Influence of spiral perforation parameters on fracture geometry near horizontal wellbores
SHAN Qinglin, JIN Yan, HAN Ling, ZHANG Ruxin
1 College of Petroleum Engineering, China University of Petroleum-Beijing, Beijing 102249, China 2 State Key Laboratory of Petroleum Resources and Engineering, Beijing 102249, China 3 Petroleum Engineering Technology Research Institute, SINOPEC Jianghan Oilfield Company, Wuhan 430000, China
During the hydraulic fracturing stimulation of a perforated well, an improper perforation policy may cause limited communication between the well and the hydraulic fractures, such as a fracture initiating from one perforation might fail to link-up with adjacent perforations or multiple fractures might initiate from one perforation or adjacent perforations. Because the complexity of near-wellbore fractures may cause a premature screen-out, leading to a failing treatment, an optimized perforation policy is required to reduce the risk of limited communication. Many numerical and experimental studies have been conducted to optimize the perforation policy of the vertical or deviated wellbores with phasing angle of 0° or 90°, and the expected fracture geometry in these studies is longitudinal fractures that grow along the axis of the wellbore. However, fewer studies have been carried out on how the perforation policy influences the geometry of transverse vertical fractures from a cased and perforated horizontal well. In this work, a combined numerical and experimental study has been carried out to investigate the sensitivity of near-well fracture geometry of spiral-perforated horizontal wellbores. First, a laboratory-scale finite element model is built to give a stress distribution near the wellbore and perforations to obtain some understanding as to which perforations act as initiation sites. Following the principle of minimum fracture initiation pressure (FIP), the minimum perforation diameter and density value have been obtained to maintain a low FIP. Based on such parameter combinations, a series of physical simulation tests for concrete samples of different perforation parameters are conducted to study the influence of increasing perforation diameter or perforation density on the fracture geometry near wellbore. This also provides a way to test the effectiveness of traditional numerical methods on the optimization of perforation policy of the spiral-perforated horizontal wellbore. The results of the tests show that the traditional finite element method (FEM) has limited applicability. For the optimized parameter combination obtained by the FEM, the large spacing of adjacent perforations leads to low probability of link-up of starter fractures. A fracture initiating from one perforation tends to propagate neglecting other perforations and fails in forming a main fracture passing through enough perforations. Hence the perforation policy optimized by traditional FEM may not enhance the continuity between the wellbore and fractures. Based on the optimization results of FEM, increasing the perforation diameter contributes to the link-up of hydraulic fractures initiating from adjacent perforations to some extent. But still there is area near wellbore where fractures overlap, and the breakdown pressure is relatively higher than that of other tests. Compared with increasing the perforation diameter, increasing perforation density can lead to much easier link-up of starter fractures and foster a main fracture passing through enough perforations. The results of this study can be used as a guide for in site execution. For both perforation diameter and perforation density influence the strength of casing. Increasing the perforation density should be first considered to reduce the complexity of near-wellbore fractures while maintaining the enough strength of the casing, leading to a successful proppant addition.
