Extreme Limited-Entry Perforating Enhances Bakken Completions
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This paper presents the evolution of a Bakken advanced completion design with the added enhancement of extreme limited entry (XLE) perforating. With this strategy, an operator has consistently stimulated more than 11 perforation clusters per stage. The high number of active clusters, or fracture-initiation points, has been measured directly with radioactive tracers and fiber-optic diagnostics and is validated through improved production relative to offset completions.
The technique, as the name suggests, pushes the level of perforation friction past 1,500 psi. Additional sources of pressure variations have been identified and can be summed up as the fracture-entry pressure. Fracture-entry pressure, defined as the pressure from immediately outside the perforations to the fracture tip, includes, but is not limited to, near-wellbore tortuosity, net pressure, stress shadowing, and fracture extension.
To advance the goal of evenly diverting stimulation fluid to all the fracture-initiation points (clusters), the designed perforation friction must moderate perforation-/cluster-level pressure variations in the fracture-entry pressure. To achieve this, the magnitude of the sources of variability should be quantified and addressed with the completion and perforating design. Additionally, one must account for the dynamic fracturing process, within which most of the fracture-entry-pressure parameters, as well as the perforations, are changing as the stimulation progresses.
The primary sources of variability to be considered when designing for XLE are as follows:
- Minimum horizontal stress variability along the lateral; 90% of a Bakken lateral is within a 750-psi range.
- Near-wellbore friction variations, stage to stage and perforation to perforation. Stepdown tests demonstrate a P50 of 625 psi for starting near-wellbore friction.
- Stress shadowing between active clusters. The magnitude of stress shadowing is dependent on formation properties, completion-design parameters, and cluster spacing. For design purposes, it is assumed to be approximately 200 psi.
- Fracture-extension-pressure variability based on changes to net pressure.
- Perforation-friction initial-condition variability may limit ability to reach design pump rate. Perforation diameters at all possible orientations must be considered, along with the number of perforations open vs. the number shot.
- Perforation friction changes during a fracturing stage of approximately 500 psi caused by rounding and erosion of perforations and possible loss of clusters because of proppant settling in the liner.
For the Bakken, a minimum 2,000-psi perforation friction is targeted at the beginning of the job, with the expectation that, as perforations round and erode, it will remain greater than 1,500 psi at the end of pumping to account for all possible fracture-entry pressure variations.
Fig. 1 introduces the injection-variability index. This is the ratio of the baseline injection rate, with zero fracture-pressure variability between initiation points (clusters), to the injection rate associated with variations in perforation friction at other initiation points intrastage. Fracture-entry pressure varies, so this pressure variation is communicated directly as an equal pressure change across the perforations. Because the pressure is constant inside the wellbore across all the clusters within a stage, the injection rate will either increase or decrease at each cluster proportionally related to the perforation friction; the rate change is magnified because it is squared relative to the pressure change. At a baseline 500-psi perforation friction, a difference of approximately 375‑psi fracture-entry pressure for different clusters in the same stage will give injection-rate differences ranging from 32 to –50% at separate clusters. Alternatively, with 1,500 psi of perforation friction, the same difference in fracture-entry pressure only translates to a rate-change difference ranging from 12 to –13%. With enough variability in fracture-entry pressure between clusters, individual cluster rates can drop to zero if the perforation friction is too low. Fig. 1 clearly demonstrates the benefit of applying this perforating strategy, because the perforations of varying fracture-entry pressures converge toward fluid-distribution parity by increasing perforation friction.
Radioactive (RA) Tracer Data: Initial Four-Stage Trial
RA tracer data was used for downhole diagnostics of perforation cluster efficiency (PCE) in early field trials of XLE in January 2016. Four stages were chosen to apply extreme values of limited entry that ranged from 2,000 psi to well over 4,000 psi. The stages with XLE showed much higher initial PCE than non-XLE stages, which had less than 1,000 psi of perforation friction. The authors believe this indicates early and continuous PCE throughout a stage without the use of solid particulate diverters even though the use of solid particulate diverters did increase the final PCE of the non-XLE stages significantly by the end of the stages traced.
RA Tracer Data: Well A
RA tracer data was used for downhole diagnostics in Well A. The PCE for all stages traced in Well A averaged 77%, but cluster count per stage varied in multiple stages that were traced, so not all of them had 15 clusters. The initial perforation friction for the stage in Well A was approximately 2,000 psi, and the final perforating friction was approximately 1,900 psi. Well A was the first full wellbore designed with XLE perforating for every stage, and the designed pump rate of 80 bbl/min could not be achieved on most stages until the perforation scheme was altered to include a few incremental clusters per stage. This is believed to have lowered the PCE, likely because of proppant transport issues in the liner.
RA Tracer Data: Well B
RA tracer data was used for downhole diagnostics in Well B, and PCE was calculated in the same manner as previously discussed. The PCE for all stages traced in Well B averaged 89%, but cluster count per stage was varied in multiple stages that were traced, so not all stages had 15 clusters. The initial perforation friction for the stage in Well B was approximately 2,200 psi, and the final perforating friction was approximately 1,400 psi. The more-significant drop in perforating friction in this stage may be the result of erosion of the perforation diameters, but a more-likely cause is a leaking plug exposing clusters from the stage below.
Post-Fracturing Fiber-Optic Warm-Back and Production Log
Post-fracturing fiber-optic diagnostics were used in two wells, a Middle Bakken well and a Three Forks well, in June of 2017. A composite carbon rod with fiber-optic strands within it was deployed to log both distributed-temperature-sensing (DTS) and distributed-acoustic-sensing data during producing and shut-in well conditions. A warm-back analysis was performed using the DTS data to analyze the fracturing-fluid distribution within each stage. The warm-back analysis demonstrated a consistent distribution of fluid as binned by thirds within each stage. This well had either 12 or 15 clusters per stage, so each intrastage bin contained four or five clusters. The same proppant per stage was pumped, so the stages with 12 clusters had 25% more proppant per cluster.
The two-well average for producing cluster efficiency was 80% for the 20 complete stages logged and analyzed.
For the Williston Basin, a 180-day cumulative oil volume is a generally accepted production metric useful for comparative analysis, and it correlates with longer-term cumulative production after 3 years. Well A and Well B had more than 1 year of production data and Well C was approaching 180 days of production data at the time of writing.
On the basis of previously published multivariate work flows that use the North Dakota Industrial Commission public database of completion parameters and production data, expectations for productivity in each geologically similar area and for a given completion design were determined. The overall results have exceeded expectations, with Well A exceeding expectations by 9% and producing more than the next-highest-performing offset operator by 170%, Well B exceeding expectations by 13% and producing more than the next-highest-performing offset operator by 250%, and Well C exceeding expectations by 14% and producing more than the next-highest-performing offset operator by 170%. The authors write that these positive results are proof of a dramatic increase in the number of effective clusters by successfully applying a high-density-perforating strategy with XLE.
- Multiple diagnostic data sets demonstrate the success of this design in driving a high PCE during the early stages of, and indeed throughout, a fracturing treatment.
- Producing PCE has been demonstrated at 80%, and thus 12 of 15 clusters per stage are contributing to production with the perforating strategy discussed.
- If the number of effective clusters for each well is known, the stage count can be right sized on the basis of PCE expectations for a given completion and perforation strategy. Development costs can be lowered by reducing completion costs and increasing productivity simultaneously. The wells presented realized a 15% reduction in development costs and have produced 170 to 250% more than offset operators in the same number of producing days.
Extreme Limited-Entry Perforating Enhances Bakken Completions
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