Refracs: Why Do They Work, and Why Do They Fail in 100 Published Field Studies?

Abstract A database has been compiled and analyzed, summarizing more than 100 field studies in which restimulation treatments (hydraulic refracs) have been performed, along with the production results. Field results demonstrate that refrac success can be attributed to many mechanisms, including: – Enlarged fracture geometry, enhancing reservoir contact – Improved pay coverage through increased fracture height in vertical wells – More thorough lateral coverage in horizontal wells or initiation of more transverse fractures – Increased fracture conductivity compared to initial frac – Restoration of fracture conductivity lost due to embedment, cyclic stress, proppant degradation, gel damage, scale, asphaltene precipitation, fines plugging, etc. – Increased conductivity in previously unpropped or inadequately propped portions of fracture – Improved production profile in well; preferentially stimulating lower permeability intervals reservoir management – Use of more suitable fracturing fluids – Re-energizing or re-inflating natural fissures – Reorientation due to stress field alterations, leading to contact of "new" rock Although less frequently published, unsuccessful restimulation treatments are also common. Documented concerns illustrated in this paper include: – Low pressured, depleted wells (especially gas wells) posing challenges with recovery of fracturing fluids – Low pressured or fault-isolated wells with limited reserves – Wells in which diagnostics indicate effective initial fractures and drainage to reservoir boundaries – Wells with undesirable existing perforations, or uncertain mechanical integrity of tubing, casing, or cement This paper will explore the common problems that lead to unsatisfactory stimulation, or initial treatments that fail over time. Guidelines for evaluating refrac candidates and improving initial treatments will be reviewed. The paper summarizes restimulation attempts in oil and gas wells in sandstone, carbonate, shale and coal formations. This organized summary of field results and references will provide significant value to readers evaluating or designing restimulation treatments.