New Results Improve Fracture Cleanup Characterization and Damage Mitigation

Abstract

It is well documented in the literature that hydraulic fracture treatments, 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. This paper presents some of the results of an investigation of fracture clean-up mechanisms. This investigation was undertaken under a Joint Industry Project (JIP) active since the year 2002. The data discussed builds on the initial results published in early 2006, which indicated that the polymer concentrates only in the filter cake and that flow along the fracture encounters significant yield stress when the filter cake cumulative thickness dominates the width of the fracture. The new results presented here demonstrate successful strategies that mitigate the effects of excessive filter cake thickness. Experimental data demonstrate that flow along the fracture would encounter lower yield stress when the breaker is delivered directly to the filter cake as opposed to randomly distributed. The data also indicate that a smaller breaker amount delivered directly into the filter cake is more effective at reducing the yield stress effects than a larger breaker amount delivered randomly in the slurry. Alternative breaker materials are explored and additional data are also presented to estimate the yield stress effect for fluid flow across the filter cake from the reservoir into the fracture. Introduction A Joint Industry Project (JIP), active since 2002, was created 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,1,2 and traditional hydraulic fracture completions show discrepancies between the placed propped length and the effective production fracture length.3 Polymer damage leading to ineffective fracture clean-up is prominent in the list of usual suspects.3–6 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.7,8 The production simulation indicated clearly that yield stress can result in only a fraction of the fracture length contributing to production for a long period of time. However, the existence of yield stress effect remained controversial until the first publication9 resulting from this JIP work was made at the Formation Damage Symposium in Lafayette, February 2006. In a series of lab experiments, yield stress measurements ranged from 0 - 17 Pa. This paper builds on the results published earlier9 and demonstrates successful strategies that mitigate the yield stress effects to help restore the effectiveness of the full length of the fracture. In addition, it presents the first data characterizing flow across the filter cake to simulate flow from the reservoir into the fracture as opposed to considering only flow along the fracture length.

Fracturing fluid cleanup by controlled release of enzymes from polyelectrolyte complex nanoparticles

“…Filter cakes with high polymer concentration form on the two faces of the fracture during the injection (Figure 1) but normally a small path in the middle of the fracture has the properties of the injected polymer unless the fracture is totally plugged by filter cakes from both faces. [3] Different exposure times to fracturing fluid, [15] and different proppant concentrations along the fracture cause local polymer concentration changes along the fracture. Thus encapsulated breakers are seldom uniformly distributed to break the concentrated fluid completely.…”

“…Filter cakes with high polymer concentration form on the two faces of the fracture during injection, but normally a small path in the middle of the fracture has the properties of the injected polymer unless the fracture is totally plugged by filter cakes. According to the latest research [3,4] , filter cakes do not form with uniform thickness and concentration along the fracture. Maximum pressure drop between a fracture and the reservoir during the production occurs through the filter cake.…”

“…Using high concentrations of enzymes as the breaker causes premature degradation while encapsulated breakers (EB) break the filter cake only if they are delivered directly into and uniformly along the filter cake. Even though a mixture of enzymes and EB [5] is normally used to break the filter cake and fracturing fluid, filter cake is reportedly [3,6,7] broken in a non-uniform manner. Barati et al [7] reported that yield stress of the filter cake and fracturing fluid, and formation damage as a result of fracturing fluid invasion into the matrix have significant effects on production of hydrocarbons especially in low permeability formations.…”

“…Incomplete cleanup of hydraulic fractures caused by gel residues [8] , width loss caused by filter cake [3,4] and fracture length loss due to unbroken fluid near the tip of the fracture [7] decreases the effective conductivity of hydraulic fractures compared to their designed conductivity. Delivering sufficient concentrations of breakers directly to the filter cake and distributing the breakers uniformly results in better cleanup of fractures.…”

“…Delivering sufficient concentrations of breakers directly to the filter cake and distributing the breakers uniformly results in better cleanup of fractures. [3,4] Enzymes have been used successfully as breakers for fracturing fluids for many years. [1] Enzymes are polymer specific, environmentally benign, easy to handle, miscible in the fluid, equipment friendly and not consumed since they act as catalysts.…”