Multistage hydraulic fracturing of horizontal wells in unconventional reservoirs is widely recognized as the driving force behind the resurgence of hydrocarbon recovery in North America land operations. However, once the hydraulic fracturing operation is complete, the flowback program can actually be the key to a well’s ultimate performance. Generally, maximizing IP has been the historic metric used to dictate well flowback procedures. This has typically resulted in flowback programs that aggressively bring the well online and minimize the time to bring the well to production, with little science or engineering applied to the flowback itself.

Investigations show that these aggressive flowback practices can contribute to a well’s completion damage, resulting in significant losses to the designed fracture half-length, and consequently reducing the well’s performance and ultimate hydrocarbon recovery.

Engineering the fracture flowback

The application of an engineered flowback program in unconventional wells has the proven ability to minimize well and completion damage following a hydraulic fracturing treatment. The Halliburton CALIBR engineered flowback service helps operators minimize completion damage and optimize the well’s performance both in terms of productivity and EUR. The CALIBR service manages the drawdown pressure in the horizontal lateral to minimize damage that could be introduced into the fractures and maximize hydrocarbon recovery. This is achieved by managing choke adjustments during the flowback using an analysis of the acquired flowback data, which are updated at regular intervals.

The flowback well test equipment shown in Figure 1, moving progressively downstream from the wellhead, includes (but is not limited to) plug parts catcher, choke manifold, wellhead desander, separator, fluid tanks and gas flare.

Halliburton flowback equipment and personnel help ensure that high-quality rate data are captured during the flowback—a crucial element of the analysis. Additionally, the Halliburton self-powered intelligent data retriever (SPIDR) system is the key component that works in collaboration with the well testing equipment to deliver the engineered component of the flowback service. The SPIDR system is a surface data acquisition device that captures high-accuracy and high-resolution pressure measurement data using a dual-quartz-crystal transducer. The SPIDR system connects to the wellhead and records pressure data every second during the flowback. These high-frequency data are necessary for the CALIBR service, providing more detail regarding the pressure behavior of the well than what is recorded on hourly flowback reports.

When the well is opened, monitoring and measurements begin immediately. Using industry rate/pressure transient analysis techniques, the CALIBR engineering team makes recommendations to the operator for the next planned choke adjustments, and the process is repeated. The subsequent recommendations for the next choke setting are progressively performed, with each transient being monitored and evaluated. Further monitoring also is performed for changes in waterhydrocarbon ratios, and such changes being accounted for when recommending any subsequent increase to the next choke size.

The culmination of the engineered flowback is dictated by one or more of several different criteria determined during prejob collaboration between Halliburton and the operator. For example, if the operator has a predetermined maximum allowable drawdown for the well, then that becomes the final target. However, if the operator has a facility constraint or a bottleneck that restricts a well’s overall productivity, then that becomes the final target. Ultimately, if a well is producing at too high of a drawdown such that hydraulic fracture conductivity is negatively impacted, then this becomes the target rate at which the well would benefit from remaining below.

Case history

Well A and Well B are two similar wells in West Texas that were completed in the same formation at similar depths. Both wells had similar stimulation designs (number of stages, proppant load and load fluid), and both wells were completed with similar horizontal lateral lengths. The primary difference between the two wells was the post-fracturing flowback. On Well A the operator did not employ an engineered flowback schedule and instead aggressively flowed back the well with multiple choke changes early in the period and no true engineering to determine the next change in the flowback schedule.

Contrary to the approach for Well A, the diagnostic analysis of the Halliburton CALIBR engineered flowback service was applied to Well B. Prejob discussions between Halliburton and the operator resulted in a flowback schedule designed to minimize well damage introduced by standard aggressive flowback practices. The flowback began with a smaller initial choke size, and smaller, calculated adjustments to the choke size were made at regular intervals based on the ongoing diagnostic analysis of the pressure transients created during the flowback. This equalized the pressure drawdown across the lateral and minimized proppant mobilization in the fractures.

Summary

Although the hydrocarbon rates are comparable, the pressure drawdown required to achieve the rates in Well A is significantly higher than in Well B, despite similar values for permeability/thickness (Figures 2 and 3). The calculation of effective system fracture half-length showed a significant reduction in Well A compared to Well B (85 m versus 232 m [279 ft versus 761 ft]). This is indicative of the damage introduced to the completion resulting from the aggressive flowback schedule pursued on Well A. Furthermore, the forecasted cumulative recovery for Well B after nearly 300 days is 13,000 bbl greater than Well A. Given the current average West Texas Intermediate oil price, that is a difference of more than $780,000 between the two wells (after less than one year).