DETERMINATION OF A SET OF RESERVOIRS FOR DUAL COMPLETION OF AN INCLINED WELL AT THE NORTH GOTURDEPE MULTI-RESERVOIR FIELD

DETERMINATION OF A SET OF RESERVOIRS FOR DUAL COMPLETION OF AN INCLINED WELL AT THE NORTH GOTURDEPE MULTI-RESERVOIR FIELD

Annaguly Deryaev

 Candidate of Technical Sciences, Researcher, Scientific Research Institute of Natural Gas  of the State Concern "Turkmengaz",

Turkmenistan, Ashgabat

Most oil and gas fields, both in our country and abroad, are multilayer. In this case, several productive layers are located floor by floor one above the other. The development of such fields by independent grids of wells drilled for each individual layer, from the point of view of rational development, is the most preferable. However, the experience of developing oil fields shows that more than half of all capital investments are spent on drilling wells. Therefore, the development of multi-layer fields with independent grids of wells for each layer requires huge capital costs and is not always economically and technologically justified. In this regard, when developing multi-layer fields, several productive layers are often combined into one production facility, which makes it possible to reduce the terms of field development, reduce capital investments in drilling wells and field development, etc.

At the same time, the simultaneous field development of several reservoirs by one object is possible only if the physical and chemical properties of oil in the combined reservoirs are the same, if the inflow of oil and gas is sufficient from each reservoir at an acceptable bottomhole pressure in the well, with close values ​​of reservoir pressure in the combined reservoirs. reservoirs, excluding oil flows between reservoirs, and similar values ​​of water cut in reservoirs. If the above conditions are not met, then multi-layer deposits are developed by the method of dual completion  (hereinafter referred to as DC) of one well. Depending on the specific geological and technical conditions for the development of deposits, the technical and operational characteristics of wells, one of the currently available DC schemes is used. Mandatory requirements for all DC schemes are the possibility of separate development and commissioning of each reservoir, measurement of oil production rates of each reservoir separately, as well as separate measurement of each reservoir for water cut, gas content and study of each reservoir for oil and gas inflow.

In order to identify groups of reservoirs in a multilayer object, the development of which is advisable to be controlled jointly, you can use the following relationship:

                                                                           (1)

where: N is the number of layers in the reservoir, index i marks the parameters of the i-th layer; m - porosity; Δσ - change in water saturation during the transition through the OWC; μ_v, μ_n are the viscosity of water and oil, respectively; ΔP - depression, k* - permeability; ω is some estimated parameter.

Expression (1) is derived from the condition of maximizing the ultimate oil recovery factor for a given water cut of the produced fluid. It is known that the well is shut down (or switched to another mode of operation) when the water cut of the produced fluid is approximately 95–98%. However, even in this case, a significant volume of oil remains in the reservoir, which can be significantly reduced with a differentiated stimulation of the reservoir. The following task was set: at what drawdowns is it necessary to exploit individual layers of a multi-layer reservoir so that the oil recovery of the entire multi-layer section at the time the specified degree of water cut of the produced fluid is reached is maximum? Relation (1) is the solution to the problem [1].

The method for chousing groups of reservoirs is as follows: we select the least permeable reservoir, set the maximum possible drawdown on it and calculate the value of the parameter ω according to (1) then, using the value of ω, using the filtration characteristics of the remaining reservoirs, we calculate drawdowns ΔPi for each reservoir according to formula (1); we build a drawdown distribution diagram for layers (see Fig. 1a) and select groups of layers in a multi-layer object for joint action with close values of ΔP.

Table.

Indicators of formations on the horizons of the field

We consider the reservoir area indicated by three open horizons during drilling of inclined well No. 147 at the North Goturdepe field. The first horizon IXd+e of the middle red sediment consists of four layers (layer I 4008-4012 m, layer II 4017-4023 m, layer III 4025-4030 m, layer IV 4040-4050 m), and the second horizon NK of the lower red-colored deposits, consisting of four layers (layer I 4150-4154 m, layer II 4165-4170 m, layer III 4173-4175 m, layer IV 4188-4193 m), the third horizon of red deposits NK2 consists of one layer (layer I 4238-4248 m ). The reservoir parameters at the horizons of the field are presented in Table In this case, N = 9.

The second layer of horizon IXd+e is the least permeable than the other layers. The maximum possible depression of this reservoir will be considered ΔP*=14 atm. Using formula (1), we calculate the estimated parameter ω for the second layer of horizon IXd+e using expression (1):

The dimension of this parameter in this case does not matter at all. According to the filtration parameters of the remaining layers, we calculate using formula (1), already written in the form:

                                                                                        (2)

Depressions in four layers of the first horizon IXd + e:

Then we get the following calculations from ΔP2≈14 atm (provided), ΔP3≈14 atm, ΔP4≈9.4 atm.

Depression in four layers of the second horizon NK1:

ΔP₁≈2,1 atm, ΔP₂≈2,1 atm, ΔP₃≈2,1 atm, ΔP₄≈2,1 atm

Depression in the reservoir of the third horizon NK2:

ΔP₁≈2,95 atm.

Figure 1a shows a diagram of the distribution of optimal drawdown across reservoirs.

According to the formula:

                                                                                           (3)

where: Δf i - oil recovery factor of the i-th reservoir; qi - active formation of the i-th layer; ql-mobile stock of the i-th layer; We calculate the maximum oil recovery factor f_opt corresponding to the operation of the facility at optimal drawdowns and the oil recovery factor for joint operation  f_joint. at ΔPi ≈ 14 atm. Let us introduce the indicator Δf, which characterizes the effect of separation by layers, corresponding to the increase in oil recovery due to a differentiated effect on the reservoir.

Figure 1. Selecting a reservoir group identification system for multilayer well No. 147 North Goturdepe for dual completion

Figure 1b shows a graph of the dependence of Δf on the degree of water cut in the produced fluid. The total average water cut in the reservoirs for the North Goturdepe field is approximately 25% (IXd + e -30%, NK1 and NK2-20%). Note that at the moment of 25% water cut, we can get the effect in the oil recovery factor of 75%.

Well No. 147 of the North Goturdepe makes it possible to divide the layers of three horizons into two systems. Four layers of the IXd+e horizon can be used in one system, four layers of the NK1 horizon and one layer of the NK2 horizon that delivered simultaneously with the second system.

The value of depressions calculated by formula (2) depends on the magnitude of depressions adopted in the reservoir with the lowest permeability horizon IXd + e, (14 atm). If the occurrence of drawdown in I and II layers of the IXd+e horizon occurs at high pressures (for example, 14 atm, and not 30, 40 and 50 atm), this will lead to a reduction in the field operation period (respectively, in high-permeability III and IV layers, the drawdown will increase ) and will allow maintaining the required recovery rate with a high oil recovery factor.

Expression (1) or the diagram in Figure 1a makes it possible to separate a multilayer object from several groups. Well No. 147 North Goturdepe allows several layers in three horizons to be divided into two systems. When choosing production systems, four layers of the IXd+e horizon are allocated to the first system by the same drawdown (I, II, III, IV), four layers of the NK1 horizon (I, II, III, IV) and one horizon layer are allocated to the second system. NK2 (I). The choice will be appropriate and this is shown in Figure 1a.

Differential impact separately on each layer in accordance with expression (1) is aDCciated with great technical difficulties. At present, the equipment allows for the operation of the field to be divided into two or, at best, three objects. For this reason, splitting by horizons was taken as a basis.

Let us set several values ​​for drawdowns at IXd + e and NK1, NK2 horizons according to formula (3) and calculate the corresponding values ​​of the oil recovery factor at the time of well watering, with 25% well watering. Figure 2 shows the dependence of the indicator Δf in relation to depressions on the horizons. This indicator indicates additional oil production when splitting into two multilayer objects (horizons IXd+e and NK1, NK2) compared to the joint operation of all reservoirs. It can be seen that when ΔP₁/ΔP₂ <1, the exponent Δf will be positive, i.e. application of WEM will lead to additional oil production.

ΔP_II/ΔP_I –  The optimal ratio of drawdowns in the layers.

Δf is the indicator of the effectiveness of differentiated stimulation with the optimal ratio of drawdowns in the reservoirs

The maximum increase in the oil recovery factor in the considered field corresponds to ΔP_II/ΔP_I≈0.5. Thus, for the maximum effect, the drawdown in the layers of the IX_d+e horizon should be approximately two times greater than in the NK₁, NK₂ horizons.

According to the calculation results, the production optimization ratio ΔP_II/ΔP_I≈0.5 must be fulfilled regardless of the absolute values of drawdowns on the horizons. This allows you to get not only an additional volume of oil, but also to choose a suitable period of development time.

For illustration, in Figure 1b, the values of the indicator Δf are entered at the optimal ratio of drawdowns on the horizons, corresponding to Figure 2.

Figure 2. Dependence of the efficiency indicator on the depressions ratio at the horizons

References:

1. Деряев А.Р.  Еседулаев Р., Основы технологии бурения при освоении нефтегазовых пластов методом ОРЭ. Научная монография. Ашгабат: Наука, 2017. Стр 98-126.