The recent slump in oil prices has given rise to a new term—drilled uncompleted (DUC) wells. In 2013 and 2014, when oil prices were above USD 100 a barrel, rates of return (ROR) from most unconventional plays were in the range of 15% to 50% depending on the quality of rock and the operator's portfolio in the basin. When oil prices fell drastically in November 2014 and continued its collapse, operators started building significant inventories of wells that were drilled but not completed. Most of these wells were drilled to honor drilling contracts, lease obligations, and promises made to investors to maintain production levels. The total number of DUC wells in the US is estimated to be greater than 5,000. The objective of this paper is to address key challenges associated with DUC completions when they are eventually fractured and brought online for production. The paper addresses four main concerns that can have significant impacts on productivity of DUC wells. These concerns include fracture hits (well interference), reservoir quality (hydrocarbon drainage), multiple horizons (zone connectivity), and well spacing (high-density drilling). In case studies, real-time observations made from wells were used to validate predictions from forward-looking fracture and production models. Four guidelines to effectively develop DUC wells emerged from the study.
First, fracture hits have been commonly observed in all unconventional plays throughout the US, with the effects on offset wells being mixed. Some fracture hits result in a positive uptick in production from offset wells whereas other fracture hits affect production negatively in the form of increased water cut, reduced wellhead pressure, etc. Understanding fracture hits and their influences on other wells is very critical to optimize completion designs in DUC wells to avoid any detrimental impacts or leverage positive effects on production.
Second, reservoir quality decides how much oil in place is available for the DUC wells to drain, which, in turn, depends on a variety of factors including, but not limited to, length of production history and parent completion geometries from offset wells. The longer the productive half-length and the number of years nearby wells have been on production, the lower the volume of oil available to produce for the DUC wells. In such cases, completion designs should be optimized to create more closely spaced, short fractures to accelerate hydrocarbon recovery.
Third, in basins where there are multiple producing horizons or formations, fracture height growth and interference between adjacent formations can result in asymmetric fracture propagation towards depleted zones. The longer these wells completed in adjacent formations have been on production, the greater the extent of asymmetry will be. Addressing this concern requires a good understanding of drainage patterns from offset wells and evaluating its impact on fracture geometries in DUC wells. Additionally, repressuring parent wells through recompletions to reduce asymmetric effects should be considered.
Fourth, in areas with high-density drilling where spacing between wells is 660 ft or less, the combination of longer production and fracture stages with multiple perforation clusters per stage can leave very little oil available to produce for the DUC well. The paper discusses the drastic decline trends in production rates in such cases due to a combination of lower pore pressure gradient (from depletion) and resistance to fluid flow due to fracture interference. All four factors described above have been demonstrated in this paper in the form of different case studies in the Williston basin to provide the readers with a much broader view of the major challenges that will be critical to understand when these DUC wells are completed.