Abstract

Multifractured horizontal wells, while enabling commercial production from unconventional gas and light-oil reservoirs, are challenging to analyze quantitatively to obtain reservoir and hydraulic-fracture properties. Production rates and flowing pressures gathered immediately after hydraulic-fracture stimulation (flowback) and over a longer time period (on-line production) can be interpreted for hydraulic-fracture properties such as fracture surface area or half-length and fracture conductivity, but dynamic fracture properties and multiphase flow during both stages of production can complicate the analysis. Recent studies have suggested that flowback data can provide early insight into these fracture properties if high-resolution fluid rates/pressures are gathered, but the physics of the process are complex, and analytic methods for interpretation are at an early stage of development. Analytic methods for longer-term production data analysis, although better established, are still limited primarily to single-phase flow and simple fracture and reservoir behavior. Rate-transient methods can be applied to both flowback and long-term production data to quantify hydraulic-fracture properties and changes in effective hydraulic-fracture length during production. For the flowback period, simplified analytic methods have been developed for before-breakthrough production of hydraulic-fracturing fluids and after-breakthrough production of hydraulic-fracture and reservoir fluids. These methods are still under development, and early applications can be illustrated. For long-term production analysis, classic rate-transient analysis techniques, such as the square-root-of-time plot, have been modified to account for multiphase flow and stress-sensitive permeability exhibited by low-permeability gas condensate and black-oil reservoirs producing below saturation pressure. A field example consisting of a multifractured horizontal well completed in a tight-oil reservoir isused to to compare hydraulic-fracture properties derived from flowback and long-term production data. Although flowback analysis yields hydraulic-fracture half-lengths consistent with hydraulic-fracture modeling results, long-term production analysis yields much smaller fracture half-lengths, possibly because of breakthrough of gas once bubble-point pressure is reached. Additional mechanisms for effective producing fracture half-length reduction can be proposed. Another important observation is that estimation of fracture properties from long-term production analysis of a reservoir producing below saturation pressure can be in significant error if the estimates are derived from techniques assuming single-phase flow. Corrections to the analytic methods for multiphase flow yield fracture half-lengths consistent with those obtained from history matching using rigorous numerical simulation. In a preliminary analysis, the techniques under development are intended to aid hydraulic-fracture evaluation and design in tight reservoirs that exhibit complex flow characteristics. Furthermore, implications of the findings will be important to assist in designing well operations to maximize well performance.

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