Hydrogen diffusion into caprock: A semi-analytical solution and a hydrogen loss criterion

Depleted gas reservoirs can provide a gigaton capacity and the necessary infrastructure for large scale hydrogen storage. Therefore, underground storage of hydrogen in these geological formations is considered a valuable option. However, the high molecular diffusion of hydrogen from these storage sites is a major concern. In this paper, the idea of hydrogen diffusion through the caprock during geological storage was investigated using a series of one- and three-dimensional numerical models. In these models, the caprock and overlying formations were considered as a semi-infinite system, while the interaction between the reservoir and the caprock was defined by the pressure and gas composition at the boundary. Hydrogen diffusion was based on chemical potential differences and the influence of pre-diffused hydrocarbon gas in the caprock was considered. Furthermore, based on the analogies between the related partial differential equations of diffusion and spontaneous imbibition processes, a simple yet accurate semi-analytical solution was proposed. The presented semi-analytical solution can be used to predict the cumulative hydrogen loss over time. This solution states that the hydrogen loss increases proportionally with the square root of time. Using the semi-analytical solution, a criterion for loss by molecular hydrogen diffusion was presented that indicates the fraction of hydrogen lost by diffusion at a given time. The results obtained based on the diffusion fluxes showed that the hydrogen loss can be underestimated if the existence of hydrocarbon gas in the caprock is ignored. The analysis also indicated that hydrogen loss is directly proportional to the interface between rock and reservoir exposed to hydrogen, the caprock porosity, the gas saturation of the caprock, the square root of the diffusion coefficient, and the square root of time. While the presence of hydrocarbon gas facilitates diffusion, the thermodynamic effects at high pressures lead to a comparatively lower molar density in the caprock than in the matrix. Thus, a lower final loss of the injected hydrogen can be expected at higher pressures.publishedVersio

Data on experimental investigation of Methyl Ester Sulphonate and nanopolystyrene for rheology improvement and filtration loss control of water-based drilling fluid

Data presented in this article focused on the application of Methyl Ester Sulphonate (MES) surfactant and nanopolystyrene in water based drilling fluid. Data from rheology study using Bingham and Power law models showed that the synergy of MES and nanopolystyrene improved the formulated drilling fluid. Filtration study under LPLT and HPHT conditions showed that MES and nanopolystyrene drilling fluid reduced filtration loss by 50.7% at LPLT and 61.1% at HPHT conditions. These filtration data were validated by filter cake permeability and scanning electron microscope images

Application of seismic attributes in structural study and fracture analysis of DQ oil field, Iran

The determination of the most unstable areas in oil fields is critical for addressing engineering problems of wellbore and sand production as well as geologic problems such as understanding dynamic constraints on hydrocarbon migration and fracture permeability. In this research work, coherency seismic attribute has been used for the determination of the most critical areas in terms of drilling stabilities in the DQ oil field, Iran. The results obtained have shown that the (1) predominant features are the SSE–NNW and N–S trends (2) the central part of the DQ structure shows the highest concentration of segment bundles, (3) the segment bundles seem to be aligned along some lineaments oriented SE–NW and SSE–NNW, and (4) on the eastern and western margins of the map there is an anomalous concentration of segments oriented E–W. It can be concluded that coherency attribute is a valuable tool for structural analysis highlighting those areas containing unstable features

A methodology for wellbore stability analysis of drilling into presalt formations: A case study from southern Iran

Drilling into presalt formations has been a long-standing issue due to the rapid changes in the diameter of the borehole during drilling operations either because of creep or wash-out dissolution. There have been many studies on characterization of salts, with many mathematical models being presented to estimate the pressure induced due to the squeezing salt sheets. However, the results of none of these models have been fully validated against real field data and some recommendations have been made based on numerical simulations. In this study, attempts were made to introduce a methodology based on damage mechanics for wellbore stability analysis of a wells drilled in the southern part of Iran. The results obtained indicated that the presence of a thick salt layer in the well has resulted in significant wellbore closure in the intervals above the reservoir section. It was also found that the salt exhibits viscoelastic behaviour during drilling due to the homogeneous temperature which has not reached the threshold limit of viscoplastic boundary. A complicated change in the stress regime was also observed which could be linked to the existence of the thick salt layer or presences of a fault crossing the well. Therefore, it is recommended to further validate this model in other wells using the methodology presented

A review on cement degradation under CO2-rich environment of sequestration projects

Global warming arising from the release of greenhouse gasses into the atmosphere is one of the biggest issues attracting a lot of attention. One of the conventional problems in sequestration projects is the degradation of Portland cement due to its exposure to supercritical CO2. This paper gives a review on the laboratory work performed to understand changes in the mechanical and transport properties of cement when it is in a CO2 rich environment. The results obtained indicated that pozzolanic material could be useful in enhancing the cement resistance against CO2, although more studies are still required to confirm this conclusion

The impact of poor cementing casing damage: A numerical simulation study

A good knowledge of the parameters causing casing damage is critically important due to vital role of casing during the life of a well. Cement sheath, which fills in the gap between the casing and wellbore wall, has a profound effect on the resistance of the casing against applied loads. Most of the empirical equations proposed to estimate the collapse resistance of casing ignore the effects of the cement sheath on collapse resistance and rather assume uniform loading on the casing. This paper aims to use numerical modeling to show how a bad cementing job may lead to casing damage. Two separate cases were simulated where the differences between good and bad cementation on casing resistance were studied. In both cases, the same values of stresses were applied at the outer boundary of the models. The results revealed that a good cementing job can provide a perfect sheath against the tangential stress induced by far-field stresses and reduce the chance of casing to be damaged

Characterization based machine learning modeling for the prediction of the rheological properties of water‑based drilling mud

The successful drilling operation depends upon the achievement of target drilling attributes within the environmental and economic constraints but this is not possible only on the basis of laboratory testing due to the limitation of time and resources. The chemistry of the mud decides its rheological potential and selection of the techniques required for recycling operations. Conductivity, pH, and photometer testing were performed for the physio-chemical characterization of the grass to be used as an environmental friendly drilling mud additive. In this study, different particle sizes (75, 150, and 300 µm) of grass powder were mixed in mud density of 8.5, 8.6, and 8.7 ppg in the measurement of gel strength and viscosity of drilling mud. The grass additive was added in different weight conditions considering no additive, 0.25, 0.5, and 1 g to assess the contribution of grass on the gel strength and viscosity of the drilling mud. The machine learning techniques (Multivariate Linear Regression Analysis, Artificial Neural Network, Support Vector Machine Regression, k-Nearest Neighbor, Decision Stump, Random Forest, and Random Tree approaches) were applied to the generated rheological data. The results of the study show that grass can be used for the improvement of the gel strength and viscosity of the drilling mud. The highest improvement of the viscosity was seen when grass powder of 150 µm was added in the 8.7 ppg drilling mud in 0.25, 0.5, and 1 g weights. The gel strength of the drilling mud was improved when the grass additive was added to the drilling mud 8.7 ppg. Random forest and Artificial Neural Network had the same results of 0.72 regression coefficient (R2) for the estimation of viscosity of the drilling mud. The random tree was found as the most effective technique for the modeling of gel strength at 10 min (GS_10min) of the drilling mud. The predictions of Artificial Neural Network had 0.92 R2 against the measured gel strength at 10 s (GS_10sec) of the drilling mud. On average, Artificial Neural Network predicted the rheological properties of the mud with the highest accuracy as compared to other machine learning approaches. The work may serve as a key source to estimate the net effect of grass additives for the improvement of the gel strength and viscosity of the drilling mud without the performance of any large number of laboratory tests.publishedVersio

Brittleness of gas shale reservoirs: A case study from the north Perth basin, Australia

Shale reservoirs have gained the attention of many in recent years due to their potential as a major gas resource. Production from this kind of formation, however, requires an accurate estimation of brittleness and employments of hydraulic fracturing. There have been many studies as to how brittleness can be estimated, but few research works were carried out so far indicating how brittleness indices vary in gas shale formations. The aim of this paper is to evaluate the variation of brittleness in one of the gas shale reservoirs located in the north Perth Basin of Australia. The results obtained indicated that the lower part of the Carynginia shale should be selected for a hydraulic fracturing job due to a high brittleness index, although a careful analysis of Total Organic Content (TOC) might be required before initiating any plans. The mineralogical report and interpretations revealed that the space created by cross-plotting the elastic parameters is able to identify dominant minerals contributing into brittleness. Performing a series of true triaxial tests, which are capable of simulating the real field condition by applying three independent principal stresses, implied that as the stress anisotropy increases, a transition takes place from brittle towards the ductile behaviours. However, when this anisotropy becomes significant, samples regain their strength. This study, therefore, recommends more studies to get a practical conclusion on brittleness under true triaxial conditions

Optimum directional well path design considering collapse and fracture pressures

Well path optimisation is often done based on the wellbore stability where the production related concerns are ignored. In fact, many of the studies carried out in the past have not included hydraulic fractures into their calculations. In fact, an optimum path for wellbore should not only provide the maximum stability during drilling but also offer a relatively low pressure to fracture the formation in the production stage. In this study, attempts are made to provide a methodology to determine an optimum well path for drilling, hydraulic fracturing and production stage using wellbore stability analysis in different stress regimes. An analytical model was proposed and used to determine the collapse pressure and fracture gradient during drilling and hydraulic fracturing at various azimuths and inclinations. The results obtained revealed that the well path does not change in a normal faulting regime during production. However, the azimuth and inclination of wells may need to be changed in the strike-slip and reverse fault regimes for a better drilling and fracturing. It was also found that deviated wells can be a better option in normal and strike-slip stress regimes, but further studies might be needed to confirm these findings.acceptedVersio

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.publishedVersio