Drawdown management optimization in unconventional wells is a challenge as well as a constant object of study. The relationship between production maximization in the early time of a well and the decrease of long term estimated ultimate recovery (EUR) is a compromise.
In 2022, YPF conducted a review of the current drawdown strategy for the fields located in the Vaca Muerta Shale Oil window in Neuquén Basin, Argentina. The premise was to analyze wells in an area with similar characteristics to quantify the drawdown effect. To achieve this objective, a new monitoring variable was proposed, along with the optimal time for choke changes. A choke schedule was suggested for new wells.
The aim of drawdown management is to improve the control over the stress on the proppant pumped during stimulation. This prevents damage to the hydraulic conductivity of the fracture and maximizes ultimate recovery. As the well is brought to production following a drawdown strategy (i.e. by changing the choke size over time), the horizontal stress decreases as the reservoir is depleted, and the bottom hole pressure (BHP) also decreases as the choke size increases. Each time the choke is changed, the BHP drop increases. If it is too aggressive, it can damage the hydraulic fracture (Hryb et al.2019). Therefore, proper drawdown management is essential to prevent hydraulic fracture damage and maintain control over the bottom hole pressure.
As the field's development progresses towards wells with different landings, longer lateral lengths and higher intensity stimulation designs, a different behavior is observed in the evaluation of drawdown curves when compared to older wells that may have been used to empirically observe initial trends.
In over-pressured reservoirs, drawdown is a major challenge to manage; the gradient pressure in the best zones of Vaca Muerta is approximately 0.9 psi/ft. Evaluating the performance at different drawdowns is very important to understand the optimal choke management.
To quantify the drawdown curves for each well, it is necessary to estimate the initial pressure (Pi) and the BHP, which is calculated by correlating the vertical flow and daily production. Given the uncertainty of these calculations, it is vital to have calibration points from dynamic gradients and reliable production measurements. Gas and water are recognized to have an important effect in this calculation, leading to wrong calculations for the BHP.