It has often amounted to little more than background noise. Now, engineers with one of the largest shale producers in the US consider the water-hammer signal to represent a potential trove of insight into stimulation performance.
ConocoPhillips has spent almost 3 years developing a new approach to water-hammer analysis and recently shared the first public details of that work in a technical paper (SPE 204154). Perhaps the core takeaway is that within a few minutes of post-stage data, a picture of reservoir and stimulation quality is available. The challenge lies in how to bring that picture into proper focus.
The water hammer is a pressure pulse that moves through the wellbore at the speed of sound in fluid, or around 5,000 ft/s, which is more than four-fold the speed of sound in air. Its name comes from the often-audible slam made on one end of the pipe when a column of moving fluid is slowed or halted.
One is usually generated as a valve is turned, or a pump slowed or stopped at the end of a fracture stage. This makes the water hammer ubiquitous in the context of hydraulic-fracturing operations the world over.
As the wave bounces up and down the wellbore it generates noticeable pressure spikes on wellhead gauges. These oscillating signatures have been the subject of industry intrigue for decades. They’ve also proven to be difficult to decipher and trust.
As such, the water hammer has not entered into the shale sector’s pantheon of fracture-design inputs. ConocoPhillips’ ongoing research and development project may help change that.
Based on a review of treatment data from more than 150 of the operator’s low-permeability wells from an unnamed field in North America, those with fast-decaying water-hammer signals amounted to the best performers, achieving an output equivalent to 94–110% of their estimated recoveries. On the other hand, wells with more prolonged or low decay rates commonly fell 10–20% below their original type curve.
“The high decay rates indicated more near-wellbore fracture surface area, which related to higher well productivity. While conversely, low decay rates mean less near-wellbore friction, longer fractures, lower well productivity,” explained Dung “Zoom” Nguyen.
Nguyen is a staff completions engineer at ConocoPhillips and coauthor of the paper that she presented at the annual SPE Hydraulic Fracturing Technology Conference in May. She elaborated that analysis on frac hits, or fracture-driven interactions (FDIs), supports the relationship between fracture geometry and water-hammer decay rates, i.e., low decay rates equate to fewer, longer, and less-complex fractures.