The so-called completion method refers to the connection between the wellbore of an oil and gas well and the oil and gas reservoir, as well as the wellbore structure, wellhead equipment, and related technical measures adopted to achieve a specific connection.

The completion methods adopted for different regions, oil and gas reservoirs, and types of oil and gas wells are different. The general principle for selecting completion methods should meet the following requirements: effectively connecting the oil and gas reservoir with the wellbore, and minimizing the resistance of oil and gas flowing into the well; It can effectively isolate oil, gas, and water layers without mutual interference, and meet the requirements of layered exploitation and management for multiple oil and gas reservoirs. Being able to overcome or reduce the impact of wellbore collapse and sand production in oil and gas reservoirs, ensuring long-term stable production of oil and gas wells, and using well completion methods that are as simple and cost-effective as possible.

According to the geological characteristics of the production layer, different completion methods are adopted:

1. Perforation completion method

The most widely used method is to drill through oil and gas layers, insert casing into the oil layer, and after cementing, perforate the production layer.

Perforation completion is the most widely and mainly used completion method both domestically and internationally. This includes casing perforation completion and tail pipe perforation completion.

(1) Casing perforation completion

Casing perforation completion is the process of drilling through the oil reservoir until the designed well depth, then running the oil reservoir casing to the bottom of the reservoir and cementing it with cement. Finally, perforation is carried out, and the perforating bullet penetrates the oil reservoir casing, cement sheath, and a certain depth of the oil reservoir, establishing a channel for oil flow. As shown on the left in Figure 1.

Casing perforation completion can selectively open oil layers with different pressures and physical properties to avoid interlayer interference, as well as avoid interlayer water, bottom water, and gas cap, and prevent interlayer collapse. It has the conditions for implementing layered injection, production, and selective fracturing or acidification operations.

(2) Tail pipe perforation completion

Tail pipe perforation completion is achieved by drilling the drill bit to the top boundary of the oil reservoir, injecting cement into the technical casing, and then using a smaller drill bit to penetrate the oil reservoir to the designed well depth. The tail pipe is then lowered and suspended on the technical casing using a drilling tool. The overlap section between the tail pipe and the technical casing is generally not less than 50m Inject cement into the tail pipe for well cementing, and then proceed with perforation.

Tail pipe perforation completion can use drilling fluid compatible with the oil reservoir to drill through the oil reservoir by balancing pressure and low equilibrium pressure, as the upper formation has been sealed by technical casing before drilling, which is beneficial for protecting the oil reservoir. In addition, this completion method can reduce the weight of casing and the amount of cement used in the oil well, thereby reducing the completion cost. Currently, most deep oil and gas wells use this method for completion. [2]

2. Naked eye completion method



The well completion method involves cementing the casing down to the top of the production layer and exposing the production section. This method is commonly used for carbonate rocks, hard sandstones, and oil reservoirs with good cementation and simple stratigraphic layers. The advantage is that the exposed area of the production layer is large, and the resistance of oil and gas flow into the well is low, but it is not suitable for multiple oil layers with different properties and pressures. There are two completion procedures for open hole completion based on the timing of drilling the production layer and casing insertion:

One is to drill the drill bit near the top boundary of the oil reservoir and then use technical casing to inject cement for well cementing. After the cement slurry returns to the predetermined design height, a smaller diameter drill bit is inserted from the technical casing to drill through the cement plug and open the oil reservoir to the design well depth for completion.

Some thick oil layers are suitable for naked eye completion, but when there is a gas cap in the upper part or a water layer near the top boundary, the technical casing can also be lowered through the oil and gas interface to seal off the upper part of the oil layer and then completed naked eye. If necessary, the oil-bearing section can be opened again, which is called the composite completion method in foreign countries. As shown in Figure 2.

Another process of open hole completion is to drill through the oil reservoir directly to the designed well depth without replacing the drill bit, and then run the technical casing near the top boundary of the oil reservoir and inject cement for well cementing. During well cementing, in order to prevent the cement slurry from damaging the oil layer below the casing shoe, sand is usually placed in the oil layer section or replaced with low water loss, high viscosity drilling fluid to prevent the mud from sinking. Alternatively, an outer casing packer and water injection joint can be installed at the bottom of the casing to support the cement slurry in the annulus and prevent it from sinking. This type of completion process is generally not used.

The main characteristic of naked eye completion is that the oil reservoir is completely exposed, thus the oil reservoir has the largest permeable area. This type of well is called a hydrodynamic perfection well, and its productivity is relatively high. Although naked eye completion has a high degree of perfection, its usage limitations are significant. Sandstone oil and gas reservoirs, as well as medium and low-permeability layers, mostly require hydraulic fracturing and cannot be carried out with bare eyes. At the same time, most sandstones contain shale interlayers, which are prone to collapse and block the wellbore when exposed to water. Carbonate oil and gas reservoirs, including fractured oil and gas reservoirs, such as many oil fields in the Middle East in the 1970s, ancient buried hill reservoirs in Renqiu Oilfield in North China, and Sichuan Gas Field, mostly use open hole completion. Due to the difficulty in implementing production enhancement measures and controlling bottom water coning and water plugging during open hole completion, as well as advances in perforation technology, it is now mostly completed through casing perforation. In the early stage of horizontal well development, in the early 1980s, most of the horizontal wells in the Cretaceous carbonate rock vertical fracture formation in Austin, USA, were completed with naked eye. Some horizontal wells in other countries also used naked eye completion, but in the late 1980s, they were mostly replaced by slotted liners or slotted liners with external packers. Especially with the current extension of horizontal well sections or the drilling of branching horizontal wells, the use of open hole completion is even less common. Because there are many technical issues that are difficult to solve in naked eye completion. [2]

3. Slotted liner completion method

Figure 3

Figure 3

(1) There are also two types of completion procedures for slotted liner well completion.

One is to drill through the oil layer with the same size drill bit, connect the lower end of the casing string to the part where the liner enters the oil layer, and seal the annular space above the top boundary of the oil layer through the casing outer packer and cement injection joint.

Due to the inability to repair or replace damaged downhole liners in this completion method, another completion process is generally adopted, which involves drilling the drill bit to the top boundary of the oil reservoir, first cementing the well with cement through the technical casing, and then drilling through the oil reservoir with a drill bit of a smaller diameter through the technical casing to the designed well depth. Finally, a pre cut liner is inserted into the oil reservoir, and the liner is suspended on the technical casing by the liner hanger (slip packer) at the top of the liner. The annular space between the liner and the casing is sealed, allowing oil and gas to flow into the wellbore through the cutting of the liner. As shown in Figure 3. This completion process will not cause damage to the oil reservoir by cementing slurry, and drilling fluids compatible with the oil reservoir or other drilling techniques that protect the oil reservoir can be used to drill through the oil reservoir. When the cutting liner wears or fails, it can also be repaired or replaced.

Figure 4

Figure 4

(2) Technical requirements for slotted lining pipes

The sand control mechanism of slotted liner allows small sand particles of a certain size that can be carried to the ground by crude oil to pass through, while blocking larger sand particles outside the liner, forming a "sand bridge" outside the liner to achieve sand control. As shown in Figure 4.

Due to the high flow velocity at the "sand bridge", small sand particles cannot stay in it. The natural sorting of sand particles enables the "sand bridge" to have good circulation capacity, while also protecting the sand skeleton of the wellbore. The shape and size of the slit eye should be determined based on the sand particle size of the skeleton.

4. Gravel filling completion method

Fill a certain size and quantity of gravel between the liner and the wellbore.

What we generally refer to as well completion refers to the completion method of oil and gas wells, which is to establish a reasonable connection channel or connection method between the oil and gas reservoir and the wellbore of the oil and gas well at the bottom of the well based on the geological characteristics of the reservoir and the technical requirements of development and exploitation.

And now the significance of well completion has expanded to include drilling completion and production completion. Production completion mainly refers to how to choose the tubing string, wellhead, etc. after drilling completion to achieve normal production of oil and gas wells.

For formations with severe cementation and loose sand, gravel filling completion method should generally be used. It first lowers the wire wrapped screen into the oil reservoir area of the well, and then uses filling fluid to pump the pre selected gravel on the ground into the annular space between the wire wrapped screen and the wellbore or between the wire wrapped screen and the casing to form a gravel filling layer, in order to block the flow of oil reservoir sand into the wellbore and achieve the purpose of protecting the wellbore wall and preventing sand from entering the well. Gravel filling completion generally uses stainless steel wrapped screen pipes instead of slotted liners. The reasons are as follows:

1) The seam width of the slotted lining pipe is limited by the strength of the cutting blade, and the minimum is 0.5mm Therefore, slotted liners are only suitable for medium and coarse sand reservoirs. The minimum gap width of the wire wrapped sieve tube can reach 0.12mm, so its applicability is much wider.

2) The wire wrapped sieve tube is a continuous gap formed by winding the wire, and there is almost no pressure drop when the fluid passes through the sieve tube. The cross-section of the wire wrapped sieve tube is trapezoidal, narrow on the outside and wide on the inside, with a certain "self-cleaning" effect. Slight blockage can be cleared by the produced fluid, and its flow area is much larger than that of the slotted liner tube.

3) The wire wrapped sieve tube is made of stainless steel wire as raw material, which has strong corrosion resistance, long service life, and high comprehensive economic benefits.

In order to meet the needs of different reservoir characteristics, both open hole completion and perforated completion can be filled with gravel, known as open hole gravel filling and casing gravel filling, respectively. [2]