A Unique Fracturing Solution in Water-Sensitive Tight Gas Formations
Authors: Yu Tianjun , Cao Yongbo , Li Ming , Wang Junfeng , Zhang Jie , Liu Yuan , Luo Yin , Yang Haihua , Wang Taiji , Fendt Andrew , Liu Peiwu 
Published: March 2023

Abstract
Water-sensitive tight gas formations pose a unique challenge for fracturing treatments. A large fracture geometry and reservoir contact are often required in such tight formations, yet the injection of a high-volume treatment fluid may result in high risk of water sensitivity or even water-block issues. Significant reduction of treatment water and improvement of proppant placement are considered as the keys to resolving this dilemma. In our study, both nitrogen-based and carbon dioxide-based options were put into the field trials. With the engineering and operation parameters pushed to the extreme, it was found that high-quality foam fracturing with nitrogen was the most cost-effective treatment method.
The S1 and H8 tight gas formations from the East Ordos basin, China, show much lower production potential compared to their neighboring blocks in the central basin. Normal fracturing practices, including gel-based fracturing, hybrid fracturing, and high-rate slickwater fracturing, were tested in those formations, and the results were not satisfactory. We discovered the root cause for the underperformance of the reservoir using evidence from lithology, magnetic resonance, and core analysis, which indicated that despite their mediocre porosity, those formations were often associated with high clay content and poor pore structures. Therefore, the effective permeability in those formations is not only lower than that of clean sandstones but is also easier to become impaired by the invasion of water. In such cases, stimulation with minimum water invasion is the key to unlocking the reservoir potential. Two different approaches were put on trial in our study; one was based on the supercritical carbon dioxide fracturing method, and the other was based on the high-quality nitrogen foam fracturing method.
The carbon dioxide option was put on trial in early phases, and its success confirmed that such formation did have potential for production. However, this option is also accompanied by the operational challenges and excessive costs, which made the nitrogen-based solution more favorable. When the engineering and operation parameters were pushed to the extreme (i.e., the pressure was reaching the limits and the average proppant concentration rose to an unprecedented level), the outcome was satisfactory. Considering the treatment safety, water reduction ratio, proppant placement volume, cost reduction, and the positive production results, the aggressive nitrogen-based fracturing option was confirmed to be the better option for water-sensitive tight gas formation from East Ordos Basin.
This paper addresses the dilemma of large-volume fracturing in water-sensitive tight gas reservoirs. Two foam options, nitrogen and carbon dioxide, were tested in the field with parameters pushed to the extreme. Although positive production results were observed from both options, operational safety and field economics were more favorable with the high-quality nitrogen option.