A modified failure criterion for transversely isotropic rocks

A modified failure criterion is proposed to determine the strength of transversely isotropic rocks. Me-chanical properties of some metamorphic and sedimentary rocks including gneiss, slate, marble, schist, shale, sandstone and limestone, which show transversely isotropic behavior, were taken into consider-ation. Afterward, introduced triaxial rock strength criterion was modified for transversely isotropic rocks. Through modification process an index was obtained that can be considered as a strength reduction parameter due to rock strength anisotropy. Comparison of the parameter with previous anisotropy in-dexes in literature showed reasonable results for the studied rock samples. The modified criterion was compared to modified Hoek-Brown and Ramamurthy criteria for different transversely isotropic rocks. It can be concluded that the modified failure criterion proposed in this study can be used for predicting the strength of transversely isotropic rocks.

An approach for wellbore failure analysis using rock cavings and image processing

There have been interests to link different cuttings/cavings to various wellbore failure types during drilling. This concept is essential when caliper and image logs are not available. Identification ofwellbore failure during drilling gives more chance of immediate actions before wireline logging program. In this paper, an approach was presented based on the image processing of ditch cuttings. This approach uses the sphericity and roundness of cuttings as input data to classify caving types and subsequently deter- mine the dominant failure type. Likewise, common definitions of cavings were discussed initially before a new criterion is suggested. This quantitative criterion was examined by observations from caliper and acoustic image logs as well. The proposed approach and criterion were implemented on ditch cuttings taken from a well in Western Australia. Results indicate that the primary failure is shear failure （breakout） due to high levels of angular cavings. However, another failure due to the fluid invasion into pre-existing fractures was also recorded by blocky cavings.

An analytical model to predict the volume of sand during drilling and production

Over the past few decades, many optical fiber sensing techniques have been developed. Among these available sensing methods, optical fiber Bragg grating（FBG） is probably the most popular one. With its unique capabilities, FBG-based geotechnical sensors can be used as a sensor array for distributive（profile） measurements, deployed under water（submersible）, for localized high resolution and/or differential measurements. The authors have developed a series of FBG-based transducers that include inclination, linear displacement and gauge/differential pore pressure sensors. Techniques that involve the field deployment of FBG inclination, extension and pore-pressure sensor arrays for automated slope stability and ground subsidence monitoring have been developed. The paper provides a background of FBG and the design concepts behind the FBG-based field monitoring sensors. Cases of field monitoring using the FBG sensor arrays are presented, and their practical implications are discussed.

A numerical study to assess the effect of heterogeneity on CO_(2) storage potential of saline aquifers

Many parameters have been indicated crucial for the selection of a saline aquifer as a carbon dioxide(CO_(2))storage site.However,less attention has been given to the impact of heterogeneity on the performance of these storage media.Thus,the heterogeneity effect was evaluated in this paper by adopting a numerical modeling approach and the existing screening criterion developed for the aquifers was updated.The updated criterion for CO_(2)storage purpose would enhance the confidence level during the selection of deep saline aquifer and thus,help to address the climate change issue.The numerical modeling was carried out via CO_(2)STORE module of Eclipse300 Simulator to evaluate the effect of different levels of heterogeneity on CO_(2)storage potential.Different degrees of heterogeneity from homogenous systems to highly heterogeneous systems in the model were incorporated through the Lorenz coefficient.In this way,simulation of nine cases was carried out for three different aquifers with different porosity values.A comparison of these results showed that heterogeneity causes the aquifer to have lower storage capacity.On the trapping potential,dissolution trapping was significant and the amount of free gas in all cases was minimum.In addition,the aquifer with the highest level of heterogeneity(HLH)had a minimum fraction of residual trapping regardless of porosity.It was also found that final pressure at the end of 30 years is the same and high for low-level heterogeneity(LLH)and medium level heterogeneity(MLH)cases and low for HLH,while the injection rate stability duration is least for HLH and maximum for LLH.Based on the results obtained,it can be concluded that low to medium level heterogeneous aquifers with a good porosity can be a suitable choice for CO_(2)storage.

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.

Analyzing the effect of steam quality and injection temperature on the performance of steam flooding

The heavy oil reservoirs are currently mainly targeted by thermal enhanced oil recovery technologies,particularly,steam flooding.Steam flooding is carried out by introducing heat into the reservoir to unlock the recovery of heavy oil by reducing oil viscosity.Several investigations were carried out to improve oil recovery by steam flooding.Most recently,high steam flooding is reported as an effective approach to improve recovery in high pressure heavy oil reservoirs.The oil recovery from steam flooding is sub-stantially affected by the steam quality and injection temperature.In this study,an attempt was made to look into the integration of parameters,i.e.steam quality and injection temperature upon steam flooding on oil recovery by using a simulation approach via ECLIPSE.The results obtained indicated that high temperature along with the moderate value of steam quality gives the best result regarding oil recovery for steam flooding in an economical way.

Assessment of parameters effectiveness in the reserve estimation methods applicable to coal bed methane reservoirs

The reserve estimation of coal bed methane(CBM)reservoirs is ascertained through the analytical methods(volumetric method,material balance equation and decline curve analysis).However,the adoption of reserve estimation methods depends on exploration stage and availability of the required parameters.This study deals with the analytical assessment of parameters that participate in effecting the reserve estimation of CBM reservoirs through the analytical techniques.The accurate measurement challenges always exist for the parameters which participate in the reserve estimation of the conventional and unconventional reservoirs because of the inclusion of limitations while measurement.Therefore,the impact of that measurement challenge must be assessed.The study specifies the impact of parametric change on the reserve estimation of CBM reservoirs so that the degree of parametric effectiveness is analyzed.Uncertain values are adopted which are associated during the evaluation of input parameters for each method to determine the overall impact on potential of CBM reserves.Results reveal that change in specific parameters considering each method provide relatively more effect on estimation of reserves.Thus,the measurement of parameters must be done accurately for assessing reserves of CBM reservoirs based on available methods.

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.

A nano-particle based approach to improve filtration control of water based muds under high pressure high temperature conditions

There have been many attempts to improve the filtration control of water based muds under High Pressure High Temperature(HPHT)condition using a cost effective approach.Nano particles are perhaps the best option considering their successful applications reported in many studies.However,they are often expensive and pose unfavourably changes on the rheology of the muds.In this paper,an attempt was made to show the application of Nano Glass Flakes(NGFs)as a cheap but effective nano particle to control the filtration of water based muds under HPHT conditions.Performing a series of rheology,filtration and conductivity tests on the mud samples with unmodified NGFs revealed that this nano particle increases the mud rheology,yield point and gel strength of the mud with a slight impact on the filtration loss.However,by modifying the surface charges of NGFs with a cationic surfactant,filtration loss was significantly reduced without any severe impacts on the mud rheology.Considering the conductivity of the mud which increases by adding the modified NGF,this nano particle might be a good choice to improve the overall performance of water based muds under HPHT conditions.

A methodology to improve nanosilica based cements used in CO2 sequestration sites

Attempts to reduce the amount of greenhouse gases released into the atmosphere in recent years have led to the development of Carbon Capture and Sequestration(CCS)technology.However,there have been many studies reporting leakages form CO2 storage sites as a result of cement degradation induced by generation of an acidic environment in the storage site.Although there are a number of approaches proposed to enhance the efficiency of the cement,the degradation issue has not been totally resolved yet perhaps due to the excessive corrosives nature of carbonic acid and supercritical CO2.The aim of this study is to propose a methodology to improve the physical and mechanical characteristics of the cement by nanomodification such that a consistent rheology,constant density and a good strength development can be achieved.A new dispersion technique was proposed to ensure that the cement formulation gives a consistent result.The results obtained indicated that unlike the literature mixing,cement slurries prepared by the new mixing technique are very consistent in their rheology,regardless of the sonication parameters chosen.The measurements of the compressive strength performed at the reservoir condition revealed that nanosilica contributes in the strength development up to a certain point.Thermogravimetric Analysis(TGA)conducted at the last stage indicated that the amount of Portlandite left in the cement by adding nanosilica is decreased due to the pozzolanic reaction,which would help the cement to have a higher chance of survival in a storage site.However,cautions must be taken to maintain a certain amount of Portlandite in the cement for slowing down the carbonation rate,as otherwise the matrix of the cement is attacked directly and the cement will be degraded very fast.

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.

An approach to improve wellbore stability in active shale formations using nanomaterials

Drilling through active shale formations has been a challenging practice in the oil and gas industry for a long period of time,given the complexity of shale structure and its interaction with Water Based Muds(WBMs).Although there have been many additives and methodologies proposed for a safe drilling through shale formations using WBMs,little success has been reported to the application of these methods once tested under different field conditions.In this paper,a new WBM formulated by nanomaterials was proposed to stabilize active shale layers during drilling.A series of rheological,density,filtration loss,bentonite dispersion and shale recovery tests were conducted on the mud samples formulated by nanosilica and Nano Glass Flakes(NGFs).The results indicated that NGF,as a cheap but effective nanomaterial,is able to significantly reduce the flirtation loss without posing any significant impacts on the density and the rheology of WBMs.It also appeared that the bentonite molecules were incapable to either hydrate or disperse in the drilling fluid system in the presence of NGFs.It seems that NGFs can stabilize clay minerals and reduce the filtration loss as remarkably efficient additive,but caution must be taken to ensure that they are properly disperse in the WBMs.

Temperature profile estimation:A study on the Boberg and Lantz steam stimulation model

Cyclic steam stimulation(CSS)is widely used for production from heavy oil reservoirs where oil viscosity is manipulated by heat.Many analytical models have been developed to predict the temperature evolution in the reservoir and estimate the oil recovery.However,they often suffer from a number of assumptions which ultimately reduce their efficiency in providing a realistic prediction.In this study,a numerical solution was proposed for two-dimensional heat conduction in heavy oil reservoirs to obtain the temperature evolution during the soaking period.For a better comparison,an industry widely accepted analytical model,knows as the Boberg and Lantz steam stimulation model,together with its modified version later proposed by Bensten and Donohue were considered to examine temperature changes in a synthetic case study.The results obtained indicated that the analytical solutions overestimate the average temperature of the reservoir by 42%after 300 days of injection while the numerical formulation can provide a close prediction.This numerical approach could be a useful tool to estimate the temperature and oil production from heavy oil reservoirs.

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.