Active horizontal development in the San Juan Basin provides valuable information about how older (parent) and newer (child) wells interact. The operator has performed a case study on the dynamics of reservoir pressure and impacts of operating strategies on horizontal Gallup Sandstone oil production. Pressure and production rate data from parent-child interactions was analyzed to develop correlations between parent well oil rates, parent well casing pressure drawdown, and child well production. Operator found that 12-month oil production from child wells improves by up to 114% when parent well pressure drawdown was 60% or less at the time of child well drilling. Observations also show that pressure management practices preserve reservoir pressure for longer periods of time and can improve individual well productivity by 21% compared to conventional production strategies. Original oil-in-place (OOIP) mapping has enabled accurate production forecasts and informed well spacing and development plans. Parent well impacts can be avoided in the San Juan Basin with thoughtful development strategies and pressure management during flowback.
There are currently 365 active horizontal oil wells producing from the Mancos Formation in the San Juan Basin. Horizontal wells target three distinct zones referred to in this manuscript as the ‘Gallup C’ (338 wells), ‘Gallup B’ (19 wells), and ‘Mancos Silt’ (8 wells) (Fig. 1a, Fig. 1b). The intermittent nature of development in the San Juan Basin, due to the oil price crash of 2014-2015 and the COVID-19 pandemic, has provided the opportunity for ideal case studies on parent and child well interactions. Authors have used practical and efficient approaches to maximize oil production, focusing on pressure management and original-oil-in-place mapping. These approaches are based on widely available data encouraging widespread adoption and cross-team collaboration. Best practices for optimizing production can only be developed when all aspects of the reservoir are considered.
San Juan Basin
The San Juan Basin is a sedimentary basin located in northwestern New Mexico and southwestern Colorado. Historically the San Juan Basin has experienced prolific natural gas production from various formations of Cretaceous age, particularly the shallow Fruitland Formation. In the San Juan Basin, an oil rim is defined by production from sandstones and silts within the Mancos Formation. Broadhead (2018) delves into much greater depth on the history of Mancos production within the San Juan Basin. Up to 6,500 ft of uplift and erosion in the San Juan Basin (Nelson and Condon 2008) during the Oligiocene has lead to shallow horizontal drilling depths within the Mancos (4,400 ft – 6,000 ft TVD) and subnormally pressured oil productive systems (̃0.38 psi/ft). Since 2012, modern horizontal drilling and hydraulic fracturing technologies have been applied to several horizons, most notably the Gallup Sandstone of the Mancos Formation. These technologies have unlocked hundreds of millions of barrels of oil reserves covering multiple horizontal drilling targets (Fig. 1b). Further north, where source rock maturity reaches 1.9% vitrinite reflectance (Ro), the Mancos is also horizontally productive as a dry gas target. This discussion will not address the Mancos dry gas window but see Nelson and Sonnenberg (2021) for an excellent in depth look at those reservoirs and their relation to other Mancos/Niobrara targets in the Rockies.