Tanstaafl. Science-fiction fans out there may remember this acronym, which stood for a central idea in Robert Heinlein’s 1966 masterpiece, The Moon is a Harsh Mistress.
“There ain’t no such thing as a free lunch” is a pillar of economics (and in academia, it’s expressed with grammatical correctness). But, it’s also true of technology. Case in point: High-density fracturing.
A useful definition. A useful definition of a frac hit is provided by Eagle Ford Training: Cross-well communication initiated while pumping a hydraulic fracturing treatment. While pumping a frac job, an offset well may become charged with pressure or sanded-in from injected frac fluids. This well has been given a frac hit, due to pressure communication. If frac sand actually enters the offset well, then it is also considered a frac’ed-into well.
The company denotes the severity levels of this problem:
- Pressure communication: For two wells to communicate, the injected frac fluids do not need to actually reach the observing well. Pressure communication only requires pressure waves, governed by the diffusivity equation, to propagate through the reservoir space.
- Sanded in (frac’ed into): When fractures, themselves, actually propagate to the producing well, then the frac fluids and sand will transmit into the producing well. This can cause damage to the completion and lift equipment.
- Loss of well control: Hydraulic fracturing is done under high rates and pressure, which requires robust well control equipment on the injecting well. When this pressure is transferred to wells that have less pressure integrity, then a failure of pressure control can occur. This can cause a blowout of either frac fluids or charged reservoir fluids.
The industry has outdone itself. With the advent of the high-rate, high-pressure, and high-volume [fracturing] technique, the generated fracture half-lengths in new wells can be greater than the well spacing (Savitski, et. al. 2013). The no-free-lunch nature of the cause is also described by Hamad, et. al., in their 2013 paper:
The ultra-tight nature of shale reservoirs has maximized the stimulated area within a section to increase production. As a result, some wells appear to interfere with one another during stimulation. This interference has been known to typically reduce (occasionally enhance) the performance of wells already in production by altering the existing fracture network, or near-well permeability, via the presence of multiple phases…Three different mechanisms are thought to be possible causes of the well-to-well interaction: depleted zones, changing stress fields, and high-permeability lithofacies.
Clearly, a frac-hit mitigation strategy is called for. Answering that call, Whitfield, et. al. (2018) propose exactly that. The authors claim first-hand experience with numerous frac-hit mitigation strategies, including shut-ins; temporary abandonments; frac-and-flow; small pre-loads; high-rate, water-defensive fracs; and refracs. They also note that another option—preventing them from occurring at all—requires a fully integrated field development plan. But this can be prohibitively expensive, and out of reach of the large number of smaller operators.
The solution that the authors propose is based on their observation that “…the damage encountered by frac hits is directly attributable to the movement of fluid in the reservoir and fracture networks from a high-pressure source (the infill frac) to a low-pressure sink (the parent wellbore). This fluid movement is allowed, due to the compressible nature of hydrocarbons, particularly gas columns in wellbores, compared to the low compressibility of the water-based frac fluid.
“This high-velocity, high-momentum fluid movement between an infill frac and existing wellbore causes disruption of the previously generated fracture network. It is capable of displacing previously displaced proppant, can damage the formation itself, and in the worst circumstances, can cause mechanical damage to the offset wells.”
A simple strategy. Based on their hypothesis, the authors propose a simple strategy: a “Pre-Load” technique to place incompressible fluid in the wellbore and generated fracture network, to prevent fluid movement in the system.
The authors say the Pre-Load technique is simple and follows a basic recipe:
- Large volume;
- Low rate;
- Pre-load two wells, regardless of distance;
- Pre-load offset wells on frac plane;
- Put online parent wells immediately after infill frac;
- Pre-load as close to the start of the frac as possible; and
- Include a chemical solvent and surfactant package.
This Pre-Load strategy has seen successful trials. More than 50 Pre-Loads have been pumped across the Eagle Ford shale, its Hawkville field in particular, and the Permian basin, with a 98% success rate in maintaining or improving parent-well production. Pre-Loads have been executed successfully on operated wells adjacent to offset operator infill activity in the Eagle Ford shale and Permian basin, where the same results have been achieved. None of the wells has required a major intervention to return to production. All infill wells next to parent wells perform in line with peer wells on the same pad, negating any production degradation, due to completion next to existing production.
There’s more to this story, which space limits telling, but one part of it is familiar: another challenge is being overcome by the insight and hard work of talented people devising elegant solutions. Well done.
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