Evaluation of Refracture Reorientation in Both Laboratory and Field Scales

Abstract

Abstract Most tight oil reservoirs need to be hydraulically fractured before putting into production. Refracure is a necessary and efficient way to control production decline after first fracturing treatments. It has been recognized that reorientation of propped refracure treatments can contact with undepleted reservoir areas and that refracure reorientation is one of the main mechanisms for production enhancement. In this paper, laboratory tests for fracture reorientation were investigated. Case histories of refracure treatments and tiltmeter mapping results were discussed. All the fracture treatments included a novel design technique of pumping paraffin balls approximately half way through the propped treatment in an attempt to initiate new fracture growth through different a perforation interval or in a different orientation. Tiltmeter mapping results indicated that fracture reorientation occurred for the majority of these refracure treatments studied. Case histories involved two tight oil reservoirs in the Daqing oilfield, China. More significant post-frac production improvements were observed for the refracure treatments with fracture reorientation. Introduction Hydraulic fracturing is very important for the economic development of many tight oil reservoirs. Refracure treatments have been used for production enhancement for decades, in attempts to create new fractures in different directions, to increase fracture dimensions, and to improve fracture conductivity. Refracuring has proved to be the most important means to enhance oil recovery in low permeability reservoirs in the Daqing oilfield. But as the frequency of refracure treatments increases, production gain from each subsequent refracure treatment declines quickly. Conventional techniques used for refracure treatments are sometimes (if not often) problematic: First, a refracure treatment tends to open an old fracture and only to regain the conductivity of the original fracture. In this case, the fracture from a re-frac treatment can not reach an undepleted reservoir area or areas that are less affected by the process of water injection and oil production. Secondly, a re-frac treatment tends to be larger than the original or previous frac treatment. As a results, it tends to increase water production in a highly water flooded reservoir in the Daqing oilfield, if the fracture created by the refrac treatment follows the same path of the original fracture. In this study, a laboratory apparatus was constructed to simulate both the fluid injection procedure and the 3-D stress field around testing samples. The experimental investigation demonstrated that a fracture could reorient itself as the stress field changed. In laboratory experiments, multiple fractures were also observed during multi-direction perforating. In addition, surface tiltmeter mapping was used to measure:fracture orientation andif the fracture orientation changed during the course of refrac treatments, andif multiple fractures were created. It was observed that multiple fractures could be created in two forms: T-shaped fractures with both horizontal and vertical components, and multiple vertical fractures with different orientations. Refrac treatments from four wells in two tight oil reservoirs in the Daqing oilfield were investigated. The results indicated that it was possible to increase production from refrac treatments in both of the two reservoirs. The target formation is at depths from 1,060 meter to 1,080 meter for one reservoir and it is from 1,550 meter to 1,610 meter deep for the other reservoir. Both the reservoirs consist of sand deposits with low porosity and very low effective permeability. The distance between the two reservoirs is about 108 km. Tiltmeter mapping results indicated that horizontal fractures and vertical fractures were created in the shallower reservoir while vertical fractures were dominative in the deeper reservoir. Tiltmeter mapping results also indicated that fracture reorientation occurred for some of these re-fracture treatments studied.