Sinopec's Post-fracture Coring Technology Leads The World

Relying on the major pilot experiments and scientific research projects of the group company, Sinopec Jianghan Oilfield formed a multi-professional joint research team to implement post-pressure coring in the three-dimensional development area of the Fuling shale gas field, unveiling the mystery of the underground artificial fracture network.

Drilling coring is very common in shale gas exploration and development, but there is no precedent in China for coring horizontal wells after fracturing or even after mining.

Starting first means encountering difficulties first. Faced with the problems of easy leakage, collapse, blowout, and collision in the complex fracture network area of shale gas after fracturing, and the difficulty in ensuring the core recovery rate, the Oil and Gas Capacity Construction Management Center of Jianghan Oilfield, in conjunction with the Oilfield Research Institute and the Engineering Institute, has carried out research and development, continuously breaking through the limits, and innovatively formed the post-fracturing coring technology of high-angle wells and horizontal wells, which has promoted the post-fracturing coring technology of Fuling shale gas field fracture area to the world's leading level.

Difficulty, risk, and difficult methods "I didn't dare to think about it before".

On April 1, in the core library, technicians were carefully studying the 122.45-meter core of Jiaoye 12-Jian 1 well, carefully depicting the direction, number, size, and width of the cracks to evaluate the fracturing effect.

As the first shale gas pressure coring well in China, the success of Jiaoye 12-Jian 1 well has a different meaning. "I didn't dare to think about coring after pressure before." Zhang Yuqiang, manager of the Engineering and Technology Management Department of Jianghan Oilfield, said frankly. Conventional coring will avoid the fracture area, and the target layer of post-pressure coring must be the post-pressure shale formation with developed artificial fractures. "The core in the artificial fracture area is broken, which makes the core easy to fall off, the harvest rate is low, and it is even possible that no core can be harvested."

In addition to the broken core, the old wells that have been put into production have serious formation pressure deficits after mining, low pressure coefficients, insufficient formation energy, and easy collapse during drilling. In addition, the drilling fluid may leak into the neighboring wells when shuttling back and forth between the well networks, causing the neighboring wells to "drowned" or the gas leakage from the neighboring wells to cause blowouts, which is very risky.

Focusing on drilling wells, Jianghan Oilfield has developed a "combination punch" of process and tool optimization, using high-performance low-density water-based drilling fluid, which effectively reduces the risk of formation leakage and collapse, as well as the pollution of the core, ensuring the integrity of basic information such as the gas content of the core, facilitating later research, while improving tools such as drill bits and screws, optimizing coring parameters such as drilling pressure and drilling speed, and continuously improving the success rate of construction.

During the drilling process, geological guidance personnel are stationed on site, carefully judge the formation level, increase the measurement frequency, determine the coordinates of the drill bit underground, calculate the anti-collision distance at any time, and ensure the accuracy of the trajectory.

Since conventional screw tools cannot be carried in the coring drill, technicians can only rely on experience and mechanical measures to control the drilling trajectory. This method has attracted many doubts, and some people think it is "blind underground extraction".

"By scientifically optimizing the position of the stabilizer and drilling parameters, we rely on mechanical measures to ensure that the trajectory is designed. Every problem in the process has been carefully considered and discussed by the team, and we are very confident in this method." Dai Yongbo introduced that this method was successfully applied in Jiaoye 66-Jian 2 Well, with more than 200 meters of continuous coring trajectory control, and the upper and lower deviations were only 1 meter.

After hitting the target layer, how to keep the core in its original shape and take it out became the biggest problem. Without experience to learn from, technicians can only grope forward.

Some experts suggested that the universal screw be installed behind the coring barrel, so that the trajectory can be controlled while coring. Technical experts rejected this proposal. They believed that disturbing the coring barrel during drilling would destroy the integrity of the core and easily jam the core. In the end, considering the contradiction between coring and trajectory control, the technical team boldly innovated and proposed a new working idea for the first time in China-coring without orientation and orientation without coring.

Whether the theory is feasible depends on practice. Jianghan Oilfield applied this idea in Jiaoye 66-Jiang 1 well, and the coring recovery rate reached 99.36%. After that, every well that successfully cored verified the correctness of the new idea.

Vertical wells, highly deviated wells, and horizontal wells continue to challenge the limits

Over the past year, following the steps of starting with the easy and then the difficult, the Oil and Gas Capacity Construction Management Center of Jianghan Oilfield has continuously challenged the limits from vertical wells to highly deviated wells to horizontal wells. Through independent innovation and iterative optimization of engineering processes, it has created a post-pressure complex fracture network area coring drilling technology and formed a post-pressure fracture network area continuous coring supporting process technology, which is far ahead in the world.

According to Lin Anguo, deputy director of the drilling management office of the center, after completing the first vertical well coring in Jiaoye 12-Jiang 1 well, they focused their attention on highly deviated wells. Jiaoye 66-Jiang 1 well, Jian 2 well, Jian 3 well, and Jian 4 well are four highly deviated coring wells with a well inclination of 66 degrees. The greater the well inclination, the easier it is for the core to break and block.

During the coring process of high-angle wells, technicians innovatively developed an inner barrel centralizer, which can keep the inner barrel of the coring in the center and reduce the disturbance to the core, thereby maintaining the original state of the core after fracturing. They also adopted high-efficiency coring drill bits, etc., which effectively improved the core recovery rate of shale fracturing transformation formations.

Jianghan Oilfield also innovated the coring wellbore utilization mode, and for the first time adopted the new method of "four wells in one". Only one wellbore is needed to complete the coring task of four wells, which can save tens of millions of yuan in costs. "After coring the Jiaoye 66-Jian 2 well, we side-drilled three times and efficiently completed the coring of the Jiaoye 66-Jian 1 well, the Jian 3 well, and the Jian 4 well. This is the first time that a shale gas well in China has completed the coring task of four wells in a single hole." Lin Anguo introduced that the cumulative coring footage of the four wells was 764.5 meters, the coring length was 760.63 meters, and the core recovery rate reached 99.45%, achieving "zero overflow in drilling, zero leakage of drilling fluid, zero collision of trajectory, and zero loss of information".

With the experience of coring in the "four-well-in-one" high-angle well, the technicians started the research on coring after pressure in horizontal wells. "Horizontal wells need to lie down to coring, and the core feeding resistance is very large. The gravity sag causes the outer tube of the coring tube to interfere with the inner tube, and the core is fragile." Dai Yongbo said, "Coring tools are also easy to disengage and break when they are long, so the accuracy of the tools is required to be higher."

Under pressure, Jianghan Oilfield deployed the first horizontal section coring well in the shale gas pressure fracture area in China-Jiaoye 66-Jian 5 well, with the purpose of clarifying the three-dimensional spatial fracture distribution characteristics of the Jiaoshiba three-dimensional development area of ​​the Fuling shale gas field through a variety of coring methods.

Jiaoye 66-Jian 5 well is designed to have a coring section of 266 meters and a horizontal section of 1957 meters. In response to the problems of the coring tool combination being difficult to lower and the inner barrel of the coring tool not being centered, the technicians optimized the coring drill tool combination, reduced the bearing clearance of the stabilizer, improved the centering of the inner barrel of the coring, and enhanced the working stability of the coring drilling. They also used shale water-based drilling fluid containing micro-nano plugging materials to effectively solve the problem of easy collapse of coring in the fracturing area. At the same time, they optimized the material of the inner barrel of the coring barrel to increase smoothness and reduce friction resistance.

Innovation never ends. The technicians continued to explore the limits of engineering, adopted the "six-barrel connection" coring method, and formed a long-barrel coring technology for drilling. In order to ensure the smooth progress of the construction, Jianghan Oilfield established a field technical team for post-pressure drilling and coring of shale gas wells, which provided on-site tracking and guidance throughout the process, and unified the management of coring tool preparation, drilling fluid performance regulation, complex trajectory control, and coring parameter optimization. The on-site personnel performed fine operations and efficiently completed tasks such as coring, core cutting, coring, ground cutting, gas testing, and digital scanning to ensure safe coring, smooth coring, complete cutting, and accurate data.

Finally, the Jiaoye 66-Jian 5 well achieved 240.2 meters of continuous coring footage in the horizontal section, 239.82 meters of horizontal section core length, and 53.7 meters of single long-barrel coring footage in the horizontal section, breaking five drilling records, including the longest total coring footage after single platform pressure, the longest continuous coring footage in the horizontal section of a single well, the longest horizontal section core length, the longest single horizontal section long-barrel coring footage, and the highest average recovery rate of horizontal section coring.

"On the basis of conducting horizontal drilling coring process tests in the same wellbore, we lowered the production casing to achieve the purpose of gas production, and for the first time realized the dual use of coring and gas production in a shale gas well." Lin Anguo introduced. At the same time, they also used the well to evaluate the distribution of residual gas in small well spacing, guide the deployment of small well spacing and dense wells in the next step, and lay a solid foundation for improving the recovery rate of multi-layer three-dimensional development and "eating up" underground resources.