Evaluating Natural Fracture Effectiveness to Optimize Well Stimulation Method for HTHP Tight Gas Reservoir

Abstract 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.