Tight and shale oil and gas reservoirs demonstrate unique geological characteristics such as extremely poor storage-flow quality, and multi-scale structure of storage and flow space. Those reservoirs are normally developed with staged fracturing of horizontal wells, which is made quite challenging by obviously different initial production rates and rapid declines. Further, uncertainty over the technical effects of drilling/completion and stimulation are significantly different. Currently, the major scientific issues that urgently need to be resolved include the requirement to reduce reservoir damage at all the exploration and development stages, to increase well production and stable production cycle, and to achieve economic and efficient development. Through the damage characteristics analysis of such reservoirs, and the main damage mechanisms summary during drilling/completion, stimulation and production, this paper introduces the basic principles and research progress of reservoir protection technologies such as the temporary plugging of physical particles and chemical filming, underbalanced drilling and completion, and interface modification, etc. The importance of damage prevention technologies in the timely discovery of tight/shale oil and gas reservoirs, correct evaluation and efficient development is elaborated with case studies. This paper also points out that integrated techniques in reservoir damage prediction and diagnosis system, multi-scale damage evaluation method, intelligent reservoir protection materials, liquid trap damage prevention measures, and reservoir protection-leakage control-permeability enhancement will be the important development trends in tight/shale oil and gas reservoir protection in the future.
In recent years, Sinopec has progressed through a series of technical improvements by means of technical research and field practices around key exploration areas, such as the Shunbei, Sichuan-Chongqing, and Ordos areas, etc. It has preliminarily formed key technologies for drilling and completion of ultra-deep wells of 8,000–9,000 m in Shunbei, key technologies for drilling and completion of shale gas wells in Sichuan and Chongqing, key technologies for marine carbonate rocks drilling in Sichuan, and key technologies for low cost drilling and completion of tight gas wells in Ordos Basin, North China. These endeavors support Sinopec’s exploration and development breakthroughs in ultra-deep oil and gas, marine carbonate oil and gas, and unconventional oil and gas resources. As exploration and development continues in increasingly deeper operations, a series of new technical problems and challenges in the key exploration areas emerges. They include the long drilling cycle, the low ROP, frequent downhole failures, and a suboptimal fracturing effect. Therefore, it is necessary to adhere to the required orientation, promote the application of mature technologies, strengthen the key technology research and vigorously implement the “quality, speed, efficiency and production improvement-focused” innovation and efficiency project, so as to provide technical supports for efficient exploration and benefit development in Sinopec key exploration areas.
Well Manshen 1 is a pre-exploration well deployed on the Manshen No. 1 fault zone of the Tabei uplift in the Tarim Basin. During the drilling process, this well suffered from both lost circulation and the collapse of Permian basalt. Further, the development of the well was challenged by poor drillability and rapid bit wear in Silurian Tataaiertage Formation, and the slanting and wellbore instability/collapse in Ordovician Santamu Formation. Through technical research, a series of technologies and interventions strategies such as Permian “2X Excellence” drilling, the Silurian vibration reduction and accelerated drilling, and the Ordovician anti-slanting/ collapse drilling were developed, which effectively solved those challenges. The application of hybrid drilling bit+ PDM fast drilling technology successfully penetrating Permian basalt in one trip; the application of polysulfonate drilling fluid system ensuring the safe drilling in Permian strata, and eliminating the downhole failures such as leakage and collapse. Compared with the adjacent wells, the ROP was increased by 265.96%; the customized PDC bit + TorkBuster torque impactor successfully penetrating Silurian strata in one trip, the torque was stable and the stick-slip vibration was weak during the drilling, and the effects of vibration reduction and speed up were clear. The application of pre-bending downhole motor BHA successfully allowed the drillbit to penetrae the Ordovician large dip-angle strata. In this, the anti-slanting effect was obvious; the application of high-performance anti-collapse water-based drilling fluid system safely drilling through the Ordovician hard and brittle mudstone, which achieved a remarkable borehole stabilization effect. This well achieved high-production industrial oil flow drilling oil testing, brought about a major breakthrough in ultra-deep oil and gas exploration of the Tarim Basin, and initially formed the ultra-deep carbonate drilling/completion technologies, which provided technical supports and best practice for the deep oil and gas exploration & development the of Tarim Oilfield.
Well Hua H50-7 is a horizontal well with an ultra-long horizontal section in Longdong area of Changqing Oilfield. The ultra-long horizontal section as well as the expected mudstone intervals and faults bring difficulties in resistance reduction, wellbore cleaning, prevention of the mudstone layer from collapsing and fault leakage prevention and plugging. The goal involves optimizing the water-based drilling fluid, the lubricity and inhibition of the water-based drilling fluid are improved to reduce friction and resistance and prevent the mudstone layer from collapsing. Different technical measures for leakage prevention and plugging were developed, depending on the severity of leakage to solve problems of fault leakage prevention and plugging. By applying the optimized water-based drilling fluid and adopting the developed leakage prevention and plugging technology, the drilling of Well Hua H50-7 had been smoothly completed. During the drilling process, the lubricity and inhibition of drilling fluid were stable. Compared with the adjacent wells using the original water-based drilling fluid, the resistance encountered in drilling string and casing running wassignificantly reduced. Lost circulations occurred were successfully plugged without the problem of borehole instability. The successful drilling of Well Hua H50-7 showed that by optimizing the formulation of water-based drilling fluid to improve its lubricity and inhibition, and adopting pertinent leakage prevention and plugging technologies, problems can be solved. Further, the problems involved in drilling the ultra-long horizontal sections of horizontal wells in Longdong Area, such as borehole instability, severe mud-making in the horizontal section, and difficulties in lost circulation control in the horizontal section could be solved effectively. At the same time, the successful completion of this well provided experience for drilling longer ultra-long horizontal section horizontal wells in the future.
.Herschel-Bulkley模型（即屈服幂率模型）可用于研究非牛顿流体的流动特性，并能在大范围剪切速率条件下得到准确的预测结果。为此，采用有限体积方法（FVM），研究了内部管柱旋转及流变参数（屈服应力τ0、稠度系数K和流性指数n）对偏心环空（E=0.5）中Herschel-Bulkley流体层流区域的轴向、切向速度剖面与压降梯度的影响。研究结果表明，内部管柱转速从100 r/min增加至400 r/min时，会引起最大轴向速度增加，增幅为120%；较低的流性指数（n=0.2）会引起偏心环空宽区域出现二次流；内部管柱转速及流变参数的变化对偏心环空宽区域切向速度剖面有不良影响；内部管柱转速从0增加至400 r/min时，会引起不同偏心环空（E=0.2, 0.4, 0.6和0.8）内幂率流体压降梯度降低，降低幅度为10%。
In order to accurately understand the change law of annular flow rate in new dual-gradient drilling under gas cut conditions, a gas-liquid two-phase flow model that considers density mutation has been established according to the theory of gas-liquid two-phase flow in the wellbore. They then analyzed the change of annular outlet flow rate under gas cut condition, as well as the influence of different factors on the change of annular outlet flow rate. The results showed that the change rate of annular outlet flow rate increased abruptly when the gas front reached the separator. Further, when the separator was located below the mud line, an abrupt increase of annular outlet flow rate would occur earlier than that of the gas at the bottom of the riser, which was helpful to the earlier identification of gas influx. Further, the influence degrees of density difference of light/heavy drilling fluids, gas influx, flow rate, separator position, well depth and wellhead back pressure on the change of annular outlet flow rate were reduced in turn. The gas-liquid two-phase flow model that considers the density mutation could accurately predict the change of annular outlet flow rate in the new-type dual-gradient drilling under gas cut condition,and provide a theoretical basis for early overflow monitoring in the new-type dual gradient drilling.
With the higher and higher environmental protection requirements for shale gas exploration and development, high-performance water-based drilling fluids have been gradually popularized in the drilling process. However,waste drilling fluid is still produced in a large amount. In order to improve the recycling efficiency of water-based drilling fluid, features of waste high-performance water-based drilling fluid, such as high content of total solid phase and micro poor solid phase, were analyzed, during which it was proposed to remove the poor solid phase in waste high-performance drilling fluid by the electro-sorption method, which had the advantages of having no addition of chemical agents, a selective removal of micro pore solid phase and no damage to the original effective components of drilling fluid. The experimental results showed that after taking the electro-sorption treatment process for two times, the ultra-fine and inferior solid phases of less than 10 μm in the waste high-performance water-based drilling fluid could be removed. Further, with the combination of electro-sorption method and centrifugal separation pretreatment, the performance of regenerated drilling fluid could be significantly improved, thus realizing the recycling of drilling fluid. The results obtained showed that the electro-absorption treatment of waste high-performance water-based drilling fluid constituted a new resource-based model for drilling fluid consumption reduction and reuse, which showed good potential for popularization and application.
To address the problems of oil-based drilling fluid, such as environmental pollution, complex environmentaltreatment process and impact on logging quality, a glycerol-based drilling fluid suitable for strong water sensitive strata was developed for low-carbon alcohols strong inhibition, high lubricity and miscibility with water in any ratio. The swelling rate of bentonite in low-carbon alcohols solution for 24 h was investigated, and glycerol was selected as the optimal inhibition lubricant. The higher the volume fraction of glycerol, the better the inhibition, lubricity and viscosity of glycerol solution. In order to reduce alcohol consumption and further improve the inhibition, a glycerol-based drilling fluid system was formed by using 30% glycerol mixed with 5% KCl. The results of the performance evaluation show that the inhibition and lubricity of glycerol-based drilling fluid system are similar to those of oil-based drilling fluid, with temperature resistance of 140 ℃ and antipollution of NaCl, CaCl2 and drill cuttings. After six months of storage, the drilling fluid has achieved stable performance and can be reused. The inhibition and lubricity of drilling fluid can be further improved by increasing the dosage of glycerol. Glycerol-based drilling fluids can be used in strong water-sensitive stratum or with high requirement of lubricity. It shows a great application potential and this study also provides a development trend in environmental friendly drilling fluids.
There are high temperature, high pressure (HTHP) formations in the Yinggehai Basin in the South China Sea, and they are considered high-risk for safety when penetrating the HTHP formations. In order to reduce the drilling risk, it is necessary to accurately predict the pressures and at each formation depth. For this reason, the seismic while drilling technology was applied in the pre-exploration Well DF-X1. The technology uses seismic data while drilling to obtain the time-depth relationship and the formation velocity, and it updates the bit position on the seismic profile in real time, and thus it determines the high pressure reservoir depth and formation pressure coefficient in front of the bit. During the actual drilling of Well DF-X1, the seismic while drilling technology was used to accurately predict the pore pressure coefficient, fracture pressure coefficient and depth of the high pressure reservoir A1 sand body. The predicted depth error of A1 sand body was only 6.00 m with the actually drilled depth, which ensured that the ϕ244.5 mm casing was run into the mudstone above the high pressure reservoir. By using seismic while drilling, the prediction accuracy of A1 sand body pore pressure coefficient achieved 3.0%, and the accuracy of formation fracture pressure coefficient up to 1.0%. Based on this, the drilling fluid density of the completion section was optimized to avoid the gas cut and lost circulation, kept the successful drilling of the well. The research results showed that the seismic while drilling technology could accurately predict the formation pressure along with the depth of the targeted high pressure reservoir, thus effectively reduce drilling risk and improving operation efficiency.
In view of the problems of high investment on acid-fracturing and unsatisfactory effect in the stimulation of fracture-cavity carbonate reservoirs, and based on the principle of pinnate branch tube stimulation, a pinnate branch tube stimulation string and the key tools (i.e., branch tube nipple) were designed. Combined with the working state of the branch tube, the stress distribution and buckling characteristics of branch tube were analyzed by the finite element method, and it was found that the maximum stress of the 316L stainless steel branch tube with a outer diameter of 8.0 and 10.0 mm was less than the yield strength of the material, which belonged to the category of elastic deformation and satisfied the requirements. Through surface tests, it was verified that the branch tube could extend from the branch tube nipple, with a starting pressure of 3.0 MPa and a maximum pressure of 3.5 MPa. By testing the performance of branch tube nipple, it was found that acid jetted out through the nozzle at high speed could make a hole in the core prior to initiating the branch tube, and the extended branch tube could enter the core to enable the branch tube run through the core. Based on carbonate reservoirs in Tahe Oilfield, the feasibility of pinnate branch tube stimulation was analyzed. It was found that the dissolution rate of carbonate rock samples from the target block reached 100%, and the rate of acid jetting to create holes reached 2.6～14.4 m/h, which possessed the conditions to implement the pinnate branch tube stimulation technology. Studies have shown that the principle of pinnate branch tube stimulation technology is feasible, and it has the advantage of low operation pressure. It is highly feasible and promising to apply this technology in the stimulation of carbonate reservoirs in Tahe Oilfield.
In order to solve problems caused by difficult re-entry of branched boreholes in CBM multi-lateral horizontal wells, such as the inability to RIH screen and the impossibility to realize the screen completion, a re-entry guide tool for screen completion with hollow structure was developed, the completion technology for multi-lateral borehole in CBM multi-lateral horizontal wells was optimized, and the steps of re-entry screen completion for multi-lateral borehole were proposed, thereby forming the re-entry screen completion technology for multi-lateral CBM horizontal wells. The technology was tested in Well Qinshi 12-1; and the test results showed that by comparing the re-entry guide tool with the borehole trajectory data measured by MWD, and it could be judged whether the re-entry was successful, and the screen could be RIH to the branched wellbore to achieve the screen completion. The results also showed that the re-entry screen completion technology for multi-lateral CBM horizontal wells solved the problems of difficult re-entry and the screen completion.
In order to understand the expansion performance of shape memory screens in the downhole environments, a system for testing screen expansion performance was developed by simulating downhole environments based on its expansion principle, by which the expansion performance of the shape memory screen prototype was tested and analyzed the influences of the circulating fluid flowrate and temperature on the response temperature, expansion rate and expansion force. The results showed that the response temperature increased and the expansion speed decreased with the increase of circulating fluid flowrate. When the circulating fluid temperature was not lower than the initial response temperature, the expansion rate increased as the circulating fluid temperature rose, which showed that the influences of circulating fluid flowrate and temperature on the final expansion force were relatively minor. The test results can provide a test basis for designing a shape memory screen and its application in well completion.
In order to further improve gas hydrate drilling and sampling technology in the South China Sea, the requirements of the South China Sea sampling environment and gas hydrate characteristics of gas hydrate drilling and sampling technology were analyzed based on pre-development and field testing, which provided a detailed introduction to the sampling technology. Through an analysis of the difficulties encountered in developing sampling tools in key aspects such as tool size, low temperature insulation, sealing valve, sampling operation mode, etc., the countermeasures were studied according to the actual needs, by which a rope pressure-temperature-preserving sampling tool that suitable for gas hydrate sampling in the South China Sea was developed, along with recommendations having to do with the future research direction of gas hydrate drilling sampling technology. The result is a technical reference and a best practices guide for the further development and improvement of gas hydrate sampling technology in the South China Sea.
The determination of“double sweet spots”(the geological and engineering sweet spot) in shale gas reservoirs is an effective method to determine whether it economically worthy of drilling, and the quantitative study of“double sweet spots”is also conducive to the optimization of drilling and fracturing, hence improving the development efficiency of shale gas. In view of the problems of numerous sweet spot parameters and low accuracy of sweet spot evaluation, the main sweet spot parameters were optimized by using the correlation coefficient method, whereby the independent weight coefficient method was adopted to quantitatively characterize both the geological sweet spot and the engineering sweet spot. Taking the strata of Jiaoshiba Block in the Fuling Shale Gas Field as the research object, five primary geological sweet spot parameters and four primary engineering sweet spot parameters were selected. The correlation coefficient of geological sweet spot was 0.89, and that of engineering sweet spot was 0.85. The research results showed that the“double sweet spots”and fracturing ability of shale gas could be used to optimize drilling and fracturing layers, horizontal drilling azimuth and casing engineering safety management. On this basis, the positive role and advantages of horizontal drilling azimuth optimization for casing safety and large volume fracturing as well as determining fracability in shale gas reservoir were analyzed and deemed feasible.
To solve the problem of frequent replacement of expandable zonal water injection string due to expansion packer damage or bottom ball valve leakage, a cylinder-type expansion packer with relatively independent internal and external pressure systems was developed in accordance with the principle of hydraulic transmission. According to field practices, a combined sand flushing valve and leak-proof device was developed to ensure positive sealing and to reduce the influence of bottom ball valve leakage on the performance of a zonal water injection string. An expandable zonal water injection string with balanced force was designed based on a cylinder type expansion packer. It combined with sand flushing valve and leak-proof device to form a long-term expandable zonal water injection technology. This technology was applied in more than 100 wells in the Shengli Oilfield, and proved to be continuously effective. The field application results show that this technology can effectively solve the problem of integral failure of expandable zonal water injection strings caused by expansion packer damage and bottom ball valve leakage, thus improving the development efficacy of water injection.
To study the dynamic sand transportation and distribution patterns of proppant within fractures during hydraulic fracturing, an experimental device simulating sand transport in fracture systems with multi-scale was self-developed. This included carrying out an experimental study on dynamic sand transportation law and proppant height distribution patterns within fractures of different sizes under different frac fluid viscosity, proppant type, pumping flow rate and proppant concentration. The experimental results showed that the viscosity of frac fluid is the most influential factor followed by particle size of the proppant, proppant concentration and flow rate. The higher the viscosity of fracturing fluid, the less proppant settlement, and the lower and the gentler the settled proppant bank profile. This is more obvious in main fractures. The larger the particle size of the proppant, the more the settled proppant, the higher the settled proppant bank profile. It is more obvious in main fractures, too. Similarly, the higher the proppant concentration, the more the settled proppant, and the higher the settled proppant bank profile. This change is even more notable in branched fractures. The higher the flow rate, the slightly less the settled proppant. Further, it is almost the same in branched fractures. The research results will provide a theoretical basis for the optimization of frac fluid, proppant, and fracturing operation parameters as well as a formulating fracturing scheme.
The current water coning variation formulae of heavy oil reservoirs with bottom water fail to take into account the changes in reservoir physical properties caused by long-term large liquid volume erosion, resulting in a relative large calculation result of the water flooding sweep volume. In order to accurately describe the change laws of water coning and water crest as well as the distribution of remaining oil in the high water cut stage, a mathematical model of water coning and water crest variations for heavy oil reservoirs with bottom water while taking into account the physical properties time-varying was established by considering the physical properties change of reservoirs caused by long-term large liquid volume erosion. Further, an equivalent seepage resistance method was used to implement the equivalent characterization of the reservoir permeability inside and outside the sweep area. The analysis showed that the reservoir physical properties that varied over time had a significant effect on water coning. Under the same water coning width, the mathematical model that took into account the factor of time-varying physical properties was 46.3% smaller than the calculated swept height that did not consider this factor. Comparing the calculation results of the former model with the results of the adjacent well logging interpretation, the relative error was only 5.3%. According to the contact relationship between the water coning and water crest of the heavy oil reservoir with bottom water considering the time-varying physical properties under the current well spacing conditions in the Bohai Q Oilfield, three types of remaining oil distribution patterns among the wells of this oilfield were summarized, and the corresponding potential trapping strategies were developed. The pilot test confirmed the reliability and effectiveness of this potential fines-trapping technology in the high water cut stage of heavy oil reservoirs with bottom water, could provide technical supports for recovering the remaining oil in those reservoirs.
It was found during the oil test of Well Gaotan-1 that the temperature of bottomhole fluids increased with production, which cannot be explained by existing test data and analysis methods. Therefore, based on the conservation equations of mass and energy and according to the seepage flow law of high-temperature fluid in the formation, the flow law in the wellbore and the heat transfer during the seepage and flow, a model of thermal flow coupling between the wellbore and reservoir was established, and a method for inversion of formation temperature in high-yield wells was proposed by analyzing transient temperature data. This method was used to invert the production pressure and temperature data of Well Gaotan-1, and the temperature curve of the inversion was highly consistent with the measured temperature curve of Well Gaotan-1, which explained the phenomenon that the bottomhole fluid temperature increased with the production. The study indicated that the proposed new method for the inversion of formation characteristic and temperature of high-yield wells could quantitatively analyze the formation thermodynamics and seepage parameters, and in that way, determine the fluids producing location of high-yield wells, and provide important basis for production string safety appraisal, field production decision-making, reservoir recognition and reserves calculation.
Resistivity imaging LWD tool can not only visually display the characteristics of micro geological bodies through borehole wall electric imaging, but also can identify the formation interface. In order to explore the logging response mechanism of the tool at the interface, this paper studies the logging response law of the azimuthal resistivity imaging LWD tool at formation interface by using the three-dimensional finite element method. In doing so, it established a quantitative calculation model of formation interface parameters according to the simulation results. The results showed that the resistivity measurement difference of different azimuthal button electrodes of the tool exhibited a good power exponent relationship with the distance from tool to the formation interface in horizontal wells. The angle between the tool and formation interface and the maximum distance between resistivity curve spikes of different azimuthal button electrodes was power exponentially in deviated wells, which is not affected by resistivity contrast of the upper and lower strata at the formation interface. The model for the formation interface parameters interpretation indicated that the tool can be recognized within 1.00 m to the horizontal interface, and the angle can be calculated quantitatively when the angle between the tool and formation interface is less than 20 degrees. The research results can provide a theoretical basis for the application of a resistivity imaging LWD tool in geological engineering.
In order to evaluate the “2X excellence” reservoirs in geology and engineering in Fuling Shale Gas Field with high quality and accuracy, the relationship between well testing data and geological/engineering evaluation parameters in the gas field was qualitatively analyzed, and then the geological/engineering parameters were selected through the European distance-squared systematic clustering method. On the basis of core test analysis data and logging data of the gas field, the logging evaluation model of geological and engineering “2X excellence” reservoir was established by using core calibration logging technology. Finally, the shale gas reservoir geological index and engineering index were calculated to establish the comprehensive evaluation chart of “2X excellence” reservoir by combining the gas producing profile logging data and using the gray correlation theory in the gas field, so as to determine the weight of the “2X excellence” reservoir parameters for production evaluation, forming the comprehensive evaluation method of such reservoir in shale gas geology and engineering. The comprehensive evaluation index of 28 wells in the Pingqiao Block of the Fuling Shale Gas Field has been evaluated by this method, and showed that the production of a single section and a single well of shale gas possessing a significant positive correlation with the comprehensive evaluation index and the crossing length in “2X Excellence” reservoir, which indicates that the shale gas reservoir could be evaluated by the proposed comprehensive logging evaluation method of shale gas geological and engineering the “2X Excellence” reservoir, and provide basis for shale gas horizontal well design and staged fracturing.