In the Huangjinba Block of the Zhaotong National Shale Gas Demonstration Zone in the periphery of the Sichuan Basin, two horizontal shale gas wells suffer severe casing deformation. In this paper, the geological and engineering characteristics of the strata around the deformed casing interval were analyzed based on the 24-arm caliper measurement of casing deformation, open-hole caliper, electrical resistivity, drilling time and gas logging, cementing CBL∖VDL and CBL imaging. Then, 24-arm caliper measurement data were analyzed and 3D imaged by 3D imaging analysis software, to figure out the morphologic characteristics of deformed casing.
Li Liuweia, Wang Gaochengb, Lian Zhanghuac, Zhang Leib, Mei Jue & He Yuloua
aSchlumberger China, Beijing 100016, China. bPetroChina Zhejiang Oilﬁeld Company, Hangzhou, Zhejiang 310023, China. cSouthwest Petroleum University, Chengdu, Sichuan 610500, China
Received 5 September 2017; accepted 25 November 2017; Available online 8 June 2018
It is shown that severe casing deformation tends to occur where structural fractures are developed. Besides, casing deformation is mainly in the form of “S”-shape bending vertically. The severely deformed casing is also characterized by obviously transverse shear deformation caused by the high-angle sliding compression of rocks. Therefore, some suggestions were proposed correspondingly. First, the countermeasures in this block shall focus on shear casing deformation caused by the sliding of rocks along the fracture face.
Wellbore trajectory shall be designed based on the structural contours map of pay zones to bypass the “ridge” and “valley bottom” area in the local high and steep structural zones. Second, the distribution information of fractured zones shall be predicted based on the ant-tracking attribute volume distribution map of pay zones, so that the wellbore trajectory can run along the strike of fractured zone. And third, it is recommended to use plug-drilling free big bore bridge plug or full-bore infinite stage completion technique, so that the conventional bridge plug milling operation after fracturing can be omitted and each fractured layer can be put into production after fracturing.
For the production wells in the Huangjinba Block of the Zhaotong National Shale Gas Demonstration Zone undertaken by PetroChina Zhejiang Oilfield Company, the three-spud-in horizontal well casing program is adopted, with Ø139.7 mm casing as the production casing. The inner diameter of the casing is 115 mm, while the outer diameter of the bridge plug for pumping in multi-layer fracturing is 108 mm. After the pumping of the bridge plug between two fracturing stages or the completion of full-hole fracturing, the working tools may be stuck due to casing deformation during the mill-out of the bridge plug in some wells, which has brought great risks and difficulties to the operations. For example, the operation cycle and complex processing costs are greatly increased, and even in serious cases the well production capacity is damaged.
A lot of analysis and research have been performed on the causes or the mechanism of casing damage of conventional oil and gas wells and water injection wells in oilfield blocks , , , , , , , , . In order to further reveal the mechanism of casing damage, many scholars regard casing-cement sheath-formation as a system and consider the uniform in-situ stress, non-uniform in-situ stress, centering and bending of casing, cement sheath distribution and cementing quality, in-tube pressure and other variable conditions, to make casing stress and deformation analysis under various simulation conditions, and realize the quantitative or qualitative interpretation of the causes of casing damage , , , , , , , , , , .
Although conventional oil and gas well casing damage assessment and analysis model theory and method are still applicable to shale gas wells, the casing in shale gas wells face more severe stress conditions, more complicated geological conditions and more widespread damage with its own characteristics. In recent years, with the increase of shale gas wells and the continuous emergence of casing damage, the research on the causes and mechanism of casing damage in shale gas wells is being strengthened.
Based on elasto-plastic mechanics, Liu Kui et al.  analyzed the mechanical behaviors of the casing-cement sheath-formation system in the horizontal segment during the fracturing of shale gas wells by means of complex function and stress field decomposition. Chen Zhaowei et al.  analyzed the correlation between casing deformation and faults, fractures, shale bedding and hydraulic fracturing in the Changning–Weiyuan shale gas demonstration area. In addition, based on the mechanical conditions of sliding faults, they proposed casing damage mechanism, that is, as the fracturing fluid enters the natural fractures along a certain channel to increase the pore pressure in the fractures, when the pore pressure reaches a critical value, natural fracture sliding will be stimulated to cause the casing deformation.
In this paper, in view of the problem of casing deformation which results in the sticking of working tools and affects the production during the well completion fracturing and the drilling and milling of bridge plug before production in horizontal wells (H1-2 and H6-7), data of drilling and logging, conventional logging, multi-armed caliper log of casing and well cementing quality evaluation were integrated to carry out a study on the casing deformation location rules and deformation characteristics, so as to reveal the location characteristics of casing deformation, the sources of power and the main factors that affect the shaping of the casing.
Positions with casing deformation
Serious casing deformation occurs in the associated fracture zone or natural fracture zone in the fault fracture zone
According to the operation records, the casing drift diameter was 112 mm (with inner diameter of 115 mm) before casing deformation at 2887 m in Well H1-2 of a platform and was less than 92 mm after deformation and less than 73 mm at 3007 m. The drift diameter of casing at 3441 m in Well H6-7 of another platform after deformation was less than 76 mm. Casing deformations at these positions are extremely severe, posing great difficulties to subsequent wellbore operations.
The electrical logging and mud logging curves of the Ø215.9 mm borehole of Well H1-2 in the third spud-in (Fig. 1) show that fault fracture zone was encountered in the well at 2780–2820 m and 2900–2945 m. The point with extremely serious casing deformation in this well was at 2887 m near the fault plane, belonging to the associated fracture development zone, where the formation of fracture resulted in the release of stratum stress and regular borehole diameter.
Fig. 1. Well logging curve and cementing quality evaluation of the interval with severely deformed casing in Well H1-2.
Multi-armed caliper log shows that vertical rock dislocation occurred at this position and the adjacent intervals. A point with very serious casing deformation at 3007 m is located in the relative development zone (2945–3035 m) of secondary fractures adjacent to the fault fracture zone at 2900–2945 m. Due to the partial development of fractures, stratum stress was released to some extent in this area, the degree of formation breaking somewhat slowed down, and diameter expansion was seen in the borehole.
Due to the existence of cracks, the show of crack gas was active. Compared with the interval of 3035–3400 m, this interval is characterized by low resistivity under the same characteristics of high gas logging value. This indicates that the gas content of the shale matrix in this interval is not high and the shale gas entering the wellbore during drilling is mainly crack gas. The point at 3007 m has a very low resistivity and the highest gas logging value, showing that cracks are most developed there.
Casing deformation was found in Well H1-2 when the 18th-stage cable bridge plug and perforating tool string were pumped to the point at 2887 m. Even though the casing deformation occurred during the 17th-stage fracturing, the Ø108 mm bridge plug was able to rush through the deformation point, showing that the initial casing deformation was not serious and the deformation was aggravating with the increase of time. After the entire fracturing was completed at 3007 m in Well H1-2, the Ø98 mm milling shoes could freely pass the position on the 82th day, and the Ø73 mm tubing body had been unable to pass the position on the 201th day, showing that the casing deformation increased with time.
Fault fracture zones are also clearly shown on the CBL and VDL curves of the cementing quality evaluation and the 8-slice CBL sound-amplitude imaging (Fig. 1). Due to the serious expansion of drilled hole caused by a fault rupture zone, channeling occurred in this interval during cementing and the cementing quality was poor.
After the full-hole fracturing was completed in Well H6-7, the pumping of bridge plug was smooth, showing that the serious casing deformation at 3441 m did not occur during the volumetric fracturing. Therefore, casing deformation at 2887 m and 3007 m in Well H1-2 and at 3441 m in Well H6-7 cannot be explained by the mechanism of casing damage proposed by Chen Zhaowei et al. . It is necessary to further explore the casing damage mechanism of shale gas wells.
The serious deformation point of Ø139.7 mm production casing in Well H6-7 was located at the depth of 3441 m. Neither the formation, which was the 4th sublayer of the Lower Silurian Longmaxi Fm, nor the lithology changed before and after the deformation point was encountered. Compared with the adjoining wells H6-1, H6-3 and H6-5 in the same platform, the well has abnormally high gas logging values at the interval of 3310–3495 m, ranging from 30% to 60%. In addition, the drilling rate of penetration of the well interval is abnormally high, with an average of 22 m/h, which is three times the normal drilling rate of penetration (ROP) (6–8 m/h) of adjacent well intervals (Fig. 2).
Fig. 2. Curves of drilling rates and the total hydrocarbon contents in gas logging at the casing deformation interval in Well H6-7.
Therefore, it is determined that the interval is an interval with large-scale fracture development. According to microseismic monitoring, the azimuths of fractures at the 7th and 8th stages are 64° and 122°, respectively. It is shown that azimuths of fractures in the natural fracture zone from 3310 m to 3495 m change from 122° to 64°. Considering that the very serious casing deformation point at 3441 m is located relatively in the middle position between the 7th and 8th stage, it is speculated that the wellbore (with azimuth of 7°–8°) at 3441 m intersects with the natural fractures at a high angle.
Casing bending deformation occurs easily at the heterogeneous position of set cement
The 24-arm caliper measurement of the Ø139.7 mm production casing at the interval of 245–2788 m in Well H1-2 shows that there is a good corresponding relationship between the place with intense casing deformation and the place with sudden change of the set cement quality of production cementing (Fig. 3), that is, along the direction of the wellbore, the position with sudden change of the set cement (good edge of set cement) is prone to have casing deformation. Under the external force of the surrounding rocks, the set cement becomes a “mold” and shapes various curved forms of casing.
Fig. 3. Well logging evaluation of interval of 2875–2930 m with casing deformation in Well H1-2.
Casing with more serious diameter shrinkage and deformation is closer to the perforation and fracturing position
There were perforation intervals of 2980.0–2980.9 m, 3000.7–3001.6 m and 3058.0–3058.9 m near the very serious casing deformation points at 2887 m and 3007 m. By comparing the maximum, average and minimum casing internal diameter curves (Fig. 4) obtained from the 24-well caliper measurement data of Well H1-2, it can be seen that the internal diameter of the casing in the interval of 2945–2978 m decreases with the increase of depth. This indicates that the diameter shrinkage and deformation is more serious as the casing is closer to the perforation and fracturing zone.
Fig. 4. Comparison of diameter changes of borehole and production casing measured in Well H1-2.
Morphology of casing deformation
Using the 24-arm caliper measurement data of production casing in Well H1-2, only the main deformation section of the casing was selected for three-dimensional imaging analysis (Fig. 5) to study the casing deformation.
Fig. 5. Three-dimensional imaging interpretation of multiple bending deformation of casing in Well H1-2.
S-shaped bending is the main casing deformation
Three-dimensional imaging analysis of 24-arm caliper measurement of production casing at the interval of 245–2978 m shows that there is casing deformation in many intervals of the well. Deformation shape is dominated by the vertical S-shaped bending.
Extremely severe deformation occurred at 2886–2889 m and 2894–2899 m, severe deformation occurred at 2904–2921 m, moderate deformation occurred at 2815–2825 m and 2970–2973 m, and slight deformation occurred at 1724–1728 m. Detailed comparative analysis shows that the most serious casing deformation (bending) is not in the interval with wellbore collapse and serious diameter expansion (Fig. 4).
Extremely serious casing deformation is characterized by vertical shear displacement deformation
Well intervals of 2886–2889 m and 2894–2899 m have extremely serious casing deformation. It can be seen from Fig. 5 that the vertical casing bending of these two intervals are large, and the diameter of the casing are significantly reduced or the casing dislocates and deviates from the original drilling hole space in some area. There is obvious shear dislocation of casing caused by the vertical dislocation and extrusion of rocks. This leads to great reduction of the drift diameter of casing, which is the main reason for the sticking of different sizes of working tools for many times.
Frequency of S-shaped bending is high (times/1.5 m)
For the interval with extremely serious casing deformation at 2886–2889 m and 2894–2899 m and the interval with serious casing deformation at 2904–2921 m, the three-dimensional images obtained through 24-arm caliper measurement are used to carry out the statistical analysis on the frequency of the casing bending deformation. The bending frequency of the interval with extremely serious casing deformation at 2886–2889 m and 2894–2899 m is about once every 1.5 m. The bending frequency of the interval with serious casing deformation at 2904–2921 m is also about once every 1.5 m. It is clear that there is certain regularity in the bending deformation of casing.
Mechanism of production casing deformation
Different from conventional sandstone horizontal well fracturing, shale gas horizontal well fracturing is characterized by small spacing (about 75 m), large fracturing fluid volume (2000 m3/stage) and large amount and complex shape of formed fractures. Fracturing with these characteristics is called volumetric fracturing in the industry.
Volumetric fracturing creates a large number of fractures in the surrounding rocks without fractures, resulting in rock failure. When the fracturing fluid encounters natural fractures, it will preferentially pass through the natural fractures. After the fractures are pinned out, the fracturing fluid continues to create fractures in the surrounding fragile strata under hydraulic pressure. When pumps are repeatedly opened during single-stage fracturing or the fluid flows through the same fracture zone during multi-stage fracturing, these rocks with natural fractures may be more prone to slip dislocation due to repeated tensions across the fracture surface.