Reservoir Quality and Well Response to Stimulation in Frontier Sandstones, Moxa Arch, Southwest Wyoming

Abstract

Abstract Reservoir quality and response to stimulation vary significantly in the Frontier Formation. Available geological and engineering data have been integrated with a core study to better understand Frontier reservoir properties and stimulation response. Second Frontier reservoir quality is best developed in marine upper shoreface/bar and fluvial channel depositional systems. Lower shoreface sandstones are generally non-reservoir-quality material. Flow study results support the use of crosslinked gelled-water frac fluids from pH 4.0 to pH 10.5. The major potential completion problem identified in the Frontier is fines migration of mixed-layer illite/smectite clay. Analysis of production and stimulation data from the Church Buttes/Bruff area indicates that foam fracs may yield higher initial flow rates, but crosslinked gelled-water treatments result in higher ultimate recoveries. Introduction Frontier Formation gas on the Moxa Arch of Southwest Wyoming occurs in mixed moderate-permeability "conventional" and low-permeability "tight" reservoirs. The quality of Frontier reservoirs drilled and the effectiveness of stimulation treatments performed over approximately forty years have varied significantly due to exploration and production covering a large geographic area of heterogeneous reservoirs. Historically, prediction of Frontier reservoir quality has been a serious problem for operators and service companies alike. Reservoir quality in Frontier sandstones is dependent upon depositional environment and subsequent rock-pore network modification during burial history. Zone thicknesses, pore geometries, and clay suites are variable, implying differences in porosity/permeability levels and potential well completion problems. Attempts to better understand Frontier stimulation and production results are complicated by differences in stimulation treatments and rock-pore network variations that may be only subtly indicated on the wireline log signatures. Geological aspects of the project include examination of regional geology, depositional environments, texture, framework mineralogy, cements, clay mineralogy, and porosity types. The analytical techniques utilized were porosity/permeability measurement, acid solubility, scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and liquid permeability testing. The engineering study included development of a 22 well routine core analysis database and a 500+ well stimulation and production history database. By integrating these studies, it becomes possible to draw some conclusions regarding reservoir quality, potential well completion problems, stimulation fluid selection, and production results. GEOLOGIC SETTING The Second Frontier Formation is a series of upper Cretaceous Age sandstone-siltstone-mudstone sequences deposited as eastward-prograding clastic wedges in the western Green River Basin. P. 715^