Rate transient analysis (RTA) of Chalk Bluff wells in Hereford field to determine stimulated formation permeability of northern DJ Basin Niobrara and Codell formations

Abstract

Shale formations have very low permeabilities, thus multistage hydraulic fracturing is used to make themeconomical. Interpretation of production data of hydraulically fractured wells brings value in evaluation of fracture treatments and well performance. It is common to use rate transient techniques to analyze production data from unconventional shale reservoirs. These techniques were based on the single-phase, linear-flow theory which uses rate-normalized pressure versus square root of time in the linear flow regime of the data. Because produced well fluids usually consist of water, oil and gas, rate transient analysis (RTA) needs to be modified to handle the multiphase flow regime. The RTA assesses the performance of hydraulically fractured horizontal wells by providing information about the product of formation effective permeability and total mobility and, similarly, the product of hydraulic fracture conductivity and total mobility. Formation effective permeability is the stimulated matrix permeability of the formation (km) modified by the newly created micro- and macro-fractures via stresses caused by the hydraulic fracture stages. In this thesis I studied the Chalk Bluff area located in the Hereford Field, which consisted of 28 horizontal wells producing from Niobrara and Codell formations, and I applied RTA to quantify reservoir performance. The flow in Niobrara and Codell formations is multiphasic. During the analysis, two observations are expected. First, effective formation permeabilities are expected to be high due to the extensive micro- and macro-fracture network as a byproduct of multistage well treatment. Otherwise, low effective formation permeabilities would indicate that stimulation was ineffective or that depletion took place in the pay zones. Second, rapid decline of production rates is expected during the initial period of production for a short amount of time followed by a smoother and gentler slope for a longer time span. The RTA interpretations are used for ranking the well performance in the study area and to relate this ranking to the geological setting of Niobrara and Codell formations. Furthermore, the RTA results will be used to assess parent-well relationships and stimulation methods. The findings of this study are intended to propose an optimal-well spacing to enhance future production performance.