Waterflooding Sandstone Reservoirs: Methods, Design and Analysis

By Jiahong Wang

This book focuses on oilfield performance analysis and development adjustment by integrating geology, applied mathematics, and other relevant theories. Based on the abundant and detailed field test and production data from Daqing and Tarim, two major oilfields in China, the regularities, characteristics, design, and adjustment of waterflooding development of sandstone reservoirs throughout the life cycle are described. Field development theories and practices are organically combined in this book, which, embracing comprehensive, systematic, and pragmatic contents, is conducive to development technicians to quickly grasp the characteristics of waterflooding and prepare adjustment plans. It is also useful as a textbook in petroleum colleges and short training courses.

• Language: English
• Publisher: Springer
• Release date: Mar 27, 2021
• ISBN: 9789811603488

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© Petroleum Industry Press 2021

J. WangWaterflooding Sandstone Reservoirs: Methods, Design and Analysishttps://doi.org/10.1007/978-981-16-0348-8_1

1. Oilfield Development Procedure

Jiahong Wang¹  

(1)

Research Institute of Petroleum Exploration and Development, Beijing, China

Jiahong Wang

Email: [e-mail]

Obvious staged features can be found in oilfield life cycle from the first planned exploratory well to the full-scale development, and even to the middle and later development adjustment. The procedure as a whole is a systematic work with different stages as its subsystems. In the subsystem, not only vertical and horizontal links among different majors exist, but also the procedures of continuous practice and understanding occur where the results of previous step determine the direction of next step. Therefore, it is necessary to follow certain work procedure in time order during oilfield development, namely oilfield development procedures. The purpose is by using advanced technology, strengthening connection between majors, and reducing repetitions, with less exploratory and test wells, to acquire basic understanding of the reservoir in a short time, to make oilfield development arrangement with strong adaptability to geological conditions, as well as to maximize oil and gas production, and further achieve efficient development. Oilfield development procedure is divided into five stages based on the oilfield development theory and practice: conceptual design stage, development plan design stage, reservoir recognition and perforation plan design stage, separate zone water injection design and adjustment stage and infilling adjustment stage.

1.1 Conceptual Design Stage

The conceptual design is based on early evaluation, including stages from the obtainment of industrial oil and gas from a wildcat at the trap, early development intervention to the accomplishment of reservoir development and conceptual design.

1.1.1 Purpose of Conceptual Design

1.1.1.1 Accelerating Development Intervention

Songliao, Junggar and Tarim basins are important petroliferous basins in China. The western region, especially, which boasts abundant oil and gas resources, is however located in desert and gobi region with tough natural and geographical environment. Most oil reservoirs there are deep-buried or super-deep-buried with various types, complexity and uncertainty. Those factors increase the difficulty for exploration and development. For example, the long drilling cycle increases the cost of drilling and oilfield construction and then investment, where the required minimum economic production of single well will become very high if it is put into extraction. Therefore, speeding up understanding of oil and gas reservoirs and improving overall economic benefits of exploration and development have become an urgent requirement for making reasonable development procedures. Specifically speaking, exploration will be combined with oil and gas development, whose work will be involved earlier than before. In this way, technicians of development engineering will catch up with the whole procedure of understanding the oil reservoir, speeding up development process and seamlessly linking up exploration with development. By making best use of the documents of discovery wells and based on the feature of oil reservoir, exploration department and development department jointly do early research and give evaluation on oil reservoir. Reservoir engineering, drilling engineering, oil production engineering, ground engineering and economic evaluation, all those work together to make a comprehensive work plan and accelerate the process of making development scheme to the greatest extent.

1.1.1.2 Investigating the Particularity of Oil Reservoir Geology and Development Difficulties and Making Technology and Economy Preparations

There are similar reservoirs in petroliferous basins, but it is difficult to find exactly the same reservoirs. Therefore, reservoir has its own specificity (see discussions in Chap. 4 about the specificity of reservoirs), which is actually one of the technical and economic difficulties of reservoir development. Only by fully understanding and intensively researching can we make a practical development strategy.

1.1.1.3 Formulating Data Acquisition Plan and Reducing Repetitive Work to Provide Scientific Evidence for Development Scheme Design

According to the data acquisition requirement in the development scheme and the state of acquired data, all additional data needed to increase understanding of oil reservoirs should be included in the data acquisition plan, especially after reservoir conceptual design.

1.1.2 Contents of Conceptual Design

1.1.2.1 Preliminarily Studying the Reservoir Geological Property and Pointing Out Problems Requiring Further Research

The investigation of reservoir geological specificity primarily includes the following four factors: general structural pattern and fault system, reservoir property and distribution, fluid property and reservoir type, and productivity. After a planed exploratory well has acquired high-yield oil and gas flow, the reservoir has three datasets generally: 2D or 3D seismic data by arranging survey grid; core and conventional physical property data and a full set of log curves and interpretation results; DST test data as well as analysis data of oil, gas, water, etc. Reservoir can be described and evaluated in the early stage using above data. Research emphases vary according to development stages. Apart from reservoir type evaluation in conceptual design stage, research should focus on what has not yet been fully understood and what data should be taken into account to increase understanding.

1.1.2.2 Establishing Conceptual Geological Model and Performing Development Mechanism Research

Geological models of a single well or well correlation sections should be established to investigate development mechanism of reservoir specificity and development difficulties.

The Donghetang Oilfield in Tarim Basin is taken as an example for the mechanism research in the development concept design.¹ Donghe 1CIII Reservoir is a thick sandstone chunk bottom-water reservoir with relatively developed inter-layers and with a linear density of 0.195 layers/m. The high production in the single well can be attributed to large thickness of oil layer (119.6 m of oil layer drilled in DH1 well), good crude oil properties (underground crude oil viscosity of 2.46 mpa·s), and large difference between reservoir pressure and saturation pressure (58 Mpa). The development mechanism focuses on whether the heavy oil sections in water cone, interlayer and bottom of oil layer restrict the high production of single well or not. To do this, numerical simulation of the above three problems is carried out using single well geological model to study: influence of rock physical property variation of the first three vertical sections on the performance, including perforation section, middle section (from bottom of the perforation to oil–water contact) and water section, influence of physical properties, position and extension of interlayer on the performance, influence of heavy oil at the bottom on the forming of water cone, etc.

The tests and experiments are carried out to investigate the specificity of reservoir geology during conceptual design stage. For example, the TZ402CIII Reservoir in Tarim Basin is a bottom-water reservoir with a condensate gas cap. The interlayer between the reservoir and the gas cap is developed while the interlayer between the reservoir and the oil-bearing layer is not. Avoiding water and avoiding gas are important principles in development technology field. The pressure interference test is carried out for the three sections in Table 1.1 to find out what should be mainly avoided, water or gas, or both. The test results are shown in the extended diagram of bottom hole pressure (Fig. 1.1).

Table 1.1

Well test results of Well TZ421 of CIII reservoir

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Fig. 1.1

The extended diagram of bottom hole pressure of inter-layers and interference among three layers in Well TZ421 CIII

The interference of the second layer to the first layer is tested. The pressure variations in the first and second layers are monitored simultaneously when the second layer is open or shut down. When the second layer is opened, the bottom hole pressure is 40.27 Mpa. After the second layer is shut down, the pressure returns to 42.34 Mpa, with a pressure difference of 2.07 Mpa, and at same time the bottom hole pressure of the first layer decreases from 44.70 to 43.76 Mpa, with a pressure drop of 0.94 Mpa.

The interference of the third layer to the second layer is tested. The pressure variations in the second and third layers are monitored simultaneously when the third layer is open or shut down. When the third layer is opened, its bottom hole pressure is 40.14 Mpa. After the third layer is shut down, the pressure returns to 42.20 Mpa, with a pressure difference of 2.06 Mpa. Meanwhile, the second layer pressure drops from 46.06 to 45.73 Mpa, with a decline of 0.33 Mpa.

The analysis of interference test results among oil layer, gas layer and water layer shows that under the same test time (80 h) and pressure difference (2.06–2.07 Mpa), the pressure of water layer drops by 0.94 Mpa, while that of gas layer only drops by 0.33 Mpa. It is preliminarily considered that the reaction of bottom water is about 2.9 times faster than that of gas top.

1.1.2.3 Compiling Development Plan

(a)

Research on the design and strategy of development

The research needs richer and more comprehensive geological data. The problems that can be solved through efforts during development scheme design can only be given some advice in the conceptual design stage, which indicates that oilfield development is a staged process. The development method, strata partition, injection-production well pattern and production capacity can be preliminarily analyzed and evaluated on the basis of preliminary understanding of controlling reserves and reservoir geology and practical experience from adjacent areas.

(b)

Determining the basis and scheme of appraisal well deployment

The appraisal well deployment is one of the important tasks and achievements of early reservoir evaluation. The purpose of deploying appraisal well is to obtain data for submitting proved reserves. The specific contents are to figure out structure, reservoir type and oil–water contact, and to quickly control oil-bearing areas. The number of appraisal wells should be minimized, and the utilization of well pattern should be considered. Therefore, the test well should not be too close to the edge, it should have drilled oil layer with certain thickness and control a large area to give sufficient flexibility for the well pattern deployment. The integration of exploration and development, reduction of data duplication and data acquisition should be based on requirement for development scheme design.

(c)

Submitting controlled reserves. This is another achievement of early reservoir evaluation.

1.1.2.4 Economic Benefits Evaluation

Another important task of developing concept design is to make a preliminary economic benefit assessment of oilfield development on the basis of integrating oil reservoir, well drilling, oil production and surface engineering, and to put forward the feasibility and development uncertainty in terms of technology and economy.

1.1.3 Data Contents and Requirements for Making Development Scheme

Reservoir is buried thousands of meters under the ground. Only core and fluid samples taken from well by drilling are oil reservoir itself, direct and key data to describe reservoir, while most data is indirect one. Acquisition of materials must be in the order of time, since some materials will never be obtained when missed. The material acquisition and the additional material used to make development scheme should be confirmed and organized in the conceptual design stage. The adoption of new or advanced testing projects and technologies, as well as the acquisition quantity, quality and standards are all highly specialized works. Therefore, it is necessary to make design scheme and standardize workflow for data acquisition, avoiding missing data. Data acquisition design includes overall design and single (well) design. The acquisition plan and arrangement should be coordinated with the research progress such as reservoir evaluation, reserve calculation, development deployment, etc.

The following six factors of information and data are required for making the development scheme.

1.1.3.1 3D Seismic Data

3D seismic data is the basis for studying the geology of oil fields, not only for researching structural system, but also for carrying out lateral reservoir prediction, which is the necessary condition for making development plans. In the conceptual design, the unknown questions of structure and fault, including two wings with little well control, especially the distribution of third-order and forth-order faults in larger fault blocks, etc., need to be clarified using 3D seismic data to improve the understanding accuracy of structure. Based on the analysis of the factors affecting the accuracy of seismic interpretation in this area, new requirements are put forward for the acquisition and interpretation of 3D seismic data.

1.1.3.2 Coring

Systematic coring to the reservoir layers is at least required from one well (conventional, sealed, pressure maintaining, oil-based drilling fluid) of the reservoir. The so-called systematic coring includes well section from 2 m above the reservoir top to 2 m below the oil bottom (30 m below the top of the bottom water reservoir), which should be completely described on-site. The sampling density of routine indoor experiment items should not be less than 10 pieces/m. For oil fields with geological reserves > 2,000 × l04 t, oil-based drilling fluid coring, sealed coring or pressure maintaining coring are conducted to obtain oil saturation, wet-ability and other data. For reservoir with developed interlayer, especially the bottom-water reservoir, all kinds of interlayer should be sampled.

1.1.3.3 Conventional Logging Series

Conventional logging series should be determined based on the requirements of logging series of exploration well in the region and geological research in study area. In addition, the study on neutron porosity, spectroscopy gamma ray, sonic wave, dip angle,and nuclear magnetic and repeated formation test (RFT) should also be carried out. Imaging log should be performed for low permeability and ultra-low permeability reservoirs.

1.1.3.4 Core Analysis

Measurement of wet-ability and oil saturation must be performed by experiment at field. Coring, sampling and experiment data collection should be carried out according to technical standards.

Conventional core analysis includes fluorescence photography, rock-mineral analysis (12 items, e.g., thin section identification), and reservoir physical property analysis (9 items, e.g., vertical permeability).

Special core analysis:

10 items, e.g., saturation, wet-ability, porosity and permeability under over burden pressure, constant rate mercury injection, etc., that are measured at field;

Sensitivity flow test of reservoir (6 items, e.g., rate sensitivity);

Geochemical analysis (7 items, e.g., group composition);

Conventional geoelectricity and core in-situ stress testing.

1.1.3.5 PVT and Surface Fluid Analysis

Sampling down in hole (middle part of oil layer) can have higher success rate for PVT analysis of black oil. Samples can be accepted under the minimum production rate in the stable well testing. Water samples for PVT should be taken underground during water test.

Surface separator should be used for sampling when down-hole sampling is not available for volatile oil reservoir. Analysis methods and items and these two sampling methods mentioned above should strictly follow technical standards to ensure that samples are qualified and that results meet the technical requirements.

1.1.3.6 Oil Test, Production Test and Injection Test

Oil Test

Testing in Drilling Process, also called Drill Stem Test (DST) is the primary approach to detect oil and gas flow in the exploration stage. DST is characterized by fast and direct, which can meet the requirements of data acquisition in the exploration stage. Its testing data is commonly used for production capacity estimation at the early stage of conceptual design evaluation.

Well completion test (including gas test and water test) is the main test method in reservoir evaluation stage. The tests can be divided into three types: effective thickness limit, oil–water/gas relationship and production capacity. The selection of oil testing layer (section) should be consistent with the purpose of oil test and the qualified well cementation.

The purpose of oil test for lower limit effective thickness is to make effective thickness interpretation chart and improve interpretation accuracy of effective thickness parameters in reserve calculation. Therefore, the layer selection should be determined based on the requirements for making log interpretation charts and the number of tested oil layers.

The purpose of oil test for oil–water/gas relationship is to provide evidence for determining oil–gas or oil–water contact, depth of oil bottom and reservoir types. Therefore, the well completion test should be generally carried out when drilling such relationship layers. For bottom oil and oil ring, it is necessary to perforate a small well segment and to get oil-bearing property by using oil test. Water testing zone chosen should contain at least one water layer to obtain the relationship between water zone and resistivity. The opened segment of the well should not be too wide, generally about 2 m (can be smaller for the oil ring), and the selected perforation position should be coordinated with the interface.

Productivity test includes the index test of oil, gas and water. In terms of productivity test of oil zone, smaller layers should be the primary choice, and combined layers should be selected appropriately to perform unstable well test. The test methods mentioned above can provide evidence for studying the production capacity of various layers.

Systematic Well Test

Systematic well test (stable well test) is different from completion test mainly in well test zones, data acquisition contents and requirements. The layer of systematic well test should be basically consistent with layer series determined by the development scheme. A set of data acquisition requirements includes the followings: (a) 4–6 working systems should be selected if possible. The lowest flow pressure is 60–80% of saturation pressure, covering all production systems as much as possible; (b) The first production system should be carried out combined with PVT sampling; (c) Oil, gas and water properties, well temperature, pressure gradient and sand content under different production systems should be collected simultaneously; (d) The previous and subsequent unstable well tests and data obtained in the productivity well test are interpreted according to the parameter requirements of modern well test; (e) The liquid production profile should be tested under different liquid production rate systems.

Production Test and Injection Test

The research on injection-production well pattern is still in the initial stage in the conceptual design stage. As a result, single well is suitable for conducting production test and injection test. The purpose of single well production test is, on one hand, to test production capacity and, on the other hand, to analyze elastic energy. No water injection occurs to a new reservoir, so it is necessary to carry out injection test. There are three main problems needed to be solved in single well injection test. The first one is to measure the water injection indicator curve and calculate start-up pressure and water absorption index under initial reservoir pressure, the second is to understand the water absorption condition of layers and to conduct water absorption profile test, and the third is the water quality standard adopted.

1.2 Development Scheme Design Stage

The evaluation well drilling at the conceptual design stage and implementation of data acquisition plan for designing development plan marks further performance of the reservoir evaluation, gradually deepens understanding of reservoir geological features and creates basic conditions for oilfield development plan design. In this stage, five tasks need to be done well.

1.2.1 Overall Plan for Making Development Scheme

The overall plan includes five parts: reservoir engineering, drilling engineering, oil production engineering, surface construction engineering and economic evaluation. Overall plan, from preliminary preparation stage to full production stage of the oilfield, should focus on the work arrangements before reviewing the overall development scheme. The specific requirements are.

1.2.1.1 Detailed Work Contents, Progress Objectives and Schedules

In the five parts mentioned above, detailed work contents, progress schedules and research objectives should be included in field and indoor research (design). The intermediate result discussion and the final scheme design review should be arranged properly.

1.2.1.2 Coordinating Work Among all Professional Fields

Coordination of the calculation of proved reserves includes seismic interpretation (structural faults and reservoir prediction), evaluation well results, four-property relationships, log interpretation charts, etc.

Casing procedures coordination covers aspects of technical casing, production casing and tubing sizes.

Coordination of cluster drilling platform optimization and target accuracy is to maintain the consistency of the best drilling footage of platform scheme with the recommended plan.

Lifting should be coordinated with water injection methods, e.g., coordination between manual lifting design parameters and reservoir engineering design production pressure difference, coordination between injected water quality technical standards and surface treatment technology, coordination of the maximum injection pressure of injection well head design, etc.

Scheme comparison, recommended scheme and scheme design index. The scheme comparison should be evaluated economically to find out and optimize the recommended scheme. The development design index and single well index will be taken as the basis of surface engineering design.

Coordination between field work and indoor research, design, ordering, etc.

Coordination among intermediate achievement discussion, scheme design review and overall work progress.

1.2.2 Developing Water Injection Test Area

If water injection development method is adopted in large- and middle-sized reservoirs, it is necessary to develop a water injection test area, which is an initial experience in water injection development in China.

1.2.2.1 Purposes

The main purpose of water injection test at the early stage is to deepen our understanding of reservoir geology, especially the distribution of various oil layers and its adaptability to oil layers. The second is to fix some basic problems in water injection, including water absorption capacity and its influencing factors in water injection wells, water injection efficiency in oil wells and the technical limits of water injection, guiding the reasonable water injection.

1.2.2.2 Design Principles

The design principle should meet the requirements of test purposes:

The location of the test area is representative for the whole oilfield, including representatives for reservoir physical properties, distribution and fluid properties.

1–4 water injection well groups are designed in the test area, all of them with a number of central wells.

Injection-production well pattern should be designed evenly, and the distances among injection-production wells should be smaller than those among production wells. When test area is in large scale, two or more injection-production well patterns or injection-production well patterns with different well spacing should be designed for comparison.

Development index of injection-production well pattern should be higher than that of production well pattern.

Experiments with thorough and feasible data acquisition plan meet the requirements of drilling engineering, oil production engineering and surface engineering.

1.2.3 Optimizing Drilling Sequence, Reducing Program Risk and Improving Injection-Production Well Pattern Adaptability

The well pattern with only exploration well and evaluation well is low in density. The biggest uncertainty in the deployment of injection-production well pattern is that the distribution and thickness variation of various oil layers are not clearly understood (of course, the tendency can also be predicted based on the seismic lateral prediction and sedimentary facies,² but the risk is still high). Therefore, with regard to the drilling procedure, measures should be taken to reduce risk, and different drilling sequence should be designed to deepen our understanding of oil layers.

1.2.3.1 Drilling Basic Well Pattern

For large- and medium-sized oilfields with multiple sets of oil-bearing series, a set of well pattern should be arranged firstly for the main oil-bearing reservoir, and other oil-bearing reservoir should be explored simultaneously. The well type should not be determined temporarily with no perforation at all. After the drilling of basic well pattern, the formal development scheme is made by re-understanding of the oil layers, where basic well pattern is modified as necessary, and then to be perforated and be put into production [1]. For example, the original basic well pattern in the Northern development zone of Daqing Saertu oilfield mainly focuses on the main oil-bearing layers in PI oil and SII oil groups, while development well patterns in the formal development plan are adjusted to two oil well patterns, Saertu and Putaohua oil formations.

1.2.3.2 First Deploying the Deep Oil-Bearing Series Well Patterns and Simultaneously Exploring the Development of Shallow Oil-Bearing Series if there are more than Two of them

Three sets of oil-bearing series are found in Tazhong-4 oilfield, Tarim Basin. The CI reservoir is the sand mudstone derived from marine-continental transitional face, where oil reservoir is unstably distributed. And many uncertain factors occur to the well pattern deployment. The CII reservoir is the carbonate condensate gas reservoir, and the CIII reservoir is the marine bulk sandstone bottom-water reservoir. The main development plan was that the CIII reservoir was drilled firstly, and then the development schemes for CI and CII reservoirs were improved and were put into practice by getting the distribution data of CI and CII reservoirs during the development of CIII reservoir.

1.2.3.3 Exploring the Edge to Control the Oil-Bearing Area First and then Drilling Wells Wholly

Monoblock reservoirs are well developed in medium-sized oil field, where exploration well and evaluation well are generally in the main position, but the edge is poorly controlled. During the deployment of development well pattern, a development well should be firstly drilled near the edge and it should be adjusted elsewhere when the oil-bearing layer is not successfully drilled. For example, during the implementation of the development scheme in 1991, the accuracy of seismic interpretation could not meet requirements due to the deep-buried TI reservoir in Lunnan Oilfield, Tarim Basin, and it was impossible to drill more evaluation wells. Since many uncertainties existed in the structure boundary, a development well was drilled firstly in the northwest corner during the development. It was found that the dip angle of the structure became steeper compared with seismic data. The edge of reservoir was drilled, avoiding more serious mistakes.

1.2.3.4 Rolling Evaluation Blocks to Build Crude Oil Production Capacity by Stages

Two sets of oil-bearing series are discovered in Hadexun Oilfield, Tarim Basin: the thin sand reservoir layer in carboniferous mudstone (CI + CII) and the Donghe sandstone reservoir (CIII). Thin sandstone reservoirs were found in HD1 and HD2 wells one after another at the beginning of 1998. Donghe sandstone reservoir of Carboniferous system was found in HD4 well at the end of November of the same year. Reservoir evaluation and detailed exploration deployment were organized under the integrating exploration and development, in which as soon as one reservoir block was explored, it would be developed and constructed, and the process was repeated and the evaluation was carried out for the next stage. Its development and construction scale and rolling evaluation were divided into four stages, with an accumulated production capacity of 200 × l04 t, realizing the goal of rapid production and efficient development.

1.2.3.5 Drilling the Control Well First to Well Understand Oil Reservoir and Make Overall Deployment

A set of development well pattern is arranged to optimize selected favorable oil-bearing block. The well pattern is rarefied to develop a regular well pattern with well spacing about several times larger than that of the development well pattern, which is filled after data is taken. This is the popular method of well pattern deployment in the development of ultra-low permeability reservoir in recent years.

Above five common practices are adopted or new practices can be created based on geology conditions and cognition.

1.2.4 Submitting Proved Reserves

Reserves are the material basis of oilfield development, and the proved reserves are essential for development scheme. Therefore, the remarkable achievement of reservoir evaluation is to submit proved reserves. The calculation of proved reserves is also regarded as an important part of overall plan.

1.2.5 Development Scheme

1.2.5.1 Principles

Oilfield development is not only controlled by technology and economy, but also is affected by other complex elements. Therefore, it is necessary to formulate oilfield development principles to guide oilfield development. The development principle should fully consider the geology property of the reservoir and the overall needs. Six requirements should be put forward: (a) relationship between the production rate and stable production. It should be considered that which one is more important, the high production rate or long stable production period; (b) relationship between natural energy utilization and water injection to maintain pressure exploitation; (c) application and adoption of new and mature technologies; (d) expectation for the assemblage of development series and well pattern density in the deployment; (e) phased recovery efficiency index or long-term one; f. economic benefit index, etc.

1.2.5.2 Research and Design Workflow

The overall development scheme includes four engineering design contents, namely reservoir, drilling, oil production and surface construction, and there are special design specifications and requirements as the reference. The four schemes share common features that they—all come from basic data of the oilfield. Geology and reservoir are the foundation of drilling, oil production and surface construction. The overall scheme design needs to go through five steps: sorting and analysis of special data, monographic study, integrating various professionals, connection between professionals, and overall evaluation across various professionals. The last three steps included in economic evaluation are the focus of the five specialties, while the first two steps are basic research needed the largest workload and manpower resource. The higher quality of development design depends on data quality and integrity, thematic setting, comprehensiveness for full reflection of every respect, data focalization and deepness, which are the most important steps.

Six datasets required for development scheme in Sect. 1.1 are based on large and various data as well as on standardized processing to meet the requirements. For example, 22 comprehensive topics are established for reservoir engineering scheme design of Tazhong 4 Oilfield in Tarim Basin, while 21 comprehensive topics are established for oil production engineering scheme design. The research quality can be improved through fitting it with reservoir geology and recovery features under the condition that all the requirements of development design are considered.

1.3 Reservoir Geology Recognition and Perforation Scheme Design Stage

It is acceptable to put the reservoir geological recognition together with the perforation scheme design in this section. Taking the perforation scheme design as an independent part is to emphasize its importance and lays a solid foundation for the next development stage.

1.3.1 Reservoir Geology Recognition

A remarkable feature of oilfield development is that it is impossible to know the reservoir in one step. Only through repeated practice and recognition can we continuously and thoroughly understand and master the geological characteristics of the reservoir.

To give advice, various parameters of reservoir are compared, analyzed, and verified on the basis of reservoir data from development wells, special data collected in implementation and uncertain questions raised in implementing requirement.

Proved reserves are reviewed, analyzing the reasons for parameter variation.

Development strategy (including development method, development series, well location, injection-production well pattern and well type) is adjusted and analyzed, based on new problems in the implementation.

Statistic and compiling work is carried out on geological basic data and maps that focus on stratigraphic correlation and sedimentary micro-face, to establish a static database, based on the vertical and horizontal distribution of various oil-bearing reservoirs.

1.3.2 Perforation Scheme Design

(a)

Purpose. It is to establish a perfect injection-production system to realize control of reservoir layer according to well pattern designed in the development scheme. The production well is connected geologically with a certain layer of a water injection well, but whether an injection-production system can be formed depends on perforation. That is to say, an injection-production system can probably be established by perforation in both a production well and a water injection well that are geologically connected.

(b)

Protocol of perforation principle, detailed rules and target horizon.

(c)

Evaluation. The evaluation parameters include: the control of well pattern to oil-bearing reservoir, the number of effective directions of water injection, the cause classification of imperfect injection-production, the proportion of low efficiency wells, etc.

(d)

Numerical simulation to predict production performance. The 3D geological model of the whole reservoir established in the development scheme is improved and modified, and the production fitting in the pre-production stage is carried out based on drilling data from development well after well completion and perforation zone planned. After the oilfield has been totally put into water injection, dynamic prediction of oilfield exploitation for next year should be carried out every year based on production fitting in the past years, to guide the annual oilfield comprehensive adjustment plan and provide an insight into the design of crude oil production and operation curve.

1.4 Design and Adjustment Stage of Layered Water Injection

Water injection development will begin as soon as the oilfield is put into full production and injection with an effective pressure gradient. The design and adjustment of layered water injection is the main production stage of water injection. What we concern is the producing feature varying from anhydrous to water cut and then to high water cut. In this stage, we should figure out the potential distribution of each water cut stage and take comprehensive measures (water plugging, fracturing, etc.) and enhance water injection to improve the production of the poorly-produced reservoirs step by step, which can slow down the decline and extend the stable production period.

1.4.1 Layered Water Injection Scheme for New Oilfields

In terms of the first water injection scheme in new oilfield, the initial volume of layered water injection is calculated based on water injection intensity that is optimized from the basis of average water injection intensity classified by interval property. The calculation process is mainly based on static data, single well production index predicted by perforation scheme and layered water injection principle.

1.4.2 Dynamic Layered Water Injection Adjustment Scheme

Dynamic layered water injection is different from the first layered water injection, primarily adjusting water injection volume according to production performance, including the whole reservoir, the property of water injection interval and the injected water volume. The so-called production performance is the changing trend of pressure, water cut and liquid production capacity. The injected water volume can directly adjust and control the pressure and water cut, while the pressure and water cut also affect oil production. Therefore, to dynamically regulate water injection, the water injection volume into each layer is adjusted—based on production changing trend, so as to maintain formation pressure, control the rising rate of water cut and slow down the decline of oil production.

Single well analysis is the basis of dynamic layered water injection. Only through single well analysis can we select water injection intervals that need to be subdivided, intervals that need to increase production and injection, small layers that need to be plugged, intervals that need to increase or reduce water injection, etc. As the basis of dynamic water injection, single well analysis is indispensable for us—to carry out dynamic water injection.

The control of the macro index of dynamic adjustment scheme for separated-layer water injection is generally based on the annual or half-year development index in this area.

1.5 Infilling and Adjustment Stage

Infilling-adjustment stage and design and adjustment of layered water injection stage share some features in common that they both occur under the condition of layered water injection process. What makes the difference is that the former is associated with adjustment of interval and injection-production well pattern, and the latter involves adjustment of all elements. Infilling-adjustment stage is the primary stage to improve water drive recovery efficiency, so that it is discussed separately.

Data