Large-scale hydraulic fracturing combined with horizontal well drilling is the key technology in shale gas development. But poor cementing quality is a crucial bottleneck restricting well completion and reservoir stimulation. In this paper, the shale gas development blocks in the Sichuan Basin were taken as the examples to explore the process and support measures that can be used to keep the mechanical integrity of cement sheath under the effect of hydraulic fracturing.
Zhao Changqinga,b,*, Hu Xiaoqianga, Zhang Yongqiangc, Liang Honga, Fang Haoa, Zhang Leic, Tang Shouyongc & Zeng Fankuna,b
aDownhole Operation Company, CNPC Chuanqing Drilling Engineering Co., Ltd., Chengdu, Sichuan 610051, China. bNational Energy Shale Gas R & D Experiment Center, Langfang, Hebei 065007, China. cPetroChina Zhejiang Oilﬁeld Company, Hangzhou, Zhejiang 310023, China
Received 20 July 2017; accepted 25 October 2017
It is indicated that the near-bit three-centralizer drifting BHA used for casing stiffness simulation can decrease the casing running difficulty in the long horizontal section of a shale gas well and increase the time efficiency and safety of casing running; that the flushing efficiency of high-efficiency oil flushing spacer fluid system is higher than 90% from room temperature to 120 °C, so it can guarantee the displacement efficiency of cement slurry to the oil-based drilling fluid and the effective cementing of borehole wall; that the performance of anti-channeling ductile cement slurry used in the cementing of long horizontal sections in this area after it is set is confirmed, with the elastic modulus of set cement being lower than 7 GPa and triaxial strength being higher than 40 MPa, so as to alleviate or avoid the damage to cement sheath in the process of fracturing; and that cementing quality is improved by applying the support technologies, e.g. drilling fluid adjustment, pre-stress cementing and ground high-pressure pumping.
During 2015–2016, these cementing technologies were applied in 85 wells in the Sichuan Basin. The average well depth of these wells is 4832 m, the average length of horizontal sections is 1560 m and the quality rate of well cementing is 89.58%. During the waiting-on-cement (WOC) time after well cementing, there is no sustained casing pressure. And gas channeling in the annulus during well drilling, completion and test is improved remarkably. It is concluded that this suite of technologies can guarantee and improve the cementing quality of long horizontal sections in shale gas wells and provides good cementing conditions for shale gas development.
With the advancement of shale gas horizontal well drilling technology, the length of horizontal section is also increasing – from 500 to 1000 m in the stage of initial shale gas development to more than 1500 m now. Some well completion technologies have become incompatible . For example, casing running is a difficult job in such a long horizontal section. In some wells, the time of casing running even exceeds 10 days. The stages of fracturing operation have increased from a few to 20–30 now. The popularization of large-scale volume fracturing technology calls for higher requirements of cementing performance, cementing quality and wellbore integrity.
It is very difficult to safely and smoothly run casing in a horizontal well
Due to the long horizontal section of shale gas horizontal well, the casing in the high angle and horizontal sections may possibly attach to the wellbore. With the increase of horizontal section length, the frictional resistance increases significantly. Because of the low effective weight of the casing, it is difficult to overcome the frictional resistance through the weight of the casing itself during the casing running process.
Well trajectory is complex. The cluster well platform established under the “factory-like” production mode of shale gas has a typical three-dimensional horizontal trajectory. Generally, the mode of “vertical-building-twisting-building-horizontal” sections is used. Due to the relatively distorted wellbore trajectory and a large rate of change at a full angle, in addition to the change in formation strata encountered during drilling and the anti-collision operation of adjacent wells, the actual wellbore trajectory is more complicated than the designed trajectory (Fig. 1).
Fig. 1. Diagram of designed and actual well trajectories of Well W204H4-6.
Shale gas wells must be fractured in multiple stages to meet the industrial production standards. However, the tool string required for completion is complex and is largely susceptible to damage during casing running. There are limited technical measures for the case when the casing running encounters resistance.
Application of oil-based drilling fluid has a great influence on the cementing quality
Oil-based drilling fluid has the characteristics of high viscosity and strong adhesion, so it requires higher displacing energy. Moreover, oil-based drilling fluid is less compatible with cement slurry. Once meeting cement slurry, it will become viscous, thus making it more difficultly to be displaced. As a result, there is higher pump pressure more potential risks.
When the wellbore stays in the oil-based drilling fluid environment for a long time, oil film is generated on (attached to) the surface of the wellbore and the casing wall, thus forming a “lipophilic and hydrophobic” environment. If the oil film cannot effectively be removed to change the wetting environment, annulus micro-gap will occur, eventually resulting in failure of hydraulic seal of cement sheath.
Cement slurry mixed with oil-based drilling fluid will seriously affect the compressive strength of set cement. Test results show that the compressive strength of set cement will reduce by 50% when the mixing ratio of cement slurry to oil-based drilling fluid is 9:1.
Wellbore integrity requirements are high when large-scale staged fracturing is carried out
Great cementing quality and set cement properties are important guarantees for the long lifecycle of shale gas wells and the effectiveness of hydraulic fracturing. Cement slurry used for cementing of shale gas horizontal well should: ① be stable without settlement, and no channeling in the horizontal section; ② have less leakage and no reservoir damage; ③ be capable of anti-gas channeling, and have properly controlled thickening time; ④ have reasonably controlled rheology and a high replacement efficiency; and ⑤ form set cements with a good mechanical performance, small elastic modulus and a high compressive strength.
Furthermore, the design of shale gas well cementing should consider not only the zone isolation, but also the requirements of wellbore integrity in subsequent large-scale staged fracturing stimulation.
Casing running in a horizontal well
A lot of researches have been done on the casing running in extended reach horizontal wells around the world , . However, these theoretical studies are still far from the actual situations in the field because of the complicated relationships between the frictional force of the string and such factors as mud cake lubricity, rock properties, borehole types, string structure and wellbore geometry. In order to ensure that the casing is safely and quickly run in hole, as the most effective practice on site, the bottom hole assembly (BHA) with simulated casing string stiffness is used for drifting and special well sections are pretreated to test the compatibility of strings and wellbore trajectory.
In the Weiyuan–Changning block, drifting operation was mainly completed based on simulated casing stiffness and special well section treatment. As to the special well section treatment, redressing was conducted at the kickoff point, point A and the bottom hole, regardless of blocking, except for the blocking points. Then a short trip was taken to verify if the casing can pass the redressing section successfully.
After the casing arrived at the bottom of the well, short trips and pulling were conducted for the well section with sticking and blocking; meanwhile, the well was flushed for at least 2 weeks. The BHAs for drifting have evolved from a single centralizer in the early stage of shale gas development, to double centralizer, and then to double centralizers with larger size and near-bit three centralizers now, as shown in Table 1. It is seen that three centralizers can still improve the efficiency of casing running under the conditions of complicated casing programs and longer horizontal well sections.
Table 1. Drifting mode and efficiency of casing running.
Appropriate centralizer spacing and type can help effectively reduce the friction for casing running . Fu Jianhong et al.  applied ANSYS to analyze the casing running of horizontal well. According to their analysis result, when one centralizer is installed for an interval of three casings, 1/3 of the casings will be in contact with the borehole walls; when one centralizer is installed for an interval of two casings, the mid-point of casings between the centralizers will be in contact with the borehole walls; when one rigid centralizer is installed for an interval of one casing, no casing will be in contact with the borehole walls.
In order to prevent the casing from contacting the borehole walls so as to reduce the friction between the casing and the sidewall, in site operations, one centralizer is installed for an interval of one casing in the high-angle and horizontal sections. Centralizers correspond to different frictions in casing running. In general, the friction coefficient is reflected in a descending order of ball centralizer, rigid centralizer, and elastic centralizer.
To reduce the friction and cost of casing running, a combination of ordinary rigid centralizer, large chamfer cyclone rigid centralizer and ball centralizer is used in actual operations. Furthermore, through the simulated calculation, reasonable spacing of centralizer is designed. A basic template for centralizer installation has been established, as shown in Table 2.
Table 2. Type collection and installation methods of centralizers.
Flushing spacer fluid technology in high-efficiency oil displacement
For the purpose of cementing interface cleaning  and isolation under the condition of oil-based drilling fluid, the system of flushing agent and flushing spacer fluid for high-efficiency oil displacement has been developed through experimental research and evaluation. The flushing agent is mainly based on surfactant compounding, namely, surfactants with different properties are optimized and combined, so that the reactions (e.g. curling, emulsifying and solubilization) between the surfactants and oil can be coordinated to realize more effective cleaning of oil pollution and oil film on surfaces.
The flushing spacer fluid system for high-efficiency oil displacement mainly consists of suspension stabilizer (XFJ-5), flushing agent (CXJ-0), water and weighting agent, with the density of 1.50–2.40 g/cm3. The overall performance of the system was evaluated through experiments, and the results are shown in Table 3, Table 4.
Table 3. Rheological parameters of spacer fluids with different densities.
Table 4. Stability of spacer fluids with different densities.
Based on the results shown in Table 3, Table 4, the density of the spacer fluid system can be arbitrarily adjusted in the range of 1.50–2.40 g/cm3, and the rheological property and stability of the slurry can be guaranteed. The slurry is cured at 90 °C for 20 min, and then placed still for 2 h. Accordingly, the density difference (Δρ2h) is less than 0.02 g/cm3, and the fluidity at room temperature is above 22 cm, which meet the requirements of safe pumping.
It can be concluded from Table 5 that the spacer fluid has a good flushing effect from room temperature to 120 °C, and is slightly lower but over 90% under the normal temperature and 120 °C, which are enough to guarantee the effective cementation between slurry and side wall.
Table 5. Flushing performance evaluation of the spacer liquid system .
According to Table 6, the viscosity of cement slurry increases after it mixes with oil-based drilling fluid, making it fail to flow, thus leading to serious slurry pollution. The addition of spacer fluid can effectively improve the rheological properties of cement slurry and drilling fluid, making them have a good compatibility both at low temperature and high temperature. Moreover, the fluidity of mixed fluid is above 18 cm and increases with the temperature.
Table 6. Anti-pollution performance evaluation of the spacer fluid system.
Another mixture with cement slurry and drilling fluid of 7:3 is used for thickening test. The thickening time at 70 °C is only 15 min. When a proportion of flushing fluid is added instead of drilling fluid to generate a mixture of 7:2:1, a three-phase pollution experiment shows that the mixed slurry is still not thick after 270 min.
To sum up, the spacer fluid can be pumped smoothly. It can effectively clean the borehole wall and improve the environment at the cementing surfaces to prevent the contact contamination between cement slurry and oil-based drilling fluid. As a result, the operation safety and cementing quality can be well guaranteed.
Application of ductile anti-channeling slurry
Large-scale volumetric fracturing technology is often used in the development of shale gas. Due to the high pumping pressure and large injection and displacement, severe pressure changes and impacts occur during the operation, and thus have a serious impact on ordinary brittle set cements. As a result, the cement sheath may generate micro clearance or even ruptures, which can destroy the sealing of the wellbore, resulting in gas channeling in annulus and restricting later development , , , , , , , .
In order to develop a cement slurry system suitable for volumetric fracturing, according to the actual well structure, stress conditions and wellhead pressure at the Changning–Weiyuan shale gas demonstration zone, and considering the initial stress state of the cement sheath and wellbore temperature changes, we used the viscoelasticity model to predict the plastic deformation and micro-annulus of the cement sheath in the whole well, and analyze the size of the micro-annulus of the cement sheath at the horizontal section and the kickoff point.
Assuming that wellhead pressure is 60 MPa, vertical stress gradient is 2.6 MPa/100 m, maximum principal stress gradient is 3.0 MPa/100 m, minimum principal stress gradient is 1.9 MPa/100 m, and Poisson’s ratio of cement sheath is 0.19, the analysis results are shown in Fig. 2.
Fig. 2. Tresca stress distribution of cement sheath with elastic modulus of 10 and 7 GPa respectively during fracturing.
It can be concluded from Fig. 2 that, in fracturing, the cement sheath of the whole well suffers from a large shear stress, so the possibility of shearing failure is high. When the wellhead pressure ranges between 60 and 70 MPa, the stress of set cements at different positions in the cementing sctione is shown in Table 7.
Table 7. Stress of set cement with different elastic modulus.
It can be seen from Table 7 that the maximum Tresca stress of ordinary cement sheath (elastic modulus of 10 GPa) in the horizontal section reaches 57.1 MPa, which is higher than the compressive strength of set cement under confining pressure. If the destruction of the cement sheath is avoided, the elastic modulus should be controlled at about 5 GPa, and the compressive strength under triaxial stress should reach 38 MPa. On the other hand, the maximum Tresca stress of cement sheath is 48.4 MPa when the elastic modulus of set cement is 10 GPa at the kickoff point. The maximum Tresca stress of the cement sheath obviously lowers.