Introduction

Due to the strategic role of horizontal wells in the production from oil reservoirs, as well as significant advances in drilling technology and well completion, the use of these wells has increased more than before and choosing the method of economic completion with the right efficiency is an art. An efficient well completion method must be done, while maintaining the mechanical integrity of the borehole, without creating significant restrictions on production capacity, and it has to be economical (Furui et al. 2004). Choosing the right method for completion in horizontal wells in thin heavy oil reservoirs plays a vital role. Therefore, optimizing the completion in horizontal wells is of great importance.

In designing the completion of horizontal wells in each field, in addition to the characteristics of the rock and fluid of the reservoir, economic considerations, etc., special attention should be paid to problems such as wellbore stability, solid/sand production and so on. The need to examine various aspects of optimizing the completion of horizontal wells complicates this task.

The field studied in this work is located in the southwest of Iran. It is currently the largest developing oil field in the world (Liu et al. 2013). This field has several reservoirs, but the most percentage of production rate comes from the Sarvak oil reservoir. Sarvak as a target reservoir has 4 m thickness, and it consists of a heavy oil of 19.95° API, 4.44–5.44 cP. viscosity and 276–441 SCF/STB GOR (Manshad et al. 2019). The reservoir drive mechanism is rock and liquid expansion and formation is homogeneous. In this reservoir, the wells are drilled with 800 meters of horizontal section. The open hole has been adapted for this field as a common completion in the horizontal section. However, this method does not have the required efficiency and after a relatively short period of production time, the rate of production has been reduced.

Sarvak is a carbonate reservoir, but it contains shale and mudstone; it may be stable in the initial phase of exploitation but the borehole wall will become unstable after acidizing or in the later phase of production. The research shows that the acid will reduce the mechanical strength of rocks (Gou et al. 2019). After a short time of production, discharging large debris of shale has significantly affected production rate. They settled in the horizontal section and cause a decline in productivity of the well.

In the case of completion method selection, having a balance between well productivity, intervention activities and cost economics is a key point (Kumar et al. 2010; Liu and Morita 2018). Accordingly, choosing a perforated or slotted liner is likely an appropriate and cost-effective completion method in this oil field. Screen liners are widely used to improve the performance of horizontal wells in sandstone (Furui et al. 2012), limestone (Zaisheng et al. 2001) and also shale reservoirs (Liu and Morita 2018). There are massive studies on the perforated and slotted liner (see Table 1).

Table 1 Review of previous studies on perforated/slotted liner
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Application of perforated and slotted liner

There are general purposes for using of the perforated liner; low cost and prevention hole collapse by formation over time (Abbassian and Parfitt 1998). This method is one of the most cost-effective solid/sand control methods (Ahsan and Etesami 2013).

Perforated and slotted liner design considerations

Following items have been considered:

  1. A.

    Maximum hole density and hole diameter from the point of inflow/outflow and stimulation with considering the hole size based on the size of produced solids and also hole density based on the flow path open area.

  2. B.

    The screen liner must be able to withstand loads of installation because holes reduce the mechanical capacity of the production string (Jorden et al. 2011).

  3. C.

    The strength of pre-drilled liner cannot be less than 80% of the same specification casing (Mantovano and Grittini 2016).

  4. D.

    The total block of holes per joint should exceed the ID cross-sectional block of the major production string but should not exceed 3% of the total surface block of the joint to be drilled. By increasing the open area to more than 3%, the collapse strength and rotational capacity of a screen liner are reduced significantly. Besides, an open area above 5% strongly affects the collapse strength and torsional capacity of the liner (Kumar et al. 2010).

Methodology of the study

Figure 1 shows the algorithm for optimizing perforated liner parameters in horizontal wells. According to an analysis of offset wells PVT data, the reservoir pressure and temperature are shown in Fig. 2 and Table 2. The formation pore pressure coefficient (FPPC) is 1.02–1.26 in Sarvak.

Fig. 1
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Flowchart for screen liner optimization

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Fig. 2
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Tested pressure (left) and tested temperature (right) in offset wells

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Table 2 Sarvak reservoir information
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The blind liner 4-1/2″/L-80/13.5 PPF was designed according to the IPS (Iranian Petroleum Standards), SY/T 5431-2008 and American Petroleum Institute (API). The stress analysis was conducted by using conventional working stress design procedures, where stresses due to various load conditions were calculated and compared to the API minimum yield strength of the tubular. Table 3 illustrates the safety design criteria used in this study. Also, Fig. 3 shows the stress check for 4 1/2″ liner.

Table 3 Minimum design factors
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Fig. 3
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Stress check for 4 1/2″ liner

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Selection of the type of screen liner

The commonly used screens are mainly divided into the perforated screen, slotted screen and punched slot liner. Two principles of screen selection must be followed: One is to ensure enough strength, and the other is the flow block that must meet the requirement of the production. As mentioned earlier, Savark is a carbonate reservoir. The wellbore is relatively stable, and the problem of sand production is not necessary to be considered. The function of the screen is to prevent formation collapses and to prevent the large debris from discharging with fluids. Therefore, we will select either perforated screen or slotted screen liner.

Casing collapse resistance strength was used to measure the capability of the casing to instability and failure (Lou et al. 2011). Therefore, finite element analysis (FEA) was run as a computerized method for predicting the collapse strength of the slotted and perforated liner with the same flow block. The results are shown in Fig. 4. The collapse strength of the slotted screen is much lower than the perforated screen, which has been proven by laboratory experiments (Figs. 5 and 6).

Fig. 4
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Finite element analysis results of the collapse strength of slotted screen and perforated screen

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Fig. 5
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Collapse resistance load of perforated liner

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Fig. 6
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Collapse resistance load of slotted screen

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Perforating screen pipes are formed by direct punching on the base pipes and will not be influenced by heat stress. They can ensure the effective overflowing block of fluids, and at the same time, reserve a relatively high strength for the largest pipe. Their tripping-in is easy in long horizontal sections. In slotted screen, the heat stress of the slotted screen pipes cannot be eliminated, the brittleness of the slotted screen pipes increases and the resistance to corrosion reduces. The maximum effective overflowing block and pipe strength cannot be ensured, the collapse resistance strength is limited, and deformation is easy to be generated during running in long horizontal sections. Tripping-in is not suitable in long horizontal sections.

Sensitivity analysis for the parameters of perforated liner

Sensitivity analysis was performed on various phase angles, hole densities, hole diameters and hole distributions to determine the pattern of the holes so that the enough strength of the liner is maintained while maintaining the maximum hole density (see Figs. 7, 8, 9, and 10).

Fig. 7
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Influence of hole diameter on collapse resistance strength at different phases

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Fig. 8
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Influence of hole density on collapse resistance strength at different phases

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Fig. 9
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Influence of different hole densities and hole diameters on collapse resistance strength factor

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Fig. 10
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Influence of different numbers of perforated lines on collapse resistance strength

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It is shown from the above figures that the influence of hole diameter on collapse resistance strength is far greater than the influence of hole density on collapse resistance strength at different phases. The sensitivity of hole diameter and hole density on collapse resistance strength are relatively the lowest at the 45° phase.

Determination of parameters of perforated liner

The calculation of collapse resistance strength

The decrease extent of the collapse resistance strength of casing pipes after perforated on casing pipes is calculated as per the following equation:

$$\alpha = \frac{{\left( {K - 1} \right)\sigma_{ \hbox{max} } }}{{\sigma_{S} }} = \frac{{\left( {0.0282 + 0.0024d + 0.0011q + 0.0002w} \right)P_{i} \left( {1 + \mu } \right)\sqrt {R_{i}^{4} + 3R_{0}^{4} } }}{{\left[ {R_{i}^{2} + R_{0}^{2} + \mu \left( {R_{i}^{2} - R_{0}^{2} } \right)} \right]\sigma_{s} }}$$
(1)

where Pi is the internal pressure of the casing pipe in MPa. The bubble pressure is 2000 psi, which is taken as the internal pressure of casing pipe, or 13.78 MPa. Ri: inner diameter of casing pipe (100.6 mm); Ro: outer diameter of casing pipe (114.3 mm); Μ: Poisson’s ratio of steel (0.25–0.3, taking 0.3); σs: Yield limit (9210 psi, taking 63.47 MPa); d: hole diameter (mm); q: hole density (holes/m); w: phase of shot (°).

It can be seen from Table 4 that the decrease extent of the collapse resistance strength of the casing pipes after perforation is limited.

Table 4 Collapse resistance strength of casing after perforated
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Calculation of tensile strength

The tensile strength after perforated in the base pipe changing is calculated as follows:

$$P_{P} = P \times \frac{P \times S}{{P \times S_{0} }}$$
(2)

where Pp: the tensile strength after perforated; P: minimum tensile strength before perforated; S: the cross-section area of the base pipe after perforated; S0: the cross-sectional area of the base pipe before perforated.

Between 8 and 12 mm hole diameter, every meter 280 holes, the perforated screen before and after drilling tensile strength was all above 80% and met the requirements of the tensile strength (Table 5). The influence of the hole diameter changing on the tensile strength is very small and thus can be ignored.

Table 5 Tensile strength of casing after perforated
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Calculation of holes per meter

To calculate the required percentage of the open area on the pipe surface, the diameter of the desired hole must first be selected. Then, the number of holes per foot of the pipe is achieved using the following equation (see the result in Table 6):

$$N = \frac{{\left( {12 \times D \times C} \right)}}{{\left( {25 \times d^{2} } \right)}}$$
(3)

where N = required holes/foot, D = outside diameter of liner (in), C = required open area (percent of surface area), d = diameter of hole (in).

Table 6 Determining holes per meter
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Calculation of loss of the flowing path

The loss of the flowing path when the crude oil flowing through the base pipe is calculated as follows:

(4)

Assuming that the daily oil production is 3000 bpd, the relative density of the crude oil is 0.90, and the local resistance factor is 0.95 according to the submerged flow (see Figs. 11, 12).

Fig. 11
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Cross-sectional area of base pipe before and after perforated

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Fig. 12
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Hole diameter Vs. the loss of flow path

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It can be seen that when crude oil flows through the small holes on the base pipes, the influence of the flow path loss on the flowing of crude oil in well holes is very small and thus can be ignored.

Comprehensively considering the collapse resistance strength and overflowing area, 4-connection–line perforated method is adopted with the hole diameter of 10 mm and hole density of 100–120 holes/m (Table 7).

Table 7 Optimum perforated liner specification for horizontal well
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Conclusion

The design must be effective in solving the specific problems of a field. The proposed design for the Sarvak oil reservoir specifically addresses the problems of formation collapses and controlling large debris from discharging with fluids. Due to the problems mentioned and also the 800-meter length of the horizontal section, two options, slotted liner and perforated liner, were selected as the best options economically and technically. In the next step, both options were analyzed by the FEA method. The FEA results show that the collapse strength of the slotted liner is much lower than the perforated liner, which has been proven by laboratory experiments. After selecting perforated liner, a sensitive analysis was performed on it. The sensitivity analysis has shown that the influence of hole diameter on collapse resistance strength is far greater than the influence of hole density on collapse resistance strength at different phases. The optimum designed perforated liner has a ratio of strength less than 8% for collapse and 5% for tensile compared to the same specification casing.