F. The strong elastic nature of the gel (G′ = 3.976 Pa at 1 Hz) formed by the partially neutralized acid system proves its suitability as a candidate for use as a diverting agent. This new acid-polymer system has significant promise for use in acid diversion to improve stimulation of carbonate reservoirs.

Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone

<p>Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. Tensile strength and strength anisotropy of rocks has been investigated for strongly layered rocks, such as shales, sandstones and gneisses, but data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites. We have performed Brazilian Disc tests on limestone samples containing pre-existing fractures and stylolites, investigating Brazilian test Strength (BtS) and fracture orientation. We used Indiana limestone samples, pre-fractured with the Brazilian Disc method, and Treuchtlinger Marmor samples which contained central stylolites. All experiments were filmed. The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ is parallel to the principal loading direction, and 0⁰ to the horizontal axis of the sample. Pre-fracturing Indiana limestone samples results in a cohesion-less planar discontinuity, whereas the stylolites in the Treuchtlinger Marmor samples are discontinuities which have some strength.</p><p>The results show that our imperfect samples with a planar discontinuity are always weaker than an intact sample. For the Indiana limestone, with a cohesion-less interface, there is 10 to 75% of weakening, which is angle-dependent. Once the angle is 30 or lower there is no influence from the initial fracture for the orientation of the new fracture. The stress-displacement pattern followed the expectation for Brazilian Disc testing. However, in the samples with a stylolite, strength is isotropic and between 25 and 65% of the strength of an intact sample. For all cases several new cracks appeared, of which the orientation is influenced by the orientation of the stylolite. The fracture pattern and associated stress drops are more complex for high angles. Interestingly, in the samples with stylolites, always more than one fracture was formed, whereas in the samples with a cohesionless interface usually only one new fracture formed, which for natural settings suggests a potential for higher fracture density when hydrofracturing a stylolite-rich interval.</p><p>A second difference between these datasets is the amplitude of the pre-existing interface. The effect of amplitude will be qualitatively investigated with a 2D Comsol model, to investigate the location of the first fracture occurring, which can then be compared to the camera data of the experiments.</p>

Compositional Modeling of Carbonate Acidizing Processes with CO2 Evolution in Aqueous Environments

Summary Several modeling studies have been conducted in carbonate acidizing, particularly in the area of aqueous environments. Yet, complete understanding of this complex subsurface process remains elusive. Characterizing the effects of evolved CO2, a product of the chemical reaction between carbonates and HCl (hydrochloric acid), has been ignored to date under the assumption that high operating pore pressures keep CO2 completely dissolved in the surrounding solution. However, the presence of CO2 in the porous media of the formation itself changes fluid-flow dynamics throughout the entire system. This paper describes a numerical simulation study to accurately model the physics of carbonate acidizing. A validation of the model is conducted by replicating experiments described in the published literature and by performing laboratory coreflood experiments of carbonate acidizing. The acid efficiency curve and initial pore pressure variations for single-phase experimental studies from the literature is matched by including the effects of evolved CO2 in the model. Two Indiana limestone cores of 6 in. length and 1.5 in. diameter were used to conduct (1) a tracer-injection study with 5 wt% KCl (potassium chloride) solution and (2) an acid-injection study with 15 wt% HCl solution. The experiments were conducted at 72°F, and 1,180 psi pore pressure. The Indiana limestone cores were characterized via computed tomography (CT) scans, and a detailed, accurate porosity profile of each core was used as input to the numerical model. The tracer fluid was used to characterize the porous environment and mechanical dispersion coefficients, and for subsequent calibration of the simulation model. From the conducted single-phase acidizing coreflood experiment, pressure drop values across the core were closely monitored with time to assess acid breakthrough, and the core effluent samples were collected at regular intervals and analyzed to determine the concentrations of calcium chloride (CaCl2) and HCl. CT scans of each core conducted post-acidizing describe its wormhole pattern. These parameters are accurately matched using the simulation model. A high pore pressure of 1,000 psi and above is not sufficient to keep all the evolved CO2 in solution during carbonate acidizing. The presence of CO2 as a separate phase hinders acid efficiency. Up to 24% by volume of pore space is shown to be occupied by the evolved CO2 that exists as a separate phase, and is located ahead of the acid front during the acidizing process, thus competing for flow with the incoming acid. The modeling of CO2 as a component for simulating the acid coreflood played a key role in acquiring a better match with experimental results, with limited dependency on empirical pore-scale parameters. In addition to wormhole propagation, the current model accurately forecasts effluent concentrations collected and quantity of rock dissolved from the acidized porous media. A new approach to accurately predict carbonate acidizing in porous media for an aqueous environment has been presented via compositional modeling using a reservoir simulator. The presented methodology can be incorporated in large field scale reservoir models.

2020 IGWS Calendar

Indiana limestone, known to geologists as the Salem Limestone,is quarried in a narrow 30-mile-long area of south-central Indiana that is alsohome to Indiana University. Gracing up to 75 percent of all limestonebuildings in North America, this stone is known for its particularstrength, durability, and ageless beauty, and clads the nation’s most eminentbuildings, including the Empire State Building, Pentagon, andNational Cathedral. For more than 100 years, this local stone hasbeen used for buildings on the Bloomingtoncampus. Beginning in 1890 with Maxwell Hall,to 2017 and the completion of Luddy Hall,home of the School of Informatics, Computing,and Engineering, nearly all academic buildingshave been built of Indiana limestone. Architectural styles on campus span threecenturies. From the highly ornate carvings andpointed arches that define Collegiate Gothicarchitecture to the streamlined Art Deco styleof the early 1900s, the beauty of the campusreflects the skills of local stone artisans. In1979, the National Register of Historic Placesadded the Old Crescent portion of campus toits list to ensure its preservation. Walking around campus, you’ll see manycarvings on the exteriors of the buildings. Fish,maize, and chemical symbols can be foundon the science buildings on the south side ofcampus, while decorative scrolls of text adornseveral of the art buildings to the east. Themost common carved figure on campus is theowl, a symbol of learning and education, andtwelve are scattered on various buildings. Today, the history of Indiana’s limestone legacyis preserved in several campus collections andarchives. Most of the photographs used for themosaic on this calendar belong to the IndianaLimestone Photograph Collection. Curated bythe Indiana Geological and Water Survey since2012, this impressive archive consists of morethan 26,000 architectural photos depictingquarries, mills, and buildings from the earlyto mid-1900s. The digitized photographs arestored on IU Libraries Image Collections Online(http://go.iu.edu/16dx). Other photos in the mosaic are from researchand outreach efforts and the collections of theIndiana Geological and Water Survey.