Investigation on in fluence of drilling unloading on wellbore stability in clay

Yi Ding · Xiang‑Jun Liu · Ping‑Ya Luo

Abstract Wellbore collapse frequently happens in the clay shale formation. To maintain wellbore stability, appropriate mud pressure is a vital factor. When clay formation is opened, drilling unloading occurs, modifying rock structure and strength at the wall of borehole, which affects the selection of mud pressure. Currently, mechanism of drilling unloading is still poorly understood which in return will bring a concern to wellbore stability. Therefore, in this study, a combination of triaxial compressive test and ultrasonic wave test has been used to simulate drilling unloading and analyze its mechanism. Results indicate that more void space is created inside the clay shale sample due to unloading. This structure change leads to a decline of strength and acoustic amplitude. Additionally, unloading in fluence is depended on varying drilling unloading parameters. Small unloading range and fast unloading rate are able to enhance stability. With various degrees of unloading impact, collapse pressure equivalent density has a clear modi fication, proving that unloading is a non-negligible in fluencing factor of wellbore stability.Besides, the unloading effect is much stronger in large con fining pressure, implying that more attention should be given to unloading when drilling is in extreme deep or high geostress formation. Findings in this paper can offer theoretical guidance for drilling in the clay shale formation.

Keywords Drilling unloading · Clay shale · Wellbore stability · Rock strength

1 Introduction

Wellbore instability is a well-known problem in the drilling operation, causing huge economic loss (Chen et al. 2003;Rahman et al. 2000). In particular, clay shale has been considered as high-risk formation since numerous wellbore collapses occur (He et al. 2016; Wilson and Wilson 2014; Abdulhadi et al. 2012). Factors affecting wellbore stability in clay shale have been widely analyzed, including hydration, mechanical property, pore pressure, in situ stress, etc. For controlling clay shale hydration, chemical research has been carried out to produce efficient inhibitors and membrane-forming agents for drilling fluid (Zhong et al.2016; Liu et al. 2014). Considering the in fluence of hydration on clay shale property, extensive hydration evaluation methods have been established (Tianshou and Chen 2014;Kang et al. 2017). Furthermore, wellbore collapse is the result of mechanical instability of rock, which raises large demands for research about rock mechanics. For instance,mechanical experiments, like triaxial, shear, tensile tests,have been applied to characterize the mechanical behavior of clay shale and find out appropriate strength criterion for the calculation of collapse pressure (Lai et al. 2007).Meanwhile, stress state of clay shale in subsurface has been discussed for wellbore stability. Studies from Han and Dusseault (2003), Roshan and Rahman (2011) and Abousleiman et al. (1997) show it is much easier for rock to have shear failure in formation with strong anisotropy and high value of in situ stress. Furthermore, because of the physicochemical reaction between clay shale and drilling fluid, pore pressure distribution around wellbore will be modi fied, affecting collapsing degree of borehole (Ghassemi et al. 2009).Correspondingly, based on Fick diffusion and Darcy law,models of pore pressure propagation have been built (Salehi et al. 2010).

The above statement gives so many in fluence factors of wellbore instability in clay shale formation, showing extreme complexity for this drilling problem. In oil field,the most direct and effective method of maintaining wellbore stability is to choose appropriate mud pressure (Xia and Moore 2006). Generally, under the in fluence of drilling fluid on rock, mud pressure has to be adjusted with drilling time. Regarding mud pressure in the process of drilling, initial mud pressure is the foundation. If initial mud pressure is too low, wellbore collapse occurs immediately, forming irregular borehole shape and causing irreversible damage to rock around the wellbore (Ding et al. 2018). Under this circumstance, adjustment of mud pressure is not so useful,further elevating the challenge of keeping wellbore stability. When it comes to initial mud pressure, unloading in fluence cannot be avoided. Once formation is opened, drilling unloading happens immediately. There is no doubt that drilling unloading can cause damage to rock around wellbore.Hence, the selection of mud pressure should include this in fluence from unloading. However, current studies about wellbore stability do not cover the unloading effect, which is not bene ficial for drilling in clay shale formation with high risk of wellbore collapsing.

Therefore, in this paper, we combine triaxial compressive test and ultrasonic wave test to simulate drilling unloading condition and to obtain rock strength and structure in the unloading process. Using these experimental data, we have analyzed the in fluence mechanism of unloading. Besides,in fluence factors of drilling unloading (unloading range,unloading rate and con fining stress) have been investigated.The aim of this work is to illustrate the mechanism of drilling unloading. Outcomes of this paper can offer theoretical reference for drilling in clay shale formation.

Fig. 1 Rock composition and structure of clay shale

2 Clay shale sample

In this study, mineral composition, clay composition and rock structure of clay shale have been presented by using XRD, visual observation and SEM (Fig. 1). Clay shale is composed by quartz, feldspar (orthoclase and plagioclase),carbonates (calcite and dolomite), clay and siderite. Meanwhile, clay consists of illite, smectite, I/S, kaolinite and chlorite. It is noted that clay is dominated in mineral composition and water-sensitive clay (smectite and I/S) is rich,suggesting strong hydration of clay shale. Strong hydration ability requires high standard for mud pressure in drilling.According to studies (Wang et al. 2006; Liu et al. 2009;Riley et al. 2012; Abass et al. 2006), from research on Qigu,Kuqa, Marcellus and Khaセi shale formation, all authors give the conclusion that hydration ability of clay shale is a signi ficant factor of affecting wellbore stability.

Furthermore, structure of clay shale shows disintegration with obvious fractures in macro- and microscale (Fig. 1c,d). This characteristic results in strong mechanical instability of clay shale, which is one of the main reasons why wellbore collapse frequently happens. Meanwhile, clay shale has a relatively high sensitivity for external stress due to this mechanical instability (Cai et al. 2007; Zhu et al. 2018).In other words, with high degree of structure disintegration, it is much easier to cause rock failure. Particularly, in the beginning of drilling, when drilling fluid replaces rock,stress state around wellbore is suddenly changed from original stress balance condition to new stress state. If this new stress state is inappropriate, stronger compressive or tensile effect will be loaded on rock around wellbore. Consequently,structure disintegration of rock is aggravated, finally leading to rock failure. Therefore, initial mud pressure has to be precisely con firmed to establish balance of stress state on rock around wellbore.

3 Experimental method

3.1 Schematic of drilling unloading

When formation is opened by drilling, unloading effect is created. Take vertical borehole as an example; its schematic of drilling unloading is illustrated in Fig. 2. Before drilling,rock in formation is controlled by in situ stress. After drilling, rock in the borehole has been replaced by drilling fluid,making mud pressure support the wall of borehole. Stress state at the wall of borehole will be modi fied due to drilling unloading. At the direction along maximum and minimum horizontal stress (σHandσh), stresses are changed fromσHandσhto mud pressure (σr), havingσH−σrandσh−σrunloading range, respectively.

Stress state of rock at the wall of borehole can be simpli fied to two stresses, which are axial stress and lateral stress (Zhang et al. 2010). With the unloading effect, lateral stress is unloaded, decreasing its bracing impact on rock.As a result, axial stress has a relatively stronger compressive effect. Meanwhile, this increasing compressive effect is able to create more damage on rock structure, incurring crack initiation and propagation (Fig. 3). Since rock structure has been changed, rock mechanical property will have the corresponding modi fication, which is the main reason why drilling unloading can affect wellbore stability.

Fig. 2 Illustration of drilling unloading

3.2 Simulation of drilling unloading

In order to simulate drilling unloading and analyze the mechanism of drilling unloading, triaxial compressive test and ultrasonic wave test have been combined, as shown in Fig. 4. In this experiment, placement of core sample is presented in Fig. 5. Besides, we use false triaxial condition(σ3=σ2) and con fining pressure is used to simulate drilling unloading. Con fining pressure is firstly loaded. Hydraulic oil in tank is pushed by nitrogen gas pressure to fill the chamber.Once the chamber is full of oil, by using the control system of con fining pressure, hydraulic power has been added into the chamber so that the con fining pressure can be increased to a certain value. On the other hand, with the release of hydraulic power, decline of con fining pressure can be finished. In a similar method, with axial pressure control system, hydraulic pressure can be added on pressure head to give axial load for core sample. Additionally, in the process of unloading test, ultrasonic source creates signal (P wave with 50 kHz), which goes through core sample from upper pressure head to lower pressure head. Finally, ultrasonic signal after passing core sample is acquired in oscilloscope and acoustic data in unloading will be saved in computer.

Fig. 3 Illustration of the unloading in fluence on rock

Fig. 4 Schematic of equipment with triaxial compressive test and ultrasonic wave test

The unloading condition is simulated by confining pressure in triaxial compressive test, and its stress path is shown in Fig. 6. In conventional triaxial test, con fining pressure is firstly increased to a certain value. Subsequently, axial stress is increased until the rock sample has a failure (Fig. 6a). Based on this process, compressive strength in con fining pressure can be acquired (Haimson and Chang 2000). To get the in fluence of unloading on rock mechanical property, unloading phase has been added into the stress path, as shown in Fig. 6b. In the beginning, the step is the same as the conventional test, loading con fining pressure to a certain value. After that, axial stress is increased to 80% of compressive strength (Point C). Then, with constant axial stress, the unloading phase starts, decreasing con fining pressure from Point A to Point B. When the unloading process ends, axial stress restarts increasing until rock failure. Compared to conventional triaxial compressive test, unloading test has an unloading phase, thus giving rock mechanical parameters containing the unloading effect.

In combination with conventional triaxial compressive test and triaxial ultrasonic test with unloading, experimental analysis of the in fluence of unloading on shale can be conducted.The whole experimental procedure is shown in Fig. 7. The experimental analysis is presented in the following section.

Fig. 5 Illustration of core sample in test

Fig. 6 Schematic of stress path in unloading simulation

4 Experimental analysis

4.1 Rock strength with drilling unloading

Based on the triaxial compressive test with and without unloading, stress–strain curves are shown in Fig. 8.In unloading condition, stress–strain curve has flat line part, which is the unloading stage. Under certain unloading range, unloading process converts rock sample from one con fined condition to another condition. Take unloading range = 20 MPa and con fining pressure = 50 MPa as an example; con fining pressure is changed from 50 to 30 MPa. Its compressive strength is located between strengths at 50 MPa and 30 MPa con fining pressure, as shown in Fig. 9. Comparing to condition withσ3= 50 MPa,sample with unloading has smaller compressive strength due to damage caused by the unloading process. On the other hand, the strength of sample with unloading (σ3from 50 to 30 MPa) is relatively higher than compressive strength withσ3= 30 MPa. That is because sample is unloaded from a higher con fining pressure (50 MPa),which means a much stronger bracing effect exists before con fining pressure decreases to 30 MPa, making relatively higher compressive strength.

Fig. 7 Schematic of experimental analysis

Fig. 8 Stress–strain of triaxial compressive test in conventional and unloading

Fig. 9 Comparison of strength property in conventional and unloading conditions

In order to acquire mechanical parameters (cohesion and internal friction angle), we chooseMPaandMPa to conduct triaxial compressive test in conventional and unloading conditions. The illustration of acquiring mechanical parameters in conventional and unloading conditions is presented in Fig. 10. As it is shown in Fig. 10,in conventional and unloading conditions, with certain confining pressures, triaxial compressive test is conducted to get stress–strain curves, respectively. Based on the compressive stress and con fining stress, two Mohr’s circles in conventional condition and two Mohr’s circles in unloading condition have been formed. By using these Mohr’s circles, Mohr–Coulomb criterion has been applied to acquire mechanical parameters in conventional and unloading conditions, as follows:

whereεandσare shear and normal stresses at failure plane,MPa;candcuare cohesion in conventional and unloading conditions, respectively, MPa;φandφuare internal friction angle in conventional and unloading conditions, respectively, degree.

4.2 Acoustic property in unloading

Acoustic property can re flect the structure of core sample.Based on that, in the unloading, ultrasonic wave test has been conducted to obtain the acoustic property of core sample, as shown in Fig. 11. For the unloading process, ultrasonic waves in the beginning and ending point (i.e., point A and B in Fig. 11a) have been acquired, respectively. It is well known that acoustic amplitude can be considered as a signal of acoustic energy (Xu et al. 2019, 2017). Besides, the development of void space (fracture or porosity) increases the number of acoustic re flections, refraction and scattering in ultrasonic wave propagation inside rock. Consequently,more acoustic energies have been absorbed in this propagation, leading to a small amplitude. Therefore, acoustic amplitude represents the evolutionary process of rock structure in unloading (Weger et al. 2009). Figure 11b shows the acoustic amplitude in unloading, showing that the amplitude has a decline after unloading, which indicates that structure damage is gradually increased and more void space is produced in unloading.

4.3 Structure of clay shale in unloading

Maximum amplitude (i.e., maximum absolute value of acoustic amplitude) is the typical feature of acoustic amplitude and can represent acoustic energy (Zou et al. 2017).Considering the relation between rock structure and acoustic amplitude, in this section, we use maximum amplitude to be an indicator of structural damage degree under external stress impact, establishing quantitative method of rock structural damage. For intact core sample of clay shale, its void space is located at small range, which is not sufficient to re flect structural change from external stress. Hence, we add variable loadings on sample to create different degrees of structure damage. Axial loadings are equal to 0% (intact sample), 30%, 60%, 90% and 100% (sample after failure)of uniaxial compressive strength (UCS). After the uniaxial loading process, porosity test and ultrasonic wave test of rock sample have been performed to acquire quantitative relation between acoustic amplitude and rock structure, as shown in Fig. 12. Its equation is given as Eq. (2). It can be found that with increasing void space (larger structure damage), acoustic maximum amplitude shows a decline, which is consistent with the theory of acoustic energy attenuation.Before 60% of UCS, increasing rate of void space is relatively slow. After that, the structural damage dramatically increases.

Fig. 10 Mohr’s circle with and without drilling unloading

whereSpis the void space, %, andAmis the P wave maximum acoustic amplitude, mV.

Unloading is one of the stress state modi fications. When the stress state of rock is changing, the corresponding change in rock structure (crack initiation and propagation, open and closure of pore and crack, etc.) will occur (Liu et al. 2006).Based on the maximum amplitude before and after drilling unloading, acoustic amplitude can be used to express structure damage. Therefore, in this paper, structure damage of unloading can be evaluated using ultrasonic wave test in the unloading stage, as follows:

Fig. 11 Ultrasonic wave in unloading stage

Fig. 12 Different structural damage of core sample

where ΔSpis the increment of void space after unloading, %;is the void space in the beginning of unloading, %;is the void space in the ending of unloading, %;is the maximum acoustic amplitude in the beginning of unloading, mV;is the maximum acoustic amplitude in the ending of unloading, mV.

5 Results and discussion

5.1 In fluence of unloading range

In drilling operation, with variable mud pressure, in situ stress and well trajectory, different unloading ranges can be formed (Gholami et al. 2015). To investigate the in fluence of unloading range, unloading test has been conducted with different unloading ranges. In this test, loading and unloading rates are both 0.2 MPa/s. The con fining pressures are 50 MPa and 30 MPa, as given in Table 1. Based on this test, rock strength parameters in different unloading ranges are shown in Fig. 13. Cohesion and internal friction angle both show clear decreasing tendency, indicating that large unloading range has a strong decline of rock strength.Meanwhile, atσ3= 50 MPa, according to acoustic maximum amplitude in the beginning and ending of unloading, increment of void space with different unloading ranges (σu) has been obtained, as shown in Fig. 14. It can be found out that high unloading range represents large increment of void space, having stronger attenuation of acoustic amplitude and larger structure damage caused by unloading.

Table 1 Testing parameters for samples with different unloading ranges

The reason of larger structural damage in high unloading range is that increasing unloading range leads to the growth of axial differential stress (σ1−σ3), which can have a stronger compressive effect along the axial direction. In this case, more damage is created under large differential stress, causing increment of void space. As a result, rock strength shows a larger decline and clay shale becomes more unstable, elevating the risk of wellbore collapse in drilling.

Fig. 13 Rock strength with unloading range

Fig. 14 Acoustic amplitude and void space with unloading range

5.2 In fluence of unloading rate

Drilling rate is an important parameter in drilling operation.Fast drilling rate means formation is open in high speed,having a large unloading rate. In this section, with different unloading rates, unloading test has been applied to investigate the in fluence of unloading rate on core sample. In this test, unloading range stays 20 MPa and con fining pressures are 50 MPa and 30 MPa, as given in Table 2. The results are illustrated in Figs. 15 and 16. It is shown that with increasing unloading rate, rock strength rises, indicating that a fast drilling rate can enhance the wellbore stability. Correspondently,at high speed of unloading, acoustic amplitude increases and void space decreases, meaning small structure damage in the unloading stage. The tendency of strength parameters with the unloading rate is typicalScurves, expressed as

Table 2 Testing parameters for samples with different unloading rates

whereVuis the unloading rate, MPa/s.

The central reason for better stability in fast rate is stress lag phenomenon. The stress lag is normally noticed in the viscoelasticity medium. With a large content of clay, clay shale can be treated as the viscoelasticity medium (Arash et al. 2018). Due to the property of viscoelasticity, stress inside rock normally lags behind stress outside rock, creating lag stress between stress inside and outside of core sample,as shown in Fig. 17. The stress outside rock is equal to the sum of stress inside rock and lag stress (Fig. 17a).

For certain point inside sample (Point B in Fig. 17a),when unloading happens, small unloading rate gives more time for outside stress to go into rock, which makes stress path of Point B much closer to outside stress. Consequently,comparatively smaller lag stressexists. On the other hand, if the unloading rate is fast, stress inside rock(Point B) does not have enough time to have the corresponding reaction with outside stress, showing larger lag stressThat is to say, with certain unloading range from outside, rock interior has small stress in high unloading rateindicating that rock interior has small damage.That is the reason why sample shows small change in acoustic amplitude and high-strength parameters in fast unloading rate.

Furthermore, when unloading rate is extremely small,outside stress can be passed through core sample and inside stress has plenty of time to modify with outside stress, causing lag stress negligible. In contrast, when unloading is fast enough, inside stress barely has to respond to outside stress (≈0 MPa), thus leading the lag stress to reach a maximum value. Due to limiting value of lag stress,rock strength, acoustic amplitude and void space all exhibitS-type curve (Figs. 15, 16f), having obvious boundary at low and high unloading rates.

Fig. 15 Rock strength with unloading rate

5.3 In fluence of con fining pressure in unloading

In drilling operation, rock at the wall of borehole is under con fined stress condition (Sayers et al. 2009). Because of different in situ stress circumstances, con fining pressure is variable. Considering different con fining pressures, the unloading effect has been analyzed in this section. It is well known that compressive strength can be used to evaluate rock strength and stability (Ashour 1988). Thus, we use compressive strength to analyze unloading in fluence with different con fining pressures. First of all, in conventional condition, compressive strength with different con fining pressures has been acquired, as shown in Fig. 18. Same as previous researches (Takahashi 2012), experimental results illustrate linear relation between compressive strength and con fining pressure. Since compressive strength is associated with con fining pressure, to purely see the in fluence of con fining pressure on unloading, each sample is given the same state, meaning the same unloading parameters except con fining pressure, as shown in Table 3.

Fig. 16 Acoustic amplitude and void space with unloading rate

Fig. 17 Schematic of stress lag for rock

With different con fining pressures, unloading in fluence is presented in Figs. 19 and Fig. 20. It is clearly seen that the strength difference between the conventional test and unloading test grows with con fining pressure. In 10 MPa con fining pressure, the difference of compressive strength(Δσc) is merely 6.8 MPa. When con fining stress reaches 50 MPa, difference has become approximately 18.8 MPa.Meanwhile, with increasing con fining pressure, structural damage and acoustic amplitude attenuation made from unloading are becoming large. These phenomena indicate that unloading in fluence is stronger in extreme deep formation or high geostress formation. Thus, drilling unloading should be given more attention when drilling operation is performed in these geological conditions.

Fig. 18 Compressive strength with different con fining pressures

Table 3 Testing parameters for samples with different con fining pressures

Fig. 19 Compressive strength in unloading with different con fining pressures

5.4 In fluence of unloading on collapse pressure

In order to clarify the in fluence of unloading in application, based on experimental analysis from the above sections, take vertical well as a case, we conduct the calculation of collapse pressure considering the in fluence of unloading. In this calculation, we assume that depth is constant (H= 2930 m) and set three typical in situ stress conditions, which are normal fault (σv＞σH＞σh), reverse fault (σH＞σh＞σv) and strike-slip fault (σH＞σv＞σh),as shown in Table 4. Besides, pore pressure is constant(pp= 32.1 MPa) and all rock mechanical parameters are acquired from the above experiments.

For vertical wellbore, stress distribution at the wall of borehole can be calculated (Lee et al. 2012):

whereσr,σθandσzare the radial, hoop and axial stress in wellbore coordinates, respectively, MPa;τzθis the component of shear stress in wellbore coordinates, MPa;ppis the pore pressure, MPa;φis the porosity, %;ais the Biot coefficient;vis the Poisson ratio; andθis the wellbore circumferential angle, degree.

Based on the stress distribution in Eq. (5), principal stresses (σi,σj,σk) can be acquired by using Eq. (6). By conducting comparison among all principal stresses,maximum and minimum principal stresses can be determined. Subsequently, with maximum and minimum principal stresses, Mohr–Coulomb criterion can be applied to acquire collapse pressure.

Considering the varying in fluence of unloading on rock strength, collapse pressure equivalent density is presented in Fig. 21. Collapse pressure equivalent density shows increment with rising unloading range. Meanwhile, too low unloading rate is detrimental for wellbore stability.These results indicate that drilling parameters are related to influencing degree of unloading. Appropriate mud pressure and drilling rate are vital for controlling unloading damage on wellbore stability. Thus, when it comes to prediction of collapse pressure, the unloading effect is non-negligible.

Fig. 20 Acoustic amplitude and void space in unloading with different con fining pressures

Table 4 In situ stress distribution

Fig. 21 In fluence of unloading on collapse pressure equivalent density

6 Conclusion

In this study, to better control wellbore stability, drilling unloading has been simulated and its effect mechanism on rock stability has been discussed. The following conclusions are drawn:

1. A new experiment combining triaxial compressive test and ultrasonic wave test has been designed to simulate drilling unloading and obtain the in fluence of drilling unloading. In the unloading stage, rock acoustic amplitude shows a decline, suggesting that structure damage has been done by unloading. Furthermore, because of this damage in the unloading stage, rock strength(cohesion and internal friction angle) after unloading is clearly lower than rock sample without unloading,indicating that unloading can cause strength decline and increase rock mechanical instability.

2. High unloading range tends to have relatively large strength decline for clay shale. This is because large unloading of con fining pressure decreases the lateral bracing effect on rock, thus causing a comparatively stronger compressive effect on rock. As a result, stronger damage and bigger strength decline are made. In addition, with increasing unloading rate, rock strength rises,suggesting that fast drilling rate can enhance wellbore stability. Meanwhile, at high speed of unloading, acoustic amplitude increases and void space decreases, meaning small structure damage in unloading. The stress lag is the reason of stronger stability in high speed of unloading. Under large unloading rate, inside stress cannot have immediate reaction for outside stress, which makes less damage inside rock and has smaller strength decline.

3. With increasing con fining pressure, the damage made from unloading is becoming large. For certain drilling unloading parameters, decline of compressive strength is large in high con fining pressure. Besides, void space is increasing with con fining pressure. These phenomena demonstrate that unloading in fluence is stronger in extreme deep formation or high geostress formation.Thus, drilling unloading is non-negligible when drilling operation is conducted in these geological conditions.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China (No. 41772151), the National Science and Technology Major Project of the Ministry of Science and Technology of China (No. 2011ZX05020-007-06) and the Application Basic Research Project of Sichuan Province (No. 2014JY0092). We appreciate their support.

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