Proppant Screen-Out During Re-Fracturing Caused by Damage of Heavy Mud in Existing Fractures: Case Study and Solutions

The reservoir pressure coefficient is over 1.80 in the Kelasu foreland area in western China. Heavy mud weighted by ultra-micro barite is used to kill the well for downhole operations. For the wells that have been fractured, the mud easily leaks into the fractures, bringing barite weighting agent into the existing fractures and damaging the well productivity. Re-fracturing is conducted in order to recover the well productivity, but abnormal pumping pressure and proppant screen-out is frequently encountered. Three typical wells that have encountered such problems were analyzed by construction data comparison and using the fracturing simulator StimPlan. The root causes were firstly theoretically analyzed and then laboratory tests were also carried out to verify our theoretical deductions. The comparison results show that the pressure for fracture extension during re-fracturing of the three wells is 20%-40% higher than the initial fracturing; the leakage of the heavy well killing fluid weighted by ultra-micro barite during downhole operations was the main cause of the abnormal pumping pressure and the proppant screen-out during re-fracturing. The laboratory test results showed that the properties of the ultra-micro barite well killing fluid is relatively good after rolling for 15 days under high temperature, but its density and rheological parameters tend to decline due to barite sedimentation; the invasion of the heavy well killing fluid into the matrix reduced the Young's modulus by 3%-5% but increased the Poisson's ratio by 102.42% maximally, which would consequently reduce the dynamic fracture width and lead to abnormally-high pumping pressure and proppant screen-out during re-fracturing. The damage of the heavy well killing fluid to the existing propped fractures was also serious. It reduced the propped fracture permeability by 50%-80% and the retained barite in the fracture was difficult to displace out. Several solutions have also been provided for wells with potential heavy mud damage. In high pressure wells where high-density well killing fluid has to be used during workover, acid-soluble solid is better to be used as the weighting agent. Besides, the re-fracturing pumping schedule should be designed with more and small proppant concentration stages if the well has been damaged by heavy mud so as to avoid screen-out. A barite removing agent was developed to remove the damage caused by barite weighting agent, which can be used for barite removal before hydraulic fracturing so as to avoid abnormal pumping pressure and proppant screen-out. The use of this agent in well BZ-D significantly mitigates the pumping pressure during re-fracturing, and a production increase of 131% was obtained. This paper digs into the impact of heavy mud residue on fracturing extension during re-fracturing through well case study. A method of avoiding abnormal pumping pressure and proppant screen-out, using barite dispersing and chelating agent before re-fracturing, was tried in a well and promising results were obtained. The understandings from this study provide reference for re-fracturing design of wells with potential heavy mud damage.