High Water Saturation Research Articles

Research and Application of EUR Prediction Method for Tight Sandstone Gas Wells: Taking Bajiaochang Shaximiao Formation Gas Reservoir as an example

Tight gas is currently an important area for the exploration and development of unconventional oil and gas resources at home and abroad. However, due to the low permeability and high-water saturation characteristics of tight gas reservoirs, gas wells exhibit low productivity, rapid decline in production, and water production in some wells. Such characteristics have reduced the single-well EUR to a certain extent, affecting the recovery rate of the gas reservoir. For new blocks, single-well EUR is the basic condition for long-term high and stable production of oil and gas wells. It is an important indicator for evaluating development effects, analyzing development potential, and formulating development-specific measures. How to quickly and accurately predict and analyze single-well EUR has always been tight Important research work for the development of gas. This paper takes the tight sandstone gas reservoir in the Bajiaochang block as an example. Based on the analysis of the production characteristics of the implemented wells, the production instability analysis method is used to analyze the variation rules and control factors of the single well EUR at different stages, and establish a single well EUR forecast analysis. plate. The analysis results show that the single-well EUR calculation result is more reliable for more than 2 years of continuous production, and can reach more than 80% of the final value. This method provides convenience for the rapid and accurate prediction of single-well EUR, and has a better field application effect.

Influence of Pore Size Distribution on the Electrokinetic Coupling Coefficient in Two-Phase Flow Conditions

Streaming potential is a promising method for a variety of hydrogeophysical applications, including the characterisation of the critical zone, contaminant transport or saline intrusion. A simple bundle of capillary tubes model that accounts for realistic pore and pore throat size distribution of porous rocks is presented in this paper to simulate the electrokinetic coupling coefficient and compared with previously published models. In contrast to previous studies, the non-monotonic pore size distribution function used in our model relies on experimental data for Berea sandstone samples. In our approach, we combined this explicit capillary size distribution with the alternating radius of each capillary tube to mimic pores and pore throats of real rocks. The simulation results obtained with our model predicts water saturation dependence of the relative electrokinetic coupling coefficient more accurately compared with previous studies. Compared with previous studies, our simulation results demonstrate that the relative coupling coefficient remains stable at higher water saturations but vanishes to zero more rapidly as water saturation approaches the irreducible value. This prediction is consistent with the published experimental data. Moreover, our model was more accurate compared with previously published studies in computing the true irreducible water saturation relative to the value reported in an experimental study on a Berea sandstone sample saturated with tap water and liquid CO2. Further modifications, including explicit modelling of the capillary trapping of the non-wetting phase, are required to improve the accuracy of the model.

New insights into the dissociation of mixed CH4/CO2 hydrates for CH4 production and CO2 storage

Recent but limited studies have shown that multistep slow depressurization based on mixed CH4/CO2 hydrate dissociation can enhance CH4 recovery and increases CO2 storage after CO2 injection into CH4 hydrate [1,2]. For the first time, the resistivity variation and gas recovery and storage variation was investigated to study the change in hydrate saturation and production/storage yield. Lab-scale CH4 and CO2 rich mixed hydrates were synthesized to mimic the production and injection well scenario. The mixed hydrates were synthesized in sandstone with moderate to high water saturation using two different CH4/CO2 gas mixtures. Furthermore, mixed CH4/CO2 hydrates were dissociated three to six steps based on cyclic depressurization. Pressure, resistivity and gas chromatography data were collected. The presence of two thermodynamic stability zones provided an opportunity for additional CH4 recovery and CO2 storage during mixed hydrate dissociation. Gas and water migration between the injection and production well caused CO2 hydrate reformation, improvement in CO2 sweep area and movement of the CO2 hydrate front toward the production well. Multiple peaks in CH4 recovery and CO2 storage suggest major dissociation and reformation. Peak values were independent of mixed hydrate type. Peaks values of CH4 rich hydrates occurred at high pressure than peak values of CO2 rich hydrates. The slight change in resistance during depressurization below pure CH4 hydrate stability pressure confirms the loss of CH4 hydrate mass recovered by the formation of CO2 hydrate mass. This study discusses the correlation between the change in resistivity and type of guest molecule and its concentration and initial water saturation. The results of this study will be useful to explore the application of slow depressurization for the dissociation of CH4/CO2 mixed hydrates to improve CH4 recovery and CO2 storage.

Production Performance Analysis of Class II Hydrate-Bearing Layers Based on an Analytic Aquifer Model

In the current numerical simulation studies, bottom water in Class II hydrate-bearing layers is represented by grids with high water saturation that significantly extends the calculation time if the volume of the bottom water is large or grid size is small. Moreover, the influence of the bottom water volume on the depressurization performance of Class II hydrate-bearing layers has not been fully investigated. In this study, the Fetkovich analytic aquifer model was coupled with a simulation model of a hydrate reservoir to accelerate the simulation of Class II hydrate-bearing layers. Then the simulation results and calculation time were compared between the coupled model and the model in which the bottom water layer is only represented by grids. Finally, the influence of the bottom water volume on the productivity of gas and water in the depressurization method was investigated and the variation of pressure, temperature, and hydrate saturation during the production process was analyzed. The results show that the coupled model can significantly reduce the simulation time of Class II hydrate-bearing layer while ensuring calculation accuracy. When the pore volume of the aquifer increases to 20 times that of the bottom water layer, the computation time of a single model in which the bottom water layer is represented by grids is 18.7 times that of the coupled model. Bottom water invasion slows down the depressurization, and therefore, the larger the aquifer, the lower the peak value of gas production, and the later it appears. However, the invading bottom water can provide heat for hydrate dissociation; therefore, the gas production rate of the hydrate-bearing layer with bottom water is higher than that of the hydrate-bearing layer without bottom water in the late development stage. Generally, the presence of bottom water reduces the cumulative gas production and increases the cumulative water production; therefore, the larger the aquifer, the more unfavorable the depressurization development of the hydrate-bearing layer.

Using clay microporosity to improve formation evaluation in potential residual oil zones: Cypress Sandstone, Illinois Basin

Clay microporosity in sandstones can cause erroneously high water saturation calculations if not properly accounted for and reduces effective pore space when evaluating a formation. We conducted a petrographic study to characterize and quantify the clay mineral microporosity of the fluvial facies of the Mississippian Cypress Sandstone in the Illinois Basin. This data helped achieve more accurate saturation calculations through core and well log analyses in potential Cypress residual oil zones. Petrographic thin sections were analyzed by scanning electron microscopy with backscatter imaging and energy dispersive X-ray spectroscopy. Clay mineral species identified included pore-filling kaolinite books, blocks, and vermicules; chlorite clusters; illite mats; illite-smectite webs; grain-coating inherited chlorite rims; and pore-bridging illite hairs. Volume percentages of microporosity particular to clay mineral species were determined by image analysis. Average microporosity values in kaolinite, chlorite, illite, and illite-smectite were 41%, 57%, 67%, and 65%, respectively. Effective clay mineral volumes, including microporosity, showed a >2-fold increase over estimates determined by X-ray diffraction, constituting an average volume of 4%. Comparing these volumes with gamma-ray log estimates proved Stieber’s work to predict clay volume most accurately. Clay volumes were used as input parameters in the dual-water method for determining water saturation, resulting in greater residual oil saturations than those predicted from Archie’s equation. Accounting for clay microporosity in helium porosimetry measurements of total porosity resulted in an ~11% decrease to effective porosity. Results from this research improve estimates of water saturation and porosity and highlight the importance of studying microscale mineralogical properties during reservoir characterization.

Microscale Evaluation of Tight Oil Mobility: Insights from Pore Network Simulation

Pore network modeling based on digital rock is employed to evaluate the mobility of shale oil in Qingshankou Formation, Songliao Basin, China. Computerized tomography technology is adopted in this work to reconstruct the digital rock of shale core. The pore network model is generated based on the computerized tomography data. We simulate the dynamics of fluid flow in a pore network model to evaluate the mobility of fluid in shale formation. The results show that the relative permeability of oil phase increases slowly in the initial stage of the displacement process, which is mainly caused by the poor continuity of the oil phase. In the later stages, with the increase in the oil phase continuity, the range of relative permeability increases. With the increase of organic matter content, the permeability of the water phase remains unchanged at low water saturation, but gradually increases at high water saturation. At the same time, it can be seen that, with the increase in organic matter content, the isosmotic point of the oil–water phase permeability shifts to the left, indicating that the wettability to water phase gradually weakens.

Experimental study of three-dimensional CO2-water drainage and fracture-matrix interactions in fractured porous media

Estimation of CO2 storage capacity in fractured porous reservoirs requires a better understanding of the CO2-water flow fundamentals in fracture-matrix systems. There are few core-flood experiments on three-dimensional CO2-water drainage in a fracture-matrix system, yet none have examined the impacts of the capillary continuity of fractures and fracture-matrix interactions. In this study, twelve drainage experiments were conducted in four fracture-matrix columns, each comprising a vertical stack of a cylindrical rock core, a filter paper serving as an analogue to a horizontal fracture, and a ceramic plate. The core sample was surrounded by open space at the top and circumferential sides to model fractures, allowing for 3-D CO2-water drainage in the rock core and displaced water draining across the horizontal fracture. X-ray computed tomography was conducted to visualize the dynamic invasion/drainage processes in four rock core samples showing contrasts in anisotropy, permeability, and heterogeneity. Experimental results show (1) the equilibrium CO2 saturations in the rock matrix vary from 0.10 to 0.60 at controlled capillary pressures up to 200 kPa, (2) the CO2 saturations in the matrix increaze with water saturation in the horizontal fracture resulting in better capillary continuity for water to drain across the fracture, and (3) the fracture water saturation can be enhanced by the non-uniform fracture capillary pressure and countercurrent flow of CO2 and water across the fracture-matrix interface. In the case of high fracture water saturation, matrix CO2 saturation largely depends on matrix anisotropy and heterogeneity. The core-scale experimental results contribute to understand the fracture-matrix interactions and CO2 storage efficiency in fractured porous media.

Reservoir characteristics and control mechanism of resistivity low-contrast oil pays in Chang 8 tight sandstone of Longdong West area, Ordos Basin

Log interpretation and evaluation of tight sandstone reservoir in Chang 8 Member of Longdong West area, Ordos Basin, China, are facing great challenges due to the co-development of normal oil pay and resistivity low-contrast oil pay. To better guide the exploration and development of oil resources in this area, the reservoir characteristics and control mechanism of resistivity low-contrast oil pay were studied. Firstly, the reservoirs were divided into resistivity low-contrast oil pay (RLP) and normal oil pay (NP) based on the relative value of the apparent resistivity increase rate. Then, the difference of reservoir characteristics between RLP and NP is analyzed by comparing a series of experimental data and real logging data in those two reservoir types. Finally, the control mechanism of RLP was studied from reservoir micro-factors and regional macro-factors, respectively. It is found that the chlorite and illite are the most abundant clay minerals in RLP and NP, respectively. Compared with NP reservoir, the average porosity of RLP is better, but the pore space is mainly composed of micropores, which lead to smaller average pore throat radius and poor pore structure. The high irreducible water saturation and high formation water salinity reduced the reservoir resistivity from micro-aspect. Besides, the difference of hydrocarbon expulsion capacity of source rock and the regional difference of formation water salinity controlled the distribution of RLP and NP. Comprehensive consideration of the reservoir micro-factors and regional macro-factors is important to carry out effective logging interpretation and evaluation.

Evaluation of Petrophysical Properties Using Well Logs of Yamama Formation in Abu Amood Oil Field, Southern Iraq

The petrophysical analysis is very important to understand the factors controlling the reservoir quality and production wells. In the current study, the petrophysical evaluation was accomplished to hydrocarbon assessment based on well log data of four wells of Early Cretaceous carbonate reservoir Yamama Formation in Abu-Amood oil field in the southern part of Iraq. The available well logs such as sonic, density, neutron, gamma ray, SP, and resistivity logs for wells AAm-1, AAm-2, AAm-3, and AAm-5 were used to delineate the reservoir characteristics of the Yamama Formation. Lithologic and mineralogic studies were performed using porosity logs combination cross plots such as density vs. neutron cross plot and M-N mineralogy plot. These cross plots show that the Yamama Formation consists mainly of limestone and the essential mineral components are dominantly calcite with small amounts of dolomite. The petrophysical characteristics such as porosity, water and hydrocarbon saturation and bulk water volume were determined and interpreted using Techlog software to carried out and building the full computer processed interpretation for reservoir properties. Based on the petrophysical properties of studied wells, the Yamama Formation is divided into six units; (YB-1, YB-2, YB-3, YC-1, YC-2 and YC-3) separated by dense non porous units (Barrier beds). The units (YB-1, YB-2, YC-2 and YC-3) represent the most important reservoir units and oil-bearing zones because these reservoir units are characterized by good petrophysical properties due to high porosity and low to moderate water saturation. The other units are not reservoirs and not oil-bearing units due to low porosity and high-water saturation.

Assessment of flag leaf water status as drought tolerance discriminating trait in durum wheat (Triticum turgidum var durum L.)

Drought is a prominent limiting factor that impacts negatively durum wheat grain yield. Ten durum wheat breeding lines were evaluated under rainfall conditions at the Field Crop Institute Agricultural Experimental Station of Setif, Algeria, during the 2016/2017 cropping season. The investigation aimed to study the ability of flag leaf water status to discriminate among varieties for drought tolerance trait. Significant variability was observed among the tested varieties for leaf dry, wilted and turgid weights, leaf relative water content, water saturation deficit and excised water loss, after three wilting periods of 30, 60 and 90 minutes dehydration at 40°C. The assessed breeding lines were differentially categorized as drought tolerant and drought sensitive based on either relative water content or water saturation deficit or excised leaf water loss genotypic mean values. Correlation, principal components and cluster analyses indicated an unwanted significant association between excised leaf water loss and relative water content and water saturation deficit and classified the assessed entries into three clusters (CI, C2 and C3). Cluster C1 had high relative water content, low water saturation deficit but high excised water loss, while C3 had low relative water content, low excised leaf water but high-water saturation deficit, C2 being intermediate. Crosses between distant clusters (C1 vs C3) are proposed to generate more variability of the targeted traits in progeny population and to break undesirable linkage between alleles controlling leaf water status, allowing to select efficiently drought tolerant genotypes.

Estimation of shear‐wave velocities in unconventional shale reservoirs

ABSTRACTAnalysis of compressional and shear‐wave sonic logs in seven organic‐rich shale reservoirs exhibiting a wide range of velocities, total organic content, thermal maturity, fluid compressibility and mineral composition indicates that compressional‐wave velocity is a strong predictor of shear‐wave velocity as is the case for conventional reservoirs. Excluding data with high water saturations, a simple linear relationship is found between the compressional and shear‐wave velocities in shale reservoirs in accordance with laboratory measurements. The relationship can be further refined for prediction purposes by local calibration or with a linear correction for total organic content. Correcting for compositional variation and fluid effects requires more complex treatment. An empirical rock physics model that explicitly incorporates these effects results in high accuracy (0.2% average mean error) in shear‐wave velocity estimation for the entire data set. The model also exhibits good precision with mean absolute error of 2%. This is significantly better than what is achieved by rock physics models that do not explicitly consider fluid properties. These results are obtained without local calibration and without requiring correction for thermal maturity, degree of lithification or the shape or distribution of the solid organic matter. Explicit consideration of mineral composition, fluid substitution effects and amount of solid organic matter was necessary in this formulation to achieve this high average prediction accuracy with near zero bias as well as good precision across all formations and without local calibration. The successful use of Gassmann's equations in this algorithm suggests that errors in shear‐wave velocity prediction in shale reservoirs due to violation of the assumptions underlying these equations are either small or self‐compensating.

Liquefaction and re-liquefaction of sediments induced by uneven loading and glacigenic earthquakes: Implications of results from the Latvian Baltic Sea coast

Seven internally-deformed layers were recognised in the shallow marine bay sediments deposited during mid Marine Isotope Stage 2 along the Latvian Baltic Sea coast bluff at Baltmuiža. Detailed sedimentological analyses of these deformed layers indicate that liquefaction was responsible for the development of soft-sediment deformation structures (SSDS) including injection structures, load casts, flame structures and pseudonodules. Traces of both initial liquefaction and re-liquefaction were recognised within at least four of the seven layers with SSDS (two different generations of pseudonodules and injection structures, clastic injection pipes, disrupted load casts). This is a unique finding, as it is difficult to re-liquefy sediment affected by previous liquefaction due to the increased packing of grains and decreased hydration after initial liquefaction. Furthermore, all layers with SSDS are characterised by eroded tops, which indicate that deformation took place when these layers were at the contemporary surface of sedimentation. We suggest that the initial liquefaction of fluidised coastal sediments was caused by various triggers (uneven loading, glacigenic earthquake), while the re-liquefaction could be related to a single trigger of mid-Late Weichselian glacigenic earthquakes at the front of the advancing last Fennoscandian Ice Sheet. Our results show that despite compaction after liquefaction, sediment re-liquefaction caused by an allogenic trigger is still possible under favourable conditions, i.e., sediment anisotropy, cyclic loading, and preserved high water saturation. Our findings are also the first to demonstrate the sedimentological imprints of re-liquefaction.

INCREASING THE EFFICIENCY OF BLASTING WORKS IN THE QUARRY OF BUILDING MATERIALS

The article is devoted to the urgent problem of increasing the efficiency of blasting operations in open pits in conditions of high water saturation of minerals.
 The idea of solving this problem is to use the effect of draining the local area of the water-saturated block of the mineral due to the ordinal explosion of the well charge. 
 The purpose of this study was to improve the efficiency of blasting operations on flooded fractured rocks in open pit mining.
 Research methodology. The proposed technique makes it possible to determine the parameters of drainage of the watered mass during the explosion of borehole charges and the parameters of an ordinal explosion. This makes it possible to improve the quality of rock crushing and reduce the consumption of explosives per unit cost.
 The main direction of research in this work is to reduce the energy intensity of the process of destruction of a water-saturated mineral by using the internal potential of water in the cracks of the massif destroyed by the explosion. The regularities and the role of the drainage funnel in the efficiency of destruction have been established.
 The novelty and originality of the work is a developed technology for removing water from the explosion zone, which increases the efficiency of explosion and destruction of rocks using special water-resistant explosives.
 The practical value of the work lies in the effectiveness of the proposed method for carrying out blasting operations, which, along with improving the quality of crushing, provides an increase in the output of blasted rock mass from one running meter of the well and reduces the cost of expensive water-resistant explosives.
 Conclusions. The introduction of research results and recommendations in open pits made it possible to increase the efficiency of blasting operations in the development of water-saturated rocks. Energy consumption for the destruction of an array of water-saturated minerals is reduced by 10%. The amount of blasted rock mass per running meter of the well increased by 14.1%, and the specific consumption of explosives decreased by (12-14) %. 
 
 Keywords: quarry, minerals, well, filtration, water-saturated massif, crushing, explosive, detonation.

Wrench faulting and trap breaching: A case study of the Kizler North Field, Lyon County, Kansas, USA

The Kizler North Field in northwest Lyon County, Kansas, is a producing field with structures associated with both uplift of the Ancestral Rockies (Pennsylvanian to early Permian) and reactivation of structures along the Proterozoic midcontinent rift system (MRS), which contributed to the current complex and poorly understood play mechanisms. The Lower Paleozoic dolomitic Simpson Group, Viola Limestone, and “Hunton Group” are the reservoir units within the field. These units have significant vuggy porosity, which is excellent for field potential; however, in places, the reservoir is inhibited by high water saturation. The seismic data show that two late-stage wrench fault events reactivated existing faults. The observed wrench faults exhibit secondary P, R’, and R Riedel shears, which likely resulted from Central Kansas uplift-MRS wrenching. The latest stage event breached reservoir caprock units during post-Mississippian to pre-Desmoinesian time and allowed for hydrocarbon migration out of the reservoirs. Future exploration models of the Kizler North and analog fields should be based on four play concepts: 1) four-way closure with wrench-fault-related traps, 2) structural highs in the Simpson Group and Viola Limestone, 3) thick “Hunton Group,” and 4) presence of a wrench fault adjacent to the well location that generates subtle closure but not directly beneath it, which causes migration out of reservoirs. In settings where complex structural styles are overprinted, particular attention should be paid to the timing of events that may cause breaches of seals in some structures but not others. Mapping the precise location and vertical throw of the reactivated wrench faults using high-resolution seismic data can help reduce the drilling risk in analog systems.

Refining Higher Modes of Rayleigh Waves Using Seismoelectric Signals Excited by a Weight‐Drop Source: Study From Numerical Simulation Aspect

AbstractEffective extraction of higher mode dispersion information has been a research hotspot in the study of surface waves. In this study, we introduce a novel approach of refining Rayleigh wave higher modes by using seismoelectric signals, namely, electromagnetic (EM) signals originating from seismoelectric conversion. Adopting a weight‐drop source, we simulate seismic and EM signals for several porous models. An extended version of the frequency‐Bessel transform method is introduced to extract dispersion information from seismic and EM signals. Results suggest that the evanescent seismoelectric waves, which are induced at subsurface interfaces penetrated by Rayleigh waves and finally are recorded nearby the ground surface after undergoing amplitude decay, can reflect dispersion characteristics of Rayleigh waves and can provide extra higher‐mode dispersion information that seismic waves cannot. We find the vadose zone with higher water saturation will cause the seismoelectric signals to have stronger strengths and change their frequency coverage of dispersion information. Electric fields obtained from potential differences between longer dipole electrodes suffer more severe distortion. In an electrically resistive environment, predicted seismoelectric signal amplitudes in the frequency band with dominant energy are well above expected background noise levels. We also find seismoelectric signals have a stronger antialiasing ability than seismic waves and their dispersion energy values are sensitive to salinity and fluid viscosity. Generally speaking, our study suggests Rayleigh wave dispersion information with abundant higher modes and good frequency‐range coverage can be obtained by using seismic and seismoelectric signals together due to their complementarity.

3D simulation of a giant oilfield in calcareous formations and scrutiny study of the interaction of the calculated parameters (Asmari formation in Maroon oilfield, Iran)

Simulation is an art to model the reservoir’s structure in the petroleum industry, which transfers all possible data digitally to the reservoir management system’s suitable computation software. The present study attempts to construct a 3D structural model using the geostatistical method to access in situ oil volume and the best oil production position. The geological area is the Maroon oilfields located at Dezful Embayment in southwest of Iran. In this oilfield, the Asmari reservoir is divided into ten zones. Several steps such as griding, kriging, and experimental variogram were carried out to access all available data to make up the model. The evaluation revealed that zones one and five were the main oil-bearing layers since it contains about 60% of total oil volume in this field. Petrophysical parameter distribution indicated that zones four and seven are good candidates for oil production since they have higher porosity and net to growth thickness than other zones. From the seventh to the tenth zone, the oil volume decreased due to high water saturation in these areas. The best zones for the future program are located in the western part. This study shows that 3D modeling will find elongation-type anticline and reservoir direction and the best location for future drilling programs based on the interaction of the petrographic parameters. The results can be used in various steps during the lifetime of the different reservoirs around the globe.

Evolution of Small Strain Soil Stiffness during Freeze-Thaw Cycle: Transition from Capillarity to Cementation Examined Using Magnetic and Piezo Crystal Sensors.

Freeze-thaw cycles caused by seasonal temperature fluctuations significantly affect the geotechnical engineering properties. This study investigated the crucial role of water distribution patterns in the characterization of elastic wave properties for the fine F-110 sand during a freeze-thaw cycle. Sand specimens with four different water distribution patterns were prepared, namely homogeneously-mixed, evaporation-driven, vertically-, and horizontally-layered specimens. The P- and S-wave signatures of the specimens were monitored using piezo crystal sensors. Results indicated the criticality of water distribution patterns in the determination of small-strain soil properties even though the specimens had identical global water saturation. The nuclear magnetic resonance-based water volume depth profiles indicated that the evaporation-driven specimens had more heterogeneous pore-invasive ice-bonding layers at a high water saturation region; by contrast, the drying process facilitated uniform meniscuses around the particle contacts near the air percolation threshold. Elastic wave measurements for laboratory-prepared specimens might over/underestimate the small-strain soil stiffness of sediments in nature, wherein the drying processes prevailed to control the water saturation. This study highlighted a clear transition from capillary-controlled to cementation-controlled elastic wave properties during temperature oscillations.

Fracture stiffness evaluation with waterless cryogenic treatment and its implication in fluid flowability of treated coals

Cryogenic fracturing using liquid nitrogen is a waterless and environmentally friendly formation stimulation method for creating complex fracture networks effectively. This study developed a non-destructive geophysical technique using seismic measurements to probe fluid flow through coal and ascertain the effectiveness of cryogenic fracturing. A theoretical model is established to determine the fracture stiffness of coal inverted from wave velocities, which serves as the nexus for correlating the hydraulic and seismic properties of fractures. In response to thermal shock and frost forces, visible cracks are observed on coal surfaces that deteriorate the mechanical properties of the coal bulk. Consequently, the wave velocity of frozen coal specimens exhibits a general decreasing trend with freezing time under both dry and saturated conditions. For a gas-filled specimen, both the normal and shear fracture stiffness decrease monotonically with freezing time as more cracks are created in the coal bulk. For a water-filled specimen, ice formation due to cryogenic treatment leads to grouting of the coal bulk. Accordingly, the fracture stiffness of wet coal increases initially and then decreases with freezing time. A coalbed with higher water saturation is preferable when applying cryogenic fracturing because fluid-filled cracks can endure larger cryogenic forces before complete failure, and the contained water aggravates coal breaking as ice pressure builds up from the volumetric expansion of the water–ice phase transition and applies additional splitting forces on pre-existing or induced fractures and cleats. Also confirmed is that the stiffness ratio is sensitive to fluid content. The measured stiffness ratio is 0.7–0.9 for dry coal but less than 0.3 for saturated coal. The present findings provide a basis for realistic estimation of coal stiffness ratio for future discrete fracture network modeling.

A new dynamic capillary-number-recovery evaluation method for tight gas reservoirs

Currently, it is difficult to accurately evaluate the recovery of tight gas reservoirs owing to its poor physical properties, strong heterogeneity, high water saturation, complex percolation laws. Conventional evaluation methods are often employed to use to calculate the gas recovery and the calculated results are frequently inaccurate in tight gas reservoirs. There is no unified recovery evaluation method for tight gas reservoirs. In this paper, a new dynamic capillary-number-recovery model was established to calculate the tight gas recovery. By using fuzzy mathematics, eight macroscopic factors that have a significant impact on recovery were selected to develop a multi-factor evaluation method for calculating the initial gas recovery. Combined with the microscopic seepage phenomenon and theoretical analysis, a matrix-fracture tight gas reservoir seepage model considering slippage effect, threshold pressure gradient and stress sensitivity effects was established. A new dynamic capillary-number-recovery model was developed to evaluate the initial gas recovery based on the seepage model and the multi-factor evaluation method. This model is more applicable to calculate gas recovery due to the consideration of macroscopic factors and microscopic seepage mechanism in tight gas reservoirs. Meanwhile, the new method was applied to evaluate the gas recovery of A block, which is a typical tight gas reservoir in Changqing gas field, China. The results indicate that the new established method is a feasible means to evaluate the gas recovery in tight gas reservoirs.

Mississippian Meramec lithologies and petrophysical property variability, stack trend, Anadarko Basin, Oklahoma

Mississippian Meramec reservoirs of the Sooner Trend in the Anadarko (Basin) in Canadian and Kingfisher Counties (STACK) play are comprised of silty limestones, calcareous siltstones, argillaceous calcareous siltstones, argillaceous siltstones, and mudstones. We found that core-defined reservoir lithologies are related to petrophysics-based rock types derived from porosity-permeability relationships using a flow-zone indicator approach. We classified lithologies and rock types in noncored wells using an artificial neural network (ANN) with overall accuracies of 93% and 70%, respectively. We observed that mudstone-rich rock type 1 exhibits high clay and relatively low calcite, whereas calcareous-rich rock type 3 has high calcite and low clay content with rock type 2 falling in between as a balance between rock types 1 and 3. Results of the ANN were applied to a suite of well logs in noncored wells in which we generated lithology and rock-type logs for the Meramec. We identified that the Meramec consists of seven stratigraphic units characterized as strike-elongate, shoaling-upward parasequences; each parasequence is capped by a marine-flooding surface. The lower three parasequences (lower Meramec) form a retrogradational parasequence set that back steps to the northwest and is capped by a maximum flooding surface. The upper Meramec is characterized by parasequences that form an aggradational to progradational stacking pattern followed again by a retrogradational trend. We predict that the parasequence stacking, associated lithology distribution, and diagenetic cements appear to control the spatial distribution of petrophysical properties (porosity, permeability, and water saturation), pore volume, and hydrocarbon pore volume (HCPV). Calcareous-rich lithologies exhibit lower porosity, permeability, HCPV, and higher water saturation. We deduced that argillaceous-rich lithologies that occur near the maximum flooding surface are the most favorable reservoir intervals because they exhibit relatively higher porosity, permeability, HCPV, and lower water saturation. Productivity could not be directly correlated to rock types as operational and completion factors as well as overpressure and oil phase play important roles on production.