The Effect of Temperature on Irreducible Water Saturation of Water-wet Cores

The conventional measurement of a relative permeability curve (RPC) is usually conducted at room temperature, which is much lower than the reservoir temperature. Previous research work on high temperature relative permeability mainly take oil-wetted cores as objective. In this paper, laboratory test and measurement are conducted using water-wet cores from the Lunnan Oilfield. Since irreducible water saturation (Swi) is a critical factor that affects and controls the relative permeability curve, special tests are conducted to measure Swi at different temperatures for water-wet cores in the course of the experiment of relative permeability. The experimental results indicate that for the water-wet cores Swi decreased with the increasing temperature from ambient to 105℃,and the relative permeability curve shifted in a low water saturation direction, i.e. moved toward the left, while it moved toward the right for oil wetness reservoirs. Seen from both macroscopic and microcosmic view, the reasons and mechanisms of relative permeability change with temperature are discussed, and factors including core wetness, viscosity force, capillary forces, contact angle, interfacial tension change are considered.

A new model for predicting irreducible water saturation in tight gas reservoirs

The irreducible water saturation(Swir) is a significant parameter for relative permeability prediction and initial hydrocarbon reserves estimation.However,the complex pore structures of the tight rocks and multiple factors of the formation conditions make the parameter difficult to be accurately predicted by the conventional methods in tight gas reservoirs.In this study,a new model was derived to calculate Swir based on the capillary model and the fractal theory.The model incorporated different types of immobile water and considered the stress effect.The dead or stationary water(DSW) was considered in this model,which described the phenomena of water trapped in the dead-end pores due to detour flow and complex pore structures.The water film,stress effect and formation temperature were also considered in the proposed model.The results calculated by the proposed model are in a good agreement with the experimental data.This proves that for tight sandstone gas reservoirs the Swir calculated from the new model is more accurate.The irreducible water saturation calculated from the new model reveals that Swir is controlled by the critical capillary radius,DSW coefficient,effective stress and formation temperature.

Evaluation of clayed silt properties on the behavior of hydrate production in South China Sea

Gas hydrate is one kind of potential energy resources that is buried under deep seafloor or frozen areas.The first trial offshore production from the silty reservoir was conducted in the South China Sea by the China Geological Survey(CGS).During this test,there were many unique characteristics different from the sand reservoir,which was believed to be related to the clayed silt physical properties.In this paper,simulation experiments,facilities analysis,and theoretical calculation were used to confirm the hydrate structure,reservoir thermo-physical property,and bond water movement rule.And the behavior of how they affected production efficiency was analyzed.The results showed that:It was reasonable to use the structure I rather than structure II methane hydrate phase equilibrium data to make the production plan;the dissociation heat absorbed by hydrate was large enough to cause hydrate self-protection or reformation depend on the reservoir thermal transfer and gas supply;clayed silt got better thermal conductivity compared to coarse grain,but poor thermal convection especially with hydrate;clayed silt sediment was easy to bond water,but the irreducible water can be exchanged to free water under high production pressure,and the most obvious pressure range of water increment was 1.9–4.9 MPa.

A new NMR-data-based method for predicting petrophysical properties of tight sandstone reservoirs

Evaluating the permeability and irreducible water saturation of tight sandstone reservoirs is challenging.This study uses distribution functions to fit measured NMR T_(2)distributions of tight sandstone reservoirs and extract parameters for characterizing pore size distribution.These parameters are then used to establish prediction models for permeability and irreducible water saturation of reservoirs.Results of comparing the fit of the T_(2)distributions by the Gauss and Weibull distribution functions show that the fitting accuracy with the Weibull distribution function is higher.The physical meaning of the statistical parameters of the Weibull distribution function is defined to establish nonlinear prediction models of permeability and irreducible water saturation using the radial basis function(RBF)method.Correlation coefficients between the predicted values by the established models and the measured values of the tight sandstone core samples are 0.944 for permeability and 0.851 for irreducible water saturation,which highlight the effectiveness of the prediction models.

Study on Geological Genesis and Sedimentary Model of Complex Low Resistivity Reservoir in Offshore Oilfield — A Case of NgIII Formation of X Oilfield in Bohai Sea

In order to study the micro genetic mechanism and main geological controlling factors of low resistivity reservoir in NgIII formation of X oilfield in Bohai sea in China, the clay mineral composition, irreducible water saturation, salinity and conductive minerals of low resistivity reservoir were studied by using the data of core, cast thin section and analysis, and compared with normal resistivity reservoir. At the same time, the control effect of sedimentary environment on low resistivity reservoir was discussed. The results show that the additional conductivity of high bound water content and high montmorillonite content in the reservoir together leads to the significant reduction of reservoir resistivity, which is the main microscopic cause of the formation of low resistance, and is mainly controlled by the sedimentary background such as paleoclimate and sedimentary cycle. During the deposition period of NgIII formation, the paleoclimate was dry and cold, and it was at the end of the water advance of the medium-term sedimentary cycle. The hydrodynamic force of the river channel was weak, the carrying capacity of the riverbed was weak, and the river channel swayed frequently, resulting in fine lithologic particle size, high shale content and complex pore structure of the reservoir, resulting in significant reduction of reservoir resistance. The research conclusion would have strong guiding significance for the development of low resistivity reservoirs in this area.

Pore structure characterization based on NMR experiment: A case from the Shanxi Formation tight sandstones in the Daning-Jixian area, eastern Ordos Basin

The study of pore structure requires consideration of important factors including pore throat size,pore radius composition,and pore-throat configuration.As the nuclear magnetic resonance(NMR)experimental results contain rich information about pore structures and fluid occurrence states,this study investigated the pore structures of the tight sandstone reservoirs of the Shanxi Formation in the Daning-Jixian area,eastern Ordos Basin.Firstly,by making the inverse cumulative curve of the NMR T2 spectrum coincide with the capillary pressure curves which were obtained by the mercury injection capillary pressure(MICP)technique,this study derived a conversion coefficient that can be used to convert the NMR T2 spectrum into the pore throat radius distribution curves based on the NMR experimental results.Subsequently,we determined the pore radius intervals corresponding to irreducible water distribution using the NMR-derived pore radius distribution curves.Finally,the NMR T2 distribution curves based on the fractal theory were analyzed and the relationships between fractal dimensions and parameters,including permeability,porosity,reservoir quality index(RQI),flow zone indicator(FZI),irreducible water saturation,RT35,and RT50,were also discussed.The NMR-derived pore throat radius distribution curves of the study area are mainly unimodal,with some curves showing slightly bimodal distributions.The irreducible water mainly occurs in small pores with a pore radius less than 100 nm.As the permeability decreases,the contribution rate of small pores to the irreducible water gradually increases.The NMR-based fractal dimensions of pores show a two-segment distribution.Small pores have small fractal dimensions and are evenly distributed,while large pores have large fractal dimensions and complex pore structures.The fractal dimension of large pores(Dmax)is poorly correlated with porosity but strongly correlated with FZI,RQI,RT35,and RT50.These results indicate that large pores are the main pore zones that determine the seepage capacity of the reservoirs.Additionally,there is a certain correlation between Dmax and the irreducible water saturation.

Reservoir Characterization and Pore Type Systems of Carbonate Low Resistivity Pay in Persian Gulf

The main focus of study is to characterize lower and upper cretaceous carbonate deposits with Low Resistivity Pay, in Persian Gulf. Four oil reservoirs in the Cretaceous including the Zubair, Buwaib, Shuaiba and Khatiyah Formations of Southern fields have been analyzed. Here is a look at that to determine main factors on decreasing resistivity in pay zone. In some intervals resistivity responses reach less than 6 to 1 ohm·m. Significant hydrocarbon accumulations are “hidden” in low resistivity Pay zone, (LRPZ). LRPZ reservoirs have been found in some formations in Persian Gulf. Causes of LRPZ reservoirs on the basis of experimental analysis include clay-coated grains, carbonate with interstitial dispersed clay. On the other side Smectite and Kaolinite of main clays types have high CEC and greater impact on lowering resistivity. Micritization and Pyritization of digenetic process have noticeable impact on LRPZ. It is mentioned that Lønøy method applied to address pore throat sizes which contain Inter crystalline porosity, Chalky Limestone, Mudstone micro porosity. Pore systems are classified at class 2 and 3 Lucia and pore size varies from 0.5 to 4 micron. NMR Core and Log results show different pore size distribution. NMR core and MRIL results explain that decreasing of resistivity in pay zone is related to texture and grain size variation not being existence of moved water. Irreducible water estimate for this reservoir was between 30% and 50%. T2 cut off estimates, for defining irreducible water saturation, 115 ms.

Petrophysical insights into pore structure in complex carbonate reservoirs using NMR data

The study delves into pore structure attributes within the complex Eocene carbonate of an Indian offshore field,encompassing pore throat,radius and their characteristics.Nuclear Magnetic Resonance(NMR)experimental data reveals crucial insights into pore structures and fluid states.This study compares the NMR T_(2) distribution curve with capillary pressure data from the Mercury Injection Capillary Pressure(MICP)technique,deriving linear and nonlinear conversion coefficients to transform NMR T_(2) spectra into equivalent pore radius distribution.Pore radius-dependent porosity partitioning,linked to permeability and the distribution of irreducible water,is conducted utilizing NMR-derived data.Following the T_(2) cut-off analysis,a two-segment fractal analysis of NMR T_(2) distribution is also carried out.This analysis unveils associations between fractal dimensions and various petrophysical parameters,including permeability,porosity,T_(2)LM,irreducible water saturation and R5o.The NMR-derived pore radius distribution is mostly unimodal,occasionally slightly bimodal.Six different pore size classes(less than 0.05μm to more than 5μm)are analysed in relation to permeability,porosity and irreducible water.Small pores(<1μm)contribute more to irreducible water with low porosity and permeability.The fractal dimension of large pores correlates strongly with porosity,permeability,T_(2)LM,irreducible water and Rso suggesting significant impact on reservoir seepage capacity.In addition to porosity partitioning,the current study demonstrates effectiveness in modelling modified permeability and correlating it with in situ permeability when applied to field NMR log data from the study area.While numerous studies focus on sandstone,our study marks the pioneering attempt at a comprehensive analysis on complex carbonate reservoirs.