Deep-buried Reservoirs Research Articles

On faults induced stress shielding to account for the formation of deep-buried reservoirs

On faults induced stress shielding to account for the formation of deep-buried reservoirs

Lithofacies logging identification for strongly heterogeneous deep-buried reservoirs based on improved Bayesian inversion: The Lower Jurassic sandstone, Central Junggar Basin, China

The strong heterogeneity characteristics of deep-buried clastic low-permeability reservoirs may lead to great risks in hydrocarbon exploration and development, which makes the accurate identification of reservoir lithofacies crucial for improving the obtained exploration results. Due to the very limited core data acquired from deep drilling, lithofacies logging identification has become the most important method for comprehensively obtaining the rock information of deep-buried reservoirs and is a fundamental task for carrying out reservoir characterization and geological modeling. In this study, a machine learning method is introduced to lithofacies logging identification, to explore an accurate lithofacies identification method for deep fluvial-delta sandstone reservoirs with frequent lithofacies changes. Here Sangonghe Formation in the Central Junggar Basin of China is taken as an example. The K-means-based synthetic minority oversampling technique (K-means SMOTE) is employed to solve the problem regarding the imbalanced lithofacies data categories used to calibrate logging data, and a probabilistic calibration method is introduced to correct the likelihood function. To address the situation in which traditional machine learning methods ignore the geological deposition process, we introduce a depositional prior for controlling the vertical spreading process based on a Markov chain and propose an improved Bayesian inversion process for training on the log data to identify lithofacies. The results of a series of experiments show that, compared with the traditional machine learning method, the new method improves the recognition accuracy by 20%, and the predicted petrographic vertical distribution results are consistent with geological constraints. In addition, SMOTE and probabilistic calibration can effectively handle data imbalance problems so that different categories can be adequately learned. Also the introduction of geological prior has a positive impact on the overall distribution, which significantly improves the accuracy and recall rate of the method. According to this comprehensive analysis, the proposed method greatly enhanced the identification of the lithofacies distributions in the Sangonghe Formation. Therefore, this method can provide a tool for logging lithofacies interpretation of deep and strongly heterogeneous clastic reservoirs in fluvial-delta and other depositional environments.

Target-oriented high-resolution elastic full-waveform inversion with an elastic redatuming method

Characterizing the elastic properties in deep-buried reservoirs beneath complex overburden structures remains challenging for seismic inversion. Elastic full-waveform inversion (FWI) is capable of quantitatively estimating the subsurface elastic properties with a reasonably high resolution. However, elastic FWI using high frequencies is computationally expensive because fine discretization is required to stabilize the wavefield simulation. In addition, it is challenging for elastic FWI to obtain the high-resolution inversion results of the deep targets due to complex overburden structures and limited energy illumination of the target of interest. To overcome these limitations, we propose a target-oriented high-resolution elastic FWI scheme by using the estimated elastic data for a virtual survey deployed only above a zone of interest. Specifically, we have developed an elastic redatuming approach to retrieve virtual elastic data by solving an iterative least-squares data-fitting problem. In addition to the elastic multicomponent data recorded at the acquisition surface, an estimate of the overburden model is required. A synthetic data example indicates that an overburden model estimated by elastic FWI with a low-frequency band enables our elastic redatuming approach to reconstruct the virtual elastic data representing the seismic reflection response from the target zone. We then perform regularized elastic FWI by using the redatumed elastic data to estimate the elastic properties in the target zone with reduced computational cost. In the numerical examples, the Marmousi2 synthetic data and 2D Volve field data are used to demonstrate the feasibility and practicality of the proposed method.

Effects of functional groups on supercritical CH4 adsorption and desorption characteristics of tectonically deformed coals

With the expansion of coalbed methane exploration and development to deep-buried reservoirs, supercritical CH4 adsorption/desorption on coals have become a key scientific issue. The evolution of functional groups of tectonically deformed coals are bound to affect supercritical CH4 adsorption/desorption characteristics of the coal. In this paper, five coal samples of three deformation series were selected. Supercritical CH4 adsorption/desorption characteristics, i.e. excess/absolute adsorption/desorption isotherms, adsorption model, maximum adsorption capacity and desorption hysteresis, were investigated. Evolution models and dynamic mechanisms of functional groups with coal deformation were discussed based on Fourier Transform Infrared Spectroscopy and 13C Nuclear Magnetic Resonance analyses. Finally, the controlling mechanisms of functional groups evolution on adsorption capacity of tectonically deformed coals were analyzed. Results show that supercritical CH4 excess adsorption and desorption isotherms of tectonically deformed coals exhibit the “single peak” change trend with a maximum value. The maximum value tends to move towards the high pressure zone with increasing adsorption capacity. The absolute adsorption and desorption branches increase monotonically with increasing pressure. Supercritical CH4 desorption hysteresis decreases with increasing adsorption capacity. Oxygen-containing functional groups (CO, COOH, OH and oxygen-bonded aliphatic/aromatic carbons) and aliphatic carbons (methyl, methylene, submethyl and quaternary carbons) are highly sensitive to stress, while aromatic CC and other aromatic carbons are difficult to change. Only some oxygen-containing functional groups and aliphatic structures of brittle deformed coals are reduced due to cataclasis and grallization, and thus adsorption capacity shows the unobvious increase. Strong ductile shear deformation results in widely competitive evolution of functional groups and increased degree of coal aromatization, which thereby induces obviously increased adsorption capacity and decreased desorption hysteresis.

Experimental study on microscopic mechanisms and displacement efficiency of N2 flooding in deep-buried clastic reservoirs

Current clastic reservoirs with burial depth of more than 5000 m are important energy sources. And microscopic initiation mechanism of residual oil after water flooding has been regarded as the key to improving oil recovery by gas flooding. In this study, an investigation was conducted in the occurrence states and distribution of residual oil of deep-buried reservoirs from microscopic pore scale. We innovatively designed a microscopic visualization experimental equipment for the first time, which broke through the limitations of visualization accuracy, temperature and pressure of the conventional experimental devices. Microscopic visualization experiments of gas injection after waterflooding at 115 °C and 55 MPa were carried out. According to the permeability and pore-throat 0 structure characteristics of deep-buried clastic reservoir, a two-dimensional glass etching micromodel was designed and fabricated. Meanwhile, the occurrence state and formation mechanism of multi-type residual oil were quantitatively characterized by image processing technology. The results showed that there were five kinds of residual oil which were corner, droplet, membranous, columnar and cluster after water flooding. In addition, it was generally concentrated in the fine pore-throats with greater resistance perpendicular to the main stream line. Two mechanisms, immiscible flooding and miscible flooding, were also found in N 2 flooding. Moreover, the injected gas carried, squeezed and dispersed the continuous clusters of residual oil, transforming them into dispersive shapes such as corner and membranous. And a certain amount of microscopic residual oil after water flooding was effectively mobilized by removing the water locking effect. The oil recovery could be improved by 11.49% in low-permeability zone, which was the highest, since the continuous cluster residual oil was decreased significantly. Membranous, columnar and cluster residual oil were well mobilized in the medium-permeability zone, and oil recovery was increased by 8.31%. In comparison, the oil recovery in high-permeability zone was only increased by 2.53%, and there was no cluster oil. Our study highlights the migration characteristic and initiation mechanism of residual oil in porous media under N 2 injection after waterflooding from the perspective of microscopic visualization, which may help to promote the development of deep-buried clastic reservoirs. • Microscopic experimental device was built to simulate 55 MPa and 115 °C. • Micromodels suitable for deep-buried clastic reservoir are designed and made. • A quantitative characterization method of micro-remaining oil occurrence state is established. • The migration characteristics and initiation mechanism during gas injection after waterflooding are clarified.

The effect of grain size, porosity and mineralogy on the compressive strength of tight sandstones: A case study from the eastern Ordos Basin, China

To better understand how textural properties and mineralogy control compressive strength, this work performed the grain size analysis, porosity measurement, X-ray diffraction, microscopic observation, and mechanical tests on deep-buried reservoir sandstones in the eastern Ordos Basin, China. The results suggest that uniaxial compressive strength (UCS) declines with increasing grain size. Meanwhile, the UCS is reduced by grains >250 μm and enhanced by grains <250 μm. Sandstones with higher fractal dimensions are poorer-sorted and have platykurtic distribution, while those with low fractal dimensions are better-sorted and show leptokurtic distribution. The former tends to have larger UCS than the latter because they have more smaller grains to distribute the load over larger grains. Both the UCS and confined compressive strength (CCS) exponentially decline with porosity. Internal friction angle and influence coefficient decrease with increasing porosity, suggesting that the compressive strength of low-porosity sandstones is more sensitive to the variation of confining pressure. With regard to mineralogy, there is a weak negative relationship between UCS and quartz content. It is because textural properties control the UCS, dominating over the effects of mineralogy, or alternatively stress concentrations and the initiation of cracks preferentially occur in brittle quartz grains rather than softer materials. • Uniaxial compressive strength is reduced by grains >250 μm and enhanced by grains <250 μm. • Poorer-sorted sandstones have larger fractal dimensions and uniaxial compressive strength. • Compression-induced cracks do not preferentially propagate along grain boundaries. • The compressive strength of lower-porosity sandstones is more sensitive to confining pressure.

Prediction and modeling of petrophysical parameters of deep-buried, low permeability glutenite reservoirs in Yubei area, Turpan-Hami Basin, China

The reservoir geological model can provide detailed reservoir characterizations for oil and gas development, thereby addressing the problems of sparse well patterns and lack of data for deep-underground reservoirs in the Yubei area. In this study, stochastic simulations based on geostatistical analyses with seismic attribute constraints were used to predict the physical properties of the Wutonggou Formation reservoirs and establish a fine 3D geological model. Log interpretation models were established to represent the longitudinal distribution of petrophysical parameters. Next, the structural framework of the strata was constructed based on seismic interpretations, and seismic attribute analysis was subsequently performed. Genetic inversion and neural network algorithms were used to obtain acoustic impedance data and predict petrophysical parameters. The planar distribution characteristics of the parameters were then clarified. Finally, petrophysical models were established using Gaussian random function simulation under the control of the facies model, which in turn was simulated via sequential indicator simulation. The model results were evaluated based on the well and production data, and the model parameters exhibited distribution patterns similar to the well data. The model parameters also corresponded well with production data, exhibiting a compliance rate of 90.98%. The model shows that the study area has a large range of reservoir depths and is generally oriented NW-SE. The Wutonggou Formation mainly developed oil-bearing reservoirs of different depositional types at the bottom of the third and first members, and the thickness of the depositions increased significantly with depth. The reservoirs, as a whole, are characterized by medium-low porosity and low permeability. Furthermore, compaction has led to significant reductions in porosity and the formation of microcracks in the glutenite of deep-buried reservoirs, which play important roles in improving reservoir permeability and hydrocarbon flow properties.

Experimental investigation on the production performance from oceanic hydrate reservoirs with different buried depths

Buried depth, as an inherent occurrence feature of hydrate reservoir, plays a significant role in fluid production during hydrate dissociation. In this study, we experimentally investigate the production performance of hydrate reservoirs at various buried depths beneath the seafloor. The hydrate-bearing system is synthesized in quartz sand with grain size varying between 100 and 500 μm in a 0.98 L reactor. Similar hydrate saturation is obtained at different prevailing pressures between 5.6 and 8.8 MPa. Depressurization experiments are designed to investigate the effect of buried depths on fluid production behavior. The results show that gas and water production increase with elevated buried depths at the same production pressure. However, based on the gas to water ratio, a deep-buried reservoir has a higher production potential at the initial stage. In contrast, a shallow-buried reservoir is an ideal candidate for fluid production in the later stage. Depressurization to the equilibrium P-T condition could lead to potential hydrate dissociation, but the rate is less intensive with less than 46.0 vol% hydrates dissociated in a prolonged time. The experimental results also reveal that both the design of depressurization and the reservoir depths have a combined effect on the overall fluid production performance.

Deep-learning assisted regularized elastic full waveform inversion using the velocity distribution information from wells

SUMMARYElastic full waveform inversion (EFWI) can, theoretically, give high-resolution estimates of the subsurface. However, in practice, the resolution and illumination of EFWI are limited by the bandwidth and aperture of seismic data. The often-present wells in developed fields as well as some exploratory regions can provide complementary information of the target area. We, thus, introduce a regularization technique, which combines the surface seismic and well log data, to help improve the resolution of EFWI. We use deep neural networks to learn the statistical relations between some selected features of the inverted model and the facies interpreted from well logs. The selected features are the means and variances of the inverted velocities defined within Gaussian windows. Using multiple fully connected layers, we train our neural networks to identify the relation between the means and variances at the well location and those from the inverted model. The network is used to map the means and variances extracted from the well to the whole model domain. We then perform another EFWI in which we fit the predicted data to the observed ones as well as fit the model over a Gaussian window to the predicted means and variances. The tests on synthetic and real seismic data demonstrate that the proposed method can effectively improve the resolution and illumination of deep-buried reservoirs, which often encounter poor illumination from seismic data.

Hydrocarbon charging and accumulation in the Permian reservoir of Wangguantun Buried Hill in Huanghua Depression, Bohai Bay Basin, China

The Permian reservoirs of Wangguantun Buried Hill are deep buried reservoirs in the Bohai Bay Basin, with a depth of more than 4000 m. The superposition of multi-stage tectonic movements also increases the complexity and unclear nature of the hydrocarbon charging and accumulation history. This study aims to address these issues by considering the structural evolution and burial–thermal history of the strata and studying the hydrocarbon expulsion process of source rocks by means of rock pyrolysis experiments. In addition, petrography, fluorescence spectroscopy, GC-MS (gas chromatography–mass spectrometry) of the oil from inclusions, laser Raman spectroscopy, and microthermometry of fluid inclusions were conducted and these analyses were coupled with cathodoluminescence tests of minerals and typical geological profiles to reveal the hydrocarbon charging process of the Permian reservoir in Wangguanshan Buried Hill. The results show that the oil of the Permian reservoirs in Wangguantun Buried Hill is generated from both upper Palaeozoic coal-measure and Paleogene lacustrine mudstone. The corresponding vitrinite reflectance (Ro) values at the expulsion thresholds of the Palaeozoic coal, Palaeozoic carbonaceous mudstone, Palaeozoic mudstone, and Paleogene mudstone are 0.68%, 0.72%, 0.72% and 0.73%, respectively. The Permian reservoirs have experienced three oil and gas charging periods. In the first period, during the late Jurassic to early Cretaceous (135–100 Ma), the reservoirs were charged primarily with less mature oil. The subsequent uplift of the strata in the late Cretaceous exposed the ancient reservoir, and most of the oil transformed into carbonaceous bitumen. In the second period, the middle and late Paleogene (45–33 Ma), the reservoirs were charged primarily with less mature oil from the Paleogene source rocks and mature to highly mature oil and gas from the coal-measure source rocks. In the third period, from the late Neogene to the present (12–0 Ma), the reservoirs were primarily charged with highly mature oil from the Paleogene mudstone and highly mature and overmature oil and gas from the coal-measure. Fractures provide the main storage space for hydrocarbons, while some were stored in intergranular and dissolved pores. Consequently, fractured reservoirs should be important targets for gas and petroleum exploration in such deep reservoirs. In addition, the regional structure evolution controlled the hydrocarbon expulsion and charging processes, which guided the formulation of the hydrocarbon accumulation model of the Permian reservoir.

High-resolution reservoir characterization using deep learning-aided elastic full-waveform inversion: The North Sea field data example

Reservoir characterization is an essential component of oil and gas production, as well as exploration. Classic reservoir characterization algorithms, deterministic and stochastic, are typically based on stacked images and rely on simplifications and approximations to the subsurface (e.g., assuming linearized reflection coefficients). Elastic full-waveform inversion (FWI), which aims to match the waveforms of prestack seismic data, potentially provides more accurate high-resolution reservoir characterization from seismic data. However, FWI can easily fail to characterize deep-buried reservoirs due to illumination limitations. We have developed a deep learning-aided elastic FWI strategy using observed seismic data and available well logs in the target area. Five facies are extracted from the well and then connected to the inverted P- and S-wave velocities using trained neural networks, which correspond to the subsurface facies distribution. Such a distribution is further converted to the desired reservoir-related parameters such as velocities and anisotropy parameters using a weighted summation. Finally, we update these estimated parameters by matching the resulting simulated wavefields to the observed seismic data, which corresponds to another round of elastic FWI aided by the a priori knowledge gained from the predictions of machine learning. A North Sea field data example, the Volve Oil Field data set, indicates that the use of facies as prior knowledge helps resolve the deep-buried reservoir target better than the use of only seismic data.

Methane in Soil Gas and Its Migration to the Atmosphere in the Dawanqi Oilfield, Tarim Basin, China

Methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the Earth’s surface, and it is quite common in hydrocarbon-prone basins. In this study, by analyzing gas concentrations and isotope composition of soil gas, the potentials of CH4 gas transferred to the surface were studied at three measurement transects in Dawanqi oilfield, Tarim Basin, China. It was found that CH4 from deep-buried reservoirs could migrate upwards to the surface through faults, fissures, and permeable rocks, during which some CH4 was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. Soil gas samples had mean concentrations of 907.1, 62.3, 21.7, 11.0, and 5.8 ppmv for CH4, C2H6, C3H8, C4H10, and C5H12, respectively. The C1/C2+ (13.3 for soil gas and 3.75 for absorbed gas) and gas wetness ratio (12% for soil gas and 26% for absorbed gas) suggested that the hydrocarbons were derived from a thermogenic process. According to isotope composition analysis, the δ13CCO2, δ13CCH4, and δDCH4 values for the soil gas from Dawanqi oilfield varied from -15.5 to -17.2‰, -11‰ to -17‰, and -150 to -189‰, respectively. The extreme 13C enrichment in CH4 is possibly because of the fractionation effects of diffusional migration and methanotrophic oxidation. Soil gas and absorbed gas showed high CH4 concentrations at the edge of the fault block, which indicated that fault was conductive to gas migration. Also, gas migrated from the surface to the atmosphere in the center region of the fault block because of the high permeability and shallow depth of the reservoir in Dawanqi oilfield.

Secondary alterations of deep buried Lower Carboniferous terrigenous rocks in some areas of the Dnieper-Donets basin

Problematics. Due to the ambiguous understanding of the nature and void space distribution within reservoir rocks, prediction of prospective deep buried and low permeable reservoirs becomes more uncertain. There are many factors influencing on the pore space, such as primary rock composition, hydrogeological regime, tectonic processes, geological time, epigenetic processes etc. Moreover, in explanation of different factors the influence of secondary reservoirs formation, systematization problems become more essential. Either, it is important to identify ‘working’ conditions of influencing factors to make the prediction of hydrocarbon deposits at great depth more certain. Finding universal criteria of prospective secondary reservoir rocks identification will allow us to enhance the discovery rate of deep buried hydrocarbon deposits. Analysis of previous studies. Many researches were devoted to epigenetic processes in the second part of XXth century. There were described major secondary alterations which may occur at great depth. Recent publications showed the expansion of previous knowledge about epigenetic processes and their influence on the secondary deep buried reservoir formation. Some scientists state that major influencing factors on reservoir quality are tectonic and primary mineral composition, others bring epigenetic imposed processes to the top of influencing factors. Purpose. The purpose of the research is to study secondary alterations and find possible zonation based on the alteration intensity of Carboniferous (Viséan and Serpukhovian) rocks within Central part of the Dnieper-Donets basin. Methods. During this research we studied 43 thin sections of Carboniferous sandstones from cores of Central part of the Dnieper-Donets basin, using polarizing microscope. Alteration stage analyses were used for determination of mineral sequence transformations. Results. Carboniferous (Viséan and Serpukhovian) sandstones discovered by deep wells within Solokhivsko-Dykanskyi and Berezivsko-Kotelevskyi archs in Central part of the Dnieper-Donets depression have been studied. Microscopic studies have shown widespread development of imposed epigenetic alterations in terrigenous rocks. The involvement of laboratory evaluation of porosity and permeability, as well as the values of formation temperatures, allowed us to determine the main trends of secondary processes effect on reservoir properties of rocks. It is shown that the development of secondary microfracturing, kaolinitization and sideritization increases the value of porosity and permeability. Dissolution, regeneration and redeposition of quartz are accompanied by calcitization and lead to the significant decrease of the reservoir properties of terrigenous rocks and formation of secondary epigenetic seals. Sideritization facilitates the development of secondary reservoirs at great depth, meanwhile calcitization leads to decrease of the pore space. Bitumoids and pyrite presence may be the evidence of hydrocarbon migration process. Combination of the research results with thermobaric data retrieved directly from borehole measurements as well as theoretically calculated on Bilske gas condensate field shows that formation of epigenetic seal in the temperature interval of 110-120 °C is caused by quartz dissolution and intensive calcitization of terrigenous rocks. Consequently, secondary reservoir rocks below the epigenetic seal have local spreading caused by microfracturing, sideritization and calcitization. Specific secondary sandy reservoirs of Berezivske gas condensate field are located within 110 and 120 °C isotherm. They occurred in acid environment leading to dissolution of calcite cement and intense sandstone kaolinitization. Such favorable conditions for secondary porosity formation may occur as a result of possible tectonic movements inversion which triggered the development of regressive epigenesis.

Hydrolysis of Pyrite: a Possible Pathway for Generation of High Concentrations of H2S Gas in Deep-buried Reservoirs?

Hydrolysis of Pyrite: a Possible Pathway for Generation of High Concentrations of H2S Gas in Deep-buried Reservoirs?

Pyrite‐hydrocarbon Interaction under Hydrothermal Conditions: an Alternative Origin of H2S and Organic Sulfur Compounds in Sedimentary Environments

AbstractSulfate rocks and organic sulfur from sedimentary organic matter are conventionally assumed as the original sulfur sources for hydrogen sulfide (H2S) in oil and gas reservoirs. However, a few recent experiments preliminarily indicate that the association of pyrite and hydrocarbons may also have implications for H2S generation, in which water effects and natural controls on the evolution of pyrite sulfur into OSCs and H2S have not been evaluated. In this study, laboratory experiments were conducted from 200 to 450°C to investigate chemical interactions between pyrite and hydrocarbons under hydrothermal conditions. Based on the experimental results, preliminary mechanism and geochemical implications were tentatively discussed. Results of the experiments showed that decomposition of pyrite produced H2S and thiophenes at as low as 330°C in the presence of water andn‐pentane. High concentrations of H2S were generated above 450°C under closed pyrolysis conditions no matter whether there is water in the designed experiments. However, much more organic sulfur compounds (OSCs) were formed in the hydrous pyrolysis than in anhydrous pyrolysis. Generally, most of sulfur liberated from pyrite at elevated temperatures was converted to H2S. Water was beneficial to breakdown of pyrite and to decomposition of alkanes into olefins but not essential to formation of large amounts of H2S, given the main hydrogen source derived from hydrocarbons. In addition, cracking of pyrite in the presence of 1‐octene under hydrous conditions was found to proceed at 200°C, producing thiols and alkyl sulfides. Unsaturated hydrocarbons would be more reactive intermediates involved in the breakdown of pyrite than alkanes. The geochemistry of OSCs is actually controlled by various geochemical factors such as thermal maturity and the carbon chain length of the alkanes. This study indicates that the scale of H2S gas generated in deep buried carbonate reservoirs via interactions between pyrite and natural gas should be much smaller than that of thermochemical sulfate reduction (TSR) due to the scarcity of pyrite in carbonate reservoirs and the limited amount of long‐chained hydrocarbons in natural gas. Nevertheless, in some cases, OSCs and/or low contents of H2S found in deep buried reservoirs may be associated with the deposited pyrite‐bearing rock and organic matters (hydrocarbons), which still needs further investigation.

Methane in soil gas and its transfer to the atmosphere in the Yakela condensed gas field in the Tarim Basin, Northwest China

In this study, by analyzing CH4 concentration and in soil-gas profiles, the potentials of CH4 gas transfer from ground to atmosphere were studied at four representative sectors in the Yakela condensed gas field in the Tarim Basin, Xinjiang, China. These are: 1) the oil-gas interface sector, 2) fault sector, 3) oil-water interface sector, 4) an external area. Variation in CH4 in soil-gas profiles showed that CH4 microseepage resulted from the migration of subsurface hydrocarbon from deep-buried reservoirs to the earth’s surface. It was found that CH4 from deep-buried reservoirs could migrate upwards to the surface through faults, fissures and permeable rocks, during which some CH4 was oxidized and the unoxidized methane remained in the soil or was emitted into the atmosphere. The lowest level of CH4 at the soil-gas profile was found at the CH4 gas-phase equilibrium point at which the CH4 migration upwards from deep-buried reservoirs and the CH4 diffusion downwards from the atmosphere met. The and ethane, propane in soil gas exhibited thermogenic characteristics, suggesting the occurrence of CH4 microseepage from deep-buried reservoirs. A linear correlation analysis between CH4 concentrations in soil gas and temperature, moisture, pH, Eh, Ec and particle size of soil indicated that both soil Eh and soil temperature could affect CH4 concentration in soil gas while soil pH could indirectly influence soil methanotrophic oxidation via impacting soil Eh.

Sour-Gas-Reservoir Exploitation in Croatia

Summary Because fossil fuels are still dominant sources of energy supply, the petroleum industry is called upon not only to provide an effective management of oil and gas reserves in order to meet rising energy demand, but also to do that in a safe and efficient manner, with as small an ecological footprint as practically possible. Consequently, also taking into account the fact that conventional oil and gas reserves are declining, petroleum companies are forced to develop and adopt new technologies to increase oil and gas recovery and to expand their upstream activities to still unexploited areas, which often implies development of deep-buried oil and natural-gas reservoirs characterized by unfavorable reservoir conditions such as high temperature and pressure and even a certain amount of impurities. Croatian experience with natural-gas production from deep-buried reservoirs is based on the development of several gas fields in the northwestern part of Croatia. The development of the largest natural-gas fields in Croatia—Molve, Stari Gradac, and Kalinovac gas fields—began at the beginning of the 1980s. The main characteristic of all the mentioned fields are extremely unfavorable reservoir conditions, with reservoir depth of more than 3000 m, high initial reservoir pressures (more than 450 bar), high temperature (180°C), and a significant share of CO2 (10 to 54%), H2S (800 ppm), and some other nonhydrocarbon compounds such as mercaptans (30 mg/m3) and mercury (1000 to 1500 μg/m3). Several other gas fields with similar reservoir conditions were discovered and developed in the last 25 years in the same region. Today, the petroleum industry in Croatia has almost 30 years of experience in processing sour natural gas with a well-established methodology of auditing processing-plant outlet-gas influences on the environment. These experiences and future plans regarding this subject will be presented in this paper.

Overview: Drilling and Cementing Technology (February 2007)

2006 confirmed foreseen changes. The demand for oil was near the offer or production capacity and the rather stable high oil prices induced new drilling programs. The result was a drilling-rig market with use near 100% and new rig construction for the offshore fleet nearing 100 rigs. The wake-up was rather sudden. If current programs use mostly conventional drilling technology while constant technology evolution improves productivity, new prospects soon will require new drilling technology. Papers highlighted here and listed for additional reading describe recent technology advances. R&amp;D programs in 2005 and 2006 anticipated the need for new technology.In the ultradeep offshore, water depth more than 10,000 ft, conventional riser architecture has reached its limit; the hyperstatic riser is one solution.In deep prospects and extended-reach wells that require multiple intermedi-ate casings, monobore architecture renders these projects feasible.In deep buried reservoirs, the effects of high temperature require careful review of the cement formulation.In some mature or shallow unconsolidated prospects with low fracture gradients, the use of ultralight cements may be required.For production from acidic environments, long-term cement-sheave durability must be studied. Noticeable efforts have been made to improve more-conventional drilling.Understanding drillstring behavior and taking into account contact forces related to string bending will improve trajectory prediction, and its monitoring will certainly increase the overall rate of penetration.Various methods of cuttings disposal exist, and the understanding of cuttings reinjection is improving.The reliability of cement-bond logs has improved with respect to zone isolation, especially when communication tests are impractical or cost-prohibitive. Drilling and Cementing Technology additional reading available at the SPE eLibrary: www.spe.org SPE 99105 "Durability of Oilwell Cement Formulations Aged in H2S-Containing Fluids" by E. Lecolier, SPE, Inst. Français du Pétrole, et al. SPE 98124 "Ultralightweight Cementing Technology Sets World Record for Liner Cementing With a 5.4-lbm/gal Slurry Density" by D. Kulakofsky, SPE, Halliburton, et al. SPE 99005 "First Hyperstatic Riser Joint Field Tested for Deep Offshore Drilling" by Y. Poirette, SPE, Inst. Français du Petróle, et al. SPE 98965 "Advancements in 3D Drillstring Mechanics: From the Bit to the Topdrive" by S. Menand, SPE, École des Mines de Paris, et al.

Overview: Drilling and Cementing Technology (February 2006)

During 2005, the oil industry paradigm changed somewhat. The media highlighted doubt concerning world oil reserves expressed in time expectancy. Is it less than the sliding 40-year scenario? There are optimistic and pessimistic scenarios. During the year, demand and supply coincided. The oil prices increased drastically as a consequence of the growth of China, India, and other countries. On the other hand, environmental specialists have alarming forecasts with respect to the greenhouse effect and temperature increase because of CO2 accumulation. Initially, the capture of CO2 probably will lead to more wells being drilled. As a result of these contradictory parameters, the drilling industry has many challenges ahead to deal with instantaneous lack of oil and future complex developments. The giant fields are reputed as discovered. Maybe the potential lies in some unexplored region (e.g., east Siberia), deep buried reservoirs (onshore and offshore), or better drainage of existing reservoirs improving the recovery factor to 65% or more. To renew the reserves, the drilling community faces many challenges. For deep reservoirs, in some instances ultradeep wells, those challenges lie in ultrahigh pressure, high temperature, and narrow mud-weight windows. Therefore, a new casing design (e.g., monobore) and new engineering and operation procedures will be required. For enhancing the life of existing reservoirs, the technology of through-tubing completions opens new perspectives. Cementing operations, often underevaluated, are of prime importance. Recent developments have led to a wider range of cement-slurry density, new well designs, and simplifying of operating procedures. Coming back to basics, with respect to destruction of rock, let's dream: Why not a revolution using laser technology? Future generations of drillers will write case-history papers on laser-drilled wells. How soon—in 2050, 2030, maybe sooner? Drilling and Cementing Technology additional reading available at the SPE eLibrary: www.spe.org SPE 92194 "Maximizing Drill Rates With Real-Time Surveillance of Mechanical Specific Energy," by Fred E. Dupriest, SPE, ExxonMobil, et al. SPE 95888 "Benchmarking Deep-Drilling Technologies," by S.W. Lambert, Schlumberger Consulting Services, et al. SPE 96927 "Why Don't We Pump More Type I/II Lightweight Slurries?" by R.R. Myers, SPE, BJ Services Co. Available at the OTC Library: www.otcnet.org OTC 17459 "Drilling Underbalanced From a Floating Unit in a 1500-m-Water-Depth Exploratory Well: Planning, Equipment, and Rig Modifications," by A.C.V.M. Lage, Petrobras

Overview: Drilling Technology (February 2005)

In the early 1900s, my grandmother ran to a field near the village of Le Bourget (future first Paris Airport) to see the trial of a pioneer aeroplane. Seventy years later, the flight time between Paris and New York was approximately 3 hours 30 minutes on the Concorde. At college, I worked with a slide rule; today, my laptop has many times the computing power of the NASA computer used when Neil Armstrong landed on the moon. Dupuis Such revolutions in drilling are not so quantifiable or spectacular; evolution improves the performance continuously. Even during crises, the drilled footage has remained constant. During the last 30 years, the number of bits manufactured has been reduced by a factor of 10, yet we now drill in water depths of 10,000 ft. Greatest progress is driven mainly by the challenge of the spatial location of a new prospect and the obligation to improve recovery, hence the deepwater extended-reach multilaterals. The new challenge is to reach the deep buried reservoir with vertical depth of approximately 30,000 ft. The borehole pressure, the narrow mud windows, and high temperature trigger new technical development and lead to reconsidering monobore design as the well architecture. The expandable-casing technology is very promising. The second aspect is the change between engineering criteria and a probabilistic approach (for instance, applied to casing design where the probability of a situation or event is rendered acceptable) by improving safety procedures and equipment. This aspect also applies to underbalanced drilling with new procedures and devices. Highlights of promising evolution include drilling-hydraulics optimization by revisiting classical drillpipe dimension and design to decrease the pressure losses (e.g., use of 4-in. drillpipe with streamline connections in a 6-in. hole); reconsideration of trajectory and drag and torque computations in directional drilling, taking into account drillstring stiff-ness, which could make future models more accurate; and future development of a low-cost 3D rotary-drilling system to improve onshore performance. We regret that if drilling models improve, there is too little evolution and willingness to acquire drilling-parameter data, the weight on hook is deducted from the tension of the dead line for a difficult assessment of the systematic error caused by line reeving, rendering friction analysis difficult. Evolution is steady, but has been rather spectacular when you look closely. Additional Drilling Technology Technical Papers Available at the SPE eLibrary: www.spe.org SPE 89959 - The Trouble-Probability Plot: A New Measure of Drilling Maturity SPE 91583 - Using Rotating Control Devices Safely in Today’s Managed-Pressure and Underbalanced-Drilling Operations SPE 87217 - Increasing Effective-Fracture Gradients by Managing Wellbore Temperatures Available at the OTC Library: www.otcnet.org OTC 16564 - Casing Drilling With Retrievable Drilling Assemblies