Effect Of The In-Situ Stress Field On Casing Failure * Tang Bo Southwest Petroleum Institute, People's Republic of China Lian Zhanghua Southwest Petroleum Institute, People's Republic of China Abstract Based on the geologic structure, seismic data, exploitation information and the data of rock mechanics tests in Qinghai oil field, the finite element mechanics model of delaminated in-situ stress with a great deal of complicated faults was set up with ANSYS software. The stress concentration area, tensile stress area and compression stress area of the oil field were determined according to the rock s Drucker-Prager rule. The distribution of casing failure in the oil field indicated that it was closely relative to those areas. By applying the study of the paper and taking some preventing measures, the casing failure was controlled and the economic benefit had been evidently growing up. This paper offers a new study method to prevent casing failure and the theoretic reference for the mechanism study of casing failure. Introduction A casing is a steel pipe string to protect the wall of oil, gas or water wells from collapsing, divide the stratum fluids and ensure the productive activities in gear. Along the complicated geologic structures, the casing stresses will change in company with the in-situ stresses. Because there are many differences in the geologic structures, geologic activities and material properties of underground rocks, it is very difficult to get the distributing law of the in-situ stresses by analytical methods and by simple measurement in shallow layers of earth crust. The best way to analyze the strata is the inversion method based on known conditions of the faults, the mechanisms of the regional seismic source, the stress-drop of the little seismic and the ambient stress field. The finite element method (FEM) is the most effective method [1~3] to deal with the complex system, although there are some problems, such as simplified geometry model, constitutive relation of rock, conditions and so on. This paper studied the effects of the in-situ stress on casing failure by FEM, based on the measured and statistical data of Yuejing II Oil Field in Caidam Basin, Qinghai province, China. The typical formation stress field was analyzed, in which the casing failure has been severely occurring. Analysis Studying formation Geologic structure The structure of Yuejing II Oil Field is a depositional trap oil reservoir controlled by structure, affected by lithology. The basic structural feature is an anticline with complicated faults. The structure s strike is 160 degree and the area is 5.7 km 2. Affected by two great reverse faults, Allar Fault and VII Fault, the secondary faults have developed abundantly. There are more than ten faults only in the main part of the structure. * This paper was supported by China National Science Foundation (Project No. 50074025) and Supported by CNPC Innovation Foundation.
Formation in-situ stress method Because of abundant faults in the oil field, 3D model will be not exact adequate for including so many secondary faults and the branches of them. In order to get the detail distribution of the area s in-situ stress and analyze the casing failure in the area, the 2D formation model was set up and analyzed in the paper. The angle of beddings varies from 10 to 14,seen as Figure 1, so the formations could be thought approximately as in different plates, of which the 3D model are made up. However, the quantities of casing failure in different deformations are difference, seen as Table 1. Figure 1. The profile in elevation of wells (II173 II146) in Yuejin II Oil Field (red lines indicate faults, top is well numbers) Table 1 The statistics of deformations occurring casing failure NO. Formation Occurring casing failure Quantity Proportion % 1 N 2 1 8 27.58 2 N 1 16 55.17 3 E 3 1 4 E 3 2 1 3.45 1 3.45 5 others 3 10.34 Table 1 indicates that the casing failure mainly takes place in formation N 1, which is just the oil stratum. The depth of the formation is from 900m to 1100m and its rock is mainly the siltstone. To analyze the relationship between the casing failure and the in-situ stress, the formation was studied in detail. Mechanics model Parameters of the faults K 5 structure belongs to formation N 1, seen as Figure 2. The figure indicates that the formation has abundant faults and branches with complicated strike. The basic parameters of those faults are listed in Table 2:
Figure 2. The cross profile of structure K 5 Table 2. Parameters of faults in structure K 5 No. Strike trend obliquity Distance, m Length, km formation Fault character 2 NE80 350 ± 50 ~60 40~80 1.5± E 3 -N 2 Reverse normal fault 4 NE80 350 ± 50 ~60 10~30± <1.0 E 3 -N 2 Reverse normal fault 6 SE80 10 ± 50 ~60 10~30± 1.5± E 3 -N 2 Reverse normal fault 8 almost EW 180 ± 70 ± 20~50± >1.0 N 1 -N 2 Reverse normal fault 10 almost EW-NE 130 ~190 60 30~50± >1.0 E 3 -N 2 Reverse normal fault 12 NE80 340 ± 60 ± 30± 1.0± E 3 -N 1 normal fault 14 almost EW N 60 ± >50 1.0± E 3 -N 2 Forward normal fault 16 NE20 290 ± 60 ± 10~20± 0.5± N 2 Normal fault 1 NW15 60 ~70 80 30~50± 1.0± E 3 -N 2 Forward normal fault 3 NW15 60 ± 70 ± >50± 1.0± E 3 -N 2 Forward normal fault 5 NW15 61 ± 70 30± 1.0± E 3 -N 2 Forward normal fault 7 almost SN W 70 ± / / N 1 Reverse normal fault 9 almost EW N 70 ± 30± <1.0 E 3 -N 2 Forward normal fault 11 NE70 160 ± 60 ± 40± 0.5± E 3 -N 1 normal fault 13 NE70 ± 340 ± 60 ± 3± <1.0 E 3 -N 1 normal fault
Geometrical model Seen as Figure 3, the geometrical model, 2100m 2100m, of K 5 structure was set up, including all of the wells of the oil field. According to the geologic data, seismic data and exploitation information, the length, width and strike of every fault were described exactly in the model. Figure 3. The geometrical model of structure K 5 For computational accuracy, the 8-node plane element was chosen, during discretization of the model, (seen as Figure 4). Figure 4. The mesh model of structure K 5 Parameters of the rocks For describing exactly the properties of rocks, Drucker-Prager yield criterion was used in the model. The yield criterion is described as [4] : where α I + J k 0 ( 1 ) 1 2 =
sinϕ α =, 2 3 3 + sin ϕ k = 3c cosϕ 3 + sin 2 ϕ and c is the cohesion value, and φ is the angle of internal friction. In fact, Drucker-Prager yield surface is a circular cone, which corresponds to the outer apices of the hexagonal Mohr-Coulomb yield surface, (seen as Figure 5). Figure 5. geometric representation of stress [4] (a) the spacial yield surface of main stresses (b) the yield curves in π plane 1- Mohr-Coulomb yield surface 2-Drucker-Prager yield surface Table 3. Parameters of rocks No. structure rocks Young's modulus (MPa) Poisson ratio Cohesion (MPa) angle of internal friction (degree) 1 Stratum N 1 siltstone 5227 0.20 1.62 52 2 faults argillaceous sandstone 1000 0.30 0.73 30 According to the data of laboratory tests on cores collected from the stratum, and considering the difference of properties of rocks between in stratum and in laboratory, parameters of the rocks were chosen as Table 3. Boundary conditions Load conditions Being 1100m depth under ground, and according to the environment, northern Allar Fault and Eastern VII Fault, the model was dealt as a plane strain problem. Hydraulic fracturing data indicate that the direction of maximum main stress of the whole oil field is almost east-west, so the press of 1.2 times of gravity of superincumbent stratum was applied on the east boundary of the model and 0.8 times on the north boundary of the model. Displacement condition Firstly, for excluding the space freedom degree of whole model, west and south boundaries was restricted in their normal directions. In addition, according to the information of exploitation and seism, the horizontal slippages (including values and directions) were applied on different faults (on the lines of the faults in the model). Figure 6 displays the all boundary conditions.
Figure 6. The boundary conditions of structure K 5 Analysis Results & Discussion Relationship between in-situ stress field and casing failure The Von Mises equivalent stress varies mostly from 15 to 40 MPa (seen as Figure 7). However, there is an area of stress concentration in the northwest part of the stratum, in which the stress varies from 40 to 70 MPa and the maximum stress reaches up to 117 MPa. The statistical data indicate 14 wells have occurred casing failure in the stress concentration area (seen as Figure 8), and the average depth of casing failure is 1051m, which is adjacent to the depth of the studied stratum (1100m). It is said that the casings in the area of stress concentration are more possible to occur failure. At the same time, the analysis is more credible. Figure 7. The Von Mises stress of structure K 5
Figure 8. The wells occurring casing failure in the stress concentration area The stress concentration occurs mainly nearby the faults, especially, at the ends of some faults which are just the directions of extending trend. During the course of the exploitation, the slippage of fault is the main reason of occurring casing failure. What do lead to the slippage? While water is injected into the stratum, the rocks with clay will be hydrated, dilate, and creep, the shear strength of rock will decrease. If the pressure of injecting is greater than the gravity of superincumbent stratum or the shear strength of the rocks, the faults will glide relatively. Relationship between main stresses and casing failure The main stresses vector diagram of the stratum (seen as Figure 9) shows that directions of main stresses have changed because of the slippage of faults, even the direction of maximum stress changes to that of minimum stress in some areas, or the directions of main stresses occur reversion in other areas. The maximum main stress is mostly tensile stress and the direction is horizontal in the whole area, but its directions accord with the strikes of faults in the stress concentration area with the effects on rock materials, faults slippage, structure s changing, etc. By contraries, the minimum main stress is mostly compression stress and the direction is vertical in the whole area, but its directions accord with the normal of faults in the stress concentration area. Figure 9. The vector of main stresses of structure K 5
The casing will be applied non-uniform loads when the maximum stress is not equal to the minimum stress. Some studies indicate that the casing collapse strength will decrease rapidly under the non-uniform loads and the curve of decreasing is a parabola with the increasing of non-uniform degree. For example, while the ratio of two main stresses is from 1.5 to 2, the casing collapse strength under non-uniform load is only from 75% to 80% of that under uniform load. It is said that the non-uniform is one of main reasons causing the casing failure, too. Especially, if one main stress is tensile stress and the other main stress is pressure stress, the casing failure is easier to occur. There are other reasons causing the casing failure from the analysis. When the tensile stress strengths are about 10 MPa, the rocks are cracked easily under tensile stress, which leads to a lot of cracks and the redistributing of stresses around the well. The faults slippage causes the decreasing of the stability of sand arc, the increasing of sand production and the server non-uniform loads on casing. Conclusions After the analysis, there are two reasons that casing failure centralizes in oil stratums. The first is that the non-uniform degree of two main stresses aggrandizement during the course of the exploitation. The second is that the collapse strength of casings in the stratum is lower than in other stratums because the casings in oil stratums are often perforated. The FEA is an effective method to deal with the casing failure under the in-situ stress. The analysis in the paper offer theoretical reference and test data for study on casing failure. References [1]. Lian Zhanghua, Han Jiangzeng, Tang Bo et al, Research on the Casing Failure Mechanism of Complex Formation by Numerical Simulation (in Chinese) [J], Natural Gas Industry, V.22, No. 1,P48~51 [2]. Wang Zhongmao, Lu Wangheng, Hu Jiangming. The Mechanism of the Prevention and Cure of Casing Failure on Oil and Water Wells in Oil Field (in Chinese)[M], Beijing, Petroleum Industry Press, 1994,P102~123 [3]. Li Zhiming, Zhang Jinzhu, The In-situ Stress and Oil & Gas Exploratory Development (in Chinese) [M], Beijing, Petroleum Industry Press, 1997:P143~177 [4]. Zheng Tianyu, The Theoretical Basis of Elastic, Plastic and Viscous Mechanics of Rock (in Chinese) [M],Beijing, Coal Industry Press, 1988