The tight gas reservoir in the central Yuanba area of Sichuan basin is one of the major breakthrough in oil and gas survey in recent years. The fractures have important influence on the migration, enrichment and production capacity of the natural gas. Based on the outcrop, core, imaging log, inclusion analysis and acoustic emission experiment data, we studied the fracture characteristics and its genesis of structural fractures in this area. The low-angle shear and high-angle shear fractures lead to the tectonic origin. The fractures have the characteristic of middle scale, minor width(sealed), high thickness, lower density and less filled degree. The high-angle and vertical fractures have better validity. It shows that there are three fracture generations in central Yuanba area: the first stage fractures are formed in late-Yanshanian tectonic movement, NW—SE and NE—SE of plane "X" conjugate shear fractures and low section shear fractures are formed under the compression of SN tectonic stress. The maximum effective principal stress of the paleostructural stress is 18.8 MPa. The fractures in the second stage are formed in middle Himalaya tectonic movement, SN and EW of plane "X" conjugate shear fractures are formed under the compression of NE tectonic stress. The maximum effective principal stress of the paleostructural stress is 27.9 MPa. The third stage are formed in late Himalaya tectonic movement. The structural stress direction is still NE, which mainly forms NW angle section shear fractures and the maximum effective principal stress of the paleostructural stress is 38.6 MPa.

Based on the study of the carbon oxygen isotopic characteristics and the observation of thin slices, we systematically studied the carbonate cementation of Chang-8 reservoir in Ordos Basin. According to the observation of the minerals under the microscope, it can be concluded that the carbonate cements were composed of the calcite, dolomite, ferrodolomite, ferrocalcite and siderite. The mineral composition and distribution are strongly influenced by the heterogeneous chemical activity of fluids. The carbonate cementation often damages the porosity and permeability. Meanwhile, the C-O isotope value was determined by the phosphoric acid method. The experimental results show that the range of carbon isotope distribution is -14.8 ‰ to -1.5 ‰, and the distribution range of oxygen isotope is -25.7 ‰ to -9.9 ‰. The Z values of the samples are mostly less than 120, indicating that the carbonate cements in the sample formed in the freshwater-brackish water environment. The paleo temperature was measured from 68 ℃ to 128 ℃ by oxygen isotope value, which shows that the cementation occurred mainly in the early diagenetic stage B to the middle diagenetic stage A. The carbonate cement content in the upper part of Chang-8 is much lower than that in the lower part, indicating that cementation was derived from carbonate deposits which were related to decarboxylation of organic acids. According to the sedimentary subfacies, the study area is divided into three areas, and the oxygen-carbon isotope gradually decreases in accordance with the half deep lacustrine subfacies-gravity flow subfacies-delta subfacies. Finally, the reasons for the present situation of isotope distribution in the basin are concluded: the organic acid moves gradually from the high potential region to the low potential region under the compaction, and the organic carbon is captured by chemical cementation after a series of chemical changes.

The fracture-cavity carbonate reservoir has large scale and discontinuous medium, and its main reservoir space is composed of fractures and caverns. Many wells are independent fractured-cavity systems, and most of them connect macrofractures to the wellbore. These wells are separate fracture-cavity cells which connect with one or two big cavities in series by fracture network, so the actual flow process of the reservoir does not match the result obtained by adopting traditional continuum theory. An analytical mathematic model is presented by considering the fracture network between well-cavity and cavity-cavity as one dimensional linear fracture, and other fracture communication flow exist in the periphery of the cavity(away from wellbore). Accordingly, the linear-in-series well test mathematic model with fracture connecting two cavity systems is established, and the model introduces the conception of transient flow and channeling flow of multiple media. The Transient conduction principle and Laplace transform are used to solve the mathematical model, and the dimensionless bottom hole pressure type curves are plotted in real space by using the improved Stehfest numerical inversion. The analysis of the sensitivity of cave parameters shows that the model can correctly reflect the relation between flow control factor and geological factor. Application examples indicate that this model can correctly explain wellbore storage factor, skin effect, the permeability of fracture system and cavity system, and the geometric parameters of fracture and cavity.

Generally speaking, the fracturing can improve the reservoir single well production, however, whether it can reduce the retrograde condensation damage in gas condensate wells or not is urgent to be discussed further. By adopting the indoor core laboratory and single well numerical simulation method, we analyze the impacts of retrograde condensation on the gas phase permeability, the retrograde gas condensate saturation near the wellbore zone, the single well production and the recovery ratio before and after fracturing. The research shows that, after fracturing, the retrograde condensation damage is more serious and the damage time is earlier. However, the fractures reduce the condensate oil critical flow saturation. In the process of the depletion exploitation, the effective permeability value of the fractured gas phase is always larger than the value before fracturing. In addition, the gas phase flow ability increases substantially. After fracturing, the gas recovery speed of the single well is fast, the formation pressure reduces rapidly and the retrograde condensation damage is more serious. Furthermore, the recovery ratio of the condensate oil reduces by 3.48 % and the recovery ratio of the natural gas improves by 6.6 %. Accordingly, it concludes that, fracturing can’t effectively reduce the degree of the retrograde condensation damage, but it can improve the flow ability of the gas phase and the gas production of the gas condensate wells.

In this artide, we analyse the oil oxidation characteristics of the light oil at different oxygen concentrations(21 % and 8 %) and in the presence of the reservoir rock by the static oil oxidation tube and thermogravimetry/derivative thermogravimetry(TG/DTG). According to the Arrhenius oxidation kinetics model, the influence of the factors on crude oil oxidation activity is characterized quantitatively. The results show that the lower oxygen concentration can reduce the oxidation reaction rate, and in addition, the reservoir rock has strong catalytic impacts on oil oxidation, which can promote the oxidation reaction. The oxygen concentration of the remaining gas drops significantly after the oxidation in a lower oxygen concentration, which greatly improve the safety of air injection displacement. The thermal analysis shows that the crude oil exhibits three stages of low temperature oxidation, fuel deposition and high temperature oxidation. The positive impact of cuttings on the crude oil oxidation is stronger than the negative impact of the lower oxygen concentration, which can significantly reduce the activation energy of crude oil at low oxygen environment. It is benefit to the application of lean oxygen injection displacement.

The calculation of the bottom hole pressure(BHP) in the high temperature and high pressure the gas wells with should consider the temperature changes inside the wellbore. The coupling of the pressure and temperature need to be solved simultaneously. The current calculation methods considering the wellbore heat transfer, and the coupling of pressure and temperature mainly apply to the steady flow. However, they don’t apply to the transition flow during the shut-in period. Meanwhile, the wellbore heat transmission of the offshore gas well in high temperature and high pressure should consider the effects of sea water. Nevertheless, there are fewer examples of this research. According to the problems above and based on the basic equation, we established a new calculation model, which considers wellbore storage, and the heat transfer among the wellbore, adjacent formation and sea water. This method is successfully applied to the BHP calculation and well testing analysis for the well M1 during the shut-in period in M gas field of the western South China Sea, thus providing the basis for the temperature and pressure calculation and well testing optimization of high temperature and high pressure offshore gas well.

In the case of the carbonate heavy oil reservoir with the well developed dissolution fracture, the percolation mechanism of dissolution fracture is different from that of fracture. The conventional well test models can not meet the needs of the well test analysis. In the heavy oil reservoir with quadruple media. According to the basic principle of the seepage flow mechanics, the quadruple-media well test mathematical model based on the fracture, dissolution fracture, dissolution pore and matrix is established in heavy oil treating as the power-law fluid. By means of the Laplace transformation and Stehfest numerical inversion, we obtained the real space solution, and drew the well test curves of three different boundary conditions. The results show that three concaves with different depths and widths occur in the pressure derivative curves of the quardrule-media reservoir and the slope of pressure derivative double-log curve is(1-n)/(3-n) in the stage of radial flow. The smaller the power-law index is, the greater the slope of pressure-derivative curve of radial flow will be. The smaller the storativity ratio is, the wider the concave will be, furthermore, the greater the interporosity flow coefficient is, the ealier the concaves will appear. Compared to the thriple-media, the change of the pressure-derivative curve affected by many parameters is more sensitive in the heavy oil reservoir with quadruple-media. The model is used to guide the interpretation and study of the well test data .

A low permeability reservoirs usually have threshold pressure gradient with extremely low permeability. The seepage mechanism of multi-stage fractured horizontal wells are complex and the productivity are affected by many factors. Based on the dual-porosity medium seepage differential equation, applying point source theory and superposition principle, a semi-analytical productivity model for the multi-stage fractured horizontal wells is established, considering the threshold pressure gradient and hydraulic fracture conductivity. Then the calculation of production and the numerical inversion are conducted, following by the analyses of the influence factors combining specific examples. The study results show that the threshold pressure gradient has great effect on productivity. The larger the threshold pressure gradient is, the lower the production will be. The angle between the hydraulic fracture and wellbore has a little influence on production, but not obvious, and the highest production appears when the angle is 90°. The conductivity of the hydraulic fracture has obvious effect on the initial production, but a little influence on that in the later period. The hydraulic fracture at both ends of the horizontal wells affect more than that in the middle, so the fractured horizontal well’s stages and the length of the fracture should be increased. The storage ratio and cross flow coefficient mainly affect the medium-term productivity. The larger the storage ratio is, the faster the production will decrease. And the larger the cross flow coefficient is, the higher the production will be. The results of this study not only contributes to the deep understanding of the multi-stage fractured horizontal well’s seepage laws in the low permeability reservoirs, but also provides some important guidance and advice for the optimization design of the multi-stage fractured horizontal wells.

The He-1 layer of Daniudi gas field belongs to the low porosity and low permeability gas reservoir. As for this, we mainly used the packer fracturing technology in the open hole, and achieved great results. In order to make full use of the low grade gas reservoirs, the horizontal wells of this gas field has been studied by the multi-cluster fracturing technology. By establishing the model of the horizontal wells productivity with the multi-cluster fracturing and using the Eclipse numerical simulation software, the geological factors and the fracture factors that affect the productivity of the horizontal wells were analyzed. Meanwhile, the orthogonal test design method was used to analyze the weight of the influence factors of the production capacity to determine the main influence factors and then optimize the design parameters of the fracturing. The application effect showed that the multi stage fracturing significantly improved the well production.

A trilinear flow model is one of the common methods solving the well test models for the multistage fractured horizontal wells. By the orthogonal test, the range analysis method and the variance analysis method, we analyzed the influence of the half fracture length, the half reservoir thickness, the width of the inner zone, the dimensionless fracture conductivity and the permeability in inner zone on the double log curves fitting of the pressure and the pressure derivative of the trilinear flow model and numerical model. According to this analysis, the factors affecting the curves of the pressure characteristic curves of two models ranged from more to less are the dimensionless fracture conductivity(F_CD), the permeability in inner zone(k_I), the half fracture length(x_f), the half reservoir thickness(x_e)and the width of inner zone(y_e). The fracture conductivity affect obviously on the fitting of the pressure characteristic curves of two models. When the fracture conductivity is less than 1, the curve fitting effect is poor, so the linear flow model is not applicable. Otherwise, when the fracture conductivity is greater than 1, the curve fitting effect is good, so the linear flow model is suitable.

In order to discuss the technical measures of the multi-directional water breakthrough, it is essential to find out the water breakthrough discipline and the understand the mechanism of the multi-round profile control technique. Therefore, we designed and the established the fracture-matrix visualization model with one-injection well for three production wells. The permeability between the different injection-production ends was inequality. By this model, we studied the migration characteristics and the plugging mechanism of the gel, the fluid diverting of subsequent water flooding and the distribution characteristics of the residual oil on the condition of the fracture-matrix. Through the core flow experiment of the shunt-wound sand-filled pipes with different permeability. And by the simulation of the interlayer heterogeneity, we compared the experimental data of the single and multi-round profile control measures. The results show that the multi-round profile control can fully play the synergistic effect to improve the performance and plug the fractured seepage channel effectively along with the promotion of the fluid diverting, and the profile control effect reduces with time of the injection. The multi-round profile control technique is better than the single profile control technique in the respect of adapting the fractured geological structure, and has a better effect on relieving the interlayer contradiction.

By using the drilling coring data, rock thin section, cast thin section, scanning electron microscope and logging data, and combined with the analysis of the test data, the diagenesis characteristics and its impact on reservoir porosity of Lenghu region of Paleogene has been carried on by the detailed study. The results showed that the maturity of the Lenghu region is low, the primary pore, secondary pore and micro cracks are both developed in the reservoir. The diagenesis types are complex, including compaction, cementation and dissolution. The compaction is the most fundamental reason for failure of reservoir. The porosity loss of the reservoirs in the study area caused of the compaction is 21.65 % in average. The cementation is an important factor for further reducing the reservoir pore loss, causing an average of 3.34 %. The dissolution of the reservoir is an important way to increase the hole, but due to the lack of dissolution fluid, its contribution is limited, increasing pores about 2.26 %. There is a positive correlation between the reservoir properties and maturity, and a negative correlation between the properties and depth, which show that the low maturity and deep buried Lulehe formation (E₁₊₂) has the strongest compaction and cementation, and the worst reservoir property. The property of lower Ganchaigou formation (E₃ ¹) was medium, and the upper of lower Ganchaigou formation(E₃ ²), which has the highest maturity and is the most shallow buried has the optimal reservoir property.

There are huge coalbed methane reserves existing in our country. These CBM reservoirs have the characteristics of low pressure, low saturation, low permeability, high heterogeneity and complex reservoir structure, with the easily broken coal rocks. The technology of the pore-creating by the hydraulic jet can avoid the collapse of the coal bed, eliminate the compaction effect caused by the conventional perforation and break the coal petrography to promote the permeability of the reservoir. By the study of the large physical models, the conclusion was that the bore diameter increases by the increasing of the nozzle diameter. The abrasive concentration and the pore diameter increases but the bore depth decreases after the increasing and the pumping pressure and the bole depth increase. Finally, the operation parameters were optimized, that is, the pumping pressure is 25 MPa, the nozzle diameter is 4 mm and the abrasive concentration is 6.6 %. These parameters are helpful for the parameter design of the site construction.

There are several methods of the production decline analysis during the shale gas reservoir development, such as the Arps model, the SEPD model, the Duong model and their composition models. Among them, the Arps model is the main method. There are two main choices of the appropriate decline methods. One is to transform the data into the linear relationship and the method with high correlation coefficient can be deemed as the better model. The other is to use the non-linear regression by the combination of the above models and choose the analysis method with high correlation coefficient. Then we proposed a new method to choose the production decline analysis model, and obtained the relation degree by comparing the linear combination of the different decline analysis models with the practical production data, furthermore, according to the degree of correlation in order, selected the production decline analysis method. This method is validated by the high fitting precision between the calculated results and the production data, which provides a reliable and reasonable way to choose the production decline models.

During the drilling process of the shale gas wells, obtaining the high quality core samples is an important prerequisite for the reservoir evaluation. Aiming at the problems such as the long coring footage, the low core recovery and the poor efficiency of the conventional coring technology, and based on the analysis of the geological characteristics of the coring layer, we optimized and improved the tools, the bit, the parameters and the technology. In the field application of the multiple shale gas wells, the average core recovery was more than 98 % and the penetration rate was up to 9 m/h. The results showed that this technology had wonderful performance in improving the core recovery and the penetration rate, and successfully resolved the various problems in the process of the coring in the shale gas wells. In the meantime it provided important support for the later development of the shale gas wells.