Study of Separate Layer Fracturing Techniques on Casing Damaged Wells
Authors: Wang Z.G. , Liu Y.S. , He J.F. , Zhang C.F. , Zhang C.J. , Guo X.W. 
Published: March 2010

Abstract
Up to 18% of the 9,600 wells in the Daqing Oil Field have been identified as having casing damage due to formation movement along various slip planes in the wellbore. The number of casing damaged wells is increasing at a rate of 600 wells annually. Therefore, it is very important to determine how to apply hydraulic fracturing techniques to those wells to recover and improve oil production in related mature reservoirs. In this paper, a new technique using a small diameter selective hydraulic fracturing technique was developed for those damaged wells for which the diameters were above 105 mm. A fracturing string with matching tools was specially designed and tested, including a packer with the packing element, sand blower and safety joint. Testing results showed that stabilized pressure time could maintain approximately 5 to 8 minutes at a testing pressure of 40 ? 45 MPa for all tools. The mathematical model that could be used to predict the stress of the casing damaged wells was developed based on the analysis of ground-bearing mechanisms under casing damaged conditions. The results from the model matched very well with practical testing results. The criteria of well and layer selection would be provided based on theoretical analysis. The technique could be used to simultaneously fracture several sub-layers hydraulically.
The technique has been successfully applied to casing damaged fractured wells and is now deployed in the Daqing Oil Field. Fourteen wells were hydraulically fractured by using this new technique. All of the treatments were successful. There were two zones being fractured on average for each well, and fluid production increased 20 tons/day/well on average with a 5.0 tons/well/day increment in oil production.

Introduction
Casing damage has become a serious problem for mature oil fields. It was estimated that more than 9,623 wells, about 18.51% of the total wells, had undergone casing damage problems by the end of 2003 in the Daqing Oil Field. Among them, 1,612 casing damaged wells are still working. Casing damage made these wells difficult to fracture downhole.
The main casing damage types in the Daqing Oil Field were deformation, shear, cracking, corrosion and so on(1). But the types occurring most frequently were casing deformation and shear, which caused 96.2% of the total casing damage. The drift diameters of 961 wells, about 59.6% of the total active casing damaged wells were larger than 105 mm and these wells could be fractured without any repairs. The drift diameters of 651 active wells were smaller than 105 mm and about 80% of them could be repaired. These wells also met with basic conditions for fracturing after they are repaired. In other words, most casing damaged wells with a drift diameter of less than 105 mm could be fractured on the condition that they were repaired first. Therefore, the technique of small diameter hydraulic fracturing was studied.

Study on Well Selection Criteria for Hydraulic Fracturing of Casing Damaged Wells
Operation safety is the primary condition for hydraulic fracturing of casing damaged wells.