1. Design methods of oilfield development

Oilfield development and design is an important part of field management, and an important basis for rational exploitation and adjustment.

1.1 Stage division of oilfield development and design

A complete oilfield development and design can be divided into three stages: early conceptual design stage of oilfield development, overall design stage of oilfield full exploitation and adjustment design stage of oilfield development in the late stage . Development and design of these three stages are critical strategic issues, and implementation plan of each stage is a tactical problem, such as drilling, completion, perforation program, injection program, allocation program etc.

1.1.1 Oilfield development and conceptual design

An oilfield which has found an well during surveying, excepting to have the condition of having oil and gas development prospect, can carry out the conceptual design. The basic task is to make full use of geophysical data as well as discovery well geological and test data, make an early reservoir evaluation, elaborate detailed prospecting evaluation steps and the implementation of the development condition, propose data which need supplementary admission and need to carry out pilot test and make the research of mechanism to reservoir development and sensitivity analysis.

In the conceptual design stage of oilfield development, there was only a few data about discovery well and evaluation well. There are still many uncertainties about understanding of reservoir. Different reservoir types and variety of information gained have great influence on design’s reliability. In times of simple structure, large oil-bearing area, multiple layers, abundant reserves and get more information about reservoir, conceptual design has relatively high reliability; For the complex fault block, lithologic structure and lithologic reservoir, must use the rolling exploration and development program.

1.1.2 The overall design plan of oilfield development

At the basic end of detailed exploration, under condition of mature development(including earthquake, drilling, coring, logging, testing, oil testing, calculation of reserves), we should start to design the overall plan of oilfield development preparation.

For full development into oilfield, the main task is to determine: strata division; well pattern and well spacing; development mode; mining speed and scale of production; reservoir dynamic monitoring system; drilling, oil production, surface construction engineering requirements and implementation program.

1.1.3 Synthesize adjustment design plan for oilfield

After oilfield is fully development, with the passage of time, through dynamic analysis of oilfield, after deepening understanding of geological reservoir, original design plan and various contradiction during oil exploration process should be adjusted.

The overall design plan of oilfield development based on rethinking of geological features and evaluation of development’s effect. The key of geological feature description is deposited on the reservoir microfacies recognition, analysis of different sedimentary microfacies in the development process of oil-water distribution characteristics, the influence of various micro interface on the development effect. The important of development’s effect evaluation is analysis of various types of reservoir condition and the impact of reserves reasons, thus giving synthesize advice to original well network, strata,  relationship between injection and production, pressure system, infill drilling and corresponding mining technology measures, It’s goal is to improve oilfield development’s effects and enhance the management level, increase the recoverable reserves and ultimate recovery.

1.2 Process and methods of design evaluation

No matter what stage of oilfield development design, must be fully digest and absorb all the information and as much as possible to achieve the information, seriously study step by step : acquisition and analysis of reservoir data, description of geological and reservoir, hydrodynamic method or study on numerical simulation, exploitation environment and policy research, mining and reservoir prediction, drilling technology and surface process design, technology, economic optimization and synthetical evaluation.

1.2.1 Acquisition and analysis of reservoir data

In oilfield development concept and overall program design stage, the more data obtained emphasis on static data, generally have geological, earthquake, logging, core, RFT, DST and fluid data(including regular, special), dynamic data generally has well test data and oil test mining. Adjustment program design stage can get more information data, especially logging, inspect coring, dynamic data obtained with different amounts of time, such as layer pressure of oil-water wells, flow, water, oil and gas and other special data. Starting from conceptual design phase of oilfield, and should emphasize perfect the database.

1.2.2 Geological and reservoir characterization

Geological and reservoir characterization roughly divided into three scales.

Large scale characterization need to make full use of seismic data processing, well logging information, hierarchical data in stratigraphic correlation, the purpose is to figure out the structure of form, fracture system, distribution of sand and effective thickness, oil-water system etc.

Mesoscale objests characterization are often rock type, sedimentary facies and its size, thin layer contrast and flow unit division, crack distribution system and development degree. Lake channel sand body with same type has its own unique heterogeneity, meandering channel sand body with different curvature, braided river sand body, straight distributary channel sand body, braided river and delta sand body are due to different deposition modes, lead to continuity, width and sand body connectivity have great difference. This is very important to establish a geological model for reservoir, play a decisive role in relevant strata and well network deployment of oilfield water injection system.

Fine scale characterization focus on internal reservoir heterogeneity changes, such as permeability distribution in different directions, variation of pore structure, clay content and distribution of microscopic interface, direction of crack distribution and density.

1.2.3 The hydrodynamic method or numerical simulation study

Conventional hydrodynamic estimates, one dimensional flow tube and two dimensional numerical simulation usually for conceptual design. Estimated recoverable reserves are frequently use in hydrodynamic method, oilfield overall development program and adjustment program design using more of  three-dimensional numerical model. Different reservoir types use different numerical models, conventional black oil deposit selection black oil model, volatile oil reservoir or gas condensate reservoir should choose component model. Numerical simulation study on key parameters required to obtain in addition to geology, lithology and fluid distribution, and usually have special analysis of reservoir properties and fluid(such as relative permeability, capillary force, PVT) and historical data of separate well layered over the past production, also need to provide such as drilling, completion, perforated interval and injection speed or information needed for these deployments study by production pressure.

1.2.4 Development environment and policy research

Oilfield development must understand the development environment(spontaneous combustion environment and political environment etc), leader’s opinion and demand of the whole national economy. At one stage, not just from technical point to measure the reasonableness of development and design, as much as possible to make development and design to reconcile technical, economic, political.

1.2.5 Mining and reservoir prediction

A development design usually make multi plans, the purpose is to predict the effects of development may be obtained by different plans, make the evaluation and comparison from the technical and economic. Mining method involves which is good the use of natural energy exploitation or maintain energy mining? How to keep the reservoir energy, is water or gas injection or miscible flooding needed? Different well pattern, well spacing, layers and mining speed can also compose many different plans. These plans design should all have degree of reserves control and prediction of recovery ratio.

1.2.6 Drilling technology and surface process

The role of drilling technology and surface process is applying modern advanced technology, realization of reservoir engineering design. Choose vertical wells, horizontal wells or cluster wells development, not only consider the applicability of reservoir, but alos consider the environmental impact. Casing series of drilling shout matched with oil production technology, oil production technology should adapt to the geological conditions of reservoir, and achieve the production target confirmed by plan design.

1.2.7 Comprehensive evaluation of technical, economic optimization

Comprehensive evaluation of technical is basis for all kinds of plan decision in oil field development design. The economic evaluation of the development plan is designed to make the selected plan not only maintain advanced and feasible in technology, but also ensure the rationality of economy.

Reservoir development plan is relatively “easy” work, but every time the well position’s encryption and adjustment should according to the current distribution of remaining oil, the difficulty and workload to figure out the remaining oil is very large.

2. Old oilfield development status

2.1 Overall situation

Oil and gas exploration have obvious periodicity, each field will experience the beginning, growth, strong to decline process. According to recovery percent of current china oil field, comprehensive water content and recent changes of oil production characteristics, consider : the old oil field in the eastern region is overall in middle-advanced development, into the reduction of production stage; The central oil field in early development, production increased steadily, but the growth rate is limited; the western region oil filed in early and middle development, the increasing potential is quite large in the future; sea oil field general in the increasing stage, output growth in large range. Future oil reserve growth is expected to remain at a high level, but the new reserves quality decline, increasing the difficulty of exploration and development.

2.2 The study of remaining oil

Our oil mostly is a continental sedimentary with multi layer reservoir, interlayer, inner and surface change greatly in permeability, and our country nearly 90% field using water injection method, due to the serious heterogeneity, each absorbent layer is different, the injected water often propulsion along a high permeability zone, make nonuniform propulsion between vertical and horizontal, cause small volume in water flooding, the water injected rush to oil wells prematurely, indented distribution of oil and water, the distribution of residual oil overall scattered, local relative concentration. Therefore, our main task in later period is to residual oil saturation as the main content, offer fine, quantitative, dynamic and prediction description of reservoir. At the same time, we should gradually focus on inter-well and study in distribution of remaining oil reservoir. In fact, study in distribution of remaining oil reservoir is also main task, need to divide a lot of manpower and material resources.

2.2.1 Remaining oil technology research status

The study of remaining oil is a worldwide problem, also a frontier research topic of geology, geophysics and reservoir engineering in different fields. Foreign research remaining oil using various method like core analysis, tracer test, numerical simulation, well logging, well testing and resistivity, recently proposed according to ”timing, location, quantitative calculation of remaining oil saturation”, design new concept of infill well position. China’s petroleum science and technology workers aster 20 years of exploration study, forming a research method for remaining oil of continental strata. The study of remaining oil geological has been analyzed by a single discipline to a multidisciplinary research, except geological study of sedimentary facies conventional subdivision, also strengthen application of petrophysics, oil and gas seepage mechanics, reservoir engineering and other relevant principles. Now mainly using small sedimentary facies analysis method, dynamic analysis, numerical reservoir simulation, reservoir engineering, C/O logging method and drilling well and coring method to study the distribution of remaining oil. Currently, our country possess the technique and tools to study the distribution of remaining oil, close to the level of technology with domestic and foreign major oil producing countries, but there are some differences in reservoir management and importance of dynamic monitoring system.

2.2.2 Main research technology of remaining oil

Main research technology of remaining oil including fine reservoir description technology, reservoir numerical simulation technology, combined technology of dynamic and static, multidisciplinary combined technology, systems analysis techniques. Reservoir description is to make a comprehensive study and evaluation on reservoir, specific task is to clarify the structural features of reservoir, type and distribution of sedimentary facies and microfacies, geometry and size of reservoir, reservoir parameter distribution and heterogeneity and microscopic features, reservoir fluid properties and distribution, and even build reservoir geological mode, calculation of oil reserves and comprehensive evaluation of reservoir. Reservoir simulation technology in the preparation of oil field development plan and determination, adjustment of production measures in oil drilling and optimization and enhance the oil recovery aspect, has gradually become an indispensable means. Combination of static and dynamic reservoir description requires integration studies on geological model and numerical simulation, based on reservoir description and oil well dynamic analysis, study macrography remaining oil distribution. Multidisciplinary research requirements to maximize using integrated information, geological, geophysics, reservoir engineering and different majors specialist share a database, taking uniform geological model as media, predict remaining oil distribution as purpose, close coordination, collaborative research. In the oil field production and development systems, oil well production dynamic is the macroscopic effects of various factors; Structural factors, sedimentary facies, reservoir property, fluid properties and water flooding situation in drilling are micro factors variation of the control system.

2.2.3 Mainly distribution pattern of remaining oil in china

China’s various types of clastic reservoir in the movable remaining oil distribution has big difference, in consideration of total reserves of original reservoir, the distribution probability of remaining oil is arranged from big to small : fluvial facies（48.6%）delta facies（27.6%）, sublacustrine fan（turbidite）facies（9.6%）, alluvial fan（alluviation-river）facies（6.9%）, fan delta facies（5.5%）and beach bar facies（1.8%).The division model of Daqing Oilfield: well uncontrollable type, poor distribution into pieces and oil layer type, injection and mining imperfect type, secondary effect type, unidirectional effect type, stagnant area type, inter-well interference type, interlayer interference type, inner non flooded type, interlayer loss type and remaining oil of fault shelter, total 10 types. Division mode of Shengli oilfield: washing area remaining oil, weak washing area remaining oil, unused thin layer, remaining oil layer caused by development engineering reasons, remaining oil in the miniature trap, remaining oil has been developed peripheral edge type, total 6 types.

2.2.4 Study on the remaining oil after tertiary oil recovery

Polymer flooding is the highest promotion tertiary oil recovery in the domestic, with the popularization of a large number of oil field polymer development technology, underground oil has higher level recovery, the remaining oil distribution is more uneven, at the same time, a large number of polymer remaining underground. Solve the mining method of polymer zone oilfield, that is how to do after polymer flooding? Whether there may take further to enhance oil recovery? What are the prospects? It is currently the focus topic of many oil’s common concern, is also category of four times oil recovery.

3. Conclusion

From the perspective of oil field development, with the oilfield development in high maturity stage, the water and oil distribution in the underground has tremendous change, the main object of exploitation turns to high dispersed and local relative enrichment, no large contiguous remaining oil, even turn to improve the microscopic oil displacement efficiency. We need a more open environment, more scientific method, more advanced technology continues to research.

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