CASE STUDY: Casing Expansion For Improved SCVF Source Identification

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Essential to successful and cost-effective Surface Casing Vent Flow shut-off is the accurate and timely identification of the source of the leaking gas and the determination of wellbore conditions on outside of the production casing.

The traditional process of sealing vent flows with perforating and cement squeezes has a low success rate. This is due in part to the intervention taking place in a wellbore interval that may not have the ideal characteristics for achieving a successful seal.

Regulations often stipulate that to stop vent flow to surface, the wellbore annulus should be sealed at or near the source of the gas. This is aspirational in the sense that this requirement does not always factor in actual wellbore conditions.

Because of the growing number of mature wellbores and society’s growing concern about fugitive methane emissions from all sources, the industry is beginning to understand how much more it would like to know about the other side of the production casing.

Most analyses of the properties of the various formations through the which the well was drilled are performed before the production casing is run and cemented. The properties of the cement are known when it is pumped, and a sample is dutifully captured if cement displacement returns reach the surface. What it looks like years later is interpreted, not measured.

Understanding the conditions and behavior of the cement and wellbore on the outside of the production casing is essential for successful SCVF shutoff. To increase success rates and reduce costs, new tools and processes are welcome and necessary.

The existing toolbox is by no means empty.

Advancements in through-casing electric logging continue. Logs for cement bond, noise/temperature, and formation properties can reveal significant and useful information about what’s on the other side of the pipe. These are used in conjunction with open-hole logs and the geological well control from offset wells and the mountains of public data available about the mature basins of Western Canada.

Because of extensive production experience, isotope analysis of leaking gas at surface provides a chemical marker of the formation from which the methane may be escaping. This has become an important element of the process of gas source verification.

Over the years operators have concluded that isotope analysis is indeed helpful. But on wells with gas inflows from multiple formations for multiple years with intermingling on their paths to surface, not always definitive.

But despite the extensive data accumulated over the past 100 years of drilling and production in the WCSB, the focus has been on the oil and gas bearing reservoirs and the inside the casing, not the non-producing zones that are ideal for sealing, or the actual quality and seal of the cement in the annulus.

The borehole-caliper component of open-hole logs is often poor or non-existent on older logs. Too many wells that are leaking today don’t have complete or modern open-hole logs.

Earlier wells did not require cement to surface. The quality of that cement is well understood as there isn’t any.

Remedial cementing to place cement in the annulus of wells that don’t have any is challenging. During primary cementing, the string is reciprocated to ensure even distribution. Now it can also be rotated to further improve cement placement. The inability to move the casing during remedial cementing creates challenges such as channeling.

More information on the physical characteristics of the annulus at any point in the wellbore has the potential to increase vent flow shutoff success rates, which is a key driver of costs.

As the Winterhawk Casing Expansion Tool (CET) is run in more wells—and surface vent pressure/flow loggers continue to improve—interesting information is being captured that has the ability to improve the economics of SCVF shutoff.

In the images below, real-time surface pressure and flow monitoring was compared with casing expansion procedures using the CET.

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This well in east-central Alberta had a high-volume vent flow from multiple sources. According to the cement bond log, this interval had the best cement in this segment of the wellbore and was above stored or source gas identified from the compensated neutron log. So it was selected as an interval with potential for casing expansion. The well contained H40 pipe which responds well to casing expansion in terms of ductility.

All five expansions resulted in immediate and measurable reductions in pressure and flow. Several conclusions can be determined from this information.

First, the expansion took place above the source of the gas. If the well were sealed at this interval, gas from downhole would be shut off.

Second, the wellbore conditions for sealing with casing expansion alone were poor. A full seal (defined as flow dropping to zero) was not achieved. This means that the problem was simply being pushed back into the reservoir.

Third, the leak was not between the cement and casing, the so-called “bond,” which is a common source of micro-annular leaks (hence the terminology). It was either through the cement because of channels or “wormholes,” leak paths between the cement and the formation, or both.

Last, because the flow never went to zero, the compressive strength of the formation at this interval appeared weak. Put another way, this is not an ideal interval for a long-lasting annular seal by any method.

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This well was completely different. It was a horizontal, drilled this century with good cement to surface. It had a low-volume, low-pressure leak that prohibited decommissioning without vent flow shutoff. Noise logs and gas isotope tests indicated the potential source of the gas. However, the well had no open-hole logs for the vertical section. The bond logs indicated good cement. It had characteristics of a micro-annular leak between the cement and casing because of high pressures during the completion process. It contained L80 pipe.

Working with open-hole logs from offset wells and the operator’s geologists, stronger shale formations likely to contain gauge or near-gauge borehole were selected for expansion

During four casing expansions the well’s annular flow went to zero. This information leads to several conclusions.

First, this interval was clearly above the source of the gas. When the cement was mechanically squeezed and the annulus was shrunk at this depth, the flow stopped.

Second, its immediate responsiveness to casing expansion indicated a micro-annular leak between the casing and cement. These are difficult to detect and inject cement into. However, knowing the exact problem and depth was a material step forward to a cost-effective solution.

Third, the formation at this depth had high compressive strength and minimal porosity and permeability. It was unlikely that gas would leak through the formation around an enhanced annular seal. This was a quality interval for a permanent seal.

Last, had the expansion been sustained, the SCVF would have been stopped. This provided Winterhawk with valuable information on how to modify its existing inventory of casing expansion tools and technologies to further advance the science.

Combined with a sensitive surface pressure/flow logger, the Winterhawk CET can now offer the industry an entirely new tool to diagnose key elements of successful SCVF shutoff. These include: