aSinopec Research Institute of Petroleum Engineering, Beijing 100101, China
bState Key Lab of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China
Because of high cost and pollution of oil-based drilling ﬂuid, the water-based drilling ﬂuid is increasingly used now. However, bedding planes and micro-cracks are rich in shale formation. When water-based drilling ﬂuid contacts formation rock, it causes the propagation of crack and invasion of drilling ﬂuid, which decrease shale strength and cause wellbore instability. In this paper, we analyzed inﬂuence of water-based drilling ﬂuid on shale strength and failure mode by mechanics experiment. Based on those experimental results, considering the effect of bedding plane and drilling time, we established modeling of wellbore stability for shale formation. The result from this model indicates that in certain azimuth of horizontal well, collapsing pressure increases dramatically due to shale failure along with bedding plane. In drilling operation, those azimuths are supposed to be avoided. This model is applicable for predication of collapsing pressure in shale formation and offers reference for choosing suitable mud weight.
As a kind of unconventional energy sources, currently shale gas plays an important role in industrialization exploration of the natural gas . In the shale gas exploration and development, the United States is leader in the world. Though China has abundant shale reserves as America, the development of shale gas is on elementary stage and faces so many challenges. Especially in drilling operation, borehole collapse has been a common problem , which increases drilling period and cost.
In view of the instability problem, normally oil-base drilling ﬂuid is the ﬁrst choice because of its relatively strong plugging and inhibitive property. But the defect of oil-base drilling ﬂuid is too expensive and harmful to environment . On the contrary, water-based drilling ﬂuid has comparatively low prize and pollution. Therefore, many scholars are trying to develop an efﬁcient water-based drilling ﬂuid for shale formation. However, shale formation has strong anisotropy and sensitive for water ﬂuid, leading to serious instability problems in drilling process.
In this paper, composition and structure of shale have been investigated by means of X-ray diffraction (XRD) and Scanning electron microscope. Meanwhile, triaxial compression test and shear test have been applied to observe its mechanics property. By those experiments, the effect of water-based drilling ﬂuid on structure and mechanics can be analyzed. Those analyses indicate that crack propagation and bedding plane are two key factors of causing borehole collapse in shale formation. According to the result, we establish a wellbore stability model for shale formation. This model is practical for analyzing wellbore stability in shale formation.
Rock samples were taken from Southern China Longmaxi formation of black shale. Bedding plane is clearly observed from standard shale cores (Fig. 1). Moreover, Scanning electron mi- croscope was applied to observe substructure morphology of shale. The result in Fig. 2 shows micro-cracks and micro-pores are rich inside shale. By measurement, width of micro-cracks ranges from 0.4μm to 3µm.
Fig. 1. Shale core samples.
XRD analysis results are presented in Table 1 and Table 2. In those samples, brittle mineral (quartz, plagioclase and calcite) is predominated and its average content is 80.77%, whereas clay mineral only averages 12.92% and ranges between 11.12% and 14.10%. In clay mineral, illite is the main content and averages 85.80%. Montmorillonite is extremely little and only averages 0.01%.
Fig. 2. Substructure morphology shale specimens.
It is noted that content of brittle mineral is very high and clay mineral is not the abundant one in the component. In addition, Montmorillonite and illite smectite, which have strong hydration and swelling property, are in low content. According to that, the shale from Southern China Longmaxi formation is typical brittle shale and has weak hydration. So hydration swelling is not the key factor for borehole collapse.
Bedding plane has huge impact on strength of shale [4,5]. To analyze this type of mechanics structure, we assume that shale rock is composed by 2 parts-rock matrix and bedding plane. We use 2 methods to get shale strength by shear test (Fig. 3) so that obtain the strength of rock matrix and bedding plane respectively. In this experiment, we can get strength of rock matrix by shearing core in the direction perpendicular to bedding plane (Fig. 3a) and strength of bedding plane by shearing core in the direction parallel to the bedding plane (Fig. 3b).
Fig. 3. Schematic of shear test for obtaining strength of rock matrix and bedding plane.
According to above methods, shear test was used to obtain the strength of matrix and bedding plane after shale was soaked with water-based drilling ﬂuid in different time. The results are presented in Fig. 4 and 5.
From results of Figs. 4 and 5, internal friction angle and cohesion of rock matrix are higher than bedding plane, which indicates bedding plane is relatively weak plane. After soaking water-based drilling ﬂuid, the strength of bedding plane declines more quickly, which means that bedding plane is affected more by drilling ﬂuid. The reason for that is the bedding plane has relatively high permeability, compared to rock matrix . Therefore, it is easier for drilling ﬂuid to penetrate inside. Consequently, bedding plane is more sensitive to water-based drilling ﬂuid. Compared with decline trend of rock matrix and bedding plane, it is similar. Both of they drop dramatically ﬁrstly and then become stable in the late stage, which means the inﬂuence of drilling ﬂuid is more obvious in initial stage.
Fig. 4. Curve of friction angle of shale bedding plane and matrix in different soaking time.
To analyze the failure mode of shale, triaxial test was conducted along with the parallel and perpendicular direction of bedding plane, as shown in Fig. 6. Broken shale cores after triaxial test are illustrated in Figs. 7 and 8. Based on the results, different compressing direction has little inﬂuence on failure mode, which is still mainly single shear failure mode. In addition, there are several cracks around shear plane. With increase of soaking time, the failure mode is still same but more cracks occur. Parts of those cracks are connected with each other, forming a crack network zone. The reason of the phenomenon is that drilling ﬂuid invades into shale after soaking, which leads to crack propagation and strength decrease. So for brittle shale with low hydration, the key point for drilling ﬂuid is its sealing capacity to prevent invasion.
Fig. 5. Curve of cohesion of shale bedding plane and matrix in different soaking time.
At present, for shale formation exploration and development, in order to obtain greater production, the horizontal well is the ﬁrst option. But horizontal well has long drilling time. In the late stage of drilling, the instability problems are getting worse. So this model takes horizontal well as case.
Fig. 6. Schematic of 2 methods of triaxial compression test.
Based on the linear elastic mechanics, the single weak-plane strength theory and Mohr-Coulomb criterion, effective stress around the borehole was obtained by coordinate conversion  (Fig. 9). The coordinate conversion equations are shown as follows:
Fig. 7. Broken shale core samples by compressing perpendicular direction in different soaking time.
Fig. 8. Broken shale core samples by compressing parallel direction in different soaking time.
As weak plane, the bedding plan has great effect on rock mechanics. Especially, the different dip and dip azimuth can make different situation for wellbore stability in shale formation. Therefore, as shown in Fig. 10, we use coordinate conversion to get state of stress on bedding plane from stress around borehole . The equations can be written as follows:
where β1 is dip angle of bedding plane, degree. α1 is dip azimuth angle of bedding plane, degree. srxx, sryy, srzz are normal stress on bedding plane, MPa. τrxy, τrxz, τryz are shear stress on bedding plane, MPa.
From coordinate conversion above, stress distribution around borehole and on bedding plane can be obtained. Based on the stress state, Mohr-Coulomb criterion is applied to determine shale failure along with rock matrix or bedding plane.
Fig. 9. Borehole coordinate system.
Fig. 10. Bedding plane coordinate system.
If shale rock failure is along with the bedding plane, according to Mohr-Coulomb criterion, the equation of failure criterion is shown as followed:
If shale rock failure is on the matrix, the equation of failure criterion can be expressed by the following equation:
where Cw is cohesion of bedding plane, MPa. C is cohesion of rock matrix, MPa. φw is internal friction angle of bedding plane, degree. φ is internal friction angle of rock matrix, degree. σ1, σ3 is the maximum principal stress and minimum principal stress respectively, MPa.
Based on above equations, if we want wellbore not collapsed, the minimum drilling ﬂuid pressure is supposed to keep shale stable in both matrix and bedding plane. So the collapse pressure can be acquired by above equations.
A case study on Southern China Longmaxi formation with our model is in the following. According to drilling date from oilﬁeld, overburden pressure is 68MPa, maximum horizontal principal stress is 62MPa and minimum horizontal principal stress is 49 MPa. Pore pressure is 30 MPa. Porosity is 7.4%. Rock mechanical parameters are taken from previous experiments.
Using the above datum into model, with the same azimuth angle of bedding plane (10°) and the same well azimuth angle (90°), we plot collapse pressure distribution with different borehole azimuth angle and different dip of bedding plane. The results are illustrated in Figs. 11-14.
Fig. 11. Collapse pressure with 0° dip in different azimuth and soaking time.
Fig. 12. Collapse pressure with 30° dip in different azimuth and soaking time.
From Figs. 11-14, with the increase time of contacting water-based drilling ﬂuid, collapse pressure grows. Besides, bedding plane only affects shale strength in low dip. When dip of bedding plane is between 0° and 30° , as shown in Figs. 11 and 12, shale is broken along with bedding plane in azimuth from 20° to 60° and from 200° to 240° . It is noted that, when shale failure along with bedding plane, the collapse pressure will grow dramatically. When dip is 60° and 90°, as shown in Figs. 13 and 14, shale formation is damaged in rock matrix and has relatively low collapse pressure compared to shale failure along with bedding plane.
Fig. 13. Collapse pressure with 60° dip in different azimuth and soaking time.
Fig. 14. Collapse pressure with 90° dip in different azimuth and soaking time.
High collapse pressure means it is easy to be collapsed in this area. Therefore, high mud density is necessary. However, too high mud density tends to cause leakage of formation. So during drilling operation, we should avoid the azimuth with high collapse pressure in order to ensure the safety of drilling. In this case, azimuth from 20° to 60° and from 200° to 240° need to be avoided in drilling.
After experiment and analyzing collapse pressure based on calculation results of model, we have conclusion in the following.
This research was supported by the United Fund Project of National Natural Science Foundation of China (Grant No. U1262209), the Open Fund (Grant No. PLN0906) of Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University) and Basic research Project of Sichuan Province (Grant No. 2014JY0092).
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