Abstract

In the last decade, many gas reservoirs with permeabilities from 0.1 to 10 mD have been developed with horizontal wells with transverse fractures. The potential negativeimpact of convergent flow in the fracture seems to have been forgotten. The widespread use of resin coated proppant (RCP) in offshore wells appears to make this problem worse. Using both case studies and reservoir simulations, we examine why RCP could make the problem of convergent flow worse compared to uncoated proppant.

A number of North Sea horizontal and deviated multi-frac gas wells that used RCP and had a significant mechanical skin are presented. Pressure buildup data confirms the presence of a near-wellbore pressure drop in the fracture. Reservoir simulation with a fine grid reproduces the observed pressure drop, due to convergent flow, using realistic proppant pack permeabilities with gel damage.

The effect of proppant production on the convergent flow skin is shown using production data before and after discrete proppant production events, demonstrating how proppant production has a beneficial effect on removing convergent flow skin. We also compare the performance of a new horizontal multi-frac well to the original discovery well in the same location, where a vertical well was fracture stimulated with uncoated proppant, and had comparable productivity.

If there is a large convergent flow skin in a fracture with uncoated proppant, this usually leads to some proppant production, which could create "infinite conductivity" channels at the perforations. This removes the convergent flow pressure drop. If RCP is used to prevent proppant flowback, such channels cannot form easily and convergent flow acts as a downhole "choke" on production. This "choke" can produce a significant positive skin. In unfavorable cases, a horizontal multi-frac well with 3 or 4 transverse fractures will produce the same as a fully perforated vertical well with a single fracture.

Based on a number of real-world incidents of proppant production during the post-fracture cleanup, we show strong evidence that a small amount of proppant production can result in an increase in well PI and decrease in apparent fracture skin. Convergent flow is the most likely mechanism to explain this. This paper highlights the potential reduction in well productivity due to using resin coated proppant for fracturing in gas wells (0.1 to 10 mD) with limited inflow area (transverse or oblique fractures) where convergent flow pressure loss is significant. We show the potential positive effect of small amounts of proppant production in such cases, forming infinite conductivity channels and removing the convergent flow skin.

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