The Chow Pressure Group: Shut-In Wells Offer Case for a New Approach to Interference Testing
Authors: Jacobs Trent 
Published: August 2020

A nearly 70-year-old equation has become the cornerstone of a newly emerged approach to reservoir diagnostics and well appraisal at two of the largest shale companies in the US.
With many North American tight-oil producers overseeing low-price-driven shut-ins of horizontal wells, this development has become increasingly relevant. As wellhead prices rise and the shut-in population is brought back into the fold, a window of opportunity exists for operators to run late-life interference tests on wells they otherwise likely would not have planned for.
One challenge though will be in selecting the ideal testing model.
Outside of the traditional approaches, the Chow Pressure Group (CPG) has been proposed as a promising candidate and one that is advantageous to all but the most price-sensitive and resource-strapped operators of shut-in assets.
Described by its adopters as a simple equation, it comes from the not-too-distant world of hydrology where it was introduced in the 1950s as a ground-breaking method to analyze the flow behavior from freshwater aquifers.
In 1987, the oil and gas industry would be introduced to CPG by one of its own academic luminaries: Erdal Ozkan, a professor of petroleum engineering at the Colorado School of Mines, an SPE technical director, and the author or coauthor of more than 100 technical papers - including SPE 16378.
In that paper, he and two other researchers showed that CPG responses are identical to pressure responses observed during a key production phase called the pseudoradial flow period. Additionally, they demonstrated how this CPG response could be plotted on a straight line when a flow regime exhibits something known as power-law behavior.
Some 30 years later, petroleum engineers at Irving, Texas-based Pioneer Natural Resources and software firm Kappa Engineering published the first of two papers (SPE 187180) that brought this work back into the fore. Using the CPG equation, they were the first to prove that the power-law behavior (and its nonstandard slope) was the norm for the majority of the operator’s wells in the Wolfcamp Shale - the most-targeted formation in the Permian Basin.
“In reality, it’s very simple. It’s the pressure change with time over the corresponding derivative slope,” said Wei-Chun Chu in offering a concise explanation of the formula of focus:
CPG=Dp/Dp’.
Chu is the lead author on the two papers which outline how the CPG was integrated into the well interference test program at Pioneer where he retired from a year ago. The equation is now a highly influential tool that Pioneer uses to guide major decisions on well spacing, completions volumes, and its grading of the results of stacked and staggered well patterns.
The veteran reservoir engineer points out that despite its simplicity, adopting this new approach requires discipline. “You have to do this very systematically,” Chu cautioned. “People will have to do their homework to get everything organized in order to understand the bigger picture.” For those who do their due diligence, the reward is a quick and accurate way to estimate the magnitude of pressure interference (MPI) between offset wells.