The selection of completion methods is mainly for single wells. Although a single well belongs to the same reservoir type, its geographical location is different, and the selected completion method is also not the same. If the reservoir has gas cap and bottom water, if it is completed by naked eye, the technical casing should seal off the gas cap before drilling through the reservoir, rather than drilling through the bottom water layer. If casing perforation is used for completion, gas cap and bottom water should be avoided. For example, if there is edge water in the reservoir, the oilfield development should fully utilize the driving effect of edge water to avoid shooting through the oil-water transition zone. Chapter 1 has explained that the selection of completion methods must be based on the characteristics of oilfield geology and reservoir engineering. This section discusses the selection of completion methods from another perspective for the current two types of completion: vertical wells, directional wells, and horizontal wells, combined with the characteristics of oilfield geology and reservoir engineering to address certain issues. The main factors to consider when selecting the completion method are shown in Figure 5. [3]

1、 Selection of vertical well completion method

The vertical well completion method has been the basic completion method since the development of petroleum at home and abroad, and it will continue to be so in the future. Vertical well completion has a wide range of adaptability, simple process technology, short well construction cycle, and low cost. According to the lithology of oil and gas well formations, they can be divided into three categories: sandstone, carbonate rock, and other lithologies, all of which can be completed by vertical well completion.

1. Sandstone oil and gas reservoir

The process diagram for selecting completion methods for sandstone oil and gas reservoirs is shown in Figure 6.

1) Sandstone is divided into layered, blocky, and lithological reservoirs. In terrestrial sedimentary strata, layered oil layers account for a large proportion. The physical properties, crude oil properties, and pressure system of block shaped or lithological oil reservoirs are generally consistent, so the completion method does not require special consideration. However, for layered oil reservoirs, especially when multiple layers are co produced in the same well, the completion method should be carefully considered. The first thing to consider is the pressure and production differences between different layers. If the differences are not significant, the same well can be used for joint production. If the differences are large, especially the interlayer pressure differences, due to the large interlayer deflection, the oil in the high-pressure layer will be injected into the low-pressure layer, and the production of multiple layers is actually lower than that of a single layer. In this case, single layer production should be adopted; But sometimes the abundance of reserves in a single layer is not sufficient for separate extraction. In this case, dual tube oil production can only be used in the same well, with each tubing string extracting one layer to eliminate interlayer interference and ensure normal production of both layers.

Although dual tube oil production solves the problem of interlayer deflection, its use has limitations. During dual tube oil production, the self injection period of the layers extracted by the two tubing columns may not be the same, and one of the layers may stop spraying first, which poses a problem of artificial lifting replacement. Due to the diameter limitation of the casing, both sets of rod pumps or electric submersible pumps cannot be simultaneously lowered into the wellbore due to the large tool diameter. If gas lift replacement is used, both sets of gas lift tubing strings can be lowered into the wellbore at the completion of the well. When one tubing stops spraying, gas lift replacement can be immediately used to maintain the advantage of dual tube production. Otherwise, in the early stage of oilfield development, dual tube production will occur, and later it will become single tube production. However, dual tube production must meet the following three conditions.

(1) Technical casing ≥ 95/8in, hanging liner ≥ 7in - two oil pipes can be inserted.

(2) Having natural gas resources is sufficient to provide gas lift oil recovery - it can take over production in a timely manner after shutdown.

(3) Production string and tools for gas lift oil production with two oil pipes inserted during well completion - regardless of which one of the two oil pipes is used

Even if the oil pipe stops spraying, gas lift can be used (because during double pipe oil production, it is unlikely to kill the well and replace the downhole tubing string when one oil pipe stops spraying).

Due to the special requirements of dual tube oil production mentioned above, it is limited to offshore oil wells and ultra deep wells on land. Due to its high single well production, underground operations are not carried out for a long time. No measures will be taken to increase production for large and medium-sized enterprises. Most of the deep and medium deep wells in onshore oil fields do not use dual tube oil production because the diameter of the casing produced by single tube oil production is relatively small, the production cost is low, the well construction cycle is short, and the method of stopping injection and switching to manual lifting can be selected according to needs. If the interlayer deflection is large, two sets of layer systems can be developed. Although an additional well is drilled, single well production management is convenient, and downhole operations and production measures are easy to implement.

2) Sandstone reservoirs can be classified into thin oil and heavy oil based on crude oil viscosity. The characteristics of terrestrial sedimentary formations are multiple layers, low permeability, and low formation energy. Most thin oil requires water injection to supplement the energy development of the formation, and multiple layers need to undergo fracturing and stimulation measures. This type of sandstone reservoir should only be completed by casing perforation, and should not be completed by naked eye or slotted liner, because naked eye or slotted liner completion cannot achieve layered water injection or layered fracturing.

As for sandstone heavy oil reservoirs, regardless of whether they are ordinary heavy oil or special or ultra heavy oil, the oil layers are mostly composed of clay and crude oil cementation, with loose cementation and sand production during the production process. Therefore, sand control measures must be taken. The sand control methods have been explained in the first section of this chapter and can be selected according to specific situations. In addition, it must be emphasized that thick oil wells should use large-diameter casing with a casing diameter of ≥ 7 inches Due to the high viscosity and flow resistance of heavy oil, it is necessary to use large-diameter casing to run large-diameter oil pipes. Chapter 3 of this book specifically discusses the issue of casing diameter in heavy oil wells, and it is repeatedly discussed.

Most of the ordinary heavy oil in sand layers is developed by water injection, such as Shengli Gudao, Gudong, Chengdong, and Shengtu Tutuo oil fields. By using casing perforation, layered control can be achieved, and resin sand fixation method can be used in injection wells; Resin sand fixation, sand control filter tube, or wire wrapped screen tube gravel filling can be used for sand control in production wells. The development experiments of the above-mentioned oil fields from the 1970s to the 1980s have proved that this completion method is suitable.

As for the ultra heavy oil in sandstone layers, steam injection is used for production. The Gaosheng Oilfield in Liaohe River is a thick and thick oil layer with gas cap and bottom water, with a thickness of 60-80mm. In the early stage, it was completed by naked eye and filled with gravel through wire mesh tube for sand control. Later, it was difficult to control the gas cap and bottom water through naked eye completion, and it was also difficult to adjust the steam absorption profile, so it was replaced by casing perforation. As for some layered or thin interbedded thick oil layers, such as the Huanxiling, Shuguang, Henan Jinglou and other oil fields in Liaohe River, as well as the sandstone and gravel oil layers in Shengli Le'an Oilfield, they were completed by casing perforation, filled with gravel through wire mesh tube or sand filter tube for sand control. The completion of the above oil fields has.

Regardless of the type of sandstone reservoir, if it is a low-permeability reservoir, fracturing stimulation measures are required; If it is a high permeability reservoir, the oil layer is loosely cemented and prone to collapse or sand production, sand control is necessary. Furthermore, thin oil reservoirs require water injection for development, while heavy oil reservoirs require steam injection for production. Moreover, it is necessary to control and adjust the water absorption, oil recovery, and steam absorption profiles layer by layer. Therefore, it is advisable to use casing perforation for completion. As for some single oil layers, without gas cap and bottom water, with moderate permeability, relying on natural energy extraction, without fracturing and production measures, it is also feasible to use a lower cut liner for well completion.

As for sandstone gas reservoirs, most of them are tight sandstones with low permeability, and fracturing measures must be taken to increase production, especially for some bottom water gas reservoirs. To prevent bottom water coning, casing perforation should be used instead of naked eye perforation. [2]

2. Carbonate reservoir

The selection process for completion of carbonate oil and gas reservoirs is shown in Figure 7.

Carbonate reservoirs can be divided into porous and fractured reservoirs or dual medium reservoirs of fractures and pores according to their permeability characteristics. For example, the pseudo creep limestone in Shengli Chunhua Oilfield is a porous reservoir, while the Wumishan reservoir in Renqiu Oilfield, North China is a dual medium reservoir mainly composed of fractures and matrix pores. Porous oil reservoirs can be completed just like sandstone oil reservoirs, as they require acidification or fracturing to increase production. Therefore, casing perforation completion is often used. Fractured or dual medium reservoirs with fractures and pores, such as the ancient buried hill reservoir in Renqiu Oilfield in North China, have gas cap and bottom water. In the early stages of development, open hole completion was adopted, and a set of open hole packers was developed for water plugging and acidification measures, but it is not as reliable as downhole action measures in casing., Later, casing perforation was used to complete the process, which greatly improved the effectiveness of controlling gas breakthrough and bottom water coning, as well as implementing acidification measures. However, if such reservoirs do not have gas cap and bottom water, open hole completion can still be used.

There are two types of carbonate gas reservoirs, just like oil reservoirs. For example, the Moxi gas field in Sichuan belongs to porous gas reservoirs, and the Jingbian gas field also belongs to this type. However, most other gas fields in Sichuan belong to fractured gas reservoirs. Most of these two types of gas reservoirs have bottom water, and porous gas reservoirs can be treated just like well completion for porous reservoirs. The measures for increasing production are the same as those for oil reservoirs, which require acidification or fracturing acidification, so casing perforation completion is often used. Fractured gas reservoirs with bottom water also require acidification and bottom water control measures, so casing perforation completion is recommended, and sometimes open hole completion can also be chosen.

3. Oil reservoirs such as igneous and metamorphic rocks

This type of oil reservoir refers to volcanic rock, andesite, volcanic rock, granite, gneiss and other oil reservoirs. These types of oil reservoirs belong to secondary buried hill oil reservoirs, which are formed by the migration of crude oil from the source layer to the fractures or pores of the above-mentioned rocks. These types of oil reservoirs are usually hard rocks and can be completed as fractured carbonate reservoirs. The process diagram for selecting completion methods for igneous and metamorphic rocks is shown in Figure 8. [3]

2、 Selection of Completion Methods for Horizontal and Directional Wells

Figure 9

Figure 9

The selection of horizontal well completion methods can be roughly divided into two categories. The process diagram for selecting the completion method for horizontal wells is shown in Figure 9.

1. Select completion method based on curvature radius

Horizontal wells with short curvature radii are currently mostly completed with naked eyes. It is mainly completed in oil reservoirs with hard vertical fractures, such as the Cretaceous strata in Austin, USA, or tight fractured sandstones, because these strata are not prone to collapse and can still maintain normal production despite being exposed to the naked eye.

Horizontal wells with medium and long curvature radii can choose completion methods based on factors such as lithology, crude oil properties, and production measures. The development of horizontal well technology is very fast nowadays, and the horizontal sections of horizontal wells are constantly increasing. In these long horizontal sections, especially in sandstone, the formation is inevitably prone to collapse during the production process. Therefore, it is not suitable to use open hole completion. Usually, slotted liner with casing external packer (ECP) completion or casing perforation completion is used.

2. Select completion method based on mining method and production increase measures

For heavy oil extraction, Canada extensively uses horizontal well steam injection to extract heavy oil in the SASKATCHEWAN area. The completion method is mostly completed using slotted liners, followed by the use of metal fiber or ceramic sand filters or other methods to prevent sand. As mentioned earlier in this section, the thick oil layer is loosely cemented and prone to collapse, making it unsuitable for open hole completion. [3]

For some horizontal wells in low-permeability oil reservoirs, fracturing measures are required, so only casing perforation can be completed. Even if a slotted liner and casing outer packer are used for completion, due to the long separation interval (length 100-200 or longer), only small-scale acidification measures can be carried out, and fracturing measures cannot be carried out. On the other hand, high-speed sand carrying fracturing fluid can enlarge or damage the gaps of the cutting pipe.

As for the selection of completion method for directional wells, since the inclination of directional wells is generally around 50 °, their completion method is basically the same as that of vertical wells.