Petroleum production from the Vaca Muerta formation represents a significant portion of Argentinian supply, so it is crucial to accurately estimate hydrocarbons reserves. The production rate of oil and gas wells in unconventional reservoirs typically declines over time. Decline curve analysis (DCA) is an empirical method that involves fitting rate line trends to history-match flow rates, to forecast future production decline, and to calculate the estimated ultimate recovery (EUR) per well. One of the limitations of modern DCA methods resides in the assumption that the choke size at the wellhead remains unchanged; this assumption is seldom true. In fact, choke size is incrementally changed by operators for two main reasons: drawdown management and hydrocarbons market supply and demand. In gas-producing wells, increasing the choke size results in a sudden increase in gas flow rate, whereas decreasing the choke size results in the opposite effect. Thereafter, changes in choke size result in a discontinuous curve of flow rate as a function of time. Under these challenging conditions, applying DCA methods to fit rate-time curves is not straightforward.
In this study, we first investigate the possibility of normalizing rates by choke size. To do so, we invoke an empirical equation that relates production flow rates to choke size. This equation is often referred to as Gilbert’s equation. Results show that choke size normalization removes the discontinuities observed in gas rates over time. A Duong model is then fitted to the normalized rate curve to forecast production and to calculate the EUR. Several examples from the Vaca Muerta shale demonstrate the successful application of this simple, fast, and practical technique in modeling hydrocarbon production and in evaluating reserves. The dataset under study consists of 16 horizontal gas-producing wells drilled in two neighboring regions. These wells targeted the lower layers of the Vaca Muerta shale that are referred to as lower Vaca Muerta and Cocina. Results show that production models change based on the location of the wells and based on the targeted shale layers. To put it in a nutshell, this paper presents a novel method for improved production forecasting and reserves estimation by considering the effects of choke size changes on hydrocarbon flow rates.