Gravel pack sand control is the earliest and most widely used mechanical sand control method. There are two commonly used gravel filling methods: one is the bare hole gravel filling used for open hole completion; The second is gravel filling inside the casing used for perforation completion. The filtration area of bare eye gravel filling is large, the gravel layer is thick, the sand control effect is good, the effective period is long, and the impact on oil reservoir productivity is small. Commonly used for early sand control in oil wells, the process is complex and requires a certain strength of the oil layer structure, with high requirements for oil layer conditions (such as a single oil layer with large thickness, no gas or water interlayers). In other cases, casing perforation is used after completion, followed by gravel filling inside the casing.

The construction design of gravel filling for sand control should comply with three basic principles: first, pay attention to the sand control effect, select the correct sand control method, and design the process parameters and steps reasonably to achieve the goal of preventing sand production from the oil reservoir; The second is to adopt advanced process technology to minimize its impact on oil well production capacity; The third is to focus on comprehensive economic benefits, improve design quality and construction success rate, and reduce costs. Sand control design should form a complete set of procedures, which is conducive to the systematization and standardization of the plan, thereby improving the quality of construction design. The general procedure is: filling method selection ->formation pretreatment design ->gravel design ->sand control column design ->sand carrying fluid design ->construction process design.

1) Selection of filling method

Select the appropriate gravel filling method based on the characteristics and design principles of the sand control reservoir and oil well, combined with the completion type.

2) Formation pretreatment design

Based on the analysis and testing results of oil reservoir sand samples and the specific situation of sand control wells, measures such as acidification and clay stabilization treatment are determined, while considering issues such as anti emulsification and prevention of new sedimentation. This step is of great significance in improving the success rate of construction and ensuring the production capacity of oil wells.

3) Gravel design

Gravel design mainly includes determining the size of gravel, controlling the quality of gravel, and determining the amount of gravel used.

(l) Selection of gravel size

After obtaining the median particle size d50 of the sand sample from the sand control well reservoir through screening experiments, the required gravel size, i.e. the median particle size D50 of the gravel, is calculated based on the formula. Currently, the Saucier formula is widely used

D50=(5-6) d50

This formula was obtained based on a large number of experiments, and the relationship curve between the gravel/sand particle size ratio and permeability measured in the experiments is shown in Figure 8-6. Figure 8-7 is a schematic diagram of the mechanism of gravel sand blocking. (a) in the figure indicates that when D50/d50<6, the interface between gravel and reservoir sand is clear, and gravel blocks the reservoir sand, resulting in no sand production in the oil and gas well; In the figure (b), it indicates that when 6<D50/d50<14, the sand part of the oil layer invades the gravel filling layer, causing gravel/sand mixing and a decrease in permeability in the gravel area. Although the oil and gas well does not produce sand, production decreases; When D50/d50>14, (c) in the figure indicates that the reservoir sand can freely pass through the gravel filling layer, making sand control ineffective.

(2) Gravel quality

To meet the requirements of sand control technology, the quality requirements for gravel mainly include: uniform particle size of sand and gravel; Good roundness and sphericity; The solubility in standard soil acid is less than 1%; The turbidity of the gravel sample after stirring in water shall not exceed 50 degrees; Microscopic observation did not reveal two or more crystalline blocks of particles; Meet the requirements of anti crushing test.

(3) Filling gravel dosage

The amount of gravel used for gravel filling sand control should be determined based on the volume of the filling area. In order to ensure construction quality, sufficient additional quantity should be considered when designing the dosage to improve its sand control effect.

4) Design of sand control column

Sand control construction columns usually include filling tools, production screens, signal screens, light pipes, stabilizers, etc.

(1) Wire winding sieve tube

Generally, fully welded stainless steel wire wound sieve tubes are selected as shown in Figure 8-9. It has anti-corrosion properties, long lifespan, high temperature resistance, large flow area, narrow external and trapezoidal internal screen gaps, and has a self-cleaning effect. The minimum screen gap that can be processed at present is 0.1mm. The above advantages are the reasons for their widespread application in industry, while the disadvantage is the high cost, which is equivalent to 2-3 times that of slotted lining pipes of the same diameter.

The design of the gap size of the sieve tube is based on the principle of blocking the smallest gravel particle size, and is considered to be 1-2/3 of the smallest filling gravel particle size.

The principle of determining the diameter of the sieve tube is to increase the flow area as much as possible while ensuring sufficient radial thickness of the gravel filling layer to ensure the sand blocking ability and stability of the filling layer. For open hole filling wells, the radial thickness of the gravel filling annulus should be greater than 50mm; The filling well inside the pipe should be greater than 25mm.

The design length of the sieve tube should exceed the upper and lower boundaries of the perforation section of the production layer by 1.0-1.5m each; The length of the screen tube for open hole completion should exceed the upper and lower boundaries of the expanded production layer by more than 1 meter, in order to ensure that the screen tube is aligned with the production layer, achieve the maximum utilization rate of the screen tube, and improve the sand control effect.

(2) Signal screen tube

Signal screen tube is a section of screen tube set up according to the needs of ground construction. For conventional low-density cyclic filling methods, an upper signal sieve tube is installed; For high-density extrusion filling methods, a lower signal screen tube is installed; The bottom signal screen tube must pass below the bottom boundary of the oil layer by more than 3 meters. Its function is: when the signal screen tube is filled with gravel and buried, the ground pump pressure will quickly rise, giving construction personnel a signal to timely move on to the next construction process. The gap and diameter size of the signal screen tube are the same as those of the production screen tube, and the length is generally 1-2 meters.

(3) Optical tube

The light tube on the sand control column is made of ordinary oil pipe or casing, located between the production screen tube and the signal screen tube, or between the production screen tube and the filling tool. Its function is to reserve a portion of filled gravel in the annular space between the light tube and the wellbore, because after the construction is completed, the filled gravel will be compacted or enter the oil layer through the borehole, causing the height of the gravel filling to sink. These reserved gravel play a supplementary role to prevent the production screen tube from being exposed and causing sand control to fail.

The diameter of the light tube is the same as the center tube diameter of the wire winding sieve tube. The length of the light tube depends on the filling method. For low-density cyclic filling, the length of the light tube is generally 20-30 meters; For high-density extrusion filling, the length of the light tube shall not be less than the length of the production screen tube. The light pipe section for open hole completion should not be designed inside the upper technical casing, but only within the open hole section. This way, the annulus between the light pipe and the wellbore can store more gravel.

(4) Stabilizer

The sand control construction column should use a stabilizer to ensure that the column is centered inside the wellbore, evenly covering the screen tube with filled gravel, and forming a reliable sand barrier. Generally, the centering degree of the pipe column is required to be no less than 67%, and a sufficient number of stabilizers need to be installed on the pipe column. For vertical wells, the spacing between centralizers should be 5-8m. For directional wells with a deviation greater than 45 °, the spacing between centralizers should be less than 3m. Moreover, the larger the inclination angle, the shorter the spacing between the stabilizers.

(5) Filling tool

According to the needs of the filling process, the commonly used gravel filling tools can be divided into three structures: one is the downward flushing filling tool. The process requires that the gravel is first put into the sand control wellbore, and then a sand control pipe column with a downward flushing nozzle is lowered. Working fluid is pumped into the oil pipe to impact the gravel and suspend it in the wellbore. The pipe column is gradually lowered, and then the gravel settles around the pipe column to form a sand barrier; The second is a reverse circulation filling tool, where the working fluid and mortar are pumped from the oil sleeve annulus into the bottom of the well, gravel remains around the screen tube, and the returned liquid enters the screen tube and returns to the ground from the oil pipe; The third is the conversion filling tool, which pumps the working fluid and mortar from the oil pipe into the bottom of the well, and uses the conversion tool to make the return liquid return from the oil sleeve annulus to the ground.

The downward flushing filling process has a simple tool, but it is prone to gravel size classification and is only suitable for shallow wells and thin layers; The reverse circulation filling process has a relatively simple tool, but the mortar is easily contaminated by debris on the inner wall of the casing, resulting in a decrease in the permeability of the filling body. Moreover, the casing column must withstand the filling pressure, with low sand carrying liquid flow rate and large liquid consumption. Therefore, it should be used as little as possible; Converting filling tools can avoid contamination of mortar, and the casing does not have to withstand filling pressure. The sand carrying fluid flow rate is high and the amount of liquid used is small, making it an ideal filling method. However, its tool structure is complex, there are many types, and the operation is difficult.

5) Selection of sand carrying fluid

The sand carrying fluid used for gravel filling requires the same requirements as other completion and repair fluids, which must not cause damage to the oil reservoir but also achieve the application purpose. Therefore, it has standards in terms of solid particle content and particle size, compatibility with oil reservoir production fluid and oil reservoir minerals, and non clogging of oil reservoir pores by reaction products. The type and amount of sand carrying fluid should be selected and designed according to the specific situation and process requirements of the oil well.

6) Construction process design

The construction process design includes various construction steps, construction process parameters, and corresponding technical requirements throughout the sand control process, as well as the specifications and quantities of construction equipment, as well as the models and quantities of construction equipment. The conventional gravel filling process inside the wellbore is as follows:

(1) Sand prevention preparation: Base fluid filtration ->acid preparation ->well killing ->perforation ->flushing of the wellbore ->wellbore scraping and hot washing.

(2) Pre treatment: Lower the pipe column ->Pressure test (to determine absorption capacity) ->Squeeze in pre liquid ->Squeeze acid liquid (or clay stabilizer) ->Pump in displacement liquid ->Close in reaction ->Pull out the pipe column.

(3) Gravel filling: lower sand control pipe column ->well washing (positive or reverse washing) ->sand addition; Sand pump ->Pump in replacement fluid ->Wash well ->Release tool and pull out pipe string.

(4) Production: Lower the production string and open the well for production.

During the construction process, relevant underground operation regulations and safety regulations should be strictly followed, and strict technical supervision should be carried out to ensure the quality and effectiveness of sand control construction.