Analyses on the formation of operational facilities in multi-layer deposits for dual completion

Known oil fields in the world are multi-layered and contain approximately 95% of oil reserves of industrial categories, which in itself determined the high importance for their rational allocation of operational facilities. Since most of the discovered fields contain several productive layers or horizons, the number of grids of production and injection wells, as well as flow rates, permissible depressions, the cost of producing tons of oil and other indicators depend primarily on the correct allocation of objects; consequently, the amount of material costs for drilling and exploitation of the field.

A systematic presentation of the conditions for combining several layers into one object of exploitation was presented in the work of N.E. Bykov [1]. He gives a list of the main geological and commercial factors that should be studied when solving the issues of allocation of operational facilities in sections of multi-layer oil fields:

– the range of oil and gas content by section; the number of productive horizons (layers); the depth of productive horizons; the thickness of clay sections and the presence of zones of fusion of productive layers; the position of oil-water contacts along the layers/horizons, the coincidence of deposits in the plan; lithological characteristics of productive horizons (layers); the range of reservoir properties, especially the permeability of layers; the difference in types deposits by horizons, formations; properties of oils and gases by section; modes of deposits and their possible change; the size of oil reserves by formations, horizons.

In the work [1], the methodological foundations for the allocation of operational facilities in the sections of multi-layer oil fields, including geological, technological and economic research, are formulated.

In connection with the discovery and development of multi-layer and multi-layer deposits in Western Siberia, the problem of the formation of operational facilities was systematically investigated by V.G. Kanalin and L.F. Dementiev [2]. The following requirements for operational facilities were formulated:

• the operational facility must contain sufficient oil reserves for its cost-effective extraction with an independent grid of wells;
• an operational object may be one powerful or several thinner oil formations separated over a significant area from the above and underlying sediments by a pack of impermeable rocks;
• the operational object must have a certain thickness, the value of which is determined by economic profitability, as well as technical capabilities;
• layers that are characterized by a similar lithological composition and approximately the same permeability and porosity should be combined into one operational object;
• one object should include formations containing oil of identical (similar) physicochemical properties and having the same oil-bearing areas;
• oil-bearing formations combined into one operational facility should be characterized by close values of the reduced reservoir pressure;
• it is desirable that there are no conditions for cork formation in the oil formations combined into one object;
• all oil formations having one surface of oil-water contact can be combined into a single operational facility;
• when allocating operational facilities, it is necessary to take into account the possibilities of methods for maintaining reservoir pressure.

In addition to these factors, the authors of the above-mentioned works also propose to take into account the regime of oil-bearing formations, saturation pressure, the following main criteria for the allocation of operational facilities:

• in multi-layer deposits with several floors of oil content, only those layers that belong mainly to one floor of oil content should be combined into an operational object;
• the combined formations must have such recoverable oil reserves per well that ensure the economic efficiency of the development of the facility;
• it is advisable to combine highly productive layers only if the pace of development of the object will differ little from the pace of their separate development;
• the unification of oil and gas deposits is allowed in conditions when the stability of the gas-oil contact is ensured by regulating the pressure in the gas cap;
• for sections with weakly cemented rocks, an additional criterion is the length of the filter, depending on the permissible depression, the intensity of cork formation and the maximum participation in the development of all interlayers;
• the development of multi-layer objects should be carried out with the implementation of all necessary measures to control it and effective measures to manage this process in order to achieve the maximum oil recovery coefficient of each of the formations included in the object separately;
• when developing multi-layer objects, water injection should be carried out separately in layers at a differentiated injection pressure;
• when developing a field with the maintenance of reservoir pressure and the use of effective means to control and regulate the development process, reservoir deposits with significantly different values of hydroconductivity, thickness, recoverable reserves and oil-bearing areas can be combined into an object.

Analyzing in detail the theory and practice of allocating operational facilities on the example of the largest deposits, it was proposed to consider 5 groups of factors: geological, hydrodynamic, technical, technological, economic.

The following are classified as geological and commercial:

– dissection of the field section, separation of productive layers; lithological characteristics of productive layers; total, effective and oil-saturated thickness of layers; reservoir properties of layers according to core and materials of field geophysics; results of testing, evaluation of filtration parameters of productive layers by hydrodynamic methods, establishment of "working" thicknesses in various modes (according to flow meters and flow meters); physical-chemical properties of oil, gas, water; thickness of intermediate thicknesses between productive layers, thickness of tires; the position of the oil-water contact and the ratio of areas within the outer contours of the oil content; oil and gas reserves in productive formations and their ratio by field section; establishment of initial reservoir pressures in deposits and their ratio by field section; hydrogeological characteristics and regime of deposits.

Hydrodynamic factors, in essence, include the main technological indicators of development, obtained on the basis of hydrodynamic calculations using various methods:

– annual oil production for each reservoir deposit; dynamics of oil production for each reservoir until the end of development; productivity, and then annual production of productive reservoirs combined into one operational facility; dynamics of oil and water production in the whole field; dynamics of well flooding, deposits and operational facilities; duration of individual stages of field development; optimal oil sampling by field, taking them into account by the deposits of each formation and by the object of operation.

Technical factors determine:

– the method and technical capabilities of operation (fountain, deep-pumping, gas lift); diameters of production columns and tubing; the possibility of using borehole equipment for simultaneous and separate operation; the possibility of selective isolation of flooded formations; the availability of devices and technologies to control the production of each formation as part of the operational facility

Technological factors include:

– selection of the grid and the system of placement of production wells for each development facility; justification of the method of maintaining reservoir pressure - reagents, systems, processes; control and regulation of the development of operational facilities in order to ensure maximum oil recovery; the possibility of using various methods to increase the final oil recovery of all layers of the operational facility.

Economic factors, according to [2], take into account:

– natural and climatic conditions of the area of the location of the field; technical and economic standards for drilling wells and field equipment; results of hydrodynamic calculations.

The tasks and the sequence of their implementation are described below [2]. In essence, the information necessary for calculating reserves and designing the development of oil fields, the technical and economic indicators of development obtained during design work, and the requirements for monitoring and managing the process of field development are listed here.

The published work by V.A. Bocharov [3] provides a brief overview of publications on this problem, as well as some of its recommendations. Here they are formulated in a slightly different wording: "it is impractical to include several layers (horizons) in one operational object:

• characterized by large recoverable oil reserves, the permeability of which varies by 2 times or more;
• when the reservoir pressure in one reservoir significantly exceeds the pressure in another, where it is close to or equal to the saturation pressure;
• in cases where oil viscosities differ by more than 4 times;
• when the productivity of one reservoir is 2 times or higher than the productivity of another."

As you can see, some quantitative criteria are already given here.

As a result of the completed review, the author [3] formulated the following 6 main criteria, the presence of which does not allow the unification of layers into one operational object:

1. The presence of reservoir waters in individual layers characterized by high pressure.
2. The presence of permeable layers that play the role of absorbing.
3. Layers belong to different types of reservoirs.
4. Complete mismatch of the contours of oil and gas potential in the formations.
5. Layers combined into one operational object should not be located on different floors of exploration.
6. There is an incommensurably large discrepancy in the composition and properties of the oils in the formations.

In addition to the above, the following 6 indicators are given:

I. The presence and size of fusion zones between productive layers.

II. Ranges of changes in porosity and permeability values.

III. The presence of fracturing.

IV. Characteristics of discontinuity of productive layers.

V. The difference in the coefficients of coverage of layers by the displacement process.

VI. A sharp difference in reservoir regimes, reservoir pressures and productivity coefficients.

Unfortunately, the listed criteria again turned out to be common, in some cases duplicating each other or even meaningless.

As you can see, the list of indicators proposed for accounting when allocating operational facilities has been expanding all the time. However, all these indicators are given without quantitative characteristics, at the descriptive level. Their abundance and lack of quantitative criteria show the complexity of the problem and the need for further research in this area.

All these requirements are extremely simplified in the work [4]. To solve the issue of combining or not combining oil reservoirs into one common operational facility with a single grid of producing and injection wells, it is proposed to apply the rationality criterion - the maximum average oil flow rate per project well during the extraction of specified (approved) recoverable oil reserves.

Important components of the general characteristics of oil reservoirs are the η_med - the average value of the well productivity coefficient, η – the number of wells for which the values of the n -productivity coefficient were determined, the average value of the square of the productivity coefficient (η²)_med, the indicator of the heterogeneity of the totality of the values of the well productivity coefficient - the square of the coefficient of variation V_n².

Another important component of the general characteristics of oil reservoirs is their calculated layer-by-layer heterogeneity in permeability V², which already includes V₁² – actual layer-by-layer heterogeneity in permeability plus V₂² – geometric heterogeneity (unevenness) of oil displacement by injected water due to the location of point sources and drains, i.e. injection and production wells. However, the analysis of the work [5] reveals unreasonable assumptions when performing it.

The following indicators were taken into account when implementing the EPR at the fields of Turkmenistan (Korpedje and Northern Goturdepe) to combine objects (horizons):

1. Geological and commercial factors.

– dissection of the field section, separation of productive layers; lithological characteristics of productive layers; total, effective and oil-saturated thickness of layers; reservoir properties of layers according to core and materials of field geophysics; results of testing, evaluation of filtration parameters of productive layers by hydrodynamic methods, establishment of "working" thicknesses in various modes (according to flow meters and flow meters); physical-chemical properties of oil, gas, water; thickness of intermediate thicknesses between productive layers, thickness of tires; the position of the oil-water contact and the ratio of areas within the outer contours of the oil content; oil and gas reserves in productive formations and their ratio by field section; establishment of initial reservoir pressures in deposits and their ratio by field section; hydrogeological characteristics and regime of deposits.

2. Technical factors.

– the method and technical capabilities of operation (fountain, deep-pumping, gas lift); the diameters of production columns and tubing; the possibility of using downhole equipment for simultaneous and separate operation; the availability of devices and technologies to control the production of each reservoir as part of the operational facility.

3. Technological factors.

– selection of the grid and the system of placement of production wells for each development facility; justification of the method of maintaining reservoir pressure - reagents, systems, processes; control and regulation of the development of operational facilities in order to ensure maximum oil recovery; the possibility of using various methods to increase the final oil recovery of all layers of the operational facility.

4. Economic factors:

– natural and climatic conditions of the field location area; technical and economic standards for drilling wells and field development; results of hydrodynamic calculations; annual oil production for each reservoir; dynamics of oil production for each reservoir until the end of development; productivity, and then annual production of productive reservoirs combined into one operational facility; dynamics of oil production and water in the field as a whole; dynamics of flooding of wells, deposits and operational facilities; duration of individual stages of field development; optimal oil sampling for the field, taking them into account for the deposits of each formation and for the object of operation.

With simultaneous separate operation of gas wells at the Korpedje field and oil wells of the Northern Goturdepe field, a high flow rate with great economic efficiency was obtained. This proves the correct choice of geological, hydrodynamic, technical, technological, economic factors influencing the implementation of the development of multi-layer deposits with the method of simultaneous separate exploitation.

Despite the long history and a large number of published works on the problem of the formation of operational facilities at multi-layer multi-storey oil fields, the discussion continues. The absence of quantitative criteria for the inclusion of heterogeneous reservoirs in one operational facility, often represented by different types of reservoirs and saturated with oil with different properties, as well as clearly formulated requirements in the Rules for the Development of oil fields leave the possibility of introducing new fields into development by creating multi-layer and even multi-horizon operational facilities with some economic advantages at the initial stage and extremely negative technological consequences after a very short service life.