Rock quality index for Niobrara horizontal well drilling and completion optimization, Wattenberg field, Colorado

Abstract

The Niobrara Formation in Wattenberg Field has been successfully drilled over the last several decades, but largely from vertical wells. Successful horizontal well drilling in Wattenberg began in 2009, where operators quickly discovered lateral wells yielded drastically greater returns on hydrocarbon production. This discovery in the field set off a new round of drilling. Subsequently, major lateral advancements have occurred within the field over the last decade, including increasing lateral lengths, stage densities, and perforation clusters for hydraulic stimulation. While fracturing the reservoir is costly, the benefits include a substantial increase in initial production within tight shale reservoirs, such as the Niobrara Formation at Wattenberg Field. By stimulating as much of the reservoir as possible along the lateral wells, operators are neglecting the conditions that exist at the wellbore, in terms of rock quality and stresses. Modern analogs in major unconventional reservoirs, including the Eagleford and Montney Shale, have discovered that initial production increases as wells are designed based on the in-situ conditions present at the wellbore. By accounting for rock quality and stresses, communication between near and far fracture networks is improved as the reservoir heterogeneity is recognized and better understood. Chalk and marl benches within the Smoky Hill Member of the Niobrara are extremely variable. I combined various parameters that define each of the Niobrara chalk and marl benches in a Rock Quality Index (RQI). This methodology was developed in order to incorporate the depositional, petrographic, and hydraulic state of the reservoir rock. The RQI is a measurement of what rock is worth targeting and where the rock is most likely to fail when hydraulically stimulated. As the RQI was developed in vertical wells with core data, lateral targets were discovered not only in the chalk benches, but also in the marl benches of the Niobrara Formation. In order to successfully apply the RQI to lateral wells where available well logs were limited, cluster analysis and neural networks were designed in order to incorporate synthetic logs within this methodology. Geometrically spaced stages and perforation clusters along lateral wells are shown to affect the completion workflow. Areas of high heterogeneity within a single stage combine differentiating rock quality and stresses. Selectively spacing the stages and perforation clusters aids in targeting zones of homogenous rock types that will exhibit similar completion parameters throughout the stage. Additionally, as the RQI was validated with the available completion and seismic data, driving mechanisms for production within the study area were explored. Lateral wells that are further spaced apart, exhibit clustered microseismic trends along the wellbore that correlate directly with increasing RQI per stage (in the absence of intersecting faults). For the case where lateral wells are tightly spaced and faults are critically stressed (as they are aligned with maximum horizontal stress), factors greater than the rock quality are controlling production. These factors consist of critically stressed faults, fractures, and pressure compartmentalization.