A real-world impact of offset frac-hits by rate transient analysis in the Bakken and Three Forks, North Dakota, USA

Abstract

Production via multi-fractured horizontal wells with specific spacing units is a common development plan in shale plays. Commonly, operators drill and complete a single well in order to hold a 1280-acre spacing unit and once the acreage is secured across the asset, infill wells at each spacing unit are drilled so, depletion from the original (parent) well can be observed within the spacing unit. This will increase the likelihood of existing wells to experience inter-communication when infill wells are hydraulically fractured which has detrimental effects. Hence, understanding the effects of well timing and spacing on the performance of the overall spacing unit is critical to choose an appropriate development plan. In this regard, rate transient analysis (RTA) is an effective way to quantify the impact of offset frac-hits, hinting the changes in reservoir properties such as stimulated reservoir volume (SRV) and well productivity. In this real-world study, we used pseudo normalized pressure versus material balance square root of time plots to determine the impact of offset frac-hits on existing wells in Williston Basin, ND. The slope of the superposition time plot is inversely proportional to the product of the contacted surface area (Ac) and the square root of reservoir permeability (k), Ack, which was a good metric to evaluate early time productivity and completion effectiveness. Additionally, production look-back performed on 71 operating wells using decline curve analysis (DCA) confirmed changes in reservoir properties and production performance to determine appropriate well spacing and infill timing. Next, economic analysis for 71 wells and 13 spacing units, using a reasonable/favorable commodity pricing, to assess the investment efficiencies of each project was carried out. Results showed that offset frac-hits cause up to 50% estimated ultimate recovery (EUR) loss, up to 80% water-cut increment, and more than 40%, Ack reduction for parent wells. Wells with multi-sleeve single ball (MSSB) completion system would have 32% lower cumulative oil production per lateral foot and 50% lower EUR than open hole sliding sleeve and cemented liner plug and perf (P-n-P). Finally, more proppant and fracturing fluid volume loading not only will not increase the production, but also deteriorates well performance. Collectively, this study indicated that 5–6 wells per 1280-acre spacing is the optimal well spacing unit within the study area to set a guideline for successful future operations. Ultimately, the very detailed field practical aspects presented here which is limited in the literature and combined with theories of RTA, can become a benchmark for further verification of mathematical models.