The Geochemistry of Carbonate Rocks in Igwe Igarra, Southwestern Nigeria

Nigeria has an abundance of valuable solid minerals and rocks which can generate revenue for the government if they are fully exploited. Carbonate rock is one of such rocks that occur prominently in Igwe Igarra area of Edo State. Five (5) carbonate rock samples (Four marble and One Calcsilicate) were subjected to geochemical analyses. A petrographic study reveals that marble and calcsilicate contain calcite, quartz, microcline, plagioclase, biotite and opaque minerals. Geochemical data shows that carbonate rocks have variable geochemical characteristics: Igwe marble is highly Calcitic (96% - 98%) and very low in dolomite Mg(CO3). Calcsilicate rock is low in CaO (27.5%) and higher in marble. The Igwe marble is pure and contains fewer impurities compared to calcsilicate rock which makes it unideal for usage. Based on these characteristics the marble from Igwe is suitable as raw materials for the productions of glass, papers, beet sugar, aggregates, lubricants and fillers.

Role of Carbonic Anhydrase as an Activator in Carbonate Rock Dissolution and Its Implication for Atmospheric CO_(2)Sink

The conversion of CO2 into H+ and is a relatively slow reaction. Hence, its kinetics may be rate determining in carbonate rock dissolution. Carbonic anhydrase (CA), which is widespread in nature, was used to catalyze the CO2 conversion process in dissolution experiments of limestone and dolomite. It was found that the rate of dissolution increases by a factor of about 10 after the addition of CA at a high CO2 partial pressure (Pco2) for limestone and about 3 at low Pco2 for dolomite. This shows that reappraisal is necessary for the importance of chemical weathering (including carbonate rock dissolution and silicate weathering) in the atmospheric CO2 sink and the mysterious missing sink in carbon cycling. It is doubtless that previous studies of weathering underestimated weathering rates due to the ignorance of CA as an activator in weathering, thus the contribution of weathering to the atmospheric CO2 sink is also underestimated. This finding also shows the need to examine the situ distribution and activity of CA in different waters and to investigate the role of CA in weathering.

A New Example of Retrograde Solubility Model for Carbonate Rocks

Objective The dissolution and precipitation of carbonate during burial diagenetic process controls the reservoir property in deep buried strata.The geological process related with it has become a research focus during recent years.The most important dissolution fluids to carbonates are probably H_2S and CO_2 as byproducts of sulfate reduction in deep-buried setting with sulfate minerals,but carbonates are more soluble in relatively low temperature,which is the so-called retrograde solubility.Several geological processes can result in the decrease of temperature,including the upward migration of thermal fluids and tectonic uplift.The Ordovician strata in the Tahe oilfield of the Tarim

Effect of overburden pressure on determination of reservoir rock types using RQI/FZI,FZI^* and Winland methods in carbonate rocks

Rock typing is an important tool in evaluation and performance prediction of reservoirs.Different techniques such as flow zone indicator(FZI),FZI~*and Winland methods are used to categorize reservoir rocks into distinct rock types.Generally,these methods are applied to petrophysical data that are measured at a pressure other than reservoir pressure.Since the pressure changes the pore structure of rock,the effect of overburden pressure on rock typing should be considered.In this study,porosity and permeability of 113 core samples were measured at five different pressures.To investigate the effect of pressure on determination of rock types,FZI,FZI~*and Winland methods were applied.Results indicated that although most of the samples remain in the same rock type when pressure changes,some of them show different trends.These are related to the mineralogy and changes in pore system of the samples due to pressure change.Additionally,the number of rock types increases with increasing pressure.Furthermore,the effect of overburden pressure on determination of rock types is more clearly observed in the Winland and FZI~*methods.Also,results revealed that a more precise reservoir dynamic simulation can be obtained by considering the reservoir rock typing process at reservoir conditions.

An Experimental Study of Mixture Corrosion Effects of Carbonate Rocks in the Transitional Zone of Littoral Karst Areas

The mechanism for development of littoral karst differs from that of inland karst, and the mixture corrosion effects are one of the most important factors that control the development of littoral karst. Through seven groups of static experiments carried out in a closed CO2-H2O system, basic conclusions can be drawn as follows: (1) the basic law of corrosion process in a transitional zone of seawater-freshwater in littoral karst areas is identical with that in the fresh water,i.e., the lithologic characteristics and rock structure are the main factors which control the development of littoral karst; (2)the mixture corrosion rate of the carbonate rock in the above transitional zone is faster than that in fresh water or seawater;(3) the mechanism for development of carbonate rocks differs at various pressures of CO2 in a transitional zone in littoral karst areas.

Autochthonous Dolomitization and Dissolution in the Microbial Carbonate Rocks of the Fengjiawan Formation in the Ordos Basin

Many euhedral dolomite crystals and related pores are found in the microbial siliceous stromatolite dolomite and siliceous oolitic dolomite in the Fengjiawan Formation of the Mesoproterozoic Jixian System in the southern Ordos Basin.With the application of the microscope,scanning electron microscope,cathodoluminescence and in-situ trace element imaging,it can be seen that different from the phase I dolomite that was damaged by silicification,the intact euhedral phase II dolomite occurred through dolomitization after silicification,concentrated mainly in the organic-rich dark laminae of the stromatolite and the dark spheres and cores of the ooids.A considerable number of phase II dolomite crystals were dissolved,giving rise to mold pores and vugs which constituted the matrix pores and also the major pore space of the Fengjiawan Formation.The formation and dissolution of the dolomite were controlled by the microenvironment favorable,respectively,for carbonate precipitation and dissolution under the influence of microbial biological activities and related biochemical reactions.The driving force and material supply of dolomitization and dissolution were confined to the fabrics enriched with microorganisms,which are highly autochthonous.This mechanism may be a key factor for the development of Precambrian dolomite and related reservoirs,in the context of the domination of microbial rocks.

Transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis:A case study of the Cretaceous of the Mesopotamia Basin in the Middle East

The transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis was stud-ied based on core,cast thin section and physical property data of Cretaceous strata in the Mesopotamia Basin,the Middle East.There are 3 ways of biological transformation of rocks:(1)The living creatures transformed formations mechanically to make the rocks looser and intergranular pores increase.(2)After formation,burrows were backfilled with coarse-grained debris,and then unsaturated fluid infiltrated into the burrows during the penecontemporaneous period,resulting in dissolution.(3)Chemical alteration occurred in abandoned burrows and dolomitization produced a large number of intercrystalline pores.The·coupling of bioturbation and dissolution occurred mainly in the penecontemporaneous phase,and was controlled by rock type,sea level decline,burrow abundance,infillings,and water environment etc.As the burrows had better physical properties than the matrix,unsaturated fluid preferentially migrated along the burrows,leading to dissolution and expansion of the burrows first and then dissolution of the matrix.The coupling of bioturbation and dolomitization occurred mainly in the burial phase.The rich organic matter and reducing bacteria in the burrow provided material basis,reducing conditions and alkaline envi-ronment for dolomitization.The metasomatism in the penecontemporaneous period had little effect on the physical properties of the burrows.When the burrows were se parated from the deposition interface,equimolar metasomatism occurred in the burrows in a closed environment,forming euhedral fine-crystalline dolomite with intercrystalline pores.The transformation degree of bioturbation to muddy carbonate reservoir depends on rock type,density,connectivity,infillings and structure of the burrows.With the increase of the carbonate mud content,the im provement to rock physical properties by bioturbation be-comes more prominent.When the burrows are filled with coarse-grained debris or fine-crystalline dolomite,the greater the density,the higher the connectivity,and the lower the tortuosity of burrows,the better the physical properties of the muddy carbonate rocks are.

Mixed carbonate rocks lithofacies features and reservoirs controlling mechanisms in a saline lacustrine basin in Yingxi area,Qaidam Basin,NW China

Taking the mixed pre-salt carbonate rocks in the upper member of Eocene Xiaganchaigou Formation(E_(3)^(2))of Yingxi area in the Qaidam Basin as an example,the lithofacies and controlling mechanisms of reservoir formation are analyzed based on a large dataset of cores,thin sections and geochemical analysis.The reservoirs in E_(3)^(2)pre-salt layers have five types of lithofacies,of them,mixed granular calcareous dolostone,massive calcareous dolostone,plaque calcareous dolostone,and laminated dolomtic limestone are of sedimentary origin,and breccia calcareous dolostone is of tectonic origin.The four types of sedimentary lithofacies are divided into two types of saline sedimentary sequence lithofacies combinations,low-energy type in the sag area and low to high-energy type in the slope and paleo-uplift zone in the depression.Affected by high-frequency supply of continental clastic material,the two types of salty sedimentary sequences are mostly incomplete subtypes of lithofacies.Lithofacies have strong impacts on pre-salt reservoirs in E_(3)^(2):(1)Lithofacies type and sedimentary sequence controlled the formation and distribution of dolomite intercrystalline pores and dissolved pores during the pene-sedimentary period.(2)The structure of laminated dolomitic limestone controlled the formation of large-scale laminated fractures and high permeability channels during the diagenetic period.(3)Granular,massive,plaque calcareous dolostones have low mud content and strong brittleness,in the late tectonic reactivation period,the distribution of the three types of lithofacies,together with their distance from the top large slip faults and secondary faults,controlled the formation and distribution of high-efficiency fracture-cave brecciaed calcareous dolostone reservoirs.The above research led to the composite lithofacies-tectonic formation model of pre-salt reservoir in E_(3)^(2)of Yingxi area.The tempo-spatial distribution of tectonic breccia calcareous dolostone reservoirs,laminated dolomitic limestone shale oil reservoirs and granular,massive calcareous dolostone dissolved-intercrystalline pore tight reservoirs in various structural belts of the studied area have been figured out.These findings gave new insights into tight-shale oil accumulation theory in mixed carbonate successions from saline lacustrine basins,aiding in high efficient exploration and development of petroleum in the studied area.

Formation of fractures in carbonate rocks by pad acid fracturing with different states of carbon dioxide

Carbonate outcrops were taken from Ma 51 sub-member in the Lower Paleozoic in the Yan’an gas field to conduct true tri-axial hydraulic fracturing experiments with water, liquid CO_(2) and supercritical CO_(2). CT scan was applied to analyze initiation and propagation laws of hydraulic fractures in carbonate rocks. The experiments show that supercritical CO_(2) has low viscosity, strong diffusivity and large filtration during fracturing, which is more liable to increase pore pressure of rocks around wellbore and decrease breakdown pressure of carbonate rocks. However, it would cost much more volume of supercritical CO_(2) than water to fracture rocks since the former increases the wellbore pressure more slowly during fracturing. For carbonate rocks with few natural fractures, tensional fractures are generated by fracturing with water and liquid CO_(2), and these fractures propagate along the maximum horizontal principal stress direction;while fracturing with supercritical CO_(2) can form shear fractures, whose morphology is rarely influenced by horizontal stress difference. Besides, the angle between propagation direction of these shear fractures near the wellbore and the maximum horizontal principal stress is 45°, and the fractures would gradually turn to propagate along the maximum horizontal principal stress when they extend to a certain distance from the wellbore, leading to an increase of fracture tortuosity compared with the former. For carbonate rocks with well-developed natural fractures, fracturing with fresh water is conducive to connect natural fractures with low approaching angle and form stepped fractures with simple morphology. The key to forming complex fractures after fracturing carbonate rocks is to connect the natural fractures with high approaching angle. It is easier for liquid CO_(2) with low viscosity to realize such connection. Multi-directional fractures with relatively complex morphology would be formed after fracturing with liquid CO_(2).

Geochemical characteristics and sedimentary environment significance of carbonate rocks in Nanfen Formation of Qingbaikou System in Tonghua,Jilin Province

In order to reveal the sedimentary environment of carbonate rocks in Nanfen Formation of Qingbaikou System in Tonghua,Jilin Province,the mineral composition and petrochemistry of carbonate rocks in Nanfen Formation were analyzed.The mineral compositions of five carbonate rock samples in Nanfen Formation mainly consist of calcite,with minor clay minerals and quartz,and the rock type is siliceous marlite.The Mn/Sr va-lues range from 1.52 to 4.08,with an average of 2.64,indicating that the carbonate rocks experienced weak diagenesis;the Sr/Ba values range from 1.26 to 2.51,with an average of 1.93,indicating marine environment;the ratio of Mg/Al ranges from 35.33 to 86.34,with an average of 62.95,indicating the seawater environment,which is consistent with the result from Sr/Ba;the MgO/CaO values range from 0.01 to 0.04,with an average of 0.02,indicating humid environment;the values of V/(V+Ni)range from 0.63 to 0.73,with an average of 0.70,indicating anoxic environment.In summary,geochemical analyses show that the Nanfen Formation carbonate rocks are marine deposits,in a warm,humid,anoxic environment with poor flow of seawater,and subsequently underwent weak diagenetic alteration.

Impact of effective stress on permeability for carbonate fractured-vuggy rocks

To gain insight into the flow mechanisms and stress sensitivity for fractured-vuggy reservoirs,several core models with different structural characteristics were designed and fabricated to investigate the impact of effective stress on permeability for carbonate fractured-vuggy rocks(CFVR).It shows that the permeability performance curves under different pore and confining pressures(i.e.altered stress conditions)for the fractured core models and the vuggy core models have similar change patterns.The ranges of permeability variation are significantly wider at high pore pressures,indicating that permeability reduction is the most significant during the early stage of development for fractured-vuggy reservoirs.Since each obtained effective stress coefficient for permeability(ESCP)varies with the changes in confining pressure and pore pressure,the effective stresses for permeability of four representative CFVR show obvious nonlinear characteristics,and the variation ranges of ESCP are all between 0 and 1.Meanwhile,a comprehensive ESCP mathematical model considering triple media,including matrix pores,fractures,and dissolved vugs,was proposed.It is proved theoretically that the ESCP of CFVR generally varies between 0 and 1.Additionally,the regression results showed that the power model ranked highest among the four empirical models mainly applied in stress sensitivity characterization,followed by the logarithmic model,exponential model,and binomial model.The concept of“permeability decline rate”was introduced to better evaluate the stress sensitivity performance for CFVR,in which the one-fracture rock is the strongest,followed by the fracture-vug rock and two-horizontalfracture rock;the through-hole rock is the weakest.In general,this study provides a theoretical basis to guide the design of development and adjustment programs for carbonate fractured-vuggy reservoirs.

Genesis and Implications of the Composition and Sedimentary Structure of Fine-Grained Carbonate Rocks in the Shulu Sag

Fine-grained carbonate rocks, which extensively occur in the Eocene strata in the Shulu sag, Bohai Bay Basin, North China, represent an unconventional, fine-grained carbonate reservoir. However, previous studies have ignored the complexity of the lithofacies components and their formation mechanisms. Fine-grained carbonate rocks are typical reservoirs in which hydrocarbons form and gather. A better understanding of the nature of these rocks is extremely important for evaluating the quality of unconventional, fine-grained carbonate reservoirs. Various lithofacies components were discriminated in this study with a combination of petrographic observations and carbon isotope analyses. These finegrained carbonate rocks comprise terrigenous, biogenic and diagenetic materials. Terrigenous input and biologically induced precipitation are the main sources of the materials in the lake. Five lithofacies were identified based on the observations of sedimentary features(core and thin section) and mineralogical data:(1) varve-like laminated calcilutite,(2) graded laminated calcilutite,(3) interlaminated calcisiltitecalcilutite,(4) massive calcilutite, and(5) massive calcisiltite-calcarenite. Their origins were recorded by various lithofacies components, which are controlled by the interactions of physical, chemical and biological processes. This study indicated that the lithology of the bedrocks was the key factor controlling carbonate accumulation. The tectonics and climate can influence the weathering and types of lithofacies. Primary productivity controlled the precipitation of the endogenic calcite. These factors jointly determined the abundant fine-grained carbonate rocks that have accumulated in the Shulu sag.

Empirical rock physics relationships on carbonate dry-frame elastic properties

Laboratory tests were conducted to analyze the ultrasonic velocity response to the pressure change in dry carbonate rocks from the Weyburn oilfield,Canada.Twenty-four samples are from seven wells with helium porosities ranging from 1%to 29%.Thin-section images,SEM and mercury intrusion porosimetry were performed to show their inner structures and pore throat size distributions.P-and S-wave velocities(V_(p) and V_(s))measurements were first done under hydrostatic loading and then while unloading,with confining pressures varying between 3 and 35 MPa.The results indicate that V_(p) and V_(s) in these samples follow a linear relation independent of the pressure change.The ratio V_(p)/V_(s) is more responsive to pressure change irrespective of the pore volume.One-third of the carbonate samples show abnormal V_(p)/V_(s) reduction with the increase in the effective pressure.The pressure dependence of velocities(PDV)of Weyburn carbonate rocks varies widely even for samples from the same formation with similar sedimentary history.Samples with loosely packed crystals and/or relatively large dominant pore diameter have higher PDV.The exponential empirical model V=A−Ce^(DPe) was tested;therein,V is the elastic wave velocity,Pe is the effective confining pressure,and A,C and D are the best fitting coefficients determined by curve fitting.The model gives good fits for most of the Weyburn carbonate samples.From a statistical point of view,there is no difference between the V_(p)-and V_(s)-derived exponential coefficient D.

Carbonate rocks resistivity determination using dual and triple porosity conductivity models

The rocks may be classified as Archie rocks or Non-Archie rocks,depending on the link between pore structure and resistivity of the rock.For carbonate rocks,keeping this factor in view,the applicability of Archie's law tends to be unreasonable and employing Archie's equation will ultimately lead to inaccurate estimates of the water saturation.Numerous investigators have reported models that forms a link between laboratory results and field well logs to determine water saturation in intricate pore systems.In case of complex systems,Archie's law fails to correctly define the resistivity-index curves.In addition,it is imperative to carry out investigations at lower saturation values in order to elaborate the non-Archie characteristics.This brings the dual and triple porosity conductivity models into the equation.As these models are essential in defining the considerable changes of the resistivity-index curves,both qualitatively and quantitatively.The linkage between the two models is depicted electrically in series or in parallel.Moreover,it is possible to have a combination of the two configurations.As a result,the experimental results can be modeled accurately using least parameters.This work presents two models to determine water saturation in carbonate rocks,containing dual and triple porosity systems.This study also investigates the impact of different influential parameters such as saturation exponents,conductivity ratios,and pore volume fraction of each network systems,on the new models.Finally,the effect of each parameter is individually studied and the sensitivity analysis on the RI curves is produced.A new approach to apply these models is also explained which requires the use of NMR log,mercury injection capillary pressure(MICP)measurement,and simple plotting techniques to determine the water saturation.The proposed new models help petrophysicists to estimate water saturation in Non-Archie rocks,containing double and triple porosity network systems.

Petrophysical properties and their influencing factors of carbonates in the fourth member of Sinian Dengying Formation, Sichuan Basin, SW China

Based on the measurements of petrological, petrophysical and elastic properties of the samples of different sedimentary facies in the fourth member of Sinian Dengying Formation (Deng 4 Member) in the Sichuan Basin, the diagenetic processes of reservoirs of different sedimentary facies and their controls on the petrophysical properties were discussed. The results show that cracks and mineral composition jointly control the petrophysical properties, and both are significantly influenced by sedimentary environment and diagenesis. The microbial dolomite of mound-shoal facies mainly experienced multi-stage dolomitization, penecontemporaneous dissolution, tectonic rupture and hydrothermal/organic acid dissolution processes, giving rise to cracks and dissolved pores. The grannular dolomite of inter-mound-shoal bottomland or dolomitic lagoon facies mainly underwent mechanical compaction, burial dolomitization and tectonic-hydrothermal action, creating cracks and intercrystalline pores. The diagenesis related to crack development increases the pressure- and saturation-dependent effects of samples, leading to significant decrease in the compressional wave impedance and Poisson's ratio. Dolomitization changes the properties of mineral particles, resulting in a Poisson's ratio close to dolomite. The muddy, siliceous and calcareous sediments in the low-energy environment lead to the decrease of impedance and the differential change of Poisson's ratio (significantly increased or decreased). The samples with both cracks and dissolved pores show high P-wave velocity dispersion characteristics, and the P-wave velocity dispersion of samples with only fractures or pores is the lowest.

Dissolution and Deformation Characteristics of Limestones Containing Different Calcite and Dolomite Content Induced by CO_(2)-Water-Rock Interaction

To investigate the impacts of mineral composition on physical and mechanical properties of carbonate rocks,limestone specimens containing different contents in calcite and dolomite are selected to perform CO_(2)-water-rock reaction experiments.The X-ray Diffraction(XRD) and Nuclear Magnetic Resonance(NMR) are carried out to examine the change characteristics of mineral dissolution and pore structure after reaction.The core flooding experiments with Fiber Bragg gratings are implemented to examine the stress sensitivity of carbonate rocks.The results show that the limestones containing pure calcite are more susceptible to acid dissolution compared to limestone containing impure dolomite.The calcite content in pure limestone decreases as the reaction undergoes.The dissolution of dolomite leads to the formation of calcite in impure limestone.Calcite dissolution leads to the formation of macropore and flow channels in pure limestone,while the effects of impure dolomite in impure limestone results in mesopore formation.When confining pressure is lower than 12 MPa,pure limestones demonstrate higher strain sensitivity coefficients compared to impure limestone containing dolomite after reaction.When confining pressure exceeds 12 MPa,the strain sensitivity coefficients of both pure and impure limestones become almost equal.

Method for identifying effective carbonate source rocks:a case study from Middle-Upper Ordovician in Tarim Basin,China

Hydrocarbon expulsion occurs only when pore fluid pressure due to hydrocarbon generation in source rock exceeds the force against migration in the adjacent carrier beds.Taking the Middle-Upper Ordovician carbonate source rock of Tarim Basin in China as an example,this paper proposes a method that identifies effective carbonate source rock based on the principles of mass balance.Data from the Well YW2 indicate that the Middle Ordovician Yijianfang Formation contains effective carbonate source rocks with low present-day TOC.Geological and geochemical analysis suggests that the hydrocarbons in the carbonate interval are likely self-generated and retained.Regular steranes from GC-MS analysis of oil extracts in this interval display similar features to those of the crude oil samples in Tabei area,indicating that the crude oil probably was migrated from the effective source rocks.By applying to other wells in the basin,the identified effective carbonate source rocks and non-source rock carbonates can be effectively identified and consistent with the actual exploration results,validating the method.Considering the contribution from the identified effective source rocks with low present-day TOC(TOC_(pd))is considered,the long-standing puzzle between the proved 3 P oil reserves and estimated resources in the basin can be reasonably explained.

Physico-Chemical and Petrographic Characterization of Carbonated Rocks of Mintom (South-Cameroon) and Their Potential Uses

The characterization of carbonated rocks of Mintom allowed identifying three features (massive limestone, banded limestone and dolomite). Samples taken in these features were subjected to the physico-chemical analyses and to the mechanical tests. The results obtained show a convergence in the mineralogical composition in particular in the calcareous features (smectite, micas and calcite);the dolomite feature being mainly constituted by dolomite associated with the smectite. The chemical composition allows finding contents raised in CaO, in all the samples. However, in the dolomite and the banded limestone, the concentrations in MgO and in SiO2 are also important. The mechanical properties showed a good compression resistance for the massive limestone and the dolomite, the mechanical behavior of the banded limestone being low. The exploitation of these results allows recommending carbonated rocks of Mintom in the production of the clinker, agricultural amendments and in the constructions of the civil engineering.

Wettability alteration and oil recovery by spontaneous imbibition of smart water and surfactants into carbonates

Naturally fractured carbonate reservoirs have very low oil recovery efficiency owing to their wettability and tightness of matrix.However,smart water can enhance oil recovery by changing the wettability of the carbonate rock surface from oilwet to water-wet,and the addition of surfactants can also change surface wettability.In the present study,the effects of a solution of modified seawater with some surfactants,namely C12 TAB,SDS,and TritonX-100(TX-100),on the wettability of carbonate rock were investigated through contact angle measurements.Oil recovery was studied using spontaneous imbibition tests at 25,70,and 90°C,followed by thermal gravity analysis to measure the amount of adsorbed material on the carbonate surface.The results indicated that Ca2+,Mg2+,and SO42-.ions may alter the carbonate rock wettability from oil-wet to water-wet,with further water wettability obtained at higher concentrations of the ions in modified seawater.Removal of NaCl from the imbibing fluid resulted in a reduced contact angle and significantly enhanced oil recovery.Low oil recoveries were obtained with modified seawater at 25 and 70°C,but once the temperature was increased to 90°C,the oil recovery in the spontaneous imbibition experiment increased dramatically.Application of smart water with C12 TAB surfactant at 0.1 wt%changed the contact angle from 161°to 52°and enhanced oil recovery to 72%,while the presence of the anionic surfactant SDS at 0.1 wt%in the smart water increased oil recovery to 64.5%.The TGA analysis results indicated that the adsorbed materials on the carbonate surface were minimal for the solution containing seawater with C12 TAB at 0.1 wt%(SW+CTAB(0.1 wt%)).Based on the experimental results,a mechanism was proposed for wettability alteration of carbonate rocks using smart water with SDS and C12 TAB surfactants.

Coping with uncertainties through an automated workflow for 3D reservoir modelling of carbonate reservoirs

Reliable 3D modelling of underground hydrocarbon reservoirs is a challenging task due to the complexity of the underground geological formations and to the availability of different types of data that are typically affected by uncertainties. In the case of geologically complex depositional environments, such as fractured hydrocarbon reservoirs, the uncertainties involved in the modelling process demand accurate analysis and quantification in order to provide a reliable confidence range of volumetric estimations. In the present work, we used a 3D model of a fractured carbonate reservoir and populated it with different lithological and petrophysical properties. The available dataset also included a discrete fracture network(DFN) property that was used to model the fracture distribution. Uncertainties affecting lithological facies, their geometry and absolute positions(related to the fault system), fracture distribution and petrophysical properties were accounted for. We included all different types of uncertainties in an automated approach using tools available in today’s modelling software packages and combining all the uncertain input parameters in a series of statistically representative geological realizations. In particular, we defined a specific workflow for the definition of the absolute permeability according to an equivalent, single porosity approach, taking into account the contribution of both the matrix and the fracture system. The results of the analyses were transferred into a 3D numerical fluid-dynamic simulator to evaluate the propagation of the uncertainties associated to the input data down to the final results, and to assess the dynamic response of the reservoir following a selected development plan. The "integrated approach" presented in this paper can be useful for all technicians involved in the construction and validation of 3D numerical models of hydrocarbon-bearing reservoirs and can potentially become part of the educational training for young geoscientists and engineers, since an integrated and well-constructed workflow is the backbone of any reservoir study.