The stress field has a great influence on fracture turning and expansion and the study on stress distribution has a guiding significance for the fracturing parameters optimization. Based on the extend finite element method, the Abaqus software was used to investigate the influence of principal stress differences and fracture inclines on normal stress and shear stress of fracture, the influence of perforation angles on fracture trajectory, shear stress and tangential displacement, and the influence of fracture spacing on fracture morphology, shear stress and pore pressure. The results show that the normal stress on the fracture decreases with the increase of fracture inclination angle, and the shear stress increases first and then decreases with the increase of fracture inclination angle, reaching the maximum in the direction of 45°. When the fracture angle keeps constant, the normal stress and shear stress on the fracture increase with the increasing perforation angle, and the radius of fracture turning decreases with the increasing perforation angle. With the condition of high principal stress difference and perforation angle of 45°, it is conducive to produce shear failure in the rock of the fracture entrance. In the simultaneous propagation of multiple fractures process, the shear stress in the disturbance region increase with the decrease of fracture spacing and the fluid distribution in the fracture at the disturbance side increase with the increasing fracture spacing. With the condition of great principal stress difference, the perforation method with small cluster spacing and 45° direction is beneficial to shear failure of rocks.
Keshen 2 gas reservoir in Tarim Basin is typical with fractured low-porosity sandstone gas reservoir with edge water. It is mainly characterized by poor petrophysical property in matrix and fracture-developed. The study found that there is great difference between the actual situation and the results obtained from the current double-medium reservoir simulation model based on traditional understanding in aspect of pressure propagating and water-invasion property. Therefore, numerical well test method was used to analyze the pressure transient well testing data, integrated with all kinds of static & performance data. And on this basis, deep analysis with numerical simulation on mechanism model is made on the response time, curve characters of interference well testing and water-invasion property. The result showed that the Keshen 2 gas reservoir is characterized by faults-fracture-matrix “triple medium” flow, and the faults in “triple medium” are the superhighway of reservoir pressure propagating and water-invasion. The non-uniformity in edge water-invasion is very serious and the water-breakthrough time is much shorter in different structural positions of Keshen 2 gas reservoir, mainly because of its faster migration rate of edge water-invasion in faults than that of fractures. The gas reservoir seepage under the faults-fracture-matrix “triple medium” new seepage mode matches well with the actual pressure propagating and water-invasion property in Keshen 2 gas reservoir. According to the mechanism study results, it is recommended to keep low gas recovery rate and cut the gas production of wells on the edge of reservoir. At the same time, dewatering gas production should be prioritized for watered-out gas wells, where there is fault developed between them and wells in the central part of reservoir. And these are the important measures for controlling edge water incursion rate along the faults and raising the recovery efficiency of gas reservoir.
For the purpose of improving the efficiency and accuracy of acid fracturing project design for the deep and high-temperature gas wells in western Sichuan Basin, an acid penetration distance of acid fracturing mathematical calculation model considering heat effects of well-bore and acid-rock reaction was established. An acid fracturing case of a field candidate well in Qixia Formation was taken as an example, the influence of different heat effects on acid penetration distance was researched and the sensitivity analysis of acid fracturing operation parameters was conducted. And the following research results were obtained. Firstly, heat transfer effect of wellbore elevated the acid temperature at the fracture inlet, which intensified the acid/rock reaction at fracture wall and decreased the acid penetration distance. Secondly, under the same injection condition, when the injected acid reached a certain volume, the increase of acid volume was limited to enhance the effective acid penetration distance. Finally, with the same injection volume condition, increasing injection rate could promote the propagation of fracture to the deep reservoir and reduced the excessive acid etching at the fracture inlet, which was beneficial to enhance acid effective penetration distance. Based on the results, a gelled acid treatment design was recommended for Well X1. The post-treatment production of Well X1 was 1.4 times that of the adjacent well, which have similar geological condition and treatment scale.
For the purpose of improving the efficiency and accuracy of acid fracturing project design for the deep and high-temperature gas wells in western Sichuan Basin， an acid penetration distance of acid fracturing mathematical calculation model considering heat effects of well-bore and acid-rock reaction was established. An acid fracturing case of a field candidate well in Qixia Formation was taken as an example， the influence of different heat effects on acid penetration distance was researched and the sensitivity analysis of acid fracturing operation parameters was conducted. And the following research results were obtained. Firstly， heat transfer effect of wellbore elevated the acid temperature at the fracture inlet， which intensified the acid/rock reaction at fracture wall and decreased the acid penetration distance. Secondly， under the same injection condition， when the injected acid reached a certain volume， the increase of acid volume was limited to enhance the effective acid penetration distance. Finally， with the same injection volume condition， increasing injection rate could promote the propagation of fracture to the deep reservoir and reduced the excessive acid etching at the fracture inlet， which was beneficial to enhance acid effective penetration distance. Based on the results， a gelled acid treatment design was recommended for Well X1. The post-treatment production of Well X1 was 1.4 times that of the adjacent well， which have similar geological condition and treatment scale.
Low-permeability gas reservoirs have low natural productivity, and usually require effective reservoir reforming methods such as hydraulic fracturing to normal production, and the influence of formation water production on gas well production cannot be ignored. Based on the analysis of the seepage law of fluids, the Darcy effect and non-Darcy effect are fully considered. The flow is divided into elliptical flow within formation and linear and radial flow within fracture. Based on the equivalent wellbore theory and the potential superposition principle, the comprehensive productivity of the flow in the formation and within the fracture was established respectively by introducing the scale factor and the two-phase pseudo-pressure function, converting the elliptical coordinate system and the rectangular coordinate system. The equation can calculate the open flow capacity of gas wells more accurately and has strong field practicability. From the sensitivity analysis, the stress sensitivity of the fracture and the technical parameters of the fracture have great influence on the productivity. There is an optimum design value of the fracture parameters, which can bring about the optimum economy while exerting the productivity of the gas well. Water production in gas wells has a great influence on gas well productivity, and it is necessary to do waterproof and water control measures.
The sedimentary facies types, distribution characteristics and plane distribution of the Upper Permian Wujiaping Formation in the northwestern Sichuan are studied by comprehensive analysis of a large number of core, section, logging, and seismic data. In the Wujiaping Formation of northwestern Sichuan Basin, there are mudstone, carbonate and pyroclastic rocks. The sedimentary facies can be divided into six species, such as the shore plain and the marsh, the ramp, the open-platform, the platform margin, the slope and the continental shelf. In the early stage of Wu 1 member, the seawater intruded on the karst ancient landform of the underlying Maokou Formation. Since the sedimentary water in the study area was relatively stagnant, the reductive environment was the shore plain and the swamp facies which formed fine-grained sedimentary rock, such as the shale at the bottom of the Wu 1 member. The relative sea level rose in the middle and late Wu 1 member, and the ramp deposition began on large scale in the study area. The southwestern part of St10-Yb7 area was the shallow ramp subphase dominated by all kinds of shoal deposits, and the north part was the deep ramp subphase which was the low energy interbank depression with low energy shoals. In the sedimentary period of Wu 2 member, the study area was affected by the local co-deposition fault and the like, the deposition range of the deep ramp was expanded to the southwest. The continuous large-scale shoals construction and the construction settlement analysis are carried out, so that during the time of the Wu 3 member, there is a thick, bright-grained limestone with microorganism-bonded rock and so on, forming the sedimentary features of the weak-rimmed platform in the St9-Lg70-Yb7 area. The southwestern part was deposited in open platform facies. The slope, and the continental shelf are deposited in northeastern part. Therefore, the Wujiaping Formation in the northwest of Sichuan has experienced the evolution of three depositional systems, such as the shore plain and the swamp, the ramp, the weak-rimmed platform / the slope / the continental shelf.
Tight sandstone gas is the focus of unconventional oil and gas exploration and development. In order to clarify the gas filling characteristics and gas-bearing characteristics of different types of tight sandstone reservoirs, 12 sandstone samples from Shanxi Formation of Yan'an gas field were selected and divided into three types according to their physical properties. Gas filling simulation experiments under different permeability, filling velocity and filling differential pressure difference were carried out respectively. The results show that the contribution of medium pore to gas saturation is greater than that of small pore in sandstone with better physical properties, while the contribution of small pore in ultra-low permeability compact sandstone to gas saturation is greater than that of medium pore. Critical injection pressure is the intrinsic property of rock, and does not change with the change of flow velocity. Gas saturation is proportional to flow velocity, and the faster the flow velocity, the higher the gas saturation. The greater the filling pressure difference, the shorter the breakthrough time of gas, the higher the gas saturation. With the increase of filling time, the change of gas saturation is very small, and finally tends to a constant value, that is, the gas saturation under the confined water condition of the core. This study can provide experimental basis for the lower limit analysis of physical properties and migration mechanism of tight sandstone reservoirs.
Braided delta front of subaqueous terminal distributary channels is an important reservoir style of Pearl Mouth Basin in the South China Sea. Although studies on the architectures of braided delta have been largely carried out by many researchers, the morphology, scale and superimposed style of low-graded architecture units have not been deeply understood in subaqueous terminal distributary channels. Taking A Oilfield of Lufeng Depression in Pearl River Mouth Basin as an example, architecture patterns of subaqueous terminal distributary channels from three hierarchies (channel complex set, channel complex, elementary channel) are studied. The results are depended on variety high-quality data of the study area. There are five wells altogether (the minimum well space is 500m) and 18.64m systematic core data. Architectures of braided river delta front system can be divided into ten hierarchies. Single channel, channel complex and channel complex set are architecture units from the 4th hierarchy to the 6th hierarchy. Elementary subaqueous terminal distributary channels are 200-500m wide and 3-7m deep. Channel complex manifests various superimposed styles, including vertical solitary style(A) , vertical swinging style(B), lateral amalgamated style(C),disordered style(D).Vertically, the channel complexes have a rule of evolution from A to D upwards.
Based on the core observation, thin sections identification, geophysical property, scanning electron microscope, cathodoluminescence tests, diagenesis and its control on physical property of the reservoirs in the 3rd member of the Liushagang Formation (El 3) in the Weixinnan Depression were analyzed by using the method of qualitative description and quantitative characterization. The research shows that the physical properties of reservoirs in study zone can be summarized as medium-low porosity and heterogeneous permeability, and the diagenetic environment of reservoir has changed from weak alkaline, acidic, alkaline to weak acidic conditions. The compaction intensity is mainly moderate-strong, and the cementation intensity is mainly moderate-weak. The compaction, which is mainly controlled by the buried depth, is the main mechanism of pore reduction in the reservoirs. The dissolution intensity is moderate, so as to improve the reservoir physical properties. When the thickness ratio of sand to mud is greater than one, the carbonate cementation and dissolution are affected by the distance to the interface of sand and mudstone. With the decrease of distance, the carbonate cementation intension enhances, the dissolution intension weakens, so the favorable reservoirs may develop in the middle of thick sandstone bodies. The structural fractures caused by the major activities can be the main migration channel of the fluid medium. The reservoirs which closing to large faults or secondary faults may cause strong corrosion.
AD Oilfield is located in the south-central Iraq, and the Khasib Formation deposited during the Late Cretaceous Turonian Age is one of the crucial oil-bearing strata. Kh2 member is the main producing layer of Khasib Formation and develops a large number of bioturbation structures. The water injection development process of Kh2 member is heavily restricted at present due to the strong reservoir heterogeneity. In aim of clarifying the correlation and genetic relationship between bioturbation and Kh2 reservoir heterogeneity, the study of ichnology characteristics, heterogeneity characteristics, types and genetics were carried out through petrography, ichnology and reservoir geology methods based on core, thin-section observation, cathodoluminescence and spot permeability tests. Results show that the Kh2 member mainly developed four ichnofabrics: Thalassinoides-Rhizocorallium, Ophiomorpha, Thalassinoides-Palaeophycus, Thalassinoides-Rhizocorallium-Planolites corresponding to mid-ramp sand beach firmground, sand beach looseground, green algae beach and beach interact, outer ramp deposition. Bioturbation led to the Kh2 reservoir heterogeneity and the reservoir spaces, oil-bearing and spot permeability distribution of burrows were improved comparing with the matrix. Bioturbation caused the composition and structure difference between burrow and matrix, and the physical and chemical properties of the two varied correspondingly. Based on the fabric difference as material basis and later superimposed by differential diagenesis including cementation, dissolution, compaction, etc., three heterogeneous types were formed and their genetic models were controlled by sedimentary environment, rock type, ichnofabric and diagenesis cooperatively. Bioturbation, sedimentation and diagenesis control the strong and multi-scale reservoir heterogeneity of Kh2 member coordinately.
Good hydrocarbon shows have been found in the Lower Cretaceous Hengtongshan Formation of the Tonghua Basin, and it has good prospects for conventional and unconventional oil and gas exploration. However, currently no systematic summary, especially the study on the characteristics of shale reservoirs in Low Cretaceous Hengtongshan Formation in the Tonghua Basin has been carried out. In this study, we took the core of Tongdi No.1 drilling well with oil and gas shows as the study object, and used the methods of organic geochemistry, X-ray diffraction, scanning electron microscopy, liquid nitrogen adsorption and other analytical methods to study the mineral composition and pore types of shale. The results show that: the brittle mineral content of shale from Hengtongshan Formation in the Tonghua Basin is 58.2%, and the quartz brittleness is 35.4%, which is conducive to fracturing; the main pore types include organic pores, microcracks, intragranular pores and intergranular pores. Among them, intragranular dissolution pores are most developed, while intergranular pores are second; the pore size distribution of shale is complex, mainly distributed between 2nm and 60nm, mainly in mesopores, followed by micropores and macropores; the mesopores provide a volume of 90.87% and a specific surface area of 85.74%, and this is the main carrier of the shale gas. In addition, this paper also discusses the factors affecting the pore structure of Hengtongshan Formation shale, It is found that the abundance of organic matter and the microscopic composition have obvious influence on the pores in the study area.
The potential of continental shale gas resources in China is abundant and the industrial shale gas has been obtained in shale reservoir in the Lower Jurassic of northeastern Sichuan Basin. The types of lithofacies and reservoir characteristics are essential to exploration and development of continental shale gas. Based on the experiments of XRD, analytical data of ordinary thin section, scanning electron microscope, high pressure mercury pressure, N2 adsorption and CO2 adsorption, a study on the continental shale of Ziliujing Formation in northeastern Sichuan Basin is carried out. According to the total organic matter content and mineral composition as classification parameters, the shale is divided into 12 different lithofacies. Four kinds of lithofacies are developed in Ziliujing Formation, which are organic-poor siliceous shale(Ⅰ3), organic-fair mixed shale(Ⅳ2), organic-fair siliceous shale(Ⅰ2) and organic-rich argillaceous shale(Ⅱ1). The reservoir characteristics of different lithofacies are quite different. The total organic matter content of Ⅱ1 is more than 2%, and the hydrocarbon generation potential is the best. The pores of organic matter and clay minerals have good connectivity and large specific surface area, and the Ⅱ1 developed laminar structure which is favorable for artificial fracturing. The organic matter content of Ⅳ2 is more than 1%. The Ⅳ2 developed high pore volume and the layered structure. However, the Ⅰ3 and the Ⅰ2 have the characteristics of massive, and developed low pore volume and specific surface area.
In recent years, shale gas of Wufeng-Longmaxi Formations in Sichuan Basin has achieved economical developments on a large scale in middle-shallow reservoirs (vertical depth<3 500 meters), and gradually expanded to the deep reservoirs (vertical depth between 3 500 meters and 4 500 meters). It has made a preliminary progress in Dazu area of western Chongqing. Through analysis and evaluation of the hydrocarbon generation potential, reservoir pores, fracturing feasibility and gas content of shale in Dazu area, the study shows that compared with Changning and Weiyuan blocks, the O3w-S1l1-1 shale reservoir of Dazu is basically equal in brittle mineral and gas content, and is slightly weaker in organic matter abundance and porosity, which means the reservoir quality is comparatively excellent; There exist pore types including organic pores, intergranular pores, intragranular pores and microfissures, among which the proportion of intragranular pores is the largest. The fissure develops on a high level, while the concentration of fissure is the largest in the Wufeng Formation. By comprehensive research, it indicates that the O3w-S1l1-1 layers in Dazu area are characterized by large thickness of types Ⅰ+Ⅱ shale reservoir, and the Wufeng Formation shows great resource potential; the pressure factor is large and the preservation condition is excellent in this area; and it shows the enrichment and high yield mode of “matrix pores+fissures”, which implies a bright future for the exploration and development of deep shale gas in that area.
The fractures and caverns of the carbonate reservoirs in the second section of the Lianglitage Formation in the eastern part of the Tazhong area are developed, and the reservoir types are diverse and heterogeneous. Various reservoir types can be clearly identified based on core and imaging logging, but it is difficult to identify wells that lack core and imaging logging data, which affects reservoir prediction and evaluation. In order to solve this problem, four reservoir types and two non-reservoir types are classified based on core, thin section and image logging data. The reservoir types include fracture reservoir, vugular reservoir, fractured-vugular reservoir and cave reservoir, and non-reservoir types includes mud filling and compact. According to these types, the conventional logging information is calibrated. Three principal factors were extracted from the six well logging parameters by factor analysis, which were interpreted as pore factors, fracture factors and mud factors. Finally, the factor score is calculated, and various reservoir and non-reservoir types can be effectively identified according to the factor score crossplots. This method is used to identify the reservoir of Well XX, and the recognition result is highly consistent with the core and imaging logging data, which provides a new technical method for logging identification of fractured-vuggy carbonate reservoirs.
Accurate shear wave logging velocity is a necessary parameter for pre-stack seismic attribute analysis and pre-stack seismic inversion. In the absence of dipole shear wave logging, it is necessary to use Krief, pride and other mathematical models to predict shear waves. In recent years, critical porosity has been well applied in calculating elastic model quantities of skeleton. Based on the critical porosity model, genetic algorithm (GA) is investigated to achieve a variable critical porosity through the borehole. The shear wave velocity is finally estimated by using the critical porosity and the GA-derived parameters. A case study shows that genetic algorithm can calculate the continuous critical variable porosity, and the predicted shear wave slowness is well consistent with DSI log. This method can be applied to tight sandstone reservoir.
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