This paper summarizes part of the results of an investigation of fracture clean-up mechanisms undertaken under a Joint Industry Project active since the year 2002. It is well documented in the literature that hydraulic fractures, although successful, often underperform: Frac and Pack completions exhibit positive skin values, and traditional hydraulic fracture completions show discrepancies between the placed propped length and the effective production fracture length. Ineffective fracture clean-up is often cited as a likely culprit.
The main results presented in this paper were obtained using a modified conductivity cell to allow polymer concentration via leakoff, and measurements of flow initiation gradients. The paper will discuss the experimental set-up and some of the artifacts that had to be removed prior to ensuring more reliable data. The results highlight the crucial role played by the filter cake and present new data that would significantly change the common industry practice of relying simply on an average polymer concentration factor.1–3
It is shown that contrary to the current method that calculates an average polymer concentration, the polymer, in practice, concentrates only in the filter cake. It is also shown that the filter cake thickness compared to the fracture thickness plays a critical role in creating significant yield stress effects, which could be either avoided through adequate design or used to estimate the resulting productivity loss.
A JIP, active since 2002, was set up with the goal of studying fracture clean-up and using the mechanisms uncovered to devise methods that would allow the production to benefit from the full length of the fracture placed. This would either boost revenue by increasing production or decrease cost by placing smaller size treatments that would still deliver the same production as the larger, less effective treatments. It is well documented in the literature that hydraulic fractures often underperform: Frac and Pack completions exhibit positive skin values,4,5 and traditional hydraulic fracture completions show discrepancies between the placed propped length and the effective production fracture length.6 Polymer concentration leading to ineffective fracture clean-up is prominent in the list of usual suspects.6–9 In addition, it was surmised that the concentrated polymer has significant yield stress and its effect on fracture fluid clean-up was modeled using a modified reservoir simulator.10 However, the existence of yield stress effect remained controversial.
This publication presents the early JIP effort that focused on the process of polymer concentration and its impact on the fracture clean-up behavior. Special effort was undertaken to identify and measure directly any flow initiation gradient (FIG) or yield stress effect. A modified conductivity cell was used to allow a series of measurements aimed at clarifying the polymer concentration process and confirming the presence, if any, of a yield stress effect.
The conductivity apparatus consists of two Ohio sandstone cores (∼ 0.2 md) confining the proppant pack, a metal cell with a movable top piston for controlling the closure stress, pressure and temperature ports and inlet and outlet ports with valves. The cores are cut to shape and potted along the edges with a rubber sealant using a mold. They are saturated under vacuum with 2% KCl prior to use. The cell construction begins with placement of the lower core, which includes a gasket on top to seal against the metal cell lip. The inlet and outlet line dead volumes are filled with water and the valves are closed. Next, the proppant is mixed with a small amount of the gel and placed on the bottom core. This slurry is leveled with a tool resembling a T-square and the remaining amount of fluid is added to the cell. The top core is inserted and air leaks out through a small hole in the mold surrounding the core. When liquid is expelled from this hole, sealant is used to plug the hole. The metal piston with O-ring is inserted and the cell is placed in the press. Fig. 1 is a schematic of the experimental apparatus. A nitrogen regulator is used when the flow initiation pressure exceeds 2 psi (approx. 5 ft of hydrostatic head in the inlet pipet).