Home FinanceEnergy & Environment Proppant movement in frac casing has been defined, but how important is it really for shale wells?

Proppant movement in frac casing has been defined, but how important is it really for shale wells?

by YAR

Proppant consists of sand-sized particles injected with fracturing fluid during a hydraulic fracturing operation. In shale oil and gas wells, the fracturing fluid is typically water with some friction reducer (such as soap) added to reduce fracturing pumping pressure. The purpose of the proppant is to prevent induced fractures in the reservoir from closing after fracking stops and the elevated pressure fades.

In shale oil and shale gas wells, the proppant used is a mixture of 100 mesh sand and 40-70 mesh sand, and these grains are less than one millimeter in diameter. These small sand particle sizes are necessary for the sand to be transported through narrow fractures in a fracture network created by the fracking operation. The larger sand would plug the net and not be injectable, that was discovered in the early days of the shale revolution.

Horizontal wells in shale are typically two miles long and are pumped with 40 separate fracking operations or stages. Each stage is approximately 250 feet long, and the metal casing contains 10 to 20 groups of perforations, with several perforations in each group. Ideally, the horizontal well is completely drilled with these holes.

The flow path of a proppant grain is elusive. First, the grain has to make a right angle bend to go from flowing along the casing to a hole. You are then faced with complex fracture geometry, perhaps a main fracture branching off into secondary fractures, like a tree trunk spreading out into branches and then into twigs.

Will the proppant grain be able to enter all of these fractures or are some of them too narrow? A 100 mesh sand grain can squeeze into a narrower fracture where a 40-70 grain cannot.

Improved oil and gas production has been documented by using proppants with a grain size less than 100 mesh, and suggests that it is worthwhile to introduce even small grains of proppant into smaller fractures to keep them open to the flow of molecules. of oil or gas. One such proppant is called DEEPROP.

New proppant flow tests out of the casing.

Recently, some new tests have been conducted investigating proppant flow through the casing itself, meaning a short length of horizontal casing that has been drilled to let out fracturing fluid. It is not an underground test: the pipe is placed in a basin on the surface and the basin collects the proppant and the fluid that comes out of the perforations.

A large number of operators have supported this project using a variety of drill sets with different perforation loads, designs and orientations. Different pumping rates, proppant sizes and sand quality have been studied.

The test hardware was as realistic as possible. The casing was standard 5.5 inches, as were the bore diameters. Pumping rates reached 90 lpm (barrels per minute), which had never before been used in proppant movement tests.

A single stage of fracturing was tested by drilling different groups along a pipeline approximately 200 feet long. Each drilling group had its own casing that directed the captured fluid and proppant to its own tank, so they could be metered.

The results were presented for two different sets of groups: 8 groups in one stage with 6 perforations in each group, or 13 groups in one stage with 3 perforations in each group. Testers used 40-70 mesh sand or 100 mesh sand transported by slippery water fluid pumped at 90 bpm.

These SPE documents report that proppant escape through the drill groups and into the vats is uneven:

· Some underpinning items, particularly larger mesh sizes such as 40-70 mesh, sail past the first few perforations in the group and do not enter the formation until later in that stage. These larger particles have more momentum.

· Smaller proppant particles, such as 100 mesh, enter cluster perforations more evenly.

· Limited entry designs have been developed using only one perforation per group at the top of the casing.

Particularly for larger proppant, perforations at the bottom of the casing attract too much proppant (gravity effect) and can be enlarged by erosion, so less proppant reaches clustered perforations along the stage of fracture.

Proppant output from coating is uneven.

All tests revealed uneven distributions of proppant output. The table shows the ratio of the largest proppant leaving a pool:smallest proppant leaving a cluster (ie, highest proppant:lowest proppant), as well as second largest proppant:second lowest proppant. These proportions are an indicator of inequality: a higher proportion means a more unequal distribution and vice versa.

Results show that 40-70 mesh proppant (larger ratios) is less uniformly distributed than 100 mesh proppant (lower ratios), in both cluster scenarios.

The interpretation given by the reports is that more of the 40-70 proppant, being larger and heavier sand grains, tends to be carried by its momentum past previous drilling clusters before exiting in the drilling clusters. later, compared to 100 mesh proppant. .

This is less than ideal because the goal is to get proppant evenly distributed throughout all perforation groups in a fracking stage. But now to the big question of how much of a difference does this make?

The challenge is to optimize the procedures so that the proppant output distributions are more uniform. From the reports, the test results were incorporated into a computational fluid dynamics model (SPE 209178). This approach has been integrated into a frac coaching program called StageCoach.

Meanwhile, reports state that “nonuniform proppant flow in the casing may be as important as formation variability and stress shading.” Let’s dive into this.

Other sources of variability of shale production.

The real question is how important an uneven distribution of proppant is to shale oil and gas production.

The great variability of shale oil and gas wells has been documented. For example, horizontal wells in the Barnett Shale of a typical length of 4,000-5,000 feet show that the bottom 10% of wells generate less than 600 Mcfd, while the top 10% generate more than 3,900 Mcfd.

Several other factors are known to contribute to the wide variability of shale oil or gas flows.

If the horizontal wellbore length and wellbore orientation are normalized to remove their variability, then fracturing stages, proppant size, and proppant amounts can be considered first-order effects. These first order effects have been prioritized and optimized in more mature shale formations.

Then there are the geological properties, such as natural fractures in the shale, in-situ stress, and fracturability of the shale rock. These are considered second-order effects because they are much more difficult to quantify. Efforts to minimize these sources of variability include horizontal wellbore logging, installation of optical cables or sonic instruments or microseismic geophones to measure fracture expansion and interaction with local geology along a horizontal wellbore.

Against these sources of variability, casing output distribution and proppant uniformity appear to be of comparable importance to other second-order effects, such as geology and stress changes along a horizontal wellbore. There is no way that the uniformity of casing output can explain the variability in production between 600 Mcfd and 3900 Mcfd, as seen in the Barnett Shale.

To put it another way, the key is to get the proppant out of the majority of the perforation groups and into the fractures created. This has been achieved by pumping very small proppant, 100 mesh or 40-70 mesh (and often both), and optimizing the concentration and amounts of proppant for a particular shale patch.

This is 90% of the goal that has been achieved with remarkable success in the shale revolution of the last 20 years. Therefore, it is difficult to see from the new surface tests that a small variability in proppant outputs from one perforation group to another could have a first-order effect on oil or gas production.

But perhaps the results of other tests, different tests, on this project will reveal more significant effects on shale production.

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