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The parameters of fractures, such as fracture geometries, placement, and conductivity, are critical for the production of horizontal wells in tight gas reservoirs. Current studies on fracture parameter optimization, especially those using the optimization algorithms, are often based on the assumptions that the fractures are uniformly placed with equal length and conductivity. However, non-uniform designs of fracture spacing have been proved to be effective in reducing fracture interference as well as increasing the well production, especially with strong rock heterogeneity. In addition, most of the current literature focuses on identifying optimal hydraulic fracture geometries without considering the fracture propagation process, which may oversimplify the evaluation of completion cost and result in fracture geometries that cannot be realized in practice. In this paper, an automatic optimization process, which could efficiently couple the fracture propagation, is proposed to optimize the fracture parameters. The genetic algorithm (GA) with penalty function method is used to optimize the constraint and non-linear problem. A modified PKN fracture propagation model is established to quickly predict the hydraulic fracture geometries. Fracture locations and fluid injection volume are sequentially optimized with consideration of heterogeneous rock properties. Model parameters are taken from typical tight gas horizontal wells in Jinqiu gas field in Sichuan Province, China. The results show the importance of permeability on both fracture length and production in tight gas well. The lower the permeability, the more significant the production increase after fracture location optimization. More fractures tend to be placed in area with small permeability and fewer fractures are required in highly permeable regions. With an increase in rock heterogeneity, the distribution of fractures becomes more uneven. Conversely, increasing the number of fractures can lead to a more even distribution of fractures. More fluid should be injected in low-permeability regions according to the optimization results. The optimization method is applied to a practical tight gas well in Sichuan Basin and the cumulative gas production can be increased by 6.5% to 8% when compared with the initial uniform designs. • A modified PKN model is proposed for quick prediction of fracture propagation, validated against numerical simulations. • GA is used to optimize fracture locations by considering rock heterogeneity's impact on fracture propagation and production. • Sequential optimization of fracture placement and injection volume is conducted with its reliability verified. [ABSTRACT FROM AUTHOR]

Evaluating reservoir storage and percolation capacities, which are closely associated with pore morphology and connectivity, is significant for the exploration and exploitation of oil and gas fields. However, ultradeep carbonate gas reservoirs have extremely high heterogeneity and complexity compared to other relatively shallow reservoirs. Therefore, investigations into all aspects of ultradeep carbonate face enormous challenges. To the best of our knowledge, no systematic study on the pore structure characteristics of ultradeep carbonate gas reservoirs has been conducted to date. In this paper, the pore structure characteristics and their effects on the gas storage and percolation capacities of the Deng IV Member gas reservoir were investigated using conventional petrophysical measurements, casting thin section analysis, scanning electron microscopy (SEM), high pressure mercury intrusion (HPMI) tests, and multiscale CT scanning analysis. The results show that the lithology of the Deng IV Member reservoir is dominated by algal clotted dolomite, algal stromatolite dolomite and algal doloarenite. The conventional petrophysical measurements indicate that the average porosity and permeability are 3.91% and 1.02 mD, respectively. However, the high porosity (8-4%) and high permeability (10–10,000 mD) intervals are developed locally. The casting thin sections, SEM and two-dimensional CT analyses identify six main types of reservoir spaces in the Deng IV Member: interparticle dissolution pores, intercrystal dissolution pores, fenestriform cavities, dissolution cavities, structural fractures and dissolution fractures. Furthermore, three types of throats, that is, necking, tubular and lamellar, were also detected. Four types of reservoirs, namely, matrix type, pore type, cavity type and fracture-cavity type, were classified according to the comprehensive analysis of HPMI data and curves. Three-dimensional pore throat network topologies were reconstructed for four types of representative core samples based on the multiscale CT scanning data and "maximum-ball" method. The distribution characteristics of pore throat radii, volumes and coordination numbers reveal the following: the reservoir space of fracture-cavity type is dominated by cavities and multiscale pore throats with the best pore-throat connectivity; the reservoir space of cavity type is primarily controlled by cavities and large-scale pore throats with moderate connectivity; the reservoir space of pore type comprises multiscale pore throats with moderate connectivity; and most areas of matrix type reservoir are occupied by rock matrix, and the pore-throat connectivity is the worst. In conclusion, the development of fractures and bedding cavities considerably improves the reservoir percolation capacity, and the development of cavities is an important supplement to the reservoir storage capacity. The reasonable combination of cavities and fractures is the basis for the efficient development of ultradeep carbonate gas reservoirs. • A variety of techniques and methods were used to characterize pore structures of a ultradeep carbonate gas reservoir. • Storage and percolation capacities of a ultradeep carbonate gas reservoir were comprehensively analyzed. • Four types of reservoirs were classified and evaluated according to the results of HPMI and multiscale CT scanning. • Reasonable combination of cavities and fractures is the key to the efficient development of ultradeep carbonate reservoirs. [ABSTRACT FROM AUTHOR]

The main target for hydrocarbon exploration of terrestrial strata in Sichuan Basin is the tight gas sandstone reservoir of the Upper Triassic Xujiahe Formation. Xujiahe Formation in YB area is a major discovery of hydrocarbon exploration in recent years, which is characterized as tight with ultralow porosity, ultralow permeability, and intensive heterogeneity. The tectonic fractures have important influence on accumulation and productivity of gas in the area. In the paper, analysis data of outcrop, core, imaging logging and other experiments were used to determine the characteristics, formation periods and genetic mechanisms of tectonic fractures. There are mainly high-angle and low-angle shear fractures in the area, which caused by tectonic stress. The length of the fractures is mainly 0–4 m from the outcrop and 10–20 cm from the cores and fracture frequencies (number per meter) is mostly 2-7 m−1, 2-4 m−1 respectively. The fracture fillings are mainly calcite, argillaceous and carbonaceous, about 60% of the fractures is unfilled or incompletely filled. Without considering the diagenetic fractures, there are three periods of the fractures: the middle-late Yanshanian tectonic movement, the early-middle Himalaya tectonic movement and the late Himalaya tectonic movement. The first period of the fractures are mainly filled with fine calcite, homogenization temperature of inclusion is 75–85 °C and the maximum effective principal stress of paleostress is 71.64 Mpa. The second period of the fractures are half-filled (or unfilled) with coarse calcite, homogenization temperature of inclusion is 150–175 °C and the maximum effective principal stress of paleostress is 74.91 Mpa. The third period of the fractures are low filling degree and cuts formed fractures clearly, the maximum effective principal stress of paleostress is 86.52 Mpa. For these tectonic movement, there are four sets of the plane "X" shear fractures in the area: NE strike fractures, NW strike fractures, NEE strike fractures, S N strike fractures. There are one set of the profile shear fractures of NWW strike. Finally, the complex fault system of nearly N S, NW and NE strike are formed in YB area. • A new method for comprehensive study of the fracture formation period is provided. • The main fracture formation dating of Xujiahe Formation in YB area as follows: 36–176 Ma and 0.2–36.4 Ma. • There are 3 periods tectonic fractures of Xujiahe Formation and formation period of different fracture types is identified. [ABSTRACT FROM AUTHOR]

Stimulation of oil and gas volcanic reservoirs is quite topical, while particular stimulation mechanisms of various fracturing technologies applied to volcanic rocks need to be clarified. This paper explores volcanic rock sample fracture surfaces' morphology, the acid etching depth, and/or proppant embedment depth under propped, acid etching, and propped-etching. The conductivity is an important evaluation index for the effect of stimulation treatment such as fracturing and acid fracturing, therefore, fracture conductivity under different conditions, such as proppant size and concentration, acid concentration and activity, the separate or joint application of acids and proppant, was evaluated with the samples from Permian igneous in the Sichuan Basin, China. The results indicate that the reaction between acid and rocks is hindered by the complexity and heterogeneity of mineral distribution, forming discrete dissolution holes in local areas instead of flow channels. The maximum embedding depth was 67 μm without acid etching, while those obtained after gelling and cross-linking acid etching were increased by 2.3–3.2 times, reaching 178 and 227 μm, respectively. The etching depth of the samples with the acid can reach 1000 μm. By the effect on conductivity, the increasing order was as follows: acid etching fracture, propped-etching fracture, and propped fracture, while there was a slight difference in conductivities of propped and propped-etching fractures. Therefore, hydraulic fracturing with proppant provides the volcanic reservoir's most effective stimulation. Acid-carrying-proppant fracturing would also be a lucrative stimulation technology if natural fractures are developed and contain acid-dissolved minerals. Acid fracturing failed to form the main fracture with high conductivity, and therefore is not recommended. This study is considered instrumental in clarifying the mechanism of different stimulation methods and substantiating the most efficient development of volcanic reservoirs. The research results can provide a reference for the optimization of stimulation method and the design of treatment parameters of volcanic reservoir. • The morphology, etching, and embedment depth of volcanic rocks were tested. • Fracture conductivity under different experimental conditions was evaluated. • The stimulation mechanism of different stimulation methods is clarified based on the experiment results. • Hydraulic fracturing is considered the most effective technique for the volcanic reservoir. [ABSTRACT FROM AUTHOR]

Unconventional oil and gas reservoirs are mostly characterized by unconnected pores and abnormal pore pressures. These conventional effective stress theories, such as Biot's theory, cannot display the quantitative relationship between unconnected pores' micro characteristics and the macro stress applied to the solid structure. These parameters related to the unconnected pores should be quantitatively incorporated in the effective stress formula. In this paper, two modified models were proposed to evaluate the solid framework's real effective stress and deformation. The effects of the unconnected pore pressure, unconnected porosity, and unconnected pore fluid's compressibility on effective stress and volumetric strain were theoretically investigated. It was found that the increase of unconnected pore pressure can decrease the effective stress and volumetric strain. For an abnormal high pore pressure formation, the growth of unconnected porosity can reduce the effective stress but increase the volumetric strain. The decrease of the unconnected pore fluid's compressibility can increase and decrease the effective stress and volumetric strain for abnormal low and high pore pressure reservoirs, respectively. Two case studies on the abnormal pore pressures induced by under-compaction and hydrocarbon generation were conducted. The results showed that the pore pressure exhibits significant increases when the unconnected pores and unconnected pore fluids are proactively compressed in the processes of under-compaction and hydrocarbon generation, respectively. The abnormal high pore pressures in these tight sandstone reservoirs in the Sichuan Basin, Tarim Basin, and Bohai Bay Basin can be theoretically generated when the pore space is compressed to 95%–84% of the initial pore space. The model can evaluate the effective stress and deformation for reservoirs with considerable unconnected porosities. In turn, it can also interpret the abnormal pressure generation induced by the changes of unconnected porosity and pore fluid's compressibility. • Two modified models are proposed to evaluate the real effective stress of the solid structure. • Effects of unconnected pores on the effective stress and volumetric strain are investigated. • Abnormal pressures under under-compaction and hydrocarbon generation are evaluated. [ABSTRACT FROM AUTHOR]

The Northwestern Sichuan Basin is a hot spot for natural gas exploration, and in recent years, many wells have obtained high-yield commercial gas flows in the Qixia Formation. However, due to the influence of foreland orogeny, the oil and gas in the Qixia Formation often have the characteristics of multisource filling, multistage accumulation and multistage transformation, which results in an unclear understanding of the gas source and the accumulation process. This paper uses the concept of sub-block research through carbon isotope and biomarkers analyses, to carry out gas-gas, gas-source and bitumen-source rock comparisons in different structural belts to determine the natural gas sources. On this basis, basin simulation and fluid inclusion analysis technology are used to determine the stage of hydrocarbon accumulation, and then the accumulation process is determined. Studies have shown that the gas source of the Qixia Formation are different in the different structural belts: (1) The natural gas from the Qixia Formation in the thrust nappe belt mainly originates from the Qiongzhusi Formation source rock; (2) The natural gas from the Qixia Formation in the hidden fault belt is a mixture of the source rock of the Qixia Formation and the source rock of the Qiongzhusi Formation; and (3) The natural gas from the Qixia Formation in the gentle belt mainly originates from the Middle Permian source rock and the Upper Permian Longtan Formation source rock. The Indosinian, Yanshania, and Himalayan movements all have great influence on the formation of the Qixia Formation reservoir in northwestern Sichuan, and the accumulation process of the different structural belts is quite different, but generally, the accumulation process of the different structural belts have experienced three stages: a primary oil pool stage, gas pool stage and reformation and definition stage. • Through sub-block research, gas source of the foreland thrust belt is studied. • Different gas sources cause differences in carbon isotopes. • Accumulation process of each structural belt is distinct. • The results have implications for exploration in complex tectonic areas. [ABSTRACT FROM AUTHOR]

The Yuanba giant gas field in the Sichuan Basin produces from Upper Permian Changxing heterogeneous reef-shoal reservoirs. It is an ultradeep tight gas reservoir with burial depth of 7000 m. This paper proposes an integrated methodology incorporating the reservoir geology description, petrophysics characterization, seismic response analysis, forward seismic modeling, pseudo-acoustic inversion, and reservoir architecture imaging to identify the geologically complex reef-shoal reservoirs and predict the distribution of the producible reservoirs. The facies of reef cap, reef core, reef base and shoal are first identified based on observations and interpretations of cores, thin sections, mud logging, wireline logs, and formation microscanner images (FMIs). High-energy shoals, reef cores, and reef caps are the most favorable reservoir microfacies. Seismic responses of reef-shoal reservoirs are then investigated through forward modeling and well-seismic calibration. Four reef belts are identified based on seismic reflections and edge detection by seismic paleogeomorphologic features. Reservoir distribution and quantitative properties are predicted using the facies-controlled postattack seismic inversion technique through the correlation between reservoir properties and seismic attributes, which is aided by pseudo-gamma acoustic inversion to exclude the shale-bearing non-reservoir and pseudo-neutron acoustic inversion based on its good relationship with porosity. The reef reservoir thicknesses range from 10 m to 130 m, and the thickest reservoir is located in the northwestern Yuanba gas field. Finally, 3-D visualization is used to characterize the spatial variation of the interior architecture, connectivity, and distribution of a good reservoir in the reef system. The accuracy rate of prediction for reservoirs and non-reservoirs reaches 88% using this method. The technique has also been successfully used for the deployment of development wells and well trajectory optimization, and the rate of encountering the predicted reservoirs is 100%. This study provides a practical approach to identify and predict the tight carbonate gas reservoir with similar petrophysical responses of density and acoustic to non-reservoirs in the complex carbonate system. • The high energy dolomitic facies is the best reservoir in the ultra-deep giant Yuanba Gas Field. • The tight reef-shoal reservoir are successfully characterized using integrated method. • Pseudo-acoustic impedance inversion is an effective tool identifying the best reservoir body. [ABSTRACT FROM AUTHOR]

The multilayered deposition is the main feature of the XC gas reservoir in the northwestern Sichuan Basin. The boundaries among layers of the XC gas reservoir is different. In this paper, we propose an extended pressure transient analysis model for vertical commingle well, considering different boundary radii and boundary types, to capture the Vertical Combined Boundary (VCB) effect on the pressure transient behavior and flow regime characteristics. The analytical solution is yielded by the Laplace transform, Bessel functions, Duhamel's superposition principle, and Stehfest inversion algorithm, and new flow regimes under different boundary combinations are identified by the pressure derivative curve. Taking the dual-layer VCB model as a type example, nine combinations of boundaries are analyzed. Compared with the current vertical commingled well model with the same boundary radii or single boundary types, this model has novel flow regimes controlled by the VCB. The research results indicate that the pressure derivative type curve during the novel flow regimes stage can diagnosis the VCB type. On the one hand, there are quantitative linear relationships between the pressure derivative feature value and interlayer heterogeneity of the VCB model with the same boundary radii. These linear relationships provide a new perspective to quickly capture interlay heterogeneity through the of the pressure derivative feature value. One the other hand, the pressure behavior and flow regimes of the VCB model with different boundary radii obey the successive superposition rule. This rule provides a method to recognize the VCB type with different boundary radii based on that of the VCB with the same boundary radii. Finally, a field case from the northwestern Sichuan Basin gas reservoir is discussed in detail to exhibit the workflow of our model application and validate the capability of our model. • An extended PTA model of vertical layered commingled reservoir with Vertical Combined Boundary (VCB) is established. • The novel VCB flow is identified and mathematical relation between pressure behavior and vertical-heterogeneity are found. • The quantitative mathematical relationships provide a fast way to capture the vertical-heterogeneity. [ABSTRACT FROM AUTHOR]

With the large-scale exploitation of marine gas reservoirs in Sichuan, a casing becomes thin, deformed, cracked or even broken after long-term downhole service, seriously restricting the working life of gas wells. To assess the safety of in-use casings, a three-level assessment model of casings is established, which provides a systematic screening and analysis method for the safety evaluation of casings in marine gas reservoirs. By means of factor analysis, feasibility demonstration and result comparison, the three-level assessment is proved to be suitable for casings. The assessment shows that the safety margin of casings rises step by step. The casing with high safety margin can be eliminated by the first level assessment to improve the efficiency and economy of the process, and then the scope of the dangerous case can be reduced by the second level assessment. Finally, the dangerous case and its real safety margin can be identified by the third level assessment to avoid increasing of the maintenance cost because of the excessive safety margin. Therefore, accurate identification and cost reduction can be achieved by three-level assessment. • This paper develops a three-level assessment for casings. • By means of factor analysis, feasibility demonstration and result comparison, it is proved that the three-level assessment is suitable for casing. • In casing three-level assessments, the safety margin of the results rises gradually. Accurate identification and cost reduction can be achieved. [ABSTRACT FROM AUTHOR]