Many engineers disregard laboratory reports demonstrating that the pressure losses across proppant samples are substantial. Similarly, it appears that the modeling studies warning of substantial productivity losses due to inadequate fracture conductivity have not been universally convincing.
Perhaps many practical frac engineers simply object to the theoretical nature of these arguments, when they quote one of the more influential orators of our time, Shania Twain,1 stating, "That don't impress me much."
Instead, many Petroleum Engineers prefer to see the economic benefit demonstrated in real reservoirs, and seemingly borrow a quotation from the film Jerry Maguire,2 stating, "Show me the money!"
The purpose of this paper is to summarize the results of over 80 field studies where well productivity was improved by increasing the fracture conductivity. The benefit of increased conductivity has been demonstrated in oil, condensate, and gas reservoirs in 50 regions around the globe.This benefit was documented by 250 authors representing over 70 companies. Increased conductivity has been shown to be beneficial in oil wells producing 2 bopd to 25,000 bopd, and in gas wells producing less than 1 MMCFD to over 100 MMCFD. Higher conductivity fractures were proven to improve the cash flow in carbonates and sandstones at depths of 2800 to 20,000 feet, and in low rate coal bed methane wells shallower than 1500 feet.
This review of industry experiences in a wide variety of reservoirs is not presented as a substitute for a comprehensive optimization study in a specific location. Instead, the following summary demonstrates that well productivity and profitability can frequently be improved with redesign of hydraulic fractures, despite the failure of many existing production models to predict those benefits.
These studies are presented to satisfy the following goals:To verify that the extreme pressure losses observed in the laboratory and predicted by current fluid flow theory are real, i.e., "Proving It".To provide a list of fields, encompassing a wide variety of reservoir types, to assist engineers searching for results from an analogous field.To provide a number of field studies to which production models can be calibrated.To summarize the collective experience of over 250 authors, and incorporate what they have learned into future fracture designs.To demonstrate the relationship between fracture conductivity, well productivity, and cash flow, i.e., "Show me the money!"
Introduction
Over 100 years ago, Forchheimer3 recognized that fluid flow through porous media obeyed Darcy's Law only at extremely low seepage velocities. At typical velocities experienced in hydraulic fractures, the pressure losses actually consist of both a frictional and an inertial component. In other engineering disciplines, this concept is well accepted. Chemical Engineers routinely use this relationship (referred to by them as the Ergun Equation4,5) to describe fluid flow through catalyst beds and media filters. Automotive Engineers similarly understand that the pressure drop within a catalytic converter is not adequately described by Darcy's Law. In most industries, it is simple to measure the pressure loss across the system and use the correct equation.
However, as Petroleum Engineers frequently pump proppant down a mile of pipe, and hundreds of feet away from the wellbore, our mistake may not be as readily apparent. Since we have yet to invent a remotely transmitting pressure gauge to place in the propped fracture, we instead choose to stick our heads in the sand and ignorantly assume Darcy's Law will adequately describe fluid flow in hydraulic fractures. We are simply wrong.
