In the Williston Basin, thin reservoirs coupled with large stimulation jobs result in large vertical hydraulic fractures and out-of-zone contribution of fluids to the wells. To understand the extent of vertical fracture growth and the source of fluids reaching the wellbore, the time-lapse elemental and isotopic composition of produced waters were compared with the in-situ pore water chemistry reconstructed from core analysis (residual salts analysis (RSA)) for a set of wells in Williams county, ND.
Residual salts analysis was performed on 28 core plugs from the Lodgepole (LP), Upper Bakken Shale (UBS), Middle Bakken (MB), Lower Bakken Shale (LBS), and Three Forks (TF). RSA data indicate that the sampled formations have distinct fingerprints, predominantly in terms of strontium abundance [Sr] and strontium isotopic compositions (87Sr/86Sr). Once baseline compositions for all formations were established, time-lapse produced water samples were taken from two lateral wells (1MB and 1TF; high-impact stimulation) proximal to the baseline RSA data. Time-lapse water chemistry from both lateral wells indicates that from initial flowback through 7 months of production >80% of the produced water is sourced predominantly from the TF with minimal water contribution from other formations. Large compositional changes in the produced water within this time-period are caused by operational disturbances and/or changes in flow rate.
Preliminary, these data suggest that high-impact stimulation results in large vertical hydraulic fractures that stay open for at least 7 months resulting in produced water being dominated by a TF source. Based on produced water data from older wells with lower-impact completions, the relative contribution of water from the TF diminishes over time indicating continued, but diminished communication with the TF. Results from this study also have implications about irreducible and critical water saturations, which both have critical impact in reservoir models. A comprehensive understanding of the origins of fluids from different subsurface storage units improves well stimulation and production programs and ultimately, well economics.
Organic-rich mudrocks (ORM) from the Brushy Canyon Formation in west Texas were deposited in the Middle Permian during the Guadalupian epoch in the Delaware Basin. Brushy Canyon ORM were examined for Re-Os isotope systematics with a goal of constraining their depositional age, the 187Os/188Os value of seawater at their time of deposition, and to examine how Re and Os partition into organic material in ORM. For these samples, Rock-Eval pyrolysis data (HI: 228-393 mg/g; OI: 16-51 mg/g) indicates predominantly Type II marine kerogen with minor contributions of Type III terrestrial organic matter. Rhenium and osmium abundances correlate positively with HI, and negatively with OI, which are proxies for organic matter type and degree of preservation. These data are consistent with previous work that indicates Re and Os abundances are controlled by the availability of chelating sites in the kerogen. Brushy Canyon Formation samples have (total organic carbon) TOC values between 0.97 and 4.04% and show a strong positive correlation with both Re and Os abundances, consistent with correlations between these parameters in other ORM suites. The positive slopes in these correlations are distinct between marine (higher slopes) and non-marine (lower slopes) lacustrine environments of deposition. The Brushy Canyon’s steep slopes are consistent with marine deposition of its organic matter and an open-ocean non-restricted setting. The relationship to other Re-Os and TOC data sets appears to be a function of the restrictivity of marine conditions, and associated variations in reducing conditions during ORM accumulation of the Delaware Basin compared with more restricted lacustrine basins with local drawdown of Re and Os.
The Re-Os isotope systematics of ORM from the Brushy Canyon Formation yields a Model 1 age of 261.3 ± 5.3 Ma (2.0% age uncertainty; MSWD = 0.82). Within the uncertainty, this agrees with the expected Guadalupian age for this formation. This Re-Os age represents the first direct, absolute age for Guadalupian organic matter in the Delaware Basin. The initial (187Os/188Os)i = 0.50 ± 0.06 obtained by isochron regression represents the 187Os/188Os of seawater at this time. This value is significantly less radiogenic than modern day seawater (~1.06). The lower 187Os/188Os of Guadalupian seawater recorded is likely caused by a decrease in the relative flux of radiogenic Os from continental weathering due to a number of local and global climatic and tectonic changes that were occurring during this time.