Design and Testing of the Stimulation Tool for Pinnate Branch Tubes

In view of the problems of high investment on acid-fracturing and unsatisfactory effect in the stimulation of fracture-cavity carbonate reservoirs, and based on the principle of pinnate branch tube stimulation, a pinnate branch tube stimulation string and the key tools (i.e., branch tube nipple) were designed. Combined with the working state of the branch tube, the stress distribution and buckling characteristics of branch tube were analyzed by the finite element method, and it was found that the maximum stress of the 316L stainless steel branch tube with a outer diameter of 8.0 and 10.0 mm was less than the yield strength of the material, which belonged to the category of elastic deformation and satisfied the requirements. Through surface tests, it was verified that the branch tube could extend from the branch tube nipple, with a starting pressure of 3.0 MPa and a maximum pressure of 3.5 MPa. By testing the performance of branch tube nipple, it was found that acid jetted out through the nozzle at high speed could make a hole in the core prior to initiating the branch tube, and the extended branch tube could enter the core to enable the branch tube run through the core. Based on carbonate reservoirs in Tahe Oilfield, the feasibility of pinnate branch tube stimulation was analyzed. It was found that the dissolution rate of carbonate rock samples from the target block reached 100%, and the rate of acid jetting to create holes reached 2.6～14.4 m/h, which possessed the conditions to implement the pinnate branch tube stimulation technology. Studies have shown that the principle of pinnate branch tube stimulation technology is feasible, and it has the advantage of low operation pressure. It is highly feasible and promising to apply this technology in the stimulation of carbonate reservoirs in Tahe Oilfield.