Proppant Movement In Frac Casing Including Runaway Perforations And Limited Entry.
Originally published on Forbes.com on June 27, 2022
Results of a surface testing protocol, and an engineering model built upon the testing, should be an asset to operators in the field trying to optimize their perforating designs.
Perforations are the tiny holes in the casing of a well that produces oil or gas. The perforations allow the oil or gas to enter the well and flow to the surface.
But most wells, before they are put on production, are fracked in an operation that pumps water or slickwater down the well and out through the perforations to create fractures in the formation that contains the oil or gas. To prop the fractures open, fine sand called proppant is pumped along with the water.
Recent studies have tried to answer the question of what’s the best proppant to use, and in what concentration, and what are the best kind of perforations to use.
Our last report discussed new tests of perforations spread over 200 feet of surface casing pipes. The focus was on horizontal wells that are used for shale oil or shale gas production. These tests revealed that proppant did not exit the perforations uniformly. Larger proppant (40-70 mesh) tended to bypass the earlier perforations and accumulate more as they exited the later perforations.
The discussion continued by addressing just how important this non-uniform distribution of proppant was by comparing with other causes of non-uniformity and the well-by-well variability in shale oil or gas production.
The same testing protocol can be used to study oriented perforating. Traditional perforating is when perforations (perfs) are shot in a helical pattern, with lots of holes all around the casing. The idea was that this guaranteed a number of decent fractures would be created in the formation.
But it’s been shown that oriented perfs, such as perfs in a straight line along the top or side of the casing, would make it easier to initiate a fracture, and would avoid interference between two or more fractures wanting to start from misaligned perf holes close to each other.
One expert argues that a fracture that is started like this wraps itself around a casing until it finds a weak stress location to kick off into the formation.
One beneficial result is that oriented perfs can lower frac pumping pressure.
Another benefit is to avoid runaway perfs, which became excessively enlarged and steal more fluid and proppant than they should, depriving other perfs along the well of needed water and sand.
How are they caused? In two ways. First, perforating in a helical pattern means hole sizes vary just a bit (by a few hundredths of an inch). For example, a perf gun at the bottom of a horizontal well will shoot a larger hole at the bottom and a smaller hole at the top of the casing.
The larger hole at the bottom will take more of the frac water rushing past (its very sensitive to hole size) and this water and proppant will erode the bottom hole more quickly than the hole at the top of the casing.
Second, gravity acts on larger proppant, even though the grains are tiny (less than 1 mm for 40-70 mesh proppant) and more of them tend to settle downward and worsen the runaway effect for perfs near the bottom of casing.
As a result, more and more of the flow enters the bottom perf and it becomes a vicious cycle that leads to a runaway perf.
This has led some operators to perforate only near the top, or at least in the top half, of the casing.
ConocoPhillips reported that the larger hole-size variations in stages perforated at multiple angles were five times more likely to have “runaway perforations that eroded so much they took the lion’s share of stimulation.”
According to Dave Cramer of ConocoPhillips, “A year from now, few people will not be doing oriented perforating; the advantages of it are clear.”
Limited entry to control induced fractures.
Limited entry refers to fewer perfs within each frac stage. Instead of 8 or 13 perf clusters discussed previously, there might be only 5 or 10. Or instead of 6 or 3 perfs per cluster, there might be just 1.
With traditional helical perf design, as frac water meets and enters the first perfs encountered, a runaway fracture can be induced beyond these perfs and suck excessive water and sand into these perfs. This has been called a heel-side bias and implies that perfs further along the well may be under-exposed in their frac treatment.
With limited entry and fewer perfs open, the frac pumping pressure rises, but this guarantees all the perfs will take fluid and proppant. There may be fewer fractures induced in the formation, and less interference between neighboring fractures, so they are more effective.
A logical conclusion is that oriented fracturing will also improve the performance of limited-entry designs.
A study on limited entry and how to optimize this in a shale oil or gas well is an important opportunity for the surface casing studies reported previously.
Microproppant effect on perforations.
Very small grained proppant, roughly ten times smaller than 100-mesh sand, can prevent runaway perforations, according to The American Oil and Gas Reporter in June 2022. By comparing frac stages completed with or without Deeprop, perforation efficiency increased from 39% to 54% in a four-well test in the Eagle Ford oil play.
The parent company, Zeosphere Ceramics, said well production rates would benefit. Bill Strobel reports another four-well test in the Delaware basin, where a Deeprop well in the Wolfcamp A outperformed a well without the microproppant by 12.3% more production. Another pair of wells targeting the Wolfcamp B gave a production boost by 15.3%. The total uplift amounted to 160,000 barrels of oil-equivalent after about a year and a half.
The detrimental effect of runaway perfs and the benefits of limited entry perforating are obvious candidates for study by the casing testing protocol discussed before.
Runaway perfs and limited entry would seem to have more substantial effects on production of shale oil or gas than non-uniformity of proppant exiting from a series of perforations due to momentum of larger proppant (40-70 mesh).
Further tests, not reported, have studied 15 perforation clusters in one stage with different charge designs and perforation orientations. This enlarged project is supported by a growing number of operators.
Altogether, results of the surface testing protocol, and the engineering model built upon the testing, should be an asset to operators in the field trying to optimize their perforating designs.