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 wells for downhole operations. For the wells that have been fractured, the mud easily leaks into fractures, bringing the 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 are 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 validate our theoretical results. 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 Young's modulus by 3%-5% but increased Poisson's ratio by 102.42% maximally, which can consequently reduce a 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 fractures was difficult to be displaced out.

Several solutions have also been provided for wells with potential heavy mud damage. In high pressure wells where a high-density well killing fluid must be used during workover, acid-soluble solid is better to be used as a weighting agent. Besides, a re-fracturing pumping schedule should be designed with more and small proppant concentration stages if a well has been damaged by heavy mud in order to avoid screen-out. A barite removing agent was developed to remove the damage caused by the barite weighting agent, which can be used for barite removal before hydraulic fracturing 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 fracture extension during re-fracturing through a well case study. A method of avoiding abnormal pumping pressure and proppant screen-out, using a barite dispersing and chelating agent before re-fracturing, was tried in a well and promising results were obtained. The understandings from this study provide a reference for a re-fracturing design of wells with potential heavy mud damage.

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