Stress Variations Controlled by Rock Composition in an Appraisal Well in the US: Evaluating Lithology, Stress Measurements, and Methods

ABSTRACT: We evaluate stress and lithology relationships, compare least-stress test methods, and assess stress-from-logs empirical equations, from a recently drilled and tested appraisal vertical well in the continental US. We perform multiple least-stress tests in several intervals spanning multiple lithologies, and including containment, injection, and producing zones. The presence of carbonates, including limestones and dolomites, is a better predictor of high fracture gradient than any other lithology. The least-stress magnitude in the salts is very close to the overburden stress. The shales exhibit the lowest least-stress magnitude among all lithologies, and when the shale intervals are compared to each other, the carbonate content seems to control the trend. The least-stress magnitude in sandstones is highly variable. The commonly used Eaton's equation performs relatively poorly predicting stresses in the non-shale lithologies, but the extended Eaton's equation, using Biot's coefficient equal to one, compares adequately to the stress calibration points in most lithologies. The relatively recent viscoelastic stress relaxation equation applied to shales performed slightly better than the Eaton's equation but laboratory data or applicable correlation data from similar lithologies/basins are needed to confirm this trend. Differences in the stress magnitudes from the least-stress test methods are probably related to poroelastic effect, test order, and operational related issues. 1. INTRODUCTION Predicting in-situ stresses in sedimentary basins has significant value to the Petroleum and Carbon Capture and Storage (CCS) industry. In particular, knowledge of the magnitude of the least principal stresses has been used to prevent fracturing the cap rock in CCS projects, identify hydraulic fracture containment in Unconventional plays, and reduce wellbore stability issues during drilling operations, among other applications. Stress contrast, in sedimentary basins, is often observed from layer to layer within intervals where the mineral composition changes. These interlayer principal stress variations have been measured and reported before (i.e., Anderson et al. 1973, Hickman et al. 1985, Warpinski 1989, Ma and Zoback 2020), and at least two explanations have been proposed: elastic properties contrast and stress relaxation within shales and clay-rich intervals. Interestingly, in some basins, shale and clay-rich formations show higher stresses than adjacent formations such as the Piceance basin in Colorado (Warpinski et al., 1985) and the Paris basin in France (Magnenet et al., 2017), but in others, the stresses in shales are comparatively low such as the Canning basin in Australia (Bailey et al., 2021).