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.

Breakdown pressure and fracture surface morphology of hydraulic fracturing in shale with H 2 O, CO 2 and N 2

“…For unconventional reservoirs such as shale with extremely low permeability, long horizontal laterals with multi-staged hydraulic fractures are necessary to deliver economic production. The introduction of hydraulic fractures significantly increases flow rate because of large surface contact area between fractures and the reservoir, enhanced permeability around the wellbore, and reduced fluid diffusion lengths (King 2010;Vincent 2010;Faraj and Brown 2010).…”

Pressure Transient Behavior of Horizontal Well with Time-Dependent Fracture Conductivity in Tight Oil Reservoirs

“…However, this operation can only alleviate the decrease rate of the fracture conductivity. The conductivity will eventually decrease [31][32][33][34]. Previous studies suggest that the fracture conductivity decreases rapidly during the first couple of days and for the rest of the time the decline degree remains very small ( Figure 1).…”

“…Although it is well documented that the fracture conductivity decreases rapidly at first and tends to converge asymptotically [31][32][33][34]37], previous studies always assumed that hydraulic fractures do not vary with time. Below, we analyze the difference between time-independent and time-dependent fractures.…”

Bayesian inversion of pressure diffusivity from microseismicity

“…With no better alternative at hand, companies have no choice but to excessively fracture (and refracture) the rock at great cost to themselves (Vincent, 2010(Vincent, , 2012LaFollette et al, 2013). Just a few important components contributing to the total cost of fracturing are as follows: water and proppant usage, pump trucks, water disposal, and public concern about triggered seismicity.…”