Abstract

The selection of proppant to provide highly conductive pathways in hydraulically-generated fractures is typically based on the proppant crush strength, permeability, availability, and cost. Extensive laboratory data determined conductivity values, obtained using API standardized methods at a variety of simulated well conditions, are available for most proppants. However, post-fracture stimulation well testing indicates that these values are often one to two orders of magnitude too high. In many fields, the productivity of fractures declines rapidly, requiring frequent re-stimulation treatments to remain economically viable. Proppant crushing and embedment, fracturing-fluid damage, and fines invasion are proppant-pack permeability damage mechanisms that have been used to explain this loss of productivity. This paper reports on recent studies that have determined that alumina-based proppant materials may promote geochemical reactions that can occur at a surprisingly rapid rate, even at moderate temperatures, resulting in the loss of porosity and permeability and the creation of fines in the proppant pack. The compatibilities of several man-made proppants ranging from lightweight ceramics to high-strength bauxites with a variety of formations are presented.

INTRODUCTION

Hydraulic-fracture stimulation (HFS) is the process of pumping fluid under pressure to fracture earth formations. These fractures are usually “propped open” by leaving them filled with a high-permeability pack of granular material (proppant) that provides a highly conductive pathway from the reservoir to the well.

  • Isolate the zone of interest (ZOI) (the zone with potential to produce oil and/or gas).

  • Perforate the well casing with shaped charge guns or hydrajet with abrasives entrained in fluid to create holes in the casing interval that crosses the ZOI.

  • Mix a viscous carrier fluid by adding gelling agents (e.g., powdered guar) to water or one of various oils (there is a movement away from using hydrocarbons, because of the risk of environmental damage).

  • Blend proppant (mined sand or man-made) with the viscous fluid (usually done “on-the-fly”) as the fluid is pumped into the wellbore.

  • Using pumps with outputs exceeding 2,000 hydraulic horsepower (HHp) each; pump the blended fracture fluid (through jointed tubing or coiled tubing) into the ZOI. Multiple pumps can be manifolded together to provide extreme pumping rates and pressures. Because the fracture fluid is sealed into the wellbore interval adjacent to the ZOI, the fluid travels through the perforations under great pressure and extends the fractures into the reservoir rock containing the hydrocarbons (oil and gas) being sought.

  • After the fracture has been initiated, proppant volumes are increased to open a wider fracture in the reservoir and fill the fracture with a permeable pack that will provide easy paths for hydrocarbons to flow into the wellbore and on to the surface for marketing.

In the typical HFS operation, massive spreads of equipment are used to:Much effort has gone into standardizing the selection of proppants to optimize stimulation cost against expected stimulation results. Many proppant materials are available, including natural sands, ceramics, sintered minerals, plastics, and composite materials.

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