Ye Chengming^1,2, Zheng Jitian², Ran Defa², Wang Jianzeng²

(1.      School of Engineering, China University of Geosciences, Wuhan Hubei 430074, China; 2. Institute of Hydrogeology and Engineering Geology Technique, CGS, Baoding Hebei 071051, China)

Abstract: With reliable effectiveness, relatively lower price, and easy installing, gravel-pre-coated screen has become the major screen used in wells in silt-fine-sand aquifers in many countries. In the 1970s, steel-based and plastic-based gravel-pre-coated screens were developed in China but not widely used due to its vulnerability of being jammed. The experimental research on double-layer gravel-pre-coated screen is aimed to solve the problem. Compared with conventional gravel-pre-coated screen, the double-layer one has a different structure: from the exterior layer to the inner layer, the gravel size diminishes. The structure was designed with some new ideas absorbed from the process of artificially well filling with gravels. The experiments show that the new structure greatly improved the permeability of the new screen as well as its ability to avoid being jammed.

Key words: double-layer gravel-pre-coated screen; experimental research;

1 Introduction

Gravel-pre-coated screen is made with filtering tube stick to certain grain filtering materials by synthetic resin. It has mane advantages for complete well through using it ,such as increasing drilling efficiency, improving well completion quality, simplifying well completion process, saving washing time ,as well as reducing the danger of sand boil in the process of well using. Gravel-pre-coated screen is one of the main well screen for silt-fine-sand aquifers in well abroad. In the 1970s, steel-based gravel-pre-coated screen were developed in China . In the 1990s,my institute has developed the plastic-based gravel-pre-coated screen based on the study of steel-based gravel-pre-coated screen. The structures of Gravel-pre-coated layers are both single-layer gravel-pre-coated .Since the success of development in china, it is widely used only in the second well completion while used less in new well completion due to the limitation of its, such as the lower inflow ability of gravel-pre-coated screen and the easier blockage of gravel-pre-coated layer. The test study of gravel-pre-coated screen was developed to point against the recently disadvantages of gravel-pre-coated screen for improving the structure of gravel-pre-coated layer in order to increasing the permeability of gravel-pre-coated layer and preventing ability for blockage that based on the mechanism of pass-water and prevent-sand as well as the process of artificially well filling with gravels. It provides basic technique support for developing double-layer gravel-pre-coated screen. In all, we acquire satisfied result through the test study.

2 Study Methods and Procedure Summarize

The test study adopts mainly methods of single-double-layer gravel-pre-coated briquette indoor testing and compare testing analyze. The study procedure as follows:

a.       Four types of filtering material(5-3mm,3-2mm,2-1mm,1-0.5mm)are selected according to performance request for filtering sand of gravel-pre-coated screen. The performance request for filtering sand of gravel-pre-coated screen is described in Table 1.

Table 1 Characteristic of Gravel

Table 2 Feature of Text Block

b.       The single-double-layer gravel-pre-coated briquette are made in the same working condition. The double-layer gravel-pre-coated briquette consists of bottom layer and surface layer. The surface layer is made with filtering materials suited for aquifer. The bottom layer is made with filtering material coarser than surface layer. The structure, size and quantity of single-double-layer gravel-pre-coated briquette are described in Table 2.

c.       The permeability testing and of single-double-layer gravel-pre-coated briquette are developed. The permeability testing involves porosity testing and permeability coefficient testing. The blocking test involves slurry blocking test and filtrate-prevent sand test.

d.       The testing and test results are be contrasted-analyzed.

3 Testing and Test Methods

3.1 Testing methods for porosity of single-double-layer gravel briquette

The testing of porosity for single-double-layer gravel-pre-coated briquette adopts the briquette method. The briquette is placed in a container loaded with certain water, and measured the volume of increasing water B. The porosity is calculated according to the following equation:

η＝＝×100％

where η is the porosity （％）,A is total volume of briquette (cm³),B is the volume of solid part of briquette (cm³),and m is the porosity volume of briquette (cm³).

The porosity testing are conducted for all single-double-layer gravel-pre-coated briquette listed in Table 2. A highest vale and a lowest valve in the testing result is taken out, and the medium porosity valve of remained four briquettes is used as the medium porosity of the briquettes .

3.2 Testing method for permeability coefficient of single-double-layer gravel-pre-coated briquette

The testing work is conducted only at the testing instrument which measured permeability coefficient of civil engineering testing machining by oneself .The effects to the contrast analyze of permeability coefficient of double-layer gravel-pre-coated layer is very little though the acquired parameter is probably not accurate. The permeability coefficient of briquettes are calculated according to equation ① under the water temperature testing condition, and the permeability coefficient under the standard water temperature (20℃) is calculated according to equation ② :

k_T=2.3× ①

where k_T is the permeability coefficient （cm/s）;α is the cross section area of change head pipe （cm²）;L is the thickness of briquette(cm）;2.3 is the alternated factor of ln and log ;t_1,t₂ are the origination time and ending time of measured-readed head, respectively（s）;H₁,H₂ are the origination and ending head（cm）,and A is the volume of briquette（cm²）.

k₂₀= k_T×η_T/η₂₀ ②

where k₂₀ is the permeability coefficient of briquette under standard temperature (cm/s);η_T is the coefficient of dynamic viscosity of water under T℃（kPa.s）,and ₂₀ is the coefficient of dynamic viscosity of water under 20℃.

As the briquettes are doused by water when testing the porosity, All briquettes are placed into electric oven under 120℃ for 24 hours before testing in order to ensure the compared property of testing data. Three briquettes are taken out randomly from each type of briquette for permeability coefficient testing. The testing of each briquettes is duplicate three times, and the medium valve of three testing results is used as the permeability coefficient of the briquette. And then, the medium valve of the permeability coefficient of three briquettes are used as medium permeability coefficient of the briquette.

3.3 Comparison of two different experiment methods of single-layered and double-layered gravel-pre-coated screen

The ①，⑧，⑨，⑩ test block in table 2 doesn’t involved in the experiment. And the others is divided into two groups. The mud jamming test and the sand jamming test are taken separately. The experiment facility is shown in Fig 1.

3.3.1 Methods of the mud jamming comparison experiment

The feature of the mud is presented as follow, viscosity is 20s, gravity is 1.05, and composition of sand is 20%. Fulfill the mud tank with mud, and use high pressured nitrogen forcing the mud coming through the test block, and the pressure is 3.5 kg/cm2. And the time needed by the mud flowing through the test block should be recorded.

          3.3.2 Methods of the Sand jamming comparison experiment

First we fill the mud tank with water, then we put sand on top of the test block. The volume of the sand and it’s mixed ratio should be no different in any test block. Next we use the high pressured nitrogen forcing the clean

water coming through the test block, and the pressure is 3.5 kg/cm². And the amount of sand getting through the block should be measured.

4 Experiment Results

The result of the experiment is presented in Table 3.

Table 3 Experiment Results

5 Comparison Analysis

5.1 Comparison results of single and

double layered gravel screen

The performance comparison of single and double layered gravel screen is taken in the same aquifer, and the result is presented in Table 4.

Table 4 Result of Comparison in Single and Double Layered Gravel Screen

5.2 Result analysis

From the table 4, it can be seen that the double layered gravel screen is better than the single one in both the Permeability and the anti-jamming ability. Here are the reasons: first, it increases the comprehensive Porosity of the gravel; second, it increases the permeability by

the decrease of anti-pervious capability. The sickness of the double layered gravel is only half of that in the single one, and it has the larger diameter-grain in the underlying layer which also increases the Permeability. Third, the trumpet-shaped structure enhances it’s anti-jamming ability. Water pass-way in the double layered gravel is large in the bottom and narrow in the external, so that, it enables the inner fine grain getting out easily. And it reduces the risk of jamming of the filter. On the other hand, thinner layer makes the grain moving route shorter, and it deduces the retention time so that it creates good condition for discharging grain.

Because of unsuitable working processing, the 3-2/2-1mm style double layered filter doesn’t work well. Onetime forming way is used in the process of the test block, that’s to say, it makes another gravel layer on top of the first one without waiting solidification of the first layer. And it causes a density area in the edge of two layers. And it causes quickly jamming of the filter by the grain in a short time.

6 Molding Process of the Double Layered Gravel Filter

The Molding process of the double layered gravel filter has little different with the single one. The different is that the molding process of the double layered gravel filter needs two times molding. The major molding process is presented as below: