In the drilling technology of extra-thick salt-gypsum layer, through the research on the three-dimensional creep pressure change law of salt-gypsum layer, the dissolution rate of salt-gypsum layer and the uneven stress on casing, we have made new understanding and breakthrough in reasonably determining drilling fluid system and density, accurately designing casing strength and comprehensive supporting technical measures, and made remarkable progress in drilling salt-gypsum layer safely and efficiently.
3.3.5. 1 Creep Law of Salt-gypsum Layer
Creep characteristics of (1) salt rock
In the typical salt rock creep curve (Figure 3- 120), creep can be divided into three stages. "A" means that the first stage is a transient creep period, and before reaching the next stage, the creep strain rate of salt rock in this stage gradually decreases, showing nonlinearity; "B" means that the second stage is a steady-state creep period, and the creep strain rate of this stage remains constant and linear; "C" indicates that the third stage is the accelerated creep stage, in which the strain rate increases until the specimen is destroyed, which is nonlinear.
Figure 3- 120 Typical Creep Curve of Salt Rock
For plastic materials such as salt rock, creep mainly shows two stages: "A" and "B", and the "B" stage lasts for a long time. For petroleum engineering, salt gypsum mainly shows two stages: transient creep and steady creep, which are mainly affected by steady creep after drilling and casing cementing.
(2) Creep equation of salt-gypsum layer
For a specific salt rock, studying its rheological characteristics is to determine the relationship between steady-state creep rate and temperature and pressure, that is, creep equation. Creep mechanism and creep equation of salt rock are related to temperature and pressure. There are many creep models of salt rock, mainly exponential and power law. Several main models reflecting the creep of salt gypsum rock are as follows.
1) power law model. The model is a purely empirical formula, and the relationship between transient creep and stress, temperature and time is expressed as follows:
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Where: εp is transient creep strain; σ is the differential stress; T is temperature; T is time; M, p and n are the indices of stress, temperature and time, respectively.
If you describe the general strain law, you should also add a steady-state term, namely:
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Where ε is the total strain; Is the steady-state strain rate. It can be expressed by Weertman dislocation slip mode:
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Where: q is the activation energy; R is the ideal gas constant; β is the stress coefficient (determined by experiment); A* is the test constant.
The power rate model explicitly expresses the relationship among stress, temperature, time and strain. The model is simple and has certain guiding significance for engineering practice, but the rheological law of salt rock is rough, so it is rarely used now.
2) Exponential temperature law. Senseny P.E et al. proposed it in 1983 to describe the rheological law of salt rock in Avery Island at high temperature (more than half of the melting temperature), and its specific expression is:
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Where: b and λ are test constants; Other symbols are the same as above.
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Power rate model and temperature index model are simple in expression and easy to use, but there are many defects, such as sometimes the steady-state creep rate is negative in data regression, which is inconsistent with the reality and cannot reflect the complex stress and temperature history well.
Through a large number of creep tests, Professor Zeng Yijin and Professor Yang Chun have studied and obtained the creep constitutive equation of salt-gypsum layer under three-dimensional conditions considering the influence of temperature:
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The last factor considers the influence of temperature. At constant room temperature, it can be expressed as:
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Among them, A2*, N, A 1, B 1*, B2 can be obtained through the creep test of the core.
(3) Calculation and analysis of creep pressure of salt-gypsum layer.
Calculation method of creep pressure of 1) salt gypsum layer. FLAC3D finite difference calculation software compiled by explicit finite difference method provides the function of simulating the creep characteristics of materials, that is, the material characteristics change with time. In FLAC3D calculation and analysis, the main difference between creep model and other constitutive models lies in the simulation of time problem.
2) Creep pressure analysis of salt-gypsum layer.
A. Creep pressure analysis of salt gypsum layers with different well depths. The buried depth of salt-gypsum layer has great influence on creep pressure. With the increase of the depth of gypsum layer, the creep pressure increases significantly; With the passage of time, the creep pressure tends to be stable, and finally it is the same as the overlying formation pressure; The deeper the salt-gypsum layer is buried, the shorter the time for the creep pressure to stabilize.
B. Creep pressure analysis of salt gypsum rock under cement retaining walls with different thicknesses. The creep pressure of salt-gypsum layer under the condition of retaining wall cement ring with certain depth and different thickness is analyzed. It is concluded that the thickness of cement sheath has an influence on the initial stress state of casing, but it is not significant.
C. creep pressure analysis of salt gypsum rocks with different thicknesses. The influence of salt and gypsum layers with different burial depth and thickness on radial compressive stress, circumferential stress and vertical stress of casing is analyzed. It is concluded that the thickness of salt layer has obvious influence on the stress state of casing in the initial stage of salt layer creep, but with the passage of time, the stress state of casing in different thickness of salt layer tends to be consistent.
D. Creep pressure analysis of salt-gypsum layer at different temperatures. The influence of temperature on the stress state of casing is very significant. The higher the temperature, the higher the creep pressure, circumferential stress and vertical stress of the casing at the initial stage of salt paste creep. With the passage of time, the creep pressure and vertical stress of casing tend to be consistent, but the circumferential stress tends to be consistent slowly.
3.3.5.2 Drilling Fluid Density Design Technology
The determination of drilling fluid density is related to the creep characteristics of salt gypsum layer and the salt saturation of drilling fluid. The determination of reasonable drilling fluid density in salt-gypsum layer is based on formation characteristics and mechanical and chemical balance.
(1) drilling fluid density spectrum
Drilling fluid density diagram is the correlation curve between hole shrinkage and drilling fluid density in salt formation. According to the creep pressure and creep experiment, the shrinkage rate of salt formation with different well depths and drilling fluid densities is calculated by FLAC3D software, and the shrinkage rate is established by fitting the synthetic curve. The drilling fluid density map can also be drawn by FLAC3D software according to the measured creep rate data of salt-gypsum layer drilling. Fig. 3- 12 1 is a drilling fluid density chart based on the measured creep rate.
(2) Drilling dissolution rate of salt-gypsum layer
During drilling, drilling fluid will dissolve the underground salt layer. The research by Professor Zeng Yijin and Professor Deng shows that there is a good logarithmic correlation between the dissolution rate of salt rock and the salt content of drilling fluid at a certain temperature.
The curve of salt rock dissolution rate with [Cl-] at different temperatures further shows that the influence of temperature on salt rock dissolution rate is not a simple linear relationship, but there is a critical point when the salt content of drilling fluid is constant. When the temperature is lower than the critical point and the salt concentration of drilling fluid remains unchanged, the dissolution rate of salt rock will increase with the increase of temperature. When the temperature is higher than the critical point and the salt concentration of drilling fluid remains unchanged, the dissolution rate of salt rock decreases with the increase of temperature (Figure 3- 122).
Figure 3- 12 1 density spectrum of drilling fluid at different well depths corresponding to shrinkage rate
Fig. 3- 122 curve of salt rock dissolution rate with [C 1-] at different temperatures.
Similarly, according to the dissolution rate of salt rock, the regression curve of the correlation between salt content of drilling fluid and hole diameter enlargement rate at different temperatures can be obtained (Figure 3- 123).
(3) Determination of drilling fluid density and salt saturation
According to the regression curve and drilling fluid density diagram of the relationship between salt rock dissolution rate and hole diameter enlargement rate and [C 1-], considering the influence of creep and dissolution, the drilling fluid density and salt saturation are determined. Firstly, according to the used drilling fluid density, find out the corresponding creep rate from the drilling fluid density diagram (Figure 3- 12 1), and then determine the salt saturation corresponding to the borehole enlargement rate from the curve of [C 1-] (Figure 3- 123) to balance the creep rate.
In addition, according to the measured creep rate of salt-gypsum layer, the salt saturation of drilling fluid and the density spectrum of drilling fluid, the drilling fluid density required for safely drilling salt-gypsum layer in an area can be determined. The basic idea of this method is that the density of drilling fluid in use can be compared with the density spectrum to determine the shrinkage rate at this density, and the difference between the measured shrinkage rate and the calculated shrinkage rate can be used as the shrinkage rate to re-calculate the corresponding drilling fluid density, which is the drilling fluid density required for safe drilling.
Fig. 3- Regression curve between hole diameter enlargement rate and [C 1-] 123.
Matching Technology of Drilling in Salt-gypsum Layer in 3.3.5.3
(1) wellbore structure design scheme
Aiming at the deep well with super-deep salt-gypsum layer, it is the key to ensure the completion safety to effectively prevent casing collapse damage caused by creep of salt-gypsum layer. At present, there are two kinds of well structures commonly used in deep salt-gypsum layers (taking Tahe Oilfield as an example).
Figure 3- 124 Casing Strength Design Table under Non-uniform External Load
1) Special seal and special play. Run 244.5mm casing to the top of the salt layer about 5000 meters, and seal the salt layer with 206.3mm casing; φ 139.7mm liner is used for cementing in the lower salt cavern. Practice has proved that this scheme is feasible for wells with clear salt layer distribution or consistent pressure system under salt, but for wells with special well conditions and multiple drilling geological purposes, the selection of well diameter and borehole extension are limited. At present, the method of adding one level to casing program is usually adopted. This scheme is more suitable for production wells familiar with various situations.
2) The scheme of exposing the salt and gypsum layer with open holes. In order to ensure the realization of drilling geological tasks, the long open-hole drilling scheme for exposing the salt-gypsum layer is optimized, that is, the long open-hole drilling scheme with large-size holes is used to expose the salt-gypsum layer in the same hole as the overlying low-pressure formation, and the salt-gypsum layer is sealed with large wall thickness and high anti-collapse casing, and the salt-gypsum layer is suspended with φ 244.5 mm or φ 273.0 mm+φ 244.5 mm combined casing. Φ177.8 mm tail pipe is used in the lower section of the salt well, and the tail pipe overlaps with the salt roof100m; Unsaturated brine drilling fluid with a density of about 1.65g/cm3 is used to reveal the salt-gypsum layer and improve the bearing capacity of the formation. Adopt reaming while drilling or hydraulic reaming technology to ensure drilling safety in salt-gypsum layer.
Compared with special sealing drilling in salt formation, long open hole drilling is more risky, and the drilling technology is generally divided into two steps, namely, sub-salt drilling technology and salt formation drilling technology. The key of drilling technology under salt is to stop drilling immediately after drilling the salt layer and re-test the formation fracture pressure to determine the bearing capacity of the open hole interval. Take one-time plugging measures for open hole intervals at low pressure points. If the formation has (or has after plugging) the ability to bear high density when drilling in the salt-gypsum layer, switch the drilling fluid system suitable for drilling in the salt-gypsum layer, and then drill in the salt-gypsum layer. If the formation can't bear the high density when drilling in the salt layer after plugging, the scheme should be adjusted and changed to a special plugging drilling scheme in the salt gypsum layer.
The advantages of this scheme are as follows: firstly, multiple pressure systems are isolated; Secondly, the casing deformation in the salt zone is avoided by overlapping the liner. Thirdly, the wellbore structure is simplified, which makes the completion hole larger and provides an extra layer of spare casing space than the special drilling sealing scheme. This scheme is more suitable for exploratory wells.
(2) Casing strength design
The key of casing design in salt-gypsum layer is the calculation of collapse strength. In the past, the maximum creep pressure of salt-gypsum layer was generally used in casing design, that is, the overlying formation pressure, and the casing was subjected to uniform load. Casing shall be hollowed out by 40% and the safety factor 1. 125 or a larger safety factor according to experience. However, this method often causes casing deformation accidents in practical application, so the non-uniform external load must be considered in the casing design of salt-gypsum layer.
1) casing strength design drawing and its application. According to the casing strength design drawing, casing strength design under non-uniform external load can be carried out. If the elliptic distribution load and its axial ratio are known, it can be judged whether the casing is safe or what kind of casing is needed to resist this load. For example, given that K=0.4 and the area around the load is 5= 1690.0MPa2, then Pc=23.2MPa If P19mm (d/t =19.3) with a wall thickness of 9.1/kloc is selected. More than the maximum limit load (Pc/σ= 65433) For the convenience of application, the relationship between the ratio of equivalent failure load Pc to the yield limit of casing material Pc/σs and the ratio of casing diameter to thickness D/t is plotted as a curve, that is, the casing strength design version (Figure 3- 124). The strength curve of casing under radial load and uniform external load is also drawn in the figure. The equivalent load of radial load is defined as the concentrated force per unit diameter length. According to the casing strength design drawing, the salt layer casing can be designed.
2) Design steps of casing strength in salt-gypsum layer.
A. According to the rheological characteristics of salt layer, in-situ stress of salt layer and hydrostatic column pressure in the well during cementing, the variation law and distribution law of creep external load of salt layer in salt layer casing with time are calculated by viscoelastic finite element calculation program, and the final stable value of external load of casing is obtained. The magnitude and inhomogeneity of this stable value are expressed by the short axis B and the long axis A of Cassini elliptic function.
B. According to the values of B and A, calculate the area S and the axial ratio K of the load on the casing, and find out the actual equivalent failure load on the casing.
C according to k and Pr, as shown in fig. 3- 124, casing strength can be designed or tested.
A. Select casing wall thickness according to K, Pr and casing steel grade (σs): firstly calculate Pr/σs, and then according to the values of K and Pr/σs, the critical diameter-thickness ratio (D/t) of casing can be found in the drawing, and the minimum wall thickness of casing can be calculated.
B, selecting casing steel grade according to K, Pr and casing wall thickness: firstly, calculate Pc/σs(Pc is the maximum effective load that casing can bear) from the chart according to K and D/t values, then remove Pc/σs with actual load Pr to obtain the minimum yield limit σs required for casing, and select casing steel grade according to σs values.
C. If the used casing steel grade (σs is known) and wall thickness (diameter-thickness ratio D/t can be calculated) are known, check the safety of casing: firstly, get Pr/σs, and then get Pc/σs when casing is damaged from the diagram according to K and D/t values. If PC/σ s is less than PR/σ s, the casing strength is insufficient, which will lead to abnormal casing damage; If PC/σ s > PR/σ s, the casing is safe.
D. When designing casing strength, it is assumed that the internal pressure of casing is zero, that is, it is calculated as total hollowing. If the internal pressure of casing is not zero, its strength against uniform external pressure will be greatly improved. However, when the external pressure of casing is uneven, the increase of casing strength with the increase of internal pressure is not obvious.
(3) Reaming technology
1) A scheme combining reaming while drilling and reaming after drilling. The upper layer of salt-gypsum layer is drilled with φ 3 1 1. 15 mm bit, and φ24 1.3mm pilot bit eccentric reaming tool is used 60m above the top boundary of salt-gypsum layer, with reaming size of 374.65 mm.
2) Hydraulic reaming scheme after drilling. First, drill with φ 3 1 1. 15 mm bit. After drilling through the salt paste layer, expand the salt layer with a hydraulic reamer. It is required that the average aperture be enlarged to φ 349.25mm..