Successful Refracturing Enhances Oil Production in Horizontal Wells: A Case Study from Daqing Oilfield, China
Authors: Wenjun Wang , He Liu , Zhaoquan Liu , Lan Zhongyu , Jianwen Yan , Lirong Zhang , Qing Liu , Liu Yongdong , Lin Zeng , Hu Yannian 
Published: December 2009

Abstract
In recent years, hundreds of horizontal wells have been drilled with cemented casing completion in Daqing oilfield, China. Some of these wells in low permeability reservoirs have been completed by using limited entry fracturing to increase the initial oil production. The production performance of some limited entry fracturing horizontal wells is lower with decline rapidly. Temperature logs of these wells indicate parts of the perforated intervals in the horizontal wellbore are not being treated enough. That is a main reason for low production. How to enhance the production of these horizontal wells? A multiple stages refracturing treatment by using packers isolated has been performed on one well. The use of more aggressive proppant schedules and intense quality control has been designed. The treating pressure of each refracturing stage shows very different that can suggest which one is not stimulated in the initial fracturing treatment. The result from the refracture treatment was encouraging with a peak post-refracture treatment production rate in excess of the peak production rate after the original completion stimulation. This possibility makes more horizontal wells as candidates for refracturing.

Introduction
Daqing oilfield, the biggest oilfield located in the Songliao basin in the northeastern of China, is a huge complex consisting of many individual oil fields. As reservoir development continuing, tighter and thinner pay zones become new targets in term of production incensement for this matured field. The production of the wells in the new blocks is as low as less than 1 ton/day for some wells. How to improve the production of these areas is a major concern. From 2003, horizontal well completion and fracture stimulation have thus become applicable to improve the production. Within the first three years, to reduce the risk of treatment in horizontal well, limited entry perforation to distribute the fractures along the horizontal lateral in single-stage treatments has proven to be an effective completion method. The technique has been applied to 24 horizontal wells in Daqing Oilfield. The average initial oil production after using the limited entry fracturing in horizontal wells is 3.2 times higher than that in vertical fracturing wells. But the production performance of some horizontal wells is still lower with decline rapidly. S92P48 well in Linjiang area located in southeast part of Daqing oil field is one of them.
Comparing the result of the temperature logging before and after fracturing treatment can show us significant about fractures distribution along the lateral. Figure 1 shows the comparing results of temperature logs in a limited entry fracturing horizontal well. In the well, we can see the four parts of significant low temperature areas by cold fracturing liquid being pumped into the hot formation, which indicates hydraulic fractures exist. But we can also find that there are just slight temperature changing at the limited entry perforation part near the end of the horizontal lateral, that indicate hydraulic fractures is not exist. Even in the three significant low temperature areas, the extents of temperature dropping are different, that show us the fractures in the horizontal wellbore are not being treated uniformity. Although perforations has been designed to hope to control the treatment fluid distribution uniformly, but sometimes we can't get the results as we want because of the heterogeneous lithology, great anisotropy of reservoir permeability along the average horizontal lateral length over 500 meters. That is a main reason for low production and decline rapidly.