In recent years, as newly-discovered oil and gas resources become more and more inferior while old oil fields in China enter into the middle-later development stages, great challenges emerge to the exploration and development. Under this circumstance, new ideas, techniques and practices are urgently needed to solve these problems. Successful large-scale development of unconventional resources in the United States has significantly promoted the integrated innovation and development which combines multiple disciplines and multiple technologies. Accordingly, the geology-engineering integration is proposed in response to the challenges induced by current low oil price and the basic requirement of “profitable exploration and development”. This model represents a new way to realize profitable exploration and development of oil and gas fields (especially the unconventional and complex oil and gas fields) in China. The organization idea and operation pattern of such geology-engineering integration have been successfully applied in developing low-porosity and low-permeability reservoirs in the Kuqa area of the Tarim Basin in western China, the Sichuan Basin (marine shale gas), and central-eastern China. This paper described the concept, connotation, and applicable scope of the geology-engineering integration, and presented three prerequisites for implementing this model. Moreover, some suggestions were put forward for promoting the development of the geology-engineering integration. To be specific, in addition to expanding the application scope and scale, learning curve should be established to develop more pertinent technologies. Innovation should be made in management model of geology-engineering integration, and market-based and cross-enterprise coordination should be applied beyond traditional systems to integrate the advantages of technologies. In this way, it is expected to substantially enhance production and profitability of complex reservoirs in China.

At present, low-quality resource is a common issue for oil and gas exploration and development in all oilfields, especially the Jilin oilfield. Due to the dual effect of low oil price and down-grading resource quality, beneficial productivity construction is more and more difficult while available productivity scale shrinks significantly. In order to deal with these challenges, the strategy of “enhancing the production and recovery percent, and reducing the investment and cost” was proposed in reference to the successful tight oil and gas development in North America. The concept of geology and engineering integration was applied to the conventional low-permeability oil reservoirs in the Xinli III block, the Jilin oilfield, and the intensive productivity construction mode was established. Moreover, a series of unconventional technologies and practices were actually used in the Jilin oilfield, including reservoir volume fracturing, unconventional energy supplement and factory-like operation. Compared with the conventional productivity construction mode, the intensive productivity construction mode is more remarkable in enhanding the production and fracture-controlled reserves and reducing the investment and operation costs. This mode is further promoted in the development of tight oil. It is indicated that, after the potential and technical systems of low-permeability oil reservoirs are re-understood sufficiently, the rational application of geology and engineering integration and unconventional technologies is inevitable for the beneficial productivity construction currently and even in the future.

In recent years, some exploration and early development tests of tight oil were satisfactorily carried out in Xinjiang Oilfield, and more practices and attempts were also implemented in technology and management. According to years of the tight oil development practices, it is found that tight oil reservoirs cannot be developed by conventional techniques due to the special reservoir properties. Instead, unconventional ideas, methods and technologies are required for unconventional reservoirs, and the model of geology-engineering integration is needed for ensuring the efficient exploration and development of such reservoirs. Accordingly, a set of practical methods was proposed after numerous practices in project management. Specifically, the core of geology-engineering integration is used in production projects. Based on various demands during different stages of project operation, the integration is followed in all steps. In this way, the composite benefits of tight oil development can be increased gradually. In 2015, based on the idea and procedure of geology-engineering integration, this set of methods was successfully applied in developing tight oil in the Mahu sag, Xinjiang Oilfield. For purpose of improving tight oil production, by means of seamless multi-discipline collaboration, combination of geological conditions and engineering technologies, and integration of management and technologies, dynamic decision-making was practically implemented in sweet-spot selection, assurance of reservoir drilling rate and optimal fracturing design, so as to promote the efficient and large-scale development of tight oil. Guided by the concept of “integration”, fine technical management was push forward, and the optimal control was realized in key links of production deployment and construction, uiming at better economic benefits. Moreover, a new model of engineering-technology service was worked out. By virtue of the engineeringtechnology service integration model, a more solid foundation was established for practical tight oil development in China onshore areas.

In the Sichuan Basin, the geologic survey on shale gas began in 2006, the construction of the state shale gas demonstration area commenced in 2012, and large-scale productivity construction was carried out in 2014. Through a series of systematic researches, the concept of integration has been actively practiced, and the innovative applicability model has been built for shale gas exploration and development. Accordingly, a geology-engineering integration platform has been created, and an intelligent shale gas field has been set up. Guided by the concept of geology-engineering integration, the Changning State Shale Gas Demonstration Area with a comprehensive productivity of 15×10⁸ m³/a has been built, which provides a new direction, new idea, new path and new prospect for CNPC's green and efficient development of shale gas. With referent to the successful unconventional oil and gas development in North America, and depending on the complex surface and subsurface conditions in the Sichuan Basin, the large-scale development of shale gas was realized, and the comprehensive benefit was continuously enhanced, through a series of deployment and exploratory attempts. In this process, a set of relatively complete and specific development model for marine shale gas in China was formed. This model has facilitated the Changning State Shale Gas Demonstration Area to become an outstanding area in China for unconventional oil and gas exploitation, with its single-well productivity gradually improved and comprehensive benefit steadily optimized.

The Zhaotong National Shale Gas Demonstration Zone is different from the explored and developed blocks in North America and the Sichuan Basin where shale gas has been successfully developed. It is characterized by complex geology, mountainous landform, high difficulty in drilling and production engineering, high operation cost, high-level safety and environment management and high risks in shale gas development. According to the development concept of “ensuring the quality and promoting the production by integrated technologies, and increasing the efficiency and improving the benefit by innovative management model”, the IPDP (Integrated Project Development by Production) high-efficiency development model which is suitable for the marine mountain shale gas in South China was proposed. The IPDP model is based on exploration-development integration, geology-engineering integration and research-production integration, and it is technically supported by the IPMP (Integrated Project Management by Production) model and geology & engineering teams. In this model, the life-cycle integrated project organization and implementation is carried out according to the working mechanism of “beneficial production as the target, engineering technology as the support, reverse thinking based design, positive organization and implementation, and factory-like operation”. In this way, the drilling and production engineering quality and well production rate of mountain shale gas are controlled all through the process from the aspects of research and evaluation, field production and implementation, and project organization and management. By virtue of the turnkey model of integrated risk project which is linked to individual-well gas production rate, the sense of responsibility for beneficial production is enhanced and the production organization is optimized, so that each link is seamlessly connected, operation efficiency is effectively increased, the cost is decreased, the benefit is increased, the development risk is avoided, and implementation result is remarkable. It provides valuable references for the high-efficient development of marine mountain shale gas in South China.

In recent years, significant breakthroughs have been realized in the exploration of the Middle-Lower Jurassic Xigou Group lowsaturation oil reservoirs in the Taibei depression, the Tuha Basin. In the Hongtai area, large-scale monoblock reserves were discovered and the productivity construction has been started promptly. Breakthrough and progress of regional expansion has been made in multiple fields. Thus, a new field of reserve enhancement and productivity construction with promising exploration prospect is uncovered and the beneficial exploration of complex oil and gas reservoirs is realized. This important discovery benefits from the transformation of exploration ideas and the technical advancement. With reference to the principle of tight oil exploration, the Tuha Oilfield is developed by certain procedures. First, promote the stimulation technology of horizontal well+volume fracturing, apply the geology-engineering integration technology to explore the low-permeability and low-saturation oil reservoirs in the Taibei depression, and deepen the investigation of hydrocarbon accumulation mechanisms to predict sweet spots of reservoirs accurately. Second, design the trajectory and orientation of horizontal wells scientifically and strengthen the engineering geosteering and trajectory tracking of horizontal sections to increase the reservoir drilling ratio of horizontal wells effectively. Third, optimize the scale and technological parameters of volume fracturing according to the characteristics of oil reservoirs, and apply the geology-engineering integration during the whole process to promote the significant exploration breakthrough. Fourth, strengthen the integration of mature technologies and enhance ROP continuously. Fifth, optimize casing program, well completion mode, pipe string design and cementing technologies. And sixth, take various measures to strengthen the cost reduction and efficiency improvement. For the low-grade reserves, it is necessary to uphold firmly the concept of beneficial exploration. For purpose of geology-engineering integration, it is essential to identify the main control factors accurately. In general, the geology-engineering integration is critical for guaranteeing the beneficial production of low-grade reserves.

Dolomite reservoirs in the YM32 Oilfield are structurally complex and the remaining oil is distributed in various patterns and can hardly be treated at the stage of high water content, resulting in challenges to the efficient and steady development of the oilfield. In this paper, the concept of geology-engineering integration was introduced to explore a practical way of “embedded” geology-engineering integration for remaining oil potential tapping by “embedding” the engineering staff into the geology research team. A series of potential tapping measures, such as the cyclic nitrogen huff and puff and the horizontal-well geosteering sidetracking, were proposed jointly for different geological patterns of remaining oil distribution. In the process of engineering implementation, the engineering technology program was adjusted and improved continuously by comprehensively analyzing multi-disciplinary knowledge and engineering data, including oil reservoir geology, geophysics, drilling and testing. Eventually, the remaining oil in fractured-porous dolomite reservoirs was developed with economic benefits. The productivity of 40×104 t/a was constructed as the schedule and the production of crude oil has been stable for 4 years. The application of geology-engineering integration in the remaining oil potential tapping of YM32 Oilfield provides the valuable reference for the efficient development of other similar oil reservoirs.

In the Sulige Gasfield with the basic geological features of low porosity, low permeability and low abundance, the development is challenging for low single-well production rate, fast pressure decline and other problems. In order to increase the single-well production rate and the recovery factor of gas reservoirs and realize efficient and high-quality development of blocks, a series of technical researches and field tests were carried out in Blocks Su 10, Su 11 and Su 53 based on the theory of geology-engineering integration. The geology-engineering integration is manifested in several aspects. Firstly, development mode of the block was determined. In Block Su 10, vertical wells and cluster wells were adopted for development, with inter-well productivity replacement. In Blocks Su 11 and Su 53, cluster wells and horizontal wells were adopted for development, with regional productivity replacement. Secondly, program design was optimized. Reservoirs were produced to the uttermost by combining geology with engineering and optimizing the orientation of horizontal wells, location of horizontal sections and reservoir stimulation modes. Thirdly, factory-like operation of horizontal wells was realized. In Block Su 53, 13 wells (including 10 horizontal wells) were selected for factory-like operation of horizontal wells. A factory-like operation mode based on “optimized program design, template engineering technology, streamline of operation, standardized operation procedure, comprehensive resource utilization and integrated team management” was proposed. Fourthly, significant breakthrough was made in horizontal well sidetracking technology. In 2015, two horizontal wells were sidetracked with average sandstone drilling ratio over 90%, average well-controlled reserves of 0.96×10⁸m³ and initial daily production rate of approximately 6.0×10⁴m³, recording the overall breakthrough from geology and engineering. And fifthly, ground process was rationalized. According to the idea of “underground prior to ground”, the overall development of horizontal wells has been realized with simplified gathering and transportation process, convenient management and decreased investment.

The stress sensitivity observed during the production of tight reservoir in western Kuqa was studied and simulated by geomechanicsrelated theories and advanced techniques. It is indicated that the stress sensitivity is essentially the complex variation and interaction among seepage field, ground stress field and fracture status during the production. In order to determine this interaction and its effects on productivity, a numerical simulation of the gas reservoir system, mechanic system and fracture system was carried out by coupling of multi-discipline data, based on the concept of geology-engineering integration. Taking a gas well in No.1 block as an example, the available data of multiple disciplines were analyzed, and reliable 3D gas reservoir model, 3D geomechanics model, and 3D discrete fracture model were built up respectively. Then, the coupling parameters between the models (i.e., stress-permeability relationship and stress-fracture aperture relationship) were determined by physical and numerical experiments. Finally, by a coupling numerical simulation, the changes of ground stress, formation pressure and seepage field with space and time in the designed development plan were identified, and the results obtained were compared with the results of simulation without considering stress sensitivity. The study results show that the permeability and fracture conductivity of stresssensitive reservoirs dropped evidently during production. Generally, the influence of the stress sensitivity on the productivity was large during early stage, decreasing during middle stage, and finally increasing again during later stage. When there was stress sensitivity, both the stable production period and the total production ratio declined greatly. The study results also show that excessive production pressure difference may lead to too fast permeability decline, and cause permanent damage which may hinder the production. Therefore, it is very important to select reasonable production pressure difference.

A shale gas field at southern margin of the Sichuan Basin commenced production in 2014. For assuring its engineering efficiency and development benefit, it is critical to accurately understand the geomechanics law and its application in various scales. Accordingly, 3D geomechanics models were built in the scales of the whole study area and the platform. These models are high-resolution models based on structure, geology, attributes and multi-scale natural fractures. Core, well logging and seismic data were used to finely describe the mechanical parameters, and a set of methods for establishing 3D pore pressure model for shale gas field was established. Advanced finite element simulator and large-scale parallel computing technology were applied to establish 3D stress field models with different planar resolutions for the whole study area and the platform. In order to accurately characterize the vertical heterogeneity of shale, the models are designed with a resolution of 0.5 m thick in target layers. Various data were utilized for quality control and calibration of these models, and new data were timely used to continually update these models. The accuracies of these models can reflect the direction, size, heterogeneity and anisotropy of the stress. The results show that in-situ stresses vary greatly at platforms, between wells and along the horizontal well sections. Such complex variations are the consequent reflections of rock textures (e.g. structural form, and multi-scale fracture system) and rock composition in various scales. These geomechanics models can meet various requirements for scale and accuracy in different applications in either the whole study area or any single well. The whole-area model can be used to optimize platform location and well location deployment, to evaluate the geologic storage capacity and resources, and to assess the mechanical stability of faults and fractured belts. The high-resolution platform model can be applied in analyzing borehole stability, managing drilling in real-time manner, optimizing fracturing design, and making post-frac comprehensive evaluation. These geomechanics models have been successfully integrated in the practices of geology-engineering integration. By iterative updating and in-time application, they facilitate the engineering efficiency and development benefits. The establishment of large-scale geomechanics models for development of shale gas fields, recording the first time in China, provides references for future operations.

Geology-engineering integration has become an indispensable exploration and development strategy along with the growing demand for production enhancement, cost control and efficiency improvement of horizontal wells in unconventional oil and gas reservoirs. Unconventional horizontal wells are faced with multiple challenges in terms of geology and engineering. In order to realize productivity breakthrough and beneficial development and meet the requirements of cost reduction and productivity construction ahead of schedule, it is common to apply horizontal wells in unconventional oil and gas reservoirs for early development. The geological uncertainties impact the drilling ratio of horizontal wells, increase operation risks and decrease drilling time efficiency. According to the concept of factorylike intensive well pattern, 3D extended reach well with complex trajectory is inevitable, making drilling operation more and more difficult. Practical operation indicates that trajectory optimization of horizontal well based on efficient geosteering method is the only way to diminish drilling risks, guarantee effective completion stimulation and realize beneficial development while ensuring the drilling ratio of sweet spots. 3D geosteering is a new-generation steering method which is developed according to the concept of geology-engineering integration, and its core lies in geosteering model reconstruction and high-precision 3D geological modeling. It maximizes the advantages of “well factory” and improves the model precision by means of horizontal wells, so as to provide the support for pre-drilling trajectory optimization, drilled formation anticipation and trajectory pre-adjustment. 3D geosteering is a process independent of LWD (logging while drilling) tool. With this technique, the drilling cost is reduced significantly, and satisfactory results are achieved in the practical drilling of shale gas or tight oil horizontal wells in China.

In the Tarim Basin, the Ordovician carbonate reservoirs are deeply buried with strong heterogeneity, and fracture-cavity reservoirs are widely distributed. Moreover, igneous rocks are extensively developed with thickness and shape greatly varying and velocity quickly changing. Subsequently, the images of underlying formations are often distorted, and the thickness characterization of low-relief structures is negatively influenced. Reverse-time migration (RTM) technology is characterized by adaptability to lateral velocity change and high imaging precision. With some key high-precision velocity modeling techniques (e.g. low-frequency velocity field establishment and update, highresolution velocity modeling, and hierarchical grid tomographic iteration), the background velocity field of the Tarim Basin is accurately inverted for reverse-time migration imaging. In this way, the pseudo-structures resulted from igneous rocks can be effectively eliminated and fracture-cavity imaging precision can be improved. As a result, the “paternoster” boundary and its spatial location can be accurately distinguished.

As the Liaohe Oilfield gradually enters the high-maturity exploration and development stage, it is difficult to research the microstructures, minor faults, small fault blocks and thin reservoirs by means of conventional seismic exploration technologies, and the seismic data is highly required in terms of signal-to-noise ratio, resolution and fidelity. In 2014, high-density digital 3D seismic acquisition technology was tested in the Qinglongtai area, the Liaohe Oilfield, for the purpose of evaluation and development of old oilfields. Based on experimental research, the high-density digital 3D seismic acquisition technology was developed with the features of small surface element, high folds, wide azimuth, dynamite source-vibrator combined excitation, single-point digital geophone receiving and low-noise receiving. Compared with the old data, the newly acquired seismic data is much better in quality. The frequency band ratio of single shot data is 21-25 Hz wider and the frequency band ratio of stack section is 12-14 Hz wider. The proposed technology provides better seismic data of the Qinglongtai area, so good evaluation and development results are obtained. It can be used as the reference for the digital 3D seismic acquisition in the future.