Pipeline transportation is the most economical and reasonable way to transport natural gas. However, there are still some technical problems in high grade gas pipeline, in which fracture control is one of the key problems. The ductile fracture for long-range expansion is the most destructive failure mode of high pressure gas pipeline. In this study, the determination and prediction methods of crack arrest toughness of high grade (X80 and above) pipe line steels were introduced. Their application range, advantages and disadvantages were analyzed. The results showed that the toughness of X80 pipe line steel can meet the needs of the pipeline crack arrest requirements. It is difficult for the ultra-high-grade (X90 and above) pipe line steel to arrest crack by their own toughness. Therefore, the crack arrester should be installed. This paper introduced the crack arrest principle, advantages and disadvantages of various kinds of crack arresters.

The main pay of the Keshen gas field is a tight sand formation with very low porosity (2% to 7%) and permeability (0.001md-0.1md). Due to the ultra-deep formation depth and superior high reservoir pressure (greater than 116 MPa) and temperatures (above 160°C), the well construction and field development costs are very high. Therefore, stimulation treatments is essential for achieving sufficiently high production rate and the basis for justifying the costs. Natural fractures (NF) are developed in the reservoir with high heterogeneity which leads to complex production mechanism due to interaction between matrix and NFs, and causes difficulties on stimulation optimization. To better understand the reservoir and optimize the well stimulation, the matrix properties, natural fracture, geomechanical data and stimulation data has been reviewed along with production data. Massive production history matching work has been done to evaluate the NFs. Other dynamic data like the well testing data and pressure data also reviewed to help understand NFs development in the full field scal. Production analysis was conducted to link the rock properties, NFs and wells' performance, and provided full understanding of production controlling factor in this reservoir. The post production analysis helped to understand the efficiency of stimulation. The production behavior for different stimulation strategy also have been illustrated by the simulation models to investigate the impaction on the stimualtion optimization strategy by the interaction between matrix and NFs. The study provided a good understanding of the production mechanism in this kind of natural fractured reservoir and NF's impact on the stimulation efficiency. The heterogeneity of NF results in very different production mechanism given similar matrix petrophysical behavior. Based on production mechanism, each well requires a customized stimulation to maximize the production potential. This study presented a case study for evaluating the production mechanism and highlighted the importance on evaluating the production mechanism on optimizing the stimulation for the naturally fractured tight reservoir.

Keshen gas field, located on the northern margin of the Tarim basin, Western China, is an unconventional sandstone tight gas reservoir with extreme reservoir conditions: ultra-deep, low porosity, low matrix permeability, high temperature and high pore pressure. In order to gain economic production most wells should be stimulated to enhance single well performance. Previous studies show that natural fractures (NF) play the most important role on productivity. Detailed studies on block KS2 found that well performance is controlled by the intersection angle (θ) between NF strike and direction of the maximum horizontal stress. When the interaction angle is small, well productivity is good. Otherwise, well productivity is poor. Based on this conclusion, different simulation options were proposed: 1) Acid hydraulic fracturing for wells with small angle, and 2) Proppant hydraulic fracturing for wells with big angle. New problems were meet when we applied this rule to other blocks, such as block KS8. In this block most wells reached a good production even if the intersection angle is big (>40°). Therefore, it is unreasonable to determine stimulation options based on the intersection angle for these wells. In order to establish alternative stimulation options, deeper analyze focused on natural fractures has been conducted. First, the tectonic history was studied to understand the NF creation and it was found that natural fractures are associated with two main tectonic phases. Most of the natural fractures developed during the earliest phase are infilled by calcite or shale(set 1), Whereas NFs developed during the last phase are open or partially open, have high permeability and are contributing to the production(set 2). Wells with reticular NFs have two sets of NFs and have high production. Advanced analyses of borehole images, including fracture classification according to the tectonic events, fracture density, and intersection angle computation, allowed us to create four main classes: class 1: presence of reticular fractures, class 2: parallel fractures with small intersection angle, class 3: parallel fractures with big intersection angle, and class 4: no natural fractures. New stimulation options were proposed based on these four classes: class 1: acidizing without hydraulic fracturing, class 2: acid hydraulic fracturing, class 3: proppant hydraulic fracturing, and class 4: it is hard to get economic production for these wells, even if the proppant hydraulic fracturing is operated. These new stimulation options have been applied in new blocks of Keshen tight gas field and provide a practical way to optimize the stimulation, to ensure well performance, and to reduce the cost associated with multiple stimulation phases in this tight gas field.