While some operators may have given up on the potential of shale gas in Poland, San Leon Energy (SLE) is invested and blazing a trail in the Baltic Basin, determined to find the sweet spots, optimize production and prove commercial viability. As is typical in unconventional reservoirs around the world, the Gdansk W Concession has thick, laterally extensive source rock 3,000 m to 4,000 m (9,843 ft to 13,123 ft) deep.
SLE has been performing a methodical vertical frack testing program in the upper and lower Ordovician target zones to determine the frack design that will allow commercially viable gas flow when effective stimulation is applied to future horizontal wells.
After testing two frack strategies in the lower Ordovician, both of which showed suboptimal proppant concentration, SLE's objective for the stimulation in the upper shale interval, an area not previously stimulated, was to prove the ability to fracture and enable sustained production. A key objective was to achieve a high proppant concentration and, if successful, the same strategies would then be applied to a multistage stimulated horizontal well in the lower Ordovician, which has superior gas saturation and porosity, with the intent to open the upper and lower intervals and deliver consistent commercial flow rates.
Stimulating the upper target zone
In November 2013 the SIGMA3 engineering team performed the stimulation design and onsite real-time fracture monitoring on the two-stage hydraulic fracture treatment conducted by United Oilfield Services (UOS) that targeted and stimulated the lower and upper Ordovician Shale.
Stage 1 was a refrack of the lower zone, and its treatment design served as a starting point for refinement based on the response of the formation. Although proppant concentration improved, the results were still not viable for sustained production.
The objective of Stage 2 was to incorporate lessons learned from the frack/refrack of the Lower Ordovician interval as well as efficiency improvements in drilling and fracturing operations that are being applied to shale wells in the U.S. to:
- Effectively stimulate the upper Ordovician;
- Evaluate the formation response during the hydraulic fracture treatment; and
- Identify opportunities for improving the stimulation strategy.
Integrated approach
By analyzing the geology, reviewing lessons learned during the frack and refrack of the lower zone and drawing from experience in other shale reservoirs, the onsite team designed a plan (Stage 2) that would best match the formation's response to treatment.
Stage 2 was pumped through a new set of perforations, with a plug above the lower zone to isolate the upper Ordovician from the previously fractured lower zone. When the perforating guns were fired, frack pressure from the earlier frack stages was communicated into the wellbore through the new perforations, indicating that previous fracks grew to at least the level of the new perforations (27 m [89 ft]).
SIGMA3 had a plan that enabled the onsite engineering team to adapt the treatment design quickly to match the formation's response, and SLE considers Stage 2 as proceeding exactly as planned.
By taking into account excessive fracture complexity and proppant type and reducing the treatment rate, Stage 2 placed far higher proppant concentrations into the formation with little pressure increase. Frack modeling indicates this stage grew down well through Stage 1's target and into the lower target zone.
“SIGMA3 provided engineering expertise for the final frack campaign of the vertical well, and it went exactly as planned,” said Joel Price, COO for SLE. “This breakthrough frack design is critical to the successful shale development in Poland's Baltic Basin as well as Europe by association. We are excited about applying these learnings to the upcoming horizontal well with the aim of achieving commercial flow rates.”
Penetrating lower target zone
SIGMA3 advised, and SLE/UOS agreed, that all sustained production is coming from the upper Ordovician zone, although simulation suggested the frack may have penetrated the lower Ordovician. The frack did not stay open, most likely due to the higher clay content layer between the upper and lower target zones. To maximize the potential to connect the lower and upper targets of the horizontal well, future completion strategies will place higher conductivity and relatively wider fractures, with particular attention given to proppant type. This type of design should help avoid proppant embedment in the higher clay content layer.
Commercially viable horizontal well
“The vertical well was the testing ground, enabling us to learn more about the well properties and change the stimulation program to achieve maximum production. We are confident that fracks can be designed in the horizontal well that will cover the entire Ordovician target zone,” Price said. “SIGMA3 and the team concluded that the production from the third frack campaign is coming from the upper half of the formation. The lower interval has even better reservoir characteristics and is our primary target. There is significant upside once the horizontal well is executed.”
SIGMA3 will be participating in frack design and analysis of the upcoming horizontal well, set to drill and frack later this year. The frack program will be 10 to 30 stages, similar to completions in the U.S., and will build on the successful aspects of the vertical well's stimulation.
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