High-resolution coupling of stratigraphic'sweet-spot'lithofacies and petrophysical properties:A multiscale study of Ordovician Goldwyer Formation,Western Australia

The identification of stratigraphic'sweet-spot'interval is significant in oil and gas formation evaluation.However,formation evaluation in macroscopic-scale merely provides low resolution and limited infor-mation,thus may lead to uncertainties in resource estimation.To accurately identify the'sweet-spot'intervals amongst heterogeneous lithofacies,we conducted a very high-resolution and quantitative analysis from in-situ macroscopic scale to laboratory microscopic scale on the Goldwyer formation of Canning Basin,Western Australia.The comprehensive advanced well logging and slim-compact micro imager(SCMI)technologies were synthetically applied in couple with the laboratory nanoscaled ex-periments.The results unveiled an extraordinarily large lithofacies heterogeneity between different rock intervals,with distinguished features shown in Goldwyer Ⅰ,Ⅱ,and Ⅲ members.The most favorable lithofacies is recognized as the laminated argillaceous thermally-matured organic matter(OM)-rich mudstone,which is widely developed in Goldwyer Ⅲ as the major attributor to'sweet-spot'intervals.Goldwyer Ⅱ is exclusively characterized by thick mudstone intervals(94.4%),interbedded with thin calcareous mudstones(5.5%),corresponding to a depositional environment of low-energy distal section of the outer ramp settings.Microscopically,the most favorable lithofacies in'sweet-spot'intervals develop numerous OM-/mineral nanopores for hydrocarbon storage.Illite-rich lithofacies develops abundant inter-particle pores from 2 to 17 nm that mainly contribute to pore volume for free gas storage capacity.OM-rich lithofacies with higher maturity have OM-pores with good connectivity,bearing large specific surface area that is beneficial for adsorbed gas capacity.

Nanopore structure comparison between shale oil and shale gas:examples from the Bakken and Longmaxi Formations

In order to analyze and compare the differences in pore structures between shale gas and shale oil formations, a few samples from the Longmaxi and Bakken Formations were collected and studied using X-ray diffraction, LECO TOC measurement, gas adsorption and field-emission scanning electron microscope. The results show that samples from the Bakken Formation have a higher TOC than those from the Longmaxi Formation. The Longmaxi Formation has higher micropore volume and larger micropore surface area and exhibited a smaller average distribution of microsize pores compared to the Bakken Formation. Both formations have similar meso-macropore volume. The Longmaxi Formation has a much larger meso-macropore surface area, which is corresponding to a smaller average meso-macropore size. CO_2 adsorption data processing shows that the pore size of the majority of the micropores in the samples from the Longmaxi Formation is less than 1 nm, while the pore size of the most of the micropores in the samples from the Bakken Formation is larger than 1 nm. Both formations have the same number of pore clusters in the 2–20 nm range, but the Bakken Formation has two additional pore size groups with mean pore size diameters larger than 20 nm. Multifractal analysis of pore size distribution curves that was derived from gas adsorption indicates that the samples from the Longmaxi Formation have more significant micropore heterogeneity and less meso-macropore heterogeneity. Abundant micropores as well as mesomacropores exist in the organic matter in the Longmaxi Formation, while the organic matter of the Bakken Formation hosts mainly micropores.

Permeability prediction using hydraulic flow units and electrofacies analysis

It is essential to characterize fluid flow in porous media to have a better understanding of petrophysical properties.Many approaches were developed to determine reservoir permeability among which the integrated analysis of hydraulic flow unit(HFU)and electrofacies(EF)is considered to be useful one.However,the application of HFU and EF analysis has not been totally understood with a limited data to develop correlation for less distance offset wells.In this study,an attempt was made to show the application of integrating HFU and EF for reliable estimation of permeability using core and wireline log data in one of the gas fields in Pakistan.The results obtained indicate that the integrated approach proposed in this study can be used,especially in less distance offset wells when a limited number of data are available for petrophysical characterization.

An Integrated Rock Typing Approach for Unraveling the Reservoir Heterogeneity of Tight Sands in the Whicher Range Field of Perth Basin, Western Australia

Tight gas sands in Whicher Range Field of Perth Basin show large heterogeneity in reservoir characteristics and production behavior related to depositional and diagenetic features. Diagenetic events (compaction and cementation) have severely affected the pore system. In order to investigate the petrophysical characteristics, reservoir sandstone facies were correlated with core porosity and permeability and their equivalent well log responses to describe hydraulic flow units and electrofacies, respectively. Thus, very tight, tight, and sub-tight sands were differentiated. To reveal the relationship between pore system properties and depositional and diagenetic characteristics in each sand type, reservoir rock types were extracted. The identified reservoir rock types are in fact a reflection of internal reservoir heterogeneity related to pore system properties. All reservoir rock types are characterized by a compacted fabric and cemented framework. But distribution and dominance of diagenetic products in each of them depend on primary depositional composition and texture. The results show that reservoir rock typing based on three aspects of reservoir sandstones (depositional properties, diagenetic features and petrophysical characteristics) is a suitable technique for depiction of reservoir heterogeneity, recognition of reservoir units and identifying factors controlling reservoir quality of tight sandstones. This methodology can be used for the other tight reservoirs.

Significant aspects of carbon capture and storage–A review

Excessive emission of greenhouse gases into the atmosphere has resulted in a progressive climate change and global warming in the past decades.There have been many approaches developed to reduce the emission of Carbon Dioxide(CO2)into the atmosphere,among which Carbon Capture and Storage(CCS)techniques has been recognized as the most promising method.This paper provides a deeper insight about the CCS technology where CO2 is captured and stored in deep geological formations for stabilization of the earth's temperature.Principles of capturing and storage for a long-term sequestration are also discussed together with the processes,mechanisms and interactions induced by supercritical CO2 upon injection into subsurface geological sites.

Integration of Core Data, Well Logs and Seismic Attributes for Identification of the Low Reservoir Quality Units with Unswept Gas in the Carbonate Rocks of the World's Largest Gas Field

Tight zones of the gas bearing Kangan and Dalan formations of the South Pars gas field contain a considerable amount of unswept gas due to their low porosity, low permeability and isolated pore types. The current study, integrates core data, rock elastic properties and 3D seismic attributes to delineate fight and low-reservoir-quality zones of the South Pars gas field. In the first step, the dynamic reservoir geomechanical parameters were calculated based on empirical relationships from well log data. The log-derived elastic moduli were validated with the available laboratory measurements of core data. Cross plots between estimated porosity and elastic parameters based on Young＇s modulus indicate that low porosity zone coincide with high values of Young＇s module. The results were validated with petro- graphic studies of the available thin sections. The core samples with low porosity and permeability are correlated with strong rocks with tight matrix frameworks and high elastic values. Subsequently, rock elastic properties including Young＇s modulus and Poisson＇s ratio along with porosity were estimated by using neural networks from a collection of 3D post-stack seismic attributes, such as acoustic impedance （ALl）, instantaneous phase of AI and apparent polarity. Distinguishing low reservoir quality areas in pay zones with unswept gas is then facilitated by locating low porosity and high elastic modulus values. An- hydrite zones are identified and eliminated as non-pay zones due to their characterization of zero porosi- ty and high Young modulus values. The methodology described has applications for unconventional re- servoirs more generally, because it is able to distinguish low porosity and permeability zones that are po- tentially productive from those unprospective zones with negligible reservoir quality.

Effect of particle size on high-pressure methane adsorption of coal

Adsorbed gas cannot be neglected in the evaluation of coalbed methane and shale gas since a significant proportion of gas is stored in the form of adsorbed gas.Adsorbed methane of coal and shale has been widely studied by high-pressure methane adsorption experiment.In sample treatment of the experiment,the sample is crushed and sieved to a particular particle size range.However,how particle size influence high-pressure methane adsorption is still unclear.In this study,low-pressure nitrogen(N_(2))and high-pressure methane adsorption have been measured on coal samples with different particle size.According to N2 sorption analysis,pore volume and surface area increase with particle size reduction.Pore size distribution of small pores(<10nm)changes among varying particle size.Pore volume proportion of small pores(<10nm)increases and pore volume proportion of big pores(>10nm)decreases with decreasing particle size.Decreasing particle size by crushing sample introduces new connectivity for closed pores to the particle surface.The responses of isotherms of high-pressure methane adsorption are different with different particle size.Methane adsorption at initial pressure(145psi)increases with decreasing particle size.Adsorption increase rate at high pressure(435-870psi)decreases with particle size reduction.This can be explained that fine sample has more pore volume and higher pore volume proportion of small pores(<10nm).Sample with particle size of 150-250μm has the highest Langmuir volume.

Sweet-spot mapping through formation evaluation and property modelling using data from the Goldwyer Formation of the Barbwire Terrace,Canning Basin

The Goldwyer Formation of the Canning Basin has been regarded as a highly prospective shale play.This study assesses the potential prospectivity of this source rock as an unconventional hydrocarbon resource.Considering the sparsity of wells penetrating the Middle Ordovician Goldwyer across the vast underexplored area of the Canning Basin,a basin-wide study of the source rock is not warranted.Goldwyer assessment of the Barbwire Terrace,a subdivision of the Canning Basin,is carried out instead.This assessment includes the estimation of key shale play properties,such as,total organic carbon,total porosity,water saturation,and brittleness index.Each property was estimated from available well data by testing multiple estimation methods.TOC values were derived from multiple regressions of different well data.A simplified Archie's equation was used to estimate water saturation.Density porosity method was primarily used for total porosity estimations.Sonic data along with density were utilized to estimate brittleness index.Each property was then modelled across the Goldwyer Formation within the terrace.This provided geostatistical estimates on the propagation of such properties.In order to generate sweet spot maps for the Barbwire Terrace,averaged maps of different properties were combined in a weighted manner.This approach attempts to simplify the complexity of unconventional resource assessment,which therefore has provided a single product evaluating the prospectivity of the Goldwyer as a hydrocarbon resource.Results have shown that TOC and porosity are mostly the deciding factors for the prospectivity of this source rock,given that their values can be too small where the Goldwyer is deemed non-prospective.Nonetheless,sweet-spot maps show that most prospective zone is the Upper Goldwyer(Goldwyer I),followed by the upper parts of the Lower Goldwyer(Goldwyer III).More specifically,southern flanks of north-western and middle regions of the Barbwire Terrace tend to be more prospective.A stricter approach where cut-off values were applied for each property showed that sweet-spot maps are only prospective in the southern flanks of the middle Barbwire Terrace of Goldwyer I.

Reservoir and lithofacies shale classification based on NMR logging

Shale gas reservoirs have fine-grained textures and high organic contents,leading to complex pore structures.Therefore,accurate well-log derived pore size distributions are difficult to acquire for this unconventional reservoir type,despite their importance.However,nuclear magnetic resonance(NMR)logging can in principle provide such information via hydrogen relaxation time measurements.Thus,in this paper,NMR response curves(of shale samples)were rigorously mathematically analyzed(with an Expectation Maximization algorithm)and categorized based on the NMR data and their geology,respectively.Thus the number of the NMR peaks,their relaxation times and amplitudes were analyzed to characterize pore size distributions and lithofacies.Seven pore size distribution classes were distinguished;these were verified independently with Pulsed-Neutron Spectrometry(PNS)well-log data.This study thus improves the interpretation of well log data in terms of pore structure and mineralogy of shale reservoirs,and consequently aids in the optimization of shale gas extraction from the subsurface.

Well selection in depleted oil and gas fields for a safe CO_(2) storage practice: A case study from Malaysia

Carbon capture and sequestration technology is recognized as a successful approach taken to mitigate the amount of greenhouse gases released into the atmosphere.However,having a successful storage practice requires wise selection of suitable wells in depleted oil or gas fields to reduce the risk of leakage and contamination of subsurface resources.The aim of this paper is to present a guideline which can be followed to provide a better understanding of sophisticated wells chosen for injection and storage practices.Reviewing recent studies carried out on different aspects of geosequestration indicated that the fracture pressure of seals and borehole conditions such as cement-sheath integrity,distance from faults and fractures together with the depth of wells are important parameters,which should be part of the analysis for well selection in depleted reservoirs.A workflow was then designed covering these aspects and it was applied to a depleted gas field in Malaysia.The results obtained indicated that Well B in the field may have the potential of being a suitable conduit for injection.Although more studies are required to consider other aspects of well selections,it is recommended to employ the formation integrity analysis as part of the caprock assessment before making any decisions.

CO_(2) storage in depleted gas reservoirs:A study on the effect of residual gas saturation

Depleted gas reservoirs are recognized as the most promising candidate for carbon dioxide storage.Primary gas production followed by injection of carbon dioxide after depletion is the strategy adopted for secondary gas recovery and storage practices.This strategy,however,depends on the injection strategy,reservoir characteristics and operational parameters.There have been many studies to-date discussing critical factors influencing the storage performance in depleted gas reservoirs while little attention was given to the effect of residual gas.In this paper,an attempt was made to highlight the importance of residual gas on the capacity,injectivity,reservoir pressurization,and trapping mechanisms of storage sites through the use of numerical simulation.The results obtained indicated that the storage performance is proportionally linked to the amount of residual gas in the medium and reservoirs with low residual fluids are a better choice for storage purposes.Therefore,it would be wise to perform the secondary recovery before storage in order to have the least amount of residual gas in the medium.Although the results of this study are useful to screen depleted gas reservoirs for the storage purpose,more studies are required to confirm the finding presented in this paper.

A new integrated approach to resolve the saturation profile using high-resolution facies in heterogenous reservoirs

The saturation calculation in complex reservoirs remains a major challenge to the oil and gas industry.In simple formations,a tendency towards simple saturation models such as Archie or Simandoux for clean and shaly reservoirs respectively is always preferable.These models were found to be working effectively in homogeneous formations within which the porosity and permeability are linked in the light of a simple facies scheme.Where the rocks show some degrees of heterogeneity,the well-logs are usually affected by different factors.This adversely results in a compromised or averaged log profiles that may affect the saturation calculations.Four wells drilled across a shaly sand of high heterogeneity have been studied in the Perth Basin,Western Australia.The aim is to resolve the hydrocarbon saturation and explain the high productivity results,despite the high water saturation,obtained through a conducted formation well test across the interested reservoir zones.A new integration technique between a suite of conventional and advanced logging tools together with the capillary pressure measurements has been carried out to generate a high-resolution reservoir saturation profile,that is lithofacies dependent.Three different independent methods were used in the studied wells to calculate the saturation and to reduce the uncertainty of the final estimated profiles.The methods are the resistivity-based saturation,the NMR-based irreducible saturation,and a new application through saturation height modeling.Furthermore,through the workflow,an effective calibration for the magnetic resonance T2 cutoff has been applied that is supported by the excellent reservoir production behavior from such complex reservoir.The methodology will help resolve the saturation calculation as one of the most challenging reservoir parameters,particularly where the resistivity logs are affected in complicated shaly sand environments.The effectiveness of the workflow shines the possibility to predict high resolution facies and saturation profiles in the lack of resistivity logs.A further possibility can complete the analysis on real time basis,which can certainly provide facies and saturation profiles extended to the uncored wells.Application of this methodology in the uncored wells has shown very encouraging results in various well trajectories,either vertical,deviated or horizontal long boreholes.

Preliminary assessment of CO_(2) injectivity in carbonate storage sites

Depleted gas reservoirs are used for a large-scale carbon dioxide(CO_(2))storage and reduction of the greenhouse gas released into the atmosphere.To identify a suitable depleted reservoir,it is essential to do a preliminary and comprehensive assessment of key storage factors such as storage capacity,injectivity,trapping mechanisms,and containment.However,there are a limited number of studies providing a preliminary assessment of CO_(2) injectivity potential in depleted gas reservoirs prior to a CO_(2) storage operation.The aim of this study is to provide a preliminary assessment of a gas field located in Malaysia for its storage potential based on subsurface characterization prior to injection.Evaluation of the reservoir interval based on the facies,cores,and wireline log data of a well located in the field indicated that the pore type and fabrics analysis is very beneficial to identify suitable locations for a successful storage practice.Although the results obtained are promising,it is recommended to combine this preliminary assessment with the fluid-mineral interactions analysis before making any judgment about reliability of storage sites.