In view of the development needs of oil and gas reservoir fracturing stimulation technology, the research status of artificial intelligence (AI) in fracturing stimulation is described, the key theoretical problems in the development of AI in fracturing are analyzed, and the future main research directions and application scenarios are prospected. At home and abroad, some progress has been made in fracturing design optimization, fracturing condition diagnosis and risk pre-warning,fracturing flowback optimization control. But it is still in the transition stage from academic research to industrial application, facing with the key theoretical problems including small sample size and lack of label data, poor model interpretability and the requirement of deep integration of datadriven and physical models. Based on the existing problems, the main research directions of artificial intelligence for fracturing in the future are prospected in this paper, such as data management and feature engineering, deep mining of fracturing data in small sample learning scenarios, interpretable fracturing intelligent algorithms based on knowledge embedding and knowledge discovery, and dynamic optimization of fracturing parameters and risk pre-warning and control methods based on reinforcement learning, etc. Three application scenarios are suggested, including intelligent optimization of fracturing design, closed-loop control of fracturing construction, and intelligent control of fracturing flowback, aiming for the high-quality balanced fracture formation and safe fracturing.

In the process of running casing in the narrow safety density window formations, the surge pressure generated by the movement of the casing string in the borehole can easily induce the complicated downhole incidents. In order to ensure the safety of running casing under narrow safety density window, a transient surge pressure calculation model for casing running in eccentric annulus was proposed. The wellbore pressure prediction and management model during the liner running was also proposed based on the wellbore pressure balance relationships. The main influencing factors of bottom hole surge pressure in running casing operation were analyzed and the case study of field running casing for managed pressure cementing was performed. The research results show that the casing running speed, casing eccentricity, and rheological parameters of drilling fluid are the main factors affecting the bottom hole surge pressure. The annulus transient surge pressure increases with the increasing of annulus fluid rheological parameters (yield value, consistency coefficient and fluidity index) and casing running speed, but decreases with the increasing of eccentricity. A case calculation was carried out by taking Well DB-X as an example, and the wellbore pressure control chart of running casing for managed pressure cementing was proposed. The calculation results show that it can effectively guarantee the bottom hole safety when keeping the liner running speed of 0.16~0.20 m/s. The data guide this well operation, effectively prevent the occurrence of downhole complications during casing running. The results of this paper can provide a new idea for velocity optimization of running casing and bottom hole pressure management during cementing, which is of great significance for reducing casing running risk and improving cementing quality in formation with narrow safety density window. 

Ultra-long horizontal well with horizontal section over 3 000 m has great advantages in improving single well production and economic benefits. With the development of this technology, it will gradually become the main direction in shale gas, tight oil and other developments. In view of the drilling difficulties of ultra-long horizontal wells of shale gas in Sichuan Basin, the key technologies for ultra-long horizontal well are developed, which include the optimization of wellbore structure, the strengthening of drilling parameters, the optimization of drilling fluid system and borehole cleaning, the geo-engineering integrated steering drilling, the casing running and cementing technology in horizontal section. This paper summarizes the practices and successful cases of horizontal shale gas wells with horizontal section over 3 000 m. On this basis,the development suggestions and directions for horizontal shale gas wells with ultra-long horizontal section in the future are pointed out, such as the further exploration and development of low-cost drilling technology, research of safe and efficient optimal fast drilling technology, efficient cooperation of geo-engineering integration, and improvement of project management under daily rate system, which is aiming at promoting the drilling technology development for horizontal shale gas wells with ultra-long horizontal section and cost-reducing and benefit increasing development in China.

Unconventional hydrocarbons have become significant resources to ensure energy security of our country, and the large scale volume fracturing technology for horizontal wells is a critical method for developing unconventional reservoirs economically. Because of the differences of such parameters as fracturing fluid, proppant, pumping rate and others between conventional and unconventional fracturing, the characteristics of proppant transport and placement are completely different. Starting from the search for the typical characteristics of proppant transportation in unconventional reservoir fracturing, the paper summarizes the domestic and international research outcomes of proppant transportation experiments and numerical simulations in recent years, expounds the laws of proppant transportation, placement and diversion in single fracture and complex fracture network, and evaluates the applicability and limitations of the numerical methods based on computational fluid dynamics. On this basis, the unconventional reservoir fracturing proppant transport simulation is discussed, and points out that the rough fracture fracturing fluid simulation experiment device, fracturing fluid-proppant velocity characterization test method, numerical simulation method for coupling of fracture-fluid-proppant, and other key issue will be the next research direction, so as to provide theoretical guidance for efficient stimulation of unconventional oil and gas reservoirs.  

In oil and gas production wells, due to the shielding influence of metal casing, the signal of electromagnetic wave wireless transmission will be greatly weakened or discontinuous, which affects the popularization and application of electromagnetic wave wireless transmission technology in production wells. Aiming at this problem, the wireless transmission mechanism of electromagnetic wave in cased well was studied in this paper, the polarization phenomenon caused by electromagnetic wave passing through casing was avoided by using magnetic current loop excitation technology to form a current loop between oil sleeves, which effectively solved the electromagnetic wave shielding problem. The field test shows that the dynamic data representing the downhole state in oil and gas production wells can be transmitted to the surface in real time through the wireless transmission technology of electromagnetic wave. The signal is clear and stable, providing technical means for production companies to master dynamic data such as downhole temperature, pressure and water cut of oil and gas production wells in real time. It provides scientific basis for reservoir rapid evaluation and production system optimization,and has broad application prospect.

At present, the cementing of deep, ultra-deep and complex wells are often faced with such challenges as long sealing sections, narrow safety density window of drilling fluid in the same open hole section,small annular clearance, low casing centralization, malignant well loss or even total loss at wellhead with conventional cementing process, cement slurry unable to return to the predetermined position, poor cementing quality and high risk of well control. In view of the narrow density window cementing in Moxi-Gaoshidi structure, Longgang structure, Shuangyushi-Hewanchang structure and Pied-Tarim structure, the paper proposes to adopt the precise managed pressure cementing technology. With?the?precise managed pressure cementing equipment?and?related?software to control the bottom hole pressure during cementing operation, realize real-time dynamic calculation of wellbore pressure to reduce the error of manual calculation, precise control downhole pressure under the condition of narrow density window with loss and blowout co-existing, ensure the wellbore to be in stable and leak-proof state without any blowout or overflow, thus improve the cementing quality. At present, this technology has been popularized and applied in Sichuan and Tarim area, has become a key technology to solve the problem of high quality cementing in such complex formations, providing a reference for improving cementing quality of similar deep and ultra-deep wells and complex formations in other regions at home and abroad.    

In Tarim basin, it is difficult to use conventional cementing to seal 4000~5000m deep and ultra-deep wells at a time, so hierarchical cementing technology is often used for construction. Due to the well developed leakage-prone formations, there are problems with conventional stage collar for two-stage cementing such as loss of primary cementing and total loss of secondary cementing, which lead to the long length of free casing section and affect the cementing quality. In order to solve the above problems, a new type stage collar with packer was developed. This tool includes a stage cementing collar，a injection valve and a hydraulic expansion packer. On the basis of the completion of the staged cementing operation, the function of packer expansion after the first stage cementing is added, which can effectively seal the annulus between casing and open hole during secondary cementing, reduce the probability of secondary cementing loss, and increase cement top. The results of laboratory tests and field application show that the tool is safe and reliable with no loss during the secondary cementing process, improving the cementing quality and saving cost. This tool provides a powerful technical guarantee for cementing the low-pressure and easy-loss long-seal section, and has a broad application prospect.

The Chang 6 reservoir is the main production layer of Huaqing oilfield, which is a typical ultra-low permeability reservoir with poor reservoir physical properties and great difficult in development. The application of horizontal well development technology has effectively increased the average single well production of the reservoir. However, in the actual development process, many horizontal wells have experienced a significant decline in production. In order to stimulate these low-yield horizontal wells, starting from the geological characteristics of the reservoir of Chang6 reservoir in Huaqing oilfield, the feasibility of the acidification process of the oil reservoir was demonstrated, and then the acidification process of the horizontal well was designed and optimized based on factors such as the production dynamics of the well. The research work of selecting the acid system, optimizing the acid formulation, designing and optimizing the acidification process parameters, and designing the acidification tools and string have been carried out, and the experimental application has been carried out in the field. The conventional stimulation and excavation process of horizontal well in Chang6 reservoir in Huaqing oilfield has been usefully explored and practiced, and the acidification process of horizontal well with Huaqing characteristics has been preliminarily formed, which provides strong support for the development and stable production of Huaqing oilfield.

Drilling automation is an inevitable trend in the development of petroleum industry, and the automatic monitoring technology of drilling fluid performance has become the focus of the industry. Currently it is basically rely on artificial drilling fluid performance monitoring, but manual sampling is with a security risk and error of manual monitoring, manual or input software analysis and calculation may lead to drilling fluid performance data for the decision-making on serious lag problems, so it is in urgent need of drilling fluid automatic monitoring equipment developed for real-time online monitoring of drilling fluid performance so as to timely perform drilling fluid performance maintenance and avoid downhole complications. In this paper, the real-time drilling fluid monitoring equipment and technology that has been successfully developed or applied in the field at home and abroad are investigated in detail. The results show that the real-time drilling fluid monitoring system at home and abroad generally has problems such as incomplete monitoring function, large volume and high price. In view of the above problems, it is proposed that the real-time drilling fluid monitoring equipment should expand its monitoring function, improve the degree of automation, optimize the monitoring method and improve the monitoring accuracy, and develop miniaturized equipment and integrated equipment in the future.The development of online monitoring technology will lay a foundation for the realization of comprehensive digitization of drilling and the intelligent development of drilling in the future.

The failure and damage of downhole tools caused by the high temperature occur frequently in Y101 block. In order to clarify the wellbore temperature field of shale gas drilling in Y101 block, based on the thermodynamic theory, a new model of wellbore temperature field of deep shale gas horizontal well drilling that comprehensively considers the influence of mechanical and hydraulic heat source terms is established, and the influencing factors are analyzed, the main control factors affecting the bottom hole temperature are clarified, and cooling measures have been proposed. The prediction results of the new model are in good agreement with the existing model and field measurements,and the error bars within ± 5% . After considering the influence of the heat source term, the maximum annulus temperature is located near the bit, and the maximum temperature is greater than the original formation temperature,and the temperature difference increases with drilling time. The temperature difference increases linearly with the increase of drilling fluid yield, weight on bit, rotating speed and injecting temperature,and increases in a power function with the increase of drilling fluid plastic viscosity and density. When the length of the horizontal section is less than 1 000 m,the ground cooling effect is good,and when exceeds 1 000 m,the decreasing of mud density is better than that of ground cooling. After applying of mud density reduction and ground cooling, the maximum bottom hole temperature is reduced by 6 ℃ , rate of penetration is increased by 67. 51% , with remarkable effect of cooling,drilling speed and efficiency improving. The research results in this paper provide theoretical support for the bottom hole cooling in the Y101 block, and provide a strong guarantee for the efficient exploration and development for deep shale gas. 

 As the rapid decline of production in horizontal shale gas wells and high cost of drilling new ones, re-fracturing can restore and increase the production of horizontal shale gas wells with good economic benefits. When conducting stimulation for the first time, the output of horizontal wells may be with low production due to improper well placement, poor completion design and damages during production. All this can be improved through re-fracturing. The re-fracturing technology for horizontal shale gas wells has been widely used in North American and is mature. This paper summarizes the technical difficulties in re-fracturing operations in horizontal shale gas wells in North America, well placement optimization, and re-fracturing technology and tools. Finally, in view of the re-fracturing for horizontal shale gas wells in Sichuan Basin, combined with the current research results and future development trend of relevant technologies, some suggestions are put forward, including carrying out re-fracturing pilot test, avoiding general fracturing to achieve precise stimulation, using supercritical CO ₂ as fracturing fluid to instead of water-based fracturing fluid. With the development of some shale gas plays gradually entering the middle and late stage, re-fracturing will become an important method for old well production enhancing.A set of shale gas horizontal well re-fracturing technology adaptive to China will also be developed, which will play a key role in reducing cost and increasing efficiency in China’s shale gas development.

LWD logging data are commonly used in formation pore pressure monitoring and calculation. However, in the process of formation pore pressure monitoring, LWD data are often undetectable due to engineering reasons or missing due to formation reasons. Therefore, Faust formula is often used to fit the LWD. Practice shows that the relative error between the fitted acoustic time difference and the measured acoustic time difference is more than 5%. Therefore, this paper improved the traditional Faust formula by removing the vertical depth parameter in the traditional Faust formula and changing the formula. The offshore drilling practice concluded that the relative error between the acoustic time data calculated by the improved formula fitting calculation and the measured acoustic data can be controlled within 5%, and the formation pressure coincidence rate calculated with the fitting of acoustic time is of over 97%, which can meet the demand of field operation, and this formula can be also used for LWD tool logs and wireline tool logs.